ChemWiki - User contributions [en]

Sleuthing for Mysteries

2013-02-10T18:02:14Z

Pm1510: Blanked the page

Sleuthing for Mysteries

2013-02-10T17:59:20Z

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==Act One==
===1. introduction===
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===2. the offer===
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===3. Gainful Employment===
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===4. Intolerable Cruelty===
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===5. Samantha's First Case===
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Sleuthing for Mysteries

2013-02-10T17:58:15Z

Pm1510:

=Characters=

=Setting=

=Plot=
==Act One==
===1. introduction===
===2. the offer===
===3. Gainful Employment===
===4. Intolerable Cruelty===
===5. Samantha's First Case===
==Act Two==
===1.===
===2.===
===3.===
===4.===
===5.===
==Act Three==
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==Act Four==
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==Act Five==
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Sleuthing for Mysteries

2013-02-10T17:54:53Z

Pm1510: Created page with "[Hello]"

[Hello]

Mesyltoe 3o11

2012-12-31T14:40:08Z

Pm1510: /* Lecture 6 */

=3.O11 Organic Synthesis Part II=

==Lecture 1==
===The Principles of Retrosynthesis===

Retrosynthesis is vital for rational and intelligent molecule design. We work backwards from the desired molecule (target molecule, TM) one step at a time. We disconnect a molecule into 'synthons', or synthetic equivalents. This means that we look at the charges required on the carbon (or other) atoms, and rationalise these into real molecules that will approximate that charge. For example, C- would rationalise to C-Li.

Disconnections often take place immediately adjacent to functional groups in the target molecule. Given functionality inevitably arises from many forward reactions, this is the best way. A good disconnection simplifies the target significantly.

===Amine synthesis===

The obvious amine synthesis, disconnecting through the C-N bond, has problems in that polyalkylation (undesirable) can occur. This can be avoided if the amine is less nucleophilic than the starting material: adding an amide to an epoxide takes advantage of this, and does not result in polyalkylation.

We can also use FGI reactions. For a primary amine, we use an azide (N3) and an organohalide which can then be reduced to an amino group with LiAlH4 or similar. For amines with an adjacent methylene, an acyl chloride can be used, as an amide is much less nucleophilic than an amine. The carbonyl can then by removed using lithal. However, this only works if there is an adjacent CH2 to hold the carbonyl.

Reductive amination is an alternative route, where there is no methylene to hold the carbonyl. This involves the creation and subsequent reduction of an imine by adding an amine into a carbonyl. This is then reduced using NaBH(OAc)3, milder than lithal, which allows the imine to be formed before all reduction occurs (so the carbonyl isn't instantly reduced, as it might be with lithal).

===The Curtius Reaction===

This reaction inserts a nitrogen into a C-C=O. A carbonyl with a leaving group is reacted with NaN3. This is then heated. N2 is released, and the nitrogen inserts between the carbonyl and the R group, giving an isocyanate. If worked up with water, an amine results, and if worked up with alcohol, a carbamate (that alcohol added as an ester) results. Carbamate is a useful protecting group. Boc is a carbamate.

==Lecture 2==

Carbonyl groups are hella important: they have a great deal of synthetic potential. They can be disconnected at the 2 and the 3 position, 2 being straightforward reaction at a carbonyl, and 3 being the reaction of an enolate. 1,3 dicarbonyls are particularly good for synthesis. We can make them by reacting an enolate with a carbonyl+LG. This is particularly easy if the molecule is symmetrical.

===The Claisen reaction and the Dieckmann reaction===

We all remember the Claisen ester condensation, I'm sure. As described above (give or take) an enolisable carbonyl is reacted with an ester to give a 1,3 dicarbonyl. When this occurs intramolecularly, it is called a Dieckmann reaction, and it works very well for the production of 5 or 6 membered rings. The driving force of this reaction is the removal of the proton between the carbonyls.

===β-Hydroxy Carbonyls and the Aldol Reaction===

These compounds also have 1,3 oxygenation, but at a lower oxidation level. The Aldol reaction is accomplished by treating carbonyl with a base. The resulting enolate attacks the other carbonyl, to give β-hydroxy carbonyl. This product is easily dehydrated by heating with acid or base, as the proton on the 2 carbon is very acidic. This forms an α,β-unsaturated carbonyl, also useful.

One problem in the aldol reaction is control. The two carbonyl reagents can both enolise, and some of these reagents can enolise at either side, resulting in a lot of undesirable side reactions. The ensure that there is a cross-enolisation (between two different compounds) we must ensure that only one of the compounds will enolise, only on one side, and that the other compound will act as the electrophile.

Intramolecularly, this tends not to be a problem: the most stable ring conformation will form, as the aldol reaction is reversible. This tends to result in six membered rings. For intermolecular reactions, we can use a component that will not enolise, where the α carbon is trisubstituted, part of an aryl system, or part of an ester.

If there is doubt as to which compound will enolise, we can turn one of the carbonyl compounds into a pre-formed enolate equivalent, such as a silyl enol ether (adding TIPS or TMS to the oxygen) or an enamine (attacking the carbonyl with nitrogen). This route means we know which compound will act as the nucleophile.

It is also possible to use an activating group to control the site of deprotonation. The addition of an ester group to the α position makes that proton much more acidic, meaning that it is much more readily lost, and that the enol is more likely to form on that side. The ester group can be removed by decarboxylation afterwards.

===The Mannich Reaction===

Formaldehyde might seem like a perfect carbonyl to use in these circumstances, as it cannot enolise, but it is too electrophilic, and will continue to react with the desired product. The Mannich reaction avoids this. It uses formaldehyde, secondary amine and acid catalyst. The amine reacts with the formaldehyde to give an iminium ion. This then acts as the electrophile in the reaction. The amine can subsequently be removed using MeI, to give an α,β-unsaturated carbonyl.

===1,3-Relationships of O and N===

It is also possible to achieve the 1,3-relationship between O and N that the Mannich reaction gives by other means: addition of an amine to an α,β-unsaturated carbonyl!

==Lecture 3==
===1,5 Dicarbonyl Disconnections & The Michael Reaction===

One reaction underpins most of the analysis of this connection, and it is the Michael reaction. This is the reaction of an enolate and an enone (Michael acceptor). We must be sure, as in the reactions above, that the correct compound acts as the nucleophile. We must also be sure that the enone reacts at the β position, not the carbonyl carbon. Using an activating group (such as an ester, seen above) on the enolate is the solution to both of these problems. The correct side of the carbonyl is thus enolised, and the soft nature of this stabilised enolate favours 1,4-attack. The result is a 1,5 dicarbonyl.

Related to the Michael reaction is the Robinson annulation. This has the enolate as part of a pre-existing ring. Using the Michael reaction, the 1,5-dicarbonyl that results can be reacted using the aldol reaction, to give a second adjoining ring.

===1,4-difunctionality===

1,5 and 1,3 difunctionality both correspond to the 'natural' polarity of carbonyl reagents. 1,4-difunctionalised compounds do not. Disconnecting in the middle gives one enolate equivalent (which is normal) and one carbonyl synthon with a positive charge at the α position, which is quite unexpected, and not what carbonyls are supposed to do at all. This is equivalent to a halocarbonyl (amongst other things). Again, it is important to ensure that the correct reagent enolises. This is accomplished, as above, using an ester activating group. This is especially important because the halide increases the acidity of the α protons, meaning they are more easily enolisable.

Allyl halides provide an alternative to α-carbonyl cations. The bromoallyl is reacted with the nucleophile (the enolate in these cases) and is then converted to a carbonyl compound via ozonolysis. This removes some of the problems with enolisation seen above. This technique is called functional group addition, because we are adding a functional group (in this case, the alkene) knowing that it is removable.

===1,4 Disconnection at a lower oxidation level===

It is also possible to disconnect in the centre of 1,4 difunctionalisation if the functional groups are carbonyl and alcohol. In this case, the carbonyl synthon is still an enol, and the alcohol sython is an epoxide. We make epoxides, as I'm sure we all remember, using mCPBA (meta-chloroperoxybenzoic acid) on an alkene.

===Alternative 1,4-disconnection===

The disconnection doesn't have to be made in the centre of the 1,4 gap, but even if it is made at the 3-4 position, unnatural polarity is still present in the synthons. One synthon has an α,β unsaturated carbonyl, which is fine, but the other has an acyl anion, with the negative charge located on the carbonyl carbon: this is quite the opposite of what we'd expect.

There are several synthetic equivalents of the acyl anion. 1,3-dithianes can be made from aldehydes, deprotonated with strong bases, and then reacted with electrophiles. The carbonyl can be regenerated by Hg2+ catalysed hydrolysis. Cyanide anions also work. They can be converted to carboxylic acids by hydrolysis, to aldehydes by DIBALH at low temp. or to a ketone by adding a Grignard followed by hydrolysis.

===Alternative Strategy: Alkynes===

Terminal alkynes can be deprotonated. The resulting anion can be reacted with electrophiles. Alkynes can be partially reduced to alkenes, so we can apply this analysis to 1,4 dioxygenated targets with an alkene in the middle as well. Using an alkyne anion on one side, our other synthon is a carbonyl cation, which is something we're all quite familiar with. We use base to deprotonate both reagents, and can then reduce for alkene functionality.

==Lecture 4==
===Alkenes===

Some of the most reliable methods for creating 1,2-difunctionality come from alkenes. They are particularly useful because of their stereospecificity.

We can functionalise alkenes using OsO4 to give syn-dihydroxylation. Epoxide ring opening can be used to give anti-dihydroxlation. The problem with epoxides is knowing which side will be attacked by the nucleophile. With good nucleophiles, the mechanism is SN2 like, and reaction will occur at the less hindered end. With weak nucleophiles, and in the presence of acid, opening occurs with a more SN1 mechanism, meaning that the most stabilised end opens.

===Strategies for Alkene Synthesis===

Elimination reactions can be stereospecific, such as E2, which requires the H and the leaving group to be app to one another. Single stereoisomers of the starting product, however, are difficult to obtain, and regiocontrol is also difficult: which β-hydrogen is eliminated?

Alkyne reduction is powerful, because the stereospecificity can be chosen: Lindlar catalyst and H2 gives cis, whilst Na/NH3 gives trans. Non-terminal alkynes are easily synthesised, which is good. The other functionalisation in the molecule has to withstand the reduction, however, and alkyne reduction gives only disubstituted alkenes.

===Carbonyl Olefination===

Carbonyl olefinations (that is, swapping a carbonyl for alkene functionality) are widely used and very useful. They are regiospecific, as the alkene ends up where the carbonyl was in the starting material, and stereocontrol can often be achieved. The reagents are a carbonyl compound and an anion with a stabilising leaving group (such as a phosphinium, phosphonate or sulfone) and the desired R groups.

===Wittig===

An organohalide is reacted with PPh3, to give a phosphonium yllid (positive charge on the P). This is reacted with the carbonyl to give the alkene product. The driving force of the reaction is the formation of the strong P=O bond. Stereocontrol depends on the R groups on the yllid. If the R groups are alkyl, the Z product predominates. If the R groups are anion stabilising groups (electron withdrawing), E predominates. Otherwise, mixtures happen.

===Julia Olefination===

To get E-alkenes when the R groups are alkyl, we use the Julia olefination. The stabilising leaving group in this case is a sulfone (sometimes a phenyl sulfone, in the classical reaction, something a heterocyclic sulfone, in the modified reaction). The sulfone and the hydroxyl are then eliminated using Na(Hg) amalgam, to give E selectivity. In the modified reaction, the amalgam is not required. The sulfone-R anion is reacted with the carbonyl compound, and eliminates spontaneously, removing the need for additional steps.

===α,β-Unsaturated Esters===

α,β-unsaturated esters are common synthetic intermediates. One reason for this is that they can be reduced to give allylic alcohols, the reagents needed for Sharpless asymmetric epoxidation. The unsaturated esters are reduced using DIBALH, and then undergo the asymmetric epoxidation.

We can use the Wittig reaction to make E isomers in this way, but the reaction rate is rather slow. A faster way of doing this is to use phosphonate as the anion-stabilising leaving group rather than phosphine. This is called the Horner-Wadsworth-Emmons reaction. They are more nucleophilic, meaning a higher rate, but still have E-selectivity.

To make Z isomers for this purpose, we can use two special phosphonates that have been designed to give this selectivity. Instead of Et groups on the phosphonate, these have electron withdrawing groups: trifluoroethyl for the Still phosphonate, and phenyl for the Ando phosphonate.

===Alkene Metathesis===

Alkene metathesis employs metal carbene complexes to switch R groups between alkenes. Ruthenium complexes, due to their air-stability and their tolerance of oxygen functional groups, are most often used.

Ring closing metathesis is a variant of this. This is an intramolecular metathesis reaction, reacting two alkenes in the same molecule to close a ring. This can often be more effective than, for example, intramolecular carbonyl olefination (such at in the Dieckmann reacton) because there is no need for selective deprotonation.

Cross-metathesis is also becoming more common. The R groups must be chosen carefully to avoid side reactions, such as homodimerisations, and control of E/Z geometry can be tricky. Reacting terminal aliphatic alkenes with acrylates (CO2R) is a successful, E-selective process.

===Trisubstituted Alkenes===

Some of the above methods work well for trisubstituted alkenes, and are widely used, such as the Horner-Wadworth-Emmons phosphonate synthesis of α,β-unsaturated esters.

==Lecture 5==
===Cross Coupling Reactions===

Cross coupling reactions are the transition metal catalysed formation of bonds to sp2 carbons. One one hand is an alkene, one substituent of which is either a metal or a leaving group. The other reagent is the group that we seek to add to the alkene, attached either to a metal or a leaving group (there must be one of each, obviously!). The reaction is catalysed by a transition metal, usually palladium. The group attached to the leaving group must not have β-hydride available for elimination (as β-hydride elimination will occur on the catalyst, and the coupling will not occur). This means that sp2 halide aryls are popular substrates.

Pd(0) is the catalytically active species in the reaction, but it's rather expensive. Pd(II) is much cheaper, and thus it needs reducing. A reducing agent, such as Et3N is used in situ to activate the catalyst. The amounts of Pd needed are very small.

===Suzuki reaction===

The Suzuki reaction, with a boronic ester/acid on one side (such as pinacolborane) and a halide on the other is very popular. The reagents aren't as nasty as in Stille coupling, and boronic acids are readily commercially available.

These reactions are stereospecific. If the reactants are both E, then the product will be E at both centres. The original stereochemistry of the reagents is preserved.

The boronic ester can be added either by metallation (using BuLi to deprotonate and then adding), hydroboration of an alkyne or crosscoupling (pinacolborane dimer with desired adduct-LG and Pd catalyst).

===Heck reaction===

The Heck reaction is a cross-coupling that involves joining a halogenated reagent with an alkene of no particular functionality. The regioselectivity is strongly influenced by the alkene structure and the Pd/ligand used. Electron poor alkenes give exclusively linear functionality. Electron neutral alkenes give more linear in the presence of triphenylphosphine, and more branched in the presence of dppp. Electron rich alkenes give mixtures in the presence of triphenylphosphine and exclusively branched in the presence of dppp. If there is only one β-hydrogen, the elimination can only go one way. If there is more than one, often the more stable alkene is predominant. Also, for elimination to occur, Pd must be syn to the hydrogen. This can effect chemistry particularly on a ring.

==Lecture 6==
===Pericyclics and Synthetic Strategy===

Pericylic reactions (reaction that proceed via a concerted cyclic mechanism) are powerful synthetic tools. They have high levels of regio- and stereoselectvity, and are stereospecific because of the orbital symmetry rules that control them. We'll look mostly at cycloadditions ([4+2], [3+2] and [2+2]) and [3,3] sigmatropic rearrangements.

===The Diels-Alder Reaction===

The Diels-Alder reaction is a [4+2] cycloaddition. It's a concerted cycloaddition between a diene (which must be able to adopt a cis conformation) and a dieneophile (an alkene or alkyne with an electron withdrawing group). The electron withdrawing group is ortho or para directing, meaning that these isomers are the major products. Because the cycloaddition is concerted, the configuration of diene and dienophile are retained in the product. It is also (often) endo-selective. The double bond in a Diels-Alder product can undergo further reactions, which makes it pretty good (such as syn hydroxylation and ozonolysis). It can also occur intramolecularly, which is a powerful way of generating two rings in a single reaction.

The Diels-Alder reaction is not confined to C-C bond creation. It can also be used to create heterocycles.

===[3+2] Cycloadditions===

These reactions, involving 1,3-dipole reagents, create five membered rings. We saw these used to create heteroaromatics.

===[2+2] Cycloadditions===

Cyclobutane can be synthesised via a [2+2] cycloaddition, which can be either thermally or photochemically activated. Ketenes can also undergo thermal [2+2] cycloadditions.

===[3,3] Sigmatropic Rearrangements===

The Claisen rearrangement is a bond moving from the 3,4 position to the 1,6 position. The 1,2 and 5,6 bonds must both be double bonds in order for this to occur. In addition, the 3 atom is an oxygen that goes from ether to carbonyl. This is usually a one-way arrangement because of the strength of the C=O bond. LDA is used to effect a Claisen rearrangement. A Claisen rearrangement can be recognised by a γ,δ-unsaturated carboxylic acid derivative.

The Cope rearrangement is the same, but there is no oxygen. This makes the reaction more reversible, as there is no strong driving force. The oxy-cope is a variation of this, where the 3 position has a hydroxy group on it. The rearrangement turns this hydroxy into an enol, which then isomerises to a ketone. The Cope rearrangement can be recognised by a δ,ε unsaturated carbonyl.

Mesyltoe 3o11

2012-12-31T14:18:21Z

Pm1510: /* Lecture 5 */

Mesyltoe 3o11

2012-12-31T13:19:50Z

Pm1510: /* α,β-Unsaturated Esters */

Mesyltoe 3o11

2012-12-31T13:00:15Z

Pm1510: /* Cross Coupling Reactions */

Mesyltoe 3o11

2012-12-31T12:39:24Z

Pm1510: /* Lecture 4 */

Mesyltoe 3o11

2012-12-31T12:28:37Z

Pm1510: /* 1,4-difunctionality */

Mesyltoe 3o11

2012-12-31T12:09:49Z

Pm1510: /* The Mannich Reaction */

Mesyltoe index

2012-12-31T11:35:16Z

Pm1510:

Mesyltoe is a student-made wiki summary of Imperial Chemistry lecture courses. It was made at Christmas, hence the pun, and is designed as a revision tool, both for the writers and the readers. Images, where uncredited, are drawn from the lecture notes distributed with the course, or created by the author.

==IIIA==

* 3.I1 Inorganic Mechanisms & Catalysis, taught by Dr. George Britovsek
* 3.I2 [https://wiki.ch.ic.ac.uk/wiki/index.php?title=Mesyltoe_3I2 Advanced Main Group Chemistry], taught by Dr. Paul Lickiss
* 3.I3 [https://wiki.ch.ic.ac.uk/wiki/index.php?title=Mesyltoe_3I3 Advanced Transition Metal Chemistry], taught by Dr.s Silvia Diez-Gonzalez and James Wilton-Ely

* 3.O3 [https://wiki.ch.ic.ac.uk/wiki/index.php?title=Mesyltoe_3O3 Polymers, The Essential Guide], taught by Dr. Joachim Steinke
* 3.O4 [https://wiki.ch.ic.ac.uk/wiki/index.php?title=Mesyltoe_3O4 Introduction to Organic Physical Chemistry], taught by Prof. Iain McCulloch
* 3.O11 [https://wiki.ch.ic.ac.uk/wiki/index.php?title=Mesyltoe_3o11 Organic Synthesis: Part 2], taught by Prof. Donald Craig
* 3.O12 An Introduction to Reaction Stereoelectronics, taught by Prof. Alan Spivey

* 3.P3 [https://wiki.ch.ic.ac.uk/wiki/index.php?title=Mesyltoe_3P3 Molecular Reaction Dynamics], taught by Dr. Laura Barter
* 3.P9 [https://wiki.ch.ic.ac.uk/wiki/index.php?title=Mesyltoe_3P9 Photochemistry], taught by Prof. James Durrant and Dr. Saif Haque
* 3.P11 Statistical Thermodynamics, taught by Dr. Fernando Bresme

Mesyltoe 3o11

2012-12-31T11:12:23Z

Pm1510: /* Lecture 2 */

Mesyltoe 3o11

2012-12-31T10:36:40Z

Pm1510: Created page with "=3.O11 Organic Synthesis Part II= ==Lecture 1== ===The Principles of Retrosynthesis=== Retrosynthesis is vital for rational and intelligent molecule design. We work backwards ..."

Mesyltoe 3I2

2012-12-30T22:38:17Z

Pm1510: /* Anions */

=3.I2 Advanced Main Group Chemistry=

A summary of a lecture course given by Dr. Paul Lickiss in Autumn 2012

==Highly Reactive Compounds==
===Radicals===

There are three ways of making radicals: homolytic bond cleavage (by heat or light, particularly of halogen or peroxide bonds), oxidation of anions or reduction of cations. Radicals can abstract hydrogens (eg. from solvent), dimerise (which often has no energetic barrier) and disproportionate, one radical becoming anionic, and one cationic (or near enough). They're implicated in polymerisation (free radical polymerisation) and the destruction of the ozone layer! They're also important in noble gas chemistry.

Radicals can be stabilised either electronically (delocalisation of the radical charge) or sterically (surrounding the reactive centre with steric bulk to prevent reaction).

===Group 14 radicals===

Carbon radicals can be planar, flexible pyramidal (allowing easy inversion about the carbon) or rigid pyramidal, and go in that order in terms of stability. In contrast, Si radicals are more stable as pyramidal structures. This is because the smaller electronegativity of Si results in higher energy MOs, meaning that orbital mixing (or hybridisation) is more favourable. We can tell this using EPR spectroscopy. Higher hyperfine coupling is observed with higher orbital s character, so we can tell that the SOMO has a large quantity of s character if it has large coupling. This shows that for radicals in solution, Si is the only group 14 element that prefers a pyramidal structure.

===Dimer radicals===

Diborane radical anions are examples of radicals stabilised by sterics. When they have mesityl substituents, the repulsion between these bulky substituents keeps the solid-phase bond legnth constant. In dialane radical anions, this steric repulsion can be sufficient to break the bond. This can also occur with phosphorus compounds.

===Interhalogens, noble gas compounds and fluorocarbons===

Radical chemistry is used in the production of CFCs. Light activated chlorine is reacted with methane, giving tetrachloromethane. This is then fluorinated (using HF and SbFCl4 catalyst) and processed further to give CFCs, which as we all know, destroy the ozone layer. This occurs by radical chlorine abstracting oxygen from ozone and trapping it in water and nitrogen dioxide. But CFCs aren't all bad! Chloro-difluoromethane is thermolysed to give tetrafluoroethylene, which when polymerised gives Teflon (PTFE). Hurrah!

Interhalogen compounds are surprising, but exist. Diatomic compounds aren't so surprising, but it's possible to have heptavalent iodine (in IF7). Many of these compounds are powerful fluorinating agents. One particularly powerful is ClF3, which is a better fluorinating agent than fluorine itself. Most of these compounds are prepared from the elements in an appropriate ratio under varying conditions. The bond strengths in these compounds reflect the electronegativity difference between the compounds, so IF is the strongest bond, and BrCl the weakest. As we might expect from this, the weaker the bond, the higher the temperature required for the reaction to occur.

The structure of the complexes conforms to VSEPR. The bonding depends upon the geometry of the complex. In complexes of trigonal bipyramidal overall geometry (regardless of whether vertices are occupied by electron pairs or not) the axial bonds are 3 centre, 4 electron bonds. The equatorial, sp2 hybridised, orbitals hold bonding or lone pair electrons. In octahedral complexes, with fewer than six substituents, the atoms in the equatorial plane are bonded by 3 centre 4 electron interactions. The axial orbitals are sp hybridised, and these orbitals hold either bond or lone pairs. In octahedral complexes with six substituents, all are bonded by 3 centre 4 electron bonds.

[[File:mesyltoe_3I2_interhalogenmo.png]]

As we can see from the MO diagram, this means that along the x axis there are two bonding electrons and two non bonding electrons. This is the origin of the 3 centre 4 electron bond.

Anionic complexes are much more common than cationic complexes, owing to the high electron affinity of the halogens. I3-, I5-, I7- and I9- are all known, and form bi-linear structures (two lines with an angle in between). They are stabilised by large soft cations, like R4N+. The cations are easier to form as the group is descended, and polyatomic iodide cations are the most common, which is as we would expect given this is the least electronegative of the halogens. Strong oxidising agents are required to form these. I5+ is linear with ends 'bent' in opposite directions. I3+ has a bent shape, like water.

===Noble Gas Chemistry===

Xe and O have a similar ionisation potential, so if it's possible to create oxygen based ionic compounds, then it should be possible to create Xe based ionic compounds. Turns out, it is! XeF2 exists and is commercially available, though it is easily hydrolysed to Xe, O2 and HF. KrF2 also exists, but is unstable at rtp.

Obviously, since noble gases already have a full valence orbital, their compounds will be hypervalent. They really bond by a process of hyperconjugation, so:

[[File:mesyltoe_3i2_hyperconjugationxef4.png]]

This means that the average Xe-F bond order is 0.5.

Xenon fluorides react with lewis acid species that can act as fluoride acceptors, to give XeFnMF5 (or similar) products. XeF6 in particular attacks glass, yielding silicon fluoride (the formation of which is the driving force) and XeOF4. Xenon fluorides when hydrolysed produce XeO3, which is highly explosive, and HF. At high pH, XeO3 becomes hydroxenonicacid (?), HXeO4-. XeF2 oxidises and fluorinates simultaneously. It is more reactive in the presence of HF. The 3c, 4e bonds in XeF2 and XeF4 are strongly polarised, and positive at Xe, making Xe susceptible to nucleophilic attack by carbon species. Substitution reactions at Xe are known, but the products tend to be thermally unstable.

Xe can act as a ligand in transition metal complexes, stabilised by weakly co-ordinating anions such as Sb2F10-. KrF2 noted above can act as a strong fluorinating agent for both Xe and Au.

==Heavier Carbene Analogues==

===Carbenes===
In a carbene, the theoretical triplet state (which is the ground state) consists of two singly occupied degenerate p-orbitals, perpendicular to the linear bonding plane. In reality, the bonding is bent, and the carbon is sp2 hybridised. One of the electrons sits in one of these orbitals, whilst the other sits in a p orbital. This means that the two electrons are not degenerate, but the energy separation is smaller than the pairing energy. The bent tripet has diradical reactivity.

The triplet state of a carbene can be stabilised by enforcing linearity, by adding sterically bulky groups, delocalising the spins and by preventing addition at the para positions by using up all the carbon's valence electrons.

===Silylene===

The silylene ground state is a singlet. It is more bent than the carbene. The lone pair has more s character, and the orbitals approach an unhybridised state as the pz orbital is empty. It is both electrophilic and nucleophilic. This means it has a lot of varied rectivity, making it very industrially useful. The singlet state is so stable for SiH2 for the same reason that the Si radicals are pyramidal: the bending and orbital mixing is more favourable due to Si's lower electronegativity. The mixing would not be possible in a linear situation: it is the breaking of symmetry via bending that allows the orbitals to be of the same symmetry, and thus to mix.

Carbene analogues in the other main groups include phosphines and borylenes. Compounds similar to NHCs can be made with selenium.

===Stabilisation and reactivity===

NH substituents can stabilise all carbenes and carbene analogues (silylene, germylene, etc.) by aromatic stabilisation. These have little effect on reactivity, but a profound effect on stabilisation.

==Heavier Multiple Bonding==

===Bonding in Main Group Elements===

Double bonds have less stability the lower down the groups you go. This is because the orbitals are more diffuse, so there is less overlap of electron density, so the bonds are longer. The bond energy includes the energy required to produce a triplet, that is, the gap between a singlet and a triplet state.

The Charles-Goddard-Malrieux-Trinquier model has two different models of double bonding. Where the bond is made up of two singlet fragments, there is a high preparation energy, and this is greater than the approximated π bond energy. This results in a trans-bent double bond, with two sp2 hybridised centres leaning together for overlap between the two p orbitals and two of the sp2 orbitals. There is a smaller HOMO-LUMO gap, and a weaker double bond. Where the bond is made of two triplet fragments, the preparation energy is less than the π bond energy. The double bond is more planar, and the centres interact as we traditionally think of bonds interacting. There can be very large substituents. Any structure distortion tends to be twisting rather than trans-bending.

These different structures affect the HOMO-LUMO gap. The bent structure (where one centre has its substituents 'up', and the other 'down') has a smaller HOMO-LUMO gap than the perfectly planar structure, but a larger HOMO-LUMO gap than the 'twisted' structure. In the twisted structure, where the two substituent planes are at right angles to one another, the HOMO and LUMO become degenerate, and they are both non-bonding, singly occupied MOs.

Disilenes are structurally flexible, and can take up any of these structures depending on circumstances, such as electron density in the solvent. The HOMO-LUMO gap affects the colour of these compounds.

===Si=Si Reactivity===

Disilenes are synthesised by elimination of halide salts: a dilithiated silicon with a dihalogenated silicon, or a dihalogenated silicon with a source of lithium. Oxygen can be added to Si=Si bonds facilely, owing to the low bond enthalpy. It adds to form a dioxygen, disilicon four-membered ring, which then rearranges so that the oxygens are bridging. The Si-Si distance is shorter than a normal bong length in this structure. To create epoxide analogues, N2O is used instead. A similar phenomenon happens with any addition reaction. Because the bond is so much weaker than its C=C equivalent, the additions are much simpler. The diffuse orbitals mean that four membered rings are possible, as there is less ring strain.

Like alkenes, disilenes can also coordinate to transition metals either mono or dihaptically. Dimetallasilanes can undergo transmetallation to the same centre to produce silane-metallacycles, or η2 co-ordinated disilanes.

Diplumbenes are also possible, and take up a trans bent structure.

===Sila- and tristanna-allenes===

Sila-allenes are arene-silicon analogues, like heteroaromatics without the additional electron density. The electropositive nature of silicon allows it to exist in these compounds with only two bonds, like a carbene. Tristanna-allene is a compound made up of a ring of three tin molecules, synthesised from distannenes.

===Group 14===

Potassium graphene can be used as a halogen abstractor in the synthesis of Si-Si triple bonds. A tribromosilane is reacted with potassium graphene. This removes 2 equivalents of bromine, and dimerises the silane. If this is repeated, another four equivalents of bromine will be eliminated, and a Si-Si triple bond results. Unlike alkynes, this has a bent geometry around either silicon. This can then be hydroborated without a catalyst, or aminated. Both methods give novel syntheses of disilenes. Bullky protecting groups are necessary for these compounds to prevent dimerisation.

Digermene and digermyne complexes also exist. It is the buildup of incipient lone pairs on Ge that gives the bending. Distannyne species have also been found. The distannyne has a reversible reaction with ethylene, forming a six membered ring with two tins and four carbons. This has a possible application in ethylene/ethane separation.

Just as the above compounds require bulky substituents for stabilisation, NHCs can also be used for stabilisation. In diborane, NHC stabilisation allows the molecule to be stable at room temperature. Diborane is made from trichloroborane, KC8 and the stabilising NHC.

Digallynes have not been isolated. The bond length of some compounds suggests a triple bond, but this is probably not the case. Complexed Na+ are sandwiched by aryl rings from the bulky substituents, artificially shortening the bond, as the sodium atoms are also complexed to the gallium atoms.

The existence of a ferrogallyne (triple bond between gallium and iron) has also been suggested, due to the short bond distance, but calculations suggest that there is no π overlap. The actual interaction is a dative covalent bond.

Alkyne analogues become increasingly bent as the group is descended due to the inert pair effect, where the electrons in the s orbital are two low in energy to take part in bonding, and so remain non-bonding. The lower down the group an atom is, the higher the likelihood of the lone pair effect.

===Group 15===

Diphosphenes are made by reacting a dichlorophosphene with Mg, causing to dimerise and eliminate magnesium chloride. The bulky groups for stabilisation are, again, necessary, and the structure is bent, owing to those pesky lone pairs. When reacted with an excess of Methyl Triflate, a disphosphenium cation results, with the triflate being the anion.

Dehydrohalogenation is used to make diarsenes. This is achieved using DBU, and involves, you've guessed it, the removal of hydrogen and halogen atoms, which complex with DBU.

A Bi=Bi compound has been made, starting from BiCl3, going via a Bi-Se 6 membered ring, and after addition of phosphine (to remove the Se), the Bi=Bi compound resulted. Like the Si=Si compound, this can be reacted with oxygen to give a compound with bridging oxygens.

===Heteronuclear Double Bonds===

Silenes and germenes are classes of compound that have a main group element double-bonded to carbon. This can be achieved by light activating a silyl-carbonyl compound, causing a 'Brook rearrangement', with one of the R groups from the Si migrating to the oxygen. It can also be achieved by eliminating a salt from a silyl-alkyl (having a halogen and a leaving group, one on C, one on Si/Ge). Heating these with BuLi lithiates the halogen position, and warming again removes the lithium with the other leaving group as a salt. The lithiated position is stabilised by the electropositivity of the α atom. These silenes and germenes can dimerise to form four-membered rings (usually 'head to tail'). ROH can be added to them to give a C-H bond and an ether bond. Oxygenation can cleave the bond to give a carbonyl and a Si=O, the latter of which can form six membered rings. Silenes can be coordinated to transition metals by β-hydride elimination or by reduction. Sila-aromatics are also possible.

Heteronuclear double bonds can be between carbon and phosphorus. A phosphene reacts with an acyl chloride, causing the loss of a TMSCl, and the formation of a single P-C bond. Another R group then migrates (as in the Brook rearrangement) to the O of the carbonyl. All the above reactivity also applies to these bonds.

Silanones are particularly difficult to prepare. Silyl alcohols are reacted with silanes, and the bonding is such that there is a Si-O-Si bond. This is then reacted with either N2O or CO2 to give a silanone.

===Group 14 to Group 16 Double Bonds===

These are more difficult to achieve, as only one end can be sterically protected. It isn't impossible though. Silicon diprotectinggroup dihalide can be lithiated, then reacted with sulfur to form the fifth member of a ring. Phosphine is then used to remove the sulfur (much as it does in the Wittig reaction) and Si=S is the result. Similar reactions are possible with germanium/tin and sulfur/selenium. Lead will react with sulfur in this way. These multiple bonds undergo Diels-Alder reactions readily, unlike carbon analogues.

==='Triple' bonds===

A phosphorus carbon triple bond has been observed, effected by elimination of a silyl ether. The possibility for triple bonding decreases as you descend group 14, due to the inert pair effect. Germanium and tin only double bond to themselves, and lead will only single bond.

===Points to Remember===

* Atoms get larger and bonds weaker on descending group
* Pi interactions get weaker down a group, and are weaker than sigma interactions.
* Oligomerisation maximises strong single bonds, and minimises weak multiple bonds.
* Steric protection is necessary for weak multiple bonds.
* Bulky groups like TMS3E or heavy substituted aryls are good steric protectors.
* Synthetic routes often involve photolysis or salt elim.
* Fewer substituents around a multiple bond means they must be larger.
* Geometry around multiple bonds is influenced by lone pair character.

A final soul-crushing point:

<pre>“The classical multiple bond indicators – bond lengths and
bond strengths – have no meaning for multiple bonds in which
elements from the higher periods are involved. However,
they are valid for an exceptional element: carbon”

Grutzmacher and Fassler, quotation from Chemical
Communications, 2000, 2175-2181.
</pre>

==Anions==

===Recap: "Carbon Anions"===

We make 'carbon anions' by metallating a haloalkane/ene/whatever, usually with lithium, but sometimes with magnesium (Grignard) or copper. The electropositivity of the metal allows the carbon to behave as though negatively charged. Organolithiums, in particular, cluster to stabilise. The degree of clustering depends on how electron-rich the solvent is. Grignard reagents undergo Schlenk equilibrium, where they dimerise (with bridging halides/carbons), exchange substituents, separate and so on.

===Grignard Compounds===

β-diketoiminate ligands (like a pyridine, but with two Ns and an H in between) can be metallated with lithium, then transmetallated to a Grignard. In the presence of potassium, Grignards dimerise, bonding through the Mg. X-ray crystallography and NMR have proved that there are no hydride bridges.

Instead of magnesium, calcium and other group 2 metals can be used. They are uncommon because the metals are relatively unreactive: CaO will not dissolve in THF or ether, so must be activated. This is done using 1,2-dibromoethane, to brominate, and then sodium, to debrominate and leave the activated Ca (called Reicke-calcium, pyrophoric). This is then reacted with an alkyl halide. Organocalcium compounds have high reactivity. They have a tendency to rearrange, and to cleave ethereal solvents.

===Mixed counterions===

Mixed counterions are when two different metals (for example, lithium and magnesium) are used to make a carbon 'anionic'. An example of this is the Schlosser base, LICKOR, produced by mixing n-BuLi with t-BuOK. Its basicity is between nBuLi and nBuK. It displays higher selectivity and 'suppresses erratic side reactions'. Another example is the so-called 'Turbo Grignard', TMP lithium mixed with MgCl2. This has enhanced reactivity and is regioselective. (TMP is tetramethylpiperidine).

===Boryl anions===

All other main group anions (give or take) satisfy the octet rule. Boryl anions, having only six electrons, do not. They can be synthesised from halo-diaminoborane, metallated, which gives a diaminoboryl anion. This anion can be stabilised using an NHC analog. Diisopropylethylenediamine is treated with magnesium. It is then reacted with BBr3, which is then lithiated, giving a five membered ring with an N-B-N series and a lithiated B. This can then be transmetallated. There is a negative charge on the boron, so it has higher nucleophilicity and no electrophilic reaction are reported.

===Anionic E=E compounds===

Disilenes can be lithiated and that chemistry taken advantage of.

===Weakly co-ordinating anions & cations===

Non-coordinating anions in the condensed phase do not exist, but a similar effect can be obtained by having many weak interactions rather than one strong reaction. This means there must be negative charge over a large area, to give spread out nucleophilicity, so there is no obvious attack point. Tetra(pentafluorophenyl)borate is an example of this.

Cryptands and crown ethers turn contact ion pairs (CIPs) into solvent-separates ion pairs (SSIPs), leaving a naked anionic centre, and increasing nucleophilicity. These become weakly co-ordinating cations.

==Cations==

===Group 14 Cations===

Olah's trimethylcarbenium ion, a positively charged t-butane (stabilised through hyperconjugation) is an example of a main group cation. It ought to be possible, given Si's electronegativity, to make a similar cation from silicon. Such ions are readily observed in mass spectra, but rarely in solutions and solids. Part of this is due to leaving group ability. Many of the leaving groups that work well with carbon do not work well with silicon, as Si-X and Si-O bonds are very strong (due to the very electropositivity that makes it so theoretically promising). This means solvolysis removing a leaving group is not an effective route. Solvolysis of perchlorates (such as Ph3SiOClO3) has been found to work, however, by solvent abstraction of the chlorate portion of the molecule.

The solvent used much have low nucleophilicity: nitriles, ketones, amide, ethers and amines all act as donors. Water and alcohols react to give silanols. Low polarity solvents, such as hexane, cannot dissolve ionic species. Toluene or chlorobenzene are the solvents of choice. The anion used must be very poorly coordinating and unreactive. Halogenated carboranes or tetra(pentafluorophenyl)borate both work. The substituents at Si must be large to protect the electrophilic centre from reaction. There must be no donor groups. The Si-H bond is weaker than the C-H bond, meaning that Ph3C+X- can abstract the hydrogen successfully. The resulting complex, with Si co-ordinated to the now positively charged solvent ring, is like a Wheland intermediate.

===Heavier Group 14 Cations===

The allyl route to group 14 cations can also be used on the heavier cations. Trimesitylsilylprop-3-ene is reacted with triethylphenylsilicon. The double bond of the propene opens to bond with the other silicon, leaving a positive charge in the chain. The Si-C bond breaks to recreate a double bond, and leave a Mes3Si+ cation. Tin can also be prepared in this way.

Reactions of very sterically hindered Si gives surprising results: intramolecular arrangement or intermolecular exchange can cause bridging SiMe2 groups.

===Borocations===

Borocations are classified by the number of neutral ligands bonded to the central atom. All three kinds are +1 in charge, but a borinium has no ligands, a borenium has one, and a boronium has two.

===NHC analogues===

Phosphorus can form NHC analogous cations as part of 5 membered rings with N-P-N cluster. The P atom carries positive charge, making it ambiphilic, unlike the Group 14 examples, which are nucleophilic. Sulfur has similar chemistry, as does selenium (though both of these carry a +2 charge). These can act as atom transfer reagents.

Mesyltoe 3I2

2012-12-30T22:37:59Z

Pm1510: /* Heavier Group 14 Cations */

=3.I2 Advanced Main Group Chemistry=

A summary of a lecture course given by Dr. Paul Lickiss in Autumn 2012

==Highly Reactive Compounds==
===Radicals===

There are three ways of making radicals: homolytic bond cleavage (by heat or light, particularly of halogen or peroxide bonds), oxidation of anions or reduction of cations. Radicals can abstract hydrogens (eg. from solvent), dimerise (which often has no energetic barrier) and disproportionate, one radical becoming anionic, and one cationic (or near enough). They're implicated in polymerisation (free radical polymerisation) and the destruction of the ozone layer! They're also important in noble gas chemistry.

Radicals can be stabilised either electronically (delocalisation of the radical charge) or sterically (surrounding the reactive centre with steric bulk to prevent reaction).

===Group 14 radicals===

Carbon radicals can be planar, flexible pyramidal (allowing easy inversion about the carbon) or rigid pyramidal, and go in that order in terms of stability. In contrast, Si radicals are more stable as pyramidal structures. This is because the smaller electronegativity of Si results in higher energy MOs, meaning that orbital mixing (or hybridisation) is more favourable. We can tell this using EPR spectroscopy. Higher hyperfine coupling is observed with higher orbital s character, so we can tell that the SOMO has a large quantity of s character if it has large coupling. This shows that for radicals in solution, Si is the only group 14 element that prefers a pyramidal structure.

===Dimer radicals===

Diborane radical anions are examples of radicals stabilised by sterics. When they have mesityl substituents, the repulsion between these bulky substituents keeps the solid-phase bond legnth constant. In dialane radical anions, this steric repulsion can be sufficient to break the bond. This can also occur with phosphorus compounds.

===Interhalogens, noble gas compounds and fluorocarbons===

Radical chemistry is used in the production of CFCs. Light activated chlorine is reacted with methane, giving tetrachloromethane. This is then fluorinated (using HF and SbFCl4 catalyst) and processed further to give CFCs, which as we all know, destroy the ozone layer. This occurs by radical chlorine abstracting oxygen from ozone and trapping it in water and nitrogen dioxide. But CFCs aren't all bad! Chloro-difluoromethane is thermolysed to give tetrafluoroethylene, which when polymerised gives Teflon (PTFE). Hurrah!

Interhalogen compounds are surprising, but exist. Diatomic compounds aren't so surprising, but it's possible to have heptavalent iodine (in IF7). Many of these compounds are powerful fluorinating agents. One particularly powerful is ClF3, which is a better fluorinating agent than fluorine itself. Most of these compounds are prepared from the elements in an appropriate ratio under varying conditions. The bond strengths in these compounds reflect the electronegativity difference between the compounds, so IF is the strongest bond, and BrCl the weakest. As we might expect from this, the weaker the bond, the higher the temperature required for the reaction to occur.

The structure of the complexes conforms to VSEPR. The bonding depends upon the geometry of the complex. In complexes of trigonal bipyramidal overall geometry (regardless of whether vertices are occupied by electron pairs or not) the axial bonds are 3 centre, 4 electron bonds. The equatorial, sp2 hybridised, orbitals hold bonding or lone pair electrons. In octahedral complexes, with fewer than six substituents, the atoms in the equatorial plane are bonded by 3 centre 4 electron interactions. The axial orbitals are sp hybridised, and these orbitals hold either bond or lone pairs. In octahedral complexes with six substituents, all are bonded by 3 centre 4 electron bonds.

[[File:mesyltoe_3I2_interhalogenmo.png]]

As we can see from the MO diagram, this means that along the x axis there are two bonding electrons and two non bonding electrons. This is the origin of the 3 centre 4 electron bond.

Anionic complexes are much more common than cationic complexes, owing to the high electron affinity of the halogens. I3-, I5-, I7- and I9- are all known, and form bi-linear structures (two lines with an angle in between). They are stabilised by large soft cations, like R4N+. The cations are easier to form as the group is descended, and polyatomic iodide cations are the most common, which is as we would expect given this is the least electronegative of the halogens. Strong oxidising agents are required to form these. I5+ is linear with ends 'bent' in opposite directions. I3+ has a bent shape, like water.

===Noble Gas Chemistry===

Xe and O have a similar ionisation potential, so if it's possible to create oxygen based ionic compounds, then it should be possible to create Xe based ionic compounds. Turns out, it is! XeF2 exists and is commercially available, though it is easily hydrolysed to Xe, O2 and HF. KrF2 also exists, but is unstable at rtp.

Obviously, since noble gases already have a full valence orbital, their compounds will be hypervalent. They really bond by a process of hyperconjugation, so:

[[File:mesyltoe_3i2_hyperconjugationxef4.png]]

This means that the average Xe-F bond order is 0.5.

Xenon fluorides react with lewis acid species that can act as fluoride acceptors, to give XeFnMF5 (or similar) products. XeF6 in particular attacks glass, yielding silicon fluoride (the formation of which is the driving force) and XeOF4. Xenon fluorides when hydrolysed produce XeO3, which is highly explosive, and HF. At high pH, XeO3 becomes hydroxenonicacid (?), HXeO4-. XeF2 oxidises and fluorinates simultaneously. It is more reactive in the presence of HF. The 3c, 4e bonds in XeF2 and XeF4 are strongly polarised, and positive at Xe, making Xe susceptible to nucleophilic attack by carbon species. Substitution reactions at Xe are known, but the products tend to be thermally unstable.

Xe can act as a ligand in transition metal complexes, stabilised by weakly co-ordinating anions such as Sb2F10-. KrF2 noted above can act as a strong fluorinating agent for both Xe and Au.

==Heavier Carbene Analogues==

===Carbenes===
In a carbene, the theoretical triplet state (which is the ground state) consists of two singly occupied degenerate p-orbitals, perpendicular to the linear bonding plane. In reality, the bonding is bent, and the carbon is sp2 hybridised. One of the electrons sits in one of these orbitals, whilst the other sits in a p orbital. This means that the two electrons are not degenerate, but the energy separation is smaller than the pairing energy. The bent tripet has diradical reactivity.

The triplet state of a carbene can be stabilised by enforcing linearity, by adding sterically bulky groups, delocalising the spins and by preventing addition at the para positions by using up all the carbon's valence electrons.

===Silylene===

The silylene ground state is a singlet. It is more bent than the carbene. The lone pair has more s character, and the orbitals approach an unhybridised state as the pz orbital is empty. It is both electrophilic and nucleophilic. This means it has a lot of varied rectivity, making it very industrially useful. The singlet state is so stable for SiH2 for the same reason that the Si radicals are pyramidal: the bending and orbital mixing is more favourable due to Si's lower electronegativity. The mixing would not be possible in a linear situation: it is the breaking of symmetry via bending that allows the orbitals to be of the same symmetry, and thus to mix.

Carbene analogues in the other main groups include phosphines and borylenes. Compounds similar to NHCs can be made with selenium.

===Stabilisation and reactivity===

NH substituents can stabilise all carbenes and carbene analogues (silylene, germylene, etc.) by aromatic stabilisation. These have little effect on reactivity, but a profound effect on stabilisation.

==Heavier Multiple Bonding==

===Bonding in Main Group Elements===

Double bonds have less stability the lower down the groups you go. This is because the orbitals are more diffuse, so there is less overlap of electron density, so the bonds are longer. The bond energy includes the energy required to produce a triplet, that is, the gap between a singlet and a triplet state.

The Charles-Goddard-Malrieux-Trinquier model has two different models of double bonding. Where the bond is made up of two singlet fragments, there is a high preparation energy, and this is greater than the approximated π bond energy. This results in a trans-bent double bond, with two sp2 hybridised centres leaning together for overlap between the two p orbitals and two of the sp2 orbitals. There is a smaller HOMO-LUMO gap, and a weaker double bond. Where the bond is made of two triplet fragments, the preparation energy is less than the π bond energy. The double bond is more planar, and the centres interact as we traditionally think of bonds interacting. There can be very large substituents. Any structure distortion tends to be twisting rather than trans-bending.

These different structures affect the HOMO-LUMO gap. The bent structure (where one centre has its substituents 'up', and the other 'down') has a smaller HOMO-LUMO gap than the perfectly planar structure, but a larger HOMO-LUMO gap than the 'twisted' structure. In the twisted structure, where the two substituent planes are at right angles to one another, the HOMO and LUMO become degenerate, and they are both non-bonding, singly occupied MOs.

Disilenes are structurally flexible, and can take up any of these structures depending on circumstances, such as electron density in the solvent. The HOMO-LUMO gap affects the colour of these compounds.

===Si=Si Reactivity===

Disilenes are synthesised by elimination of halide salts: a dilithiated silicon with a dihalogenated silicon, or a dihalogenated silicon with a source of lithium. Oxygen can be added to Si=Si bonds facilely, owing to the low bond enthalpy. It adds to form a dioxygen, disilicon four-membered ring, which then rearranges so that the oxygens are bridging. The Si-Si distance is shorter than a normal bong length in this structure. To create epoxide analogues, N2O is used instead. A similar phenomenon happens with any addition reaction. Because the bond is so much weaker than its C=C equivalent, the additions are much simpler. The diffuse orbitals mean that four membered rings are possible, as there is less ring strain.

Like alkenes, disilenes can also coordinate to transition metals either mono or dihaptically. Dimetallasilanes can undergo transmetallation to the same centre to produce silane-metallacycles, or η2 co-ordinated disilanes.

Diplumbenes are also possible, and take up a trans bent structure.

===Sila- and tristanna-allenes===

Sila-allenes are arene-silicon analogues, like heteroaromatics without the additional electron density. The electropositive nature of silicon allows it to exist in these compounds with only two bonds, like a carbene. Tristanna-allene is a compound made up of a ring of three tin molecules, synthesised from distannenes.

===Group 14===

Potassium graphene can be used as a halogen abstractor in the synthesis of Si-Si triple bonds. A tribromosilane is reacted with potassium graphene. This removes 2 equivalents of bromine, and dimerises the silane. If this is repeated, another four equivalents of bromine will be eliminated, and a Si-Si triple bond results. Unlike alkynes, this has a bent geometry around either silicon. This can then be hydroborated without a catalyst, or aminated. Both methods give novel syntheses of disilenes. Bullky protecting groups are necessary for these compounds to prevent dimerisation.

Digermene and digermyne complexes also exist. It is the buildup of incipient lone pairs on Ge that gives the bending. Distannyne species have also been found. The distannyne has a reversible reaction with ethylene, forming a six membered ring with two tins and four carbons. This has a possible application in ethylene/ethane separation.

Just as the above compounds require bulky substituents for stabilisation, NHCs can also be used for stabilisation. In diborane, NHC stabilisation allows the molecule to be stable at room temperature. Diborane is made from trichloroborane, KC8 and the stabilising NHC.

Digallynes have not been isolated. The bond length of some compounds suggests a triple bond, but this is probably not the case. Complexed Na+ are sandwiched by aryl rings from the bulky substituents, artificially shortening the bond, as the sodium atoms are also complexed to the gallium atoms.

The existence of a ferrogallyne (triple bond between gallium and iron) has also been suggested, due to the short bond distance, but calculations suggest that there is no π overlap. The actual interaction is a dative covalent bond.

Alkyne analogues become increasingly bent as the group is descended due to the inert pair effect, where the electrons in the s orbital are two low in energy to take part in bonding, and so remain non-bonding. The lower down the group an atom is, the higher the likelihood of the lone pair effect.

===Group 15===

Diphosphenes are made by reacting a dichlorophosphene with Mg, causing to dimerise and eliminate magnesium chloride. The bulky groups for stabilisation are, again, necessary, and the structure is bent, owing to those pesky lone pairs. When reacted with an excess of Methyl Triflate, a disphosphenium cation results, with the triflate being the anion.

Dehydrohalogenation is used to make diarsenes. This is achieved using DBU, and involves, you've guessed it, the removal of hydrogen and halogen atoms, which complex with DBU.

A Bi=Bi compound has been made, starting from BiCl3, going via a Bi-Se 6 membered ring, and after addition of phosphine (to remove the Se), the Bi=Bi compound resulted. Like the Si=Si compound, this can be reacted with oxygen to give a compound with bridging oxygens.

===Heteronuclear Double Bonds===

Silenes and germenes are classes of compound that have a main group element double-bonded to carbon. This can be achieved by light activating a silyl-carbonyl compound, causing a 'Brook rearrangement', with one of the R groups from the Si migrating to the oxygen. It can also be achieved by eliminating a salt from a silyl-alkyl (having a halogen and a leaving group, one on C, one on Si/Ge). Heating these with BuLi lithiates the halogen position, and warming again removes the lithium with the other leaving group as a salt. The lithiated position is stabilised by the electropositivity of the α atom. These silenes and germenes can dimerise to form four-membered rings (usually 'head to tail'). ROH can be added to them to give a C-H bond and an ether bond. Oxygenation can cleave the bond to give a carbonyl and a Si=O, the latter of which can form six membered rings. Silenes can be coordinated to transition metals by β-hydride elimination or by reduction. Sila-aromatics are also possible.

Heteronuclear double bonds can be between carbon and phosphorus. A phosphene reacts with an acyl chloride, causing the loss of a TMSCl, and the formation of a single P-C bond. Another R group then migrates (as in the Brook rearrangement) to the O of the carbonyl. All the above reactivity also applies to these bonds.

Silanones are particularly difficult to prepare. Silyl alcohols are reacted with silanes, and the bonding is such that there is a Si-O-Si bond. This is then reacted with either N2O or CO2 to give a silanone.

===Group 14 to Group 16 Double Bonds===

These are more difficult to achieve, as only one end can be sterically protected. It isn't impossible though. Silicon diprotectinggroup dihalide can be lithiated, then reacted with sulfur to form the fifth member of a ring. Phosphine is then used to remove the sulfur (much as it does in the Wittig reaction) and Si=S is the result. Similar reactions are possible with germanium/tin and sulfur/selenium. Lead will react with sulfur in this way. These multiple bonds undergo Diels-Alder reactions readily, unlike carbon analogues.

==='Triple' bonds===

A phosphorus carbon triple bond has been observed, effected by elimination of a silyl ether. The possibility for triple bonding decreases as you descend group 14, due to the inert pair effect. Germanium and tin only double bond to themselves, and lead will only single bond.

===Points to Remember===

* Atoms get larger and bonds weaker on descending group
* Pi interactions get weaker down a group, and are weaker than sigma interactions.
* Oligomerisation maximises strong single bonds, and minimises weak multiple bonds.
* Steric protection is necessary for weak multiple bonds.
* Bulky groups like TMS3E or heavy substituted aryls are good steric protectors.
* Synthetic routes often involve photolysis or salt elim.
* Fewer substituents around a multiple bond means they must be larger.
* Geometry around multiple bonds is influenced by lone pair character.

A final soul-crushing point:

<pre>“The classical multiple bond indicators – bond lengths and
bond strengths – have no meaning for multiple bonds in which
elements from the higher periods are involved. However,
they are valid for an exceptional element: carbon”

Grutzmacher and Fassler, quotation from Chemical
Communications, 2000, 2175-2181.
</pre>

===Anions===
===Recap: "Carbon Anions"===

We make 'carbon anions' by metallating a haloalkane/ene/whatever, usually with lithium, but sometimes with magnesium (Grignard) or copper. The electropositivity of the metal allows the carbon to behave as though negatively charged. Organolithiums, in particular, cluster to stabilise. The degree of clustering depends on how electron-rich the solvent is. Grignard reagents undergo Schlenk equilibrium, where they dimerise (with bridging halides/carbons), exchange substituents, separate and so on.

===Grignard Compounds===

β-diketoiminate ligands (like a pyridine, but with two Ns and an H in between) can be metallated with lithium, then transmetallated to a Grignard. In the presence of potassium, Grignards dimerise, bonding through the Mg. X-ray crystallography and NMR have proved that there are no hydride bridges.

Instead of magnesium, calcium and other group 2 metals can be used. They are uncommon because the metals are relatively unreactive: CaO will not dissolve in THF or ether, so must be activated. This is done using 1,2-dibromoethane, to brominate, and then sodium, to debrominate and leave the activated Ca (called Reicke-calcium, pyrophoric). This is then reacted with an alkyl halide. Organocalcium compounds have high reactivity. They have a tendency to rearrange, and to cleave ethereal solvents.

===Mixed counterions===

Mixed counterions are when two different metals (for example, lithium and magnesium) are used to make a carbon 'anionic'. An example of this is the Schlosser base, LICKOR, produced by mixing n-BuLi with t-BuOK. Its basicity is between nBuLi and nBuK. It displays higher selectivity and 'suppresses erratic side reactions'. Another example is the so-called 'Turbo Grignard', TMP lithium mixed with MgCl2. This has enhanced reactivity and is regioselective. (TMP is tetramethylpiperidine).

===Boryl anions===

All other main group anions (give or take) satisfy the octet rule. Boryl anions, having only six electrons, do not. They can be synthesised from halo-diaminoborane, metallated, which gives a diaminoboryl anion. This anion can be stabilised using an NHC analog. Diisopropylethylenediamine is treated with magnesium. It is then reacted with BBr3, which is then lithiated, giving a five membered ring with an N-B-N series and a lithiated B. This can then be transmetallated. There is a negative charge on the boron, so it has higher nucleophilicity and no electrophilic reaction are reported.

===Anionic E=E compounds===

Disilenes can be lithiated and that chemistry taken advantage of.

===Weakly co-ordinating anions & cations===

Non-coordinating anions in the condensed phase do not exist, but a similar effect can be obtained by having many weak interactions rather than one strong reaction. This means there must be negative charge over a large area, to give spread out nucleophilicity, so there is no obvious attack point. Tetra(pentafluorophenyl)borate is an example of this.

Cryptands and crown ethers turn contact ion pairs (CIPs) into solvent-separates ion pairs (SSIPs), leaving a naked anionic centre, and increasing nucleophilicity. These become weakly co-ordinating cations.

==Cations==

===Group 14 Cations===

Olah's trimethylcarbenium ion, a positively charged t-butane (stabilised through hyperconjugation) is an example of a main group cation. It ought to be possible, given Si's electronegativity, to make a similar cation from silicon. Such ions are readily observed in mass spectra, but rarely in solutions and solids. Part of this is due to leaving group ability. Many of the leaving groups that work well with carbon do not work well with silicon, as Si-X and Si-O bonds are very strong (due to the very electropositivity that makes it so theoretically promising). This means solvolysis removing a leaving group is not an effective route. Solvolysis of perchlorates (such as Ph3SiOClO3) has been found to work, however, by solvent abstraction of the chlorate portion of the molecule.

The solvent used much have low nucleophilicity: nitriles, ketones, amide, ethers and amines all act as donors. Water and alcohols react to give silanols. Low polarity solvents, such as hexane, cannot dissolve ionic species. Toluene or chlorobenzene are the solvents of choice. The anion used must be very poorly coordinating and unreactive. Halogenated carboranes or tetra(pentafluorophenyl)borate both work. The substituents at Si must be large to protect the electrophilic centre from reaction. There must be no donor groups. The Si-H bond is weaker than the C-H bond, meaning that Ph3C+X- can abstract the hydrogen successfully. The resulting complex, with Si co-ordinated to the now positively charged solvent ring, is like a Wheland intermediate.

===Heavier Group 14 Cations===

The allyl route to group 14 cations can also be used on the heavier cations. Trimesitylsilylprop-3-ene is reacted with triethylphenylsilicon. The double bond of the propene opens to bond with the other silicon, leaving a positive charge in the chain. The Si-C bond breaks to recreate a double bond, and leave a Mes3Si+ cation. Tin can also be prepared in this way.

Reactions of very sterically hindered Si gives surprising results: intramolecular arrangement or intermolecular exchange can cause bridging SiMe2 groups.

===Borocations===

Borocations are classified by the number of neutral ligands bonded to the central atom. All three kinds are +1 in charge, but a borinium has no ligands, a borenium has one, and a boronium has two.

===NHC analogues===

Phosphorus can form NHC analogous cations as part of 5 membered rings with N-P-N cluster. The P atom carries positive charge, making it ambiphilic, unlike the Group 14 examples, which are nucleophilic. Sulfur has similar chemistry, as does selenium (though both of these carry a +2 charge). These can act as atom transfer reagents.

Mesyltoe 3I2

2012-12-30T22:23:01Z

Pm1510: /* Weakly co-ordinating anions & cations */

=3.I2 Advanced Main Group Chemistry=

A summary of a lecture course given by Dr. Paul Lickiss in Autumn 2012

==Highly Reactive Compounds==
===Radicals===

There are three ways of making radicals: homolytic bond cleavage (by heat or light, particularly of halogen or peroxide bonds), oxidation of anions or reduction of cations. Radicals can abstract hydrogens (eg. from solvent), dimerise (which often has no energetic barrier) and disproportionate, one radical becoming anionic, and one cationic (or near enough). They're implicated in polymerisation (free radical polymerisation) and the destruction of the ozone layer! They're also important in noble gas chemistry.

Radicals can be stabilised either electronically (delocalisation of the radical charge) or sterically (surrounding the reactive centre with steric bulk to prevent reaction).

===Group 14 radicals===

Carbon radicals can be planar, flexible pyramidal (allowing easy inversion about the carbon) or rigid pyramidal, and go in that order in terms of stability. In contrast, Si radicals are more stable as pyramidal structures. This is because the smaller electronegativity of Si results in higher energy MOs, meaning that orbital mixing (or hybridisation) is more favourable. We can tell this using EPR spectroscopy. Higher hyperfine coupling is observed with higher orbital s character, so we can tell that the SOMO has a large quantity of s character if it has large coupling. This shows that for radicals in solution, Si is the only group 14 element that prefers a pyramidal structure.

===Dimer radicals===

Diborane radical anions are examples of radicals stabilised by sterics. When they have mesityl substituents, the repulsion between these bulky substituents keeps the solid-phase bond legnth constant. In dialane radical anions, this steric repulsion can be sufficient to break the bond. This can also occur with phosphorus compounds.

===Interhalogens, noble gas compounds and fluorocarbons===

Radical chemistry is used in the production of CFCs. Light activated chlorine is reacted with methane, giving tetrachloromethane. This is then fluorinated (using HF and SbFCl4 catalyst) and processed further to give CFCs, which as we all know, destroy the ozone layer. This occurs by radical chlorine abstracting oxygen from ozone and trapping it in water and nitrogen dioxide. But CFCs aren't all bad! Chloro-difluoromethane is thermolysed to give tetrafluoroethylene, which when polymerised gives Teflon (PTFE). Hurrah!

Interhalogen compounds are surprising, but exist. Diatomic compounds aren't so surprising, but it's possible to have heptavalent iodine (in IF7). Many of these compounds are powerful fluorinating agents. One particularly powerful is ClF3, which is a better fluorinating agent than fluorine itself. Most of these compounds are prepared from the elements in an appropriate ratio under varying conditions. The bond strengths in these compounds reflect the electronegativity difference between the compounds, so IF is the strongest bond, and BrCl the weakest. As we might expect from this, the weaker the bond, the higher the temperature required for the reaction to occur.

The structure of the complexes conforms to VSEPR. The bonding depends upon the geometry of the complex. In complexes of trigonal bipyramidal overall geometry (regardless of whether vertices are occupied by electron pairs or not) the axial bonds are 3 centre, 4 electron bonds. The equatorial, sp2 hybridised, orbitals hold bonding or lone pair electrons. In octahedral complexes, with fewer than six substituents, the atoms in the equatorial plane are bonded by 3 centre 4 electron interactions. The axial orbitals are sp hybridised, and these orbitals hold either bond or lone pairs. In octahedral complexes with six substituents, all are bonded by 3 centre 4 electron bonds.

[[File:mesyltoe_3I2_interhalogenmo.png]]

As we can see from the MO diagram, this means that along the x axis there are two bonding electrons and two non bonding electrons. This is the origin of the 3 centre 4 electron bond.

Anionic complexes are much more common than cationic complexes, owing to the high electron affinity of the halogens. I3-, I5-, I7- and I9- are all known, and form bi-linear structures (two lines with an angle in between). They are stabilised by large soft cations, like R4N+. The cations are easier to form as the group is descended, and polyatomic iodide cations are the most common, which is as we would expect given this is the least electronegative of the halogens. Strong oxidising agents are required to form these. I5+ is linear with ends 'bent' in opposite directions. I3+ has a bent shape, like water.

===Noble Gas Chemistry===

Xe and O have a similar ionisation potential, so if it's possible to create oxygen based ionic compounds, then it should be possible to create Xe based ionic compounds. Turns out, it is! XeF2 exists and is commercially available, though it is easily hydrolysed to Xe, O2 and HF. KrF2 also exists, but is unstable at rtp.

Obviously, since noble gases already have a full valence orbital, their compounds will be hypervalent. They really bond by a process of hyperconjugation, so:

[[File:mesyltoe_3i2_hyperconjugationxef4.png]]

This means that the average Xe-F bond order is 0.5.

Xenon fluorides react with lewis acid species that can act as fluoride acceptors, to give XeFnMF5 (or similar) products. XeF6 in particular attacks glass, yielding silicon fluoride (the formation of which is the driving force) and XeOF4. Xenon fluorides when hydrolysed produce XeO3, which is highly explosive, and HF. At high pH, XeO3 becomes hydroxenonicacid (?), HXeO4-. XeF2 oxidises and fluorinates simultaneously. It is more reactive in the presence of HF. The 3c, 4e bonds in XeF2 and XeF4 are strongly polarised, and positive at Xe, making Xe susceptible to nucleophilic attack by carbon species. Substitution reactions at Xe are known, but the products tend to be thermally unstable.

Xe can act as a ligand in transition metal complexes, stabilised by weakly co-ordinating anions such as Sb2F10-. KrF2 noted above can act as a strong fluorinating agent for both Xe and Au.

==Heavier Carbene Analogues==

===Carbenes===
In a carbene, the theoretical triplet state (which is the ground state) consists of two singly occupied degenerate p-orbitals, perpendicular to the linear bonding plane. In reality, the bonding is bent, and the carbon is sp2 hybridised. One of the electrons sits in one of these orbitals, whilst the other sits in a p orbital. This means that the two electrons are not degenerate, but the energy separation is smaller than the pairing energy. The bent tripet has diradical reactivity.

The triplet state of a carbene can be stabilised by enforcing linearity, by adding sterically bulky groups, delocalising the spins and by preventing addition at the para positions by using up all the carbon's valence electrons.

===Silylene===

The silylene ground state is a singlet. It is more bent than the carbene. The lone pair has more s character, and the orbitals approach an unhybridised state as the pz orbital is empty. It is both electrophilic and nucleophilic. This means it has a lot of varied rectivity, making it very industrially useful. The singlet state is so stable for SiH2 for the same reason that the Si radicals are pyramidal: the bending and orbital mixing is more favourable due to Si's lower electronegativity. The mixing would not be possible in a linear situation: it is the breaking of symmetry via bending that allows the orbitals to be of the same symmetry, and thus to mix.

Carbene analogues in the other main groups include phosphines and borylenes. Compounds similar to NHCs can be made with selenium.

===Stabilisation and reactivity===

NH substituents can stabilise all carbenes and carbene analogues (silylene, germylene, etc.) by aromatic stabilisation. These have little effect on reactivity, but a profound effect on stabilisation.

==Heavier Multiple Bonding==

===Bonding in Main Group Elements===

Double bonds have less stability the lower down the groups you go. This is because the orbitals are more diffuse, so there is less overlap of electron density, so the bonds are longer. The bond energy includes the energy required to produce a triplet, that is, the gap between a singlet and a triplet state.

The Charles-Goddard-Malrieux-Trinquier model has two different models of double bonding. Where the bond is made up of two singlet fragments, there is a high preparation energy, and this is greater than the approximated π bond energy. This results in a trans-bent double bond, with two sp2 hybridised centres leaning together for overlap between the two p orbitals and two of the sp2 orbitals. There is a smaller HOMO-LUMO gap, and a weaker double bond. Where the bond is made of two triplet fragments, the preparation energy is less than the π bond energy. The double bond is more planar, and the centres interact as we traditionally think of bonds interacting. There can be very large substituents. Any structure distortion tends to be twisting rather than trans-bending.

These different structures affect the HOMO-LUMO gap. The bent structure (where one centre has its substituents 'up', and the other 'down') has a smaller HOMO-LUMO gap than the perfectly planar structure, but a larger HOMO-LUMO gap than the 'twisted' structure. In the twisted structure, where the two substituent planes are at right angles to one another, the HOMO and LUMO become degenerate, and they are both non-bonding, singly occupied MOs.

Disilenes are structurally flexible, and can take up any of these structures depending on circumstances, such as electron density in the solvent. The HOMO-LUMO gap affects the colour of these compounds.

===Si=Si Reactivity===

Disilenes are synthesised by elimination of halide salts: a dilithiated silicon with a dihalogenated silicon, or a dihalogenated silicon with a source of lithium. Oxygen can be added to Si=Si bonds facilely, owing to the low bond enthalpy. It adds to form a dioxygen, disilicon four-membered ring, which then rearranges so that the oxygens are bridging. The Si-Si distance is shorter than a normal bong length in this structure. To create epoxide analogues, N2O is used instead. A similar phenomenon happens with any addition reaction. Because the bond is so much weaker than its C=C equivalent, the additions are much simpler. The diffuse orbitals mean that four membered rings are possible, as there is less ring strain.

Like alkenes, disilenes can also coordinate to transition metals either mono or dihaptically. Dimetallasilanes can undergo transmetallation to the same centre to produce silane-metallacycles, or η2 co-ordinated disilanes.

Diplumbenes are also possible, and take up a trans bent structure.

===Sila- and tristanna-allenes===

Sila-allenes are arene-silicon analogues, like heteroaromatics without the additional electron density. The electropositive nature of silicon allows it to exist in these compounds with only two bonds, like a carbene. Tristanna-allene is a compound made up of a ring of three tin molecules, synthesised from distannenes.

===Group 14===

Potassium graphene can be used as a halogen abstractor in the synthesis of Si-Si triple bonds. A tribromosilane is reacted with potassium graphene. This removes 2 equivalents of bromine, and dimerises the silane. If this is repeated, another four equivalents of bromine will be eliminated, and a Si-Si triple bond results. Unlike alkynes, this has a bent geometry around either silicon. This can then be hydroborated without a catalyst, or aminated. Both methods give novel syntheses of disilenes. Bullky protecting groups are necessary for these compounds to prevent dimerisation.

Digermene and digermyne complexes also exist. It is the buildup of incipient lone pairs on Ge that gives the bending. Distannyne species have also been found. The distannyne has a reversible reaction with ethylene, forming a six membered ring with two tins and four carbons. This has a possible application in ethylene/ethane separation.

Just as the above compounds require bulky substituents for stabilisation, NHCs can also be used for stabilisation. In diborane, NHC stabilisation allows the molecule to be stable at room temperature. Diborane is made from trichloroborane, KC8 and the stabilising NHC.

Digallynes have not been isolated. The bond length of some compounds suggests a triple bond, but this is probably not the case. Complexed Na+ are sandwiched by aryl rings from the bulky substituents, artificially shortening the bond, as the sodium atoms are also complexed to the gallium atoms.

The existence of a ferrogallyne (triple bond between gallium and iron) has also been suggested, due to the short bond distance, but calculations suggest that there is no π overlap. The actual interaction is a dative covalent bond.

Alkyne analogues become increasingly bent as the group is descended due to the inert pair effect, where the electrons in the s orbital are two low in energy to take part in bonding, and so remain non-bonding. The lower down the group an atom is, the higher the likelihood of the lone pair effect.

===Group 15===

Diphosphenes are made by reacting a dichlorophosphene with Mg, causing to dimerise and eliminate magnesium chloride. The bulky groups for stabilisation are, again, necessary, and the structure is bent, owing to those pesky lone pairs. When reacted with an excess of Methyl Triflate, a disphosphenium cation results, with the triflate being the anion.

Dehydrohalogenation is used to make diarsenes. This is achieved using DBU, and involves, you've guessed it, the removal of hydrogen and halogen atoms, which complex with DBU.

A Bi=Bi compound has been made, starting from BiCl3, going via a Bi-Se 6 membered ring, and after addition of phosphine (to remove the Se), the Bi=Bi compound resulted. Like the Si=Si compound, this can be reacted with oxygen to give a compound with bridging oxygens.

===Heteronuclear Double Bonds===

Silenes and germenes are classes of compound that have a main group element double-bonded to carbon. This can be achieved by light activating a silyl-carbonyl compound, causing a 'Brook rearrangement', with one of the R groups from the Si migrating to the oxygen. It can also be achieved by eliminating a salt from a silyl-alkyl (having a halogen and a leaving group, one on C, one on Si/Ge). Heating these with BuLi lithiates the halogen position, and warming again removes the lithium with the other leaving group as a salt. The lithiated position is stabilised by the electropositivity of the α atom. These silenes and germenes can dimerise to form four-membered rings (usually 'head to tail'). ROH can be added to them to give a C-H bond and an ether bond. Oxygenation can cleave the bond to give a carbonyl and a Si=O, the latter of which can form six membered rings. Silenes can be coordinated to transition metals by β-hydride elimination or by reduction. Sila-aromatics are also possible.

Heteronuclear double bonds can be between carbon and phosphorus. A phosphene reacts with an acyl chloride, causing the loss of a TMSCl, and the formation of a single P-C bond. Another R group then migrates (as in the Brook rearrangement) to the O of the carbonyl. All the above reactivity also applies to these bonds.

Silanones are particularly difficult to prepare. Silyl alcohols are reacted with silanes, and the bonding is such that there is a Si-O-Si bond. This is then reacted with either N2O or CO2 to give a silanone.

===Group 14 to Group 16 Double Bonds===

These are more difficult to achieve, as only one end can be sterically protected. It isn't impossible though. Silicon diprotectinggroup dihalide can be lithiated, then reacted with sulfur to form the fifth member of a ring. Phosphine is then used to remove the sulfur (much as it does in the Wittig reaction) and Si=S is the result. Similar reactions are possible with germanium/tin and sulfur/selenium. Lead will react with sulfur in this way. These multiple bonds undergo Diels-Alder reactions readily, unlike carbon analogues.

==='Triple' bonds===

A phosphorus carbon triple bond has been observed, effected by elimination of a silyl ether. The possibility for triple bonding decreases as you descend group 14, due to the inert pair effect. Germanium and tin only double bond to themselves, and lead will only single bond.

===Points to Remember===

* Atoms get larger and bonds weaker on descending group
* Pi interactions get weaker down a group, and are weaker than sigma interactions.
* Oligomerisation maximises strong single bonds, and minimises weak multiple bonds.
* Steric protection is necessary for weak multiple bonds.
* Bulky groups like TMS3E or heavy substituted aryls are good steric protectors.
* Synthetic routes often involve photolysis or salt elim.
* Fewer substituents around a multiple bond means they must be larger.
* Geometry around multiple bonds is influenced by lone pair character.

A final soul-crushing point:

<pre>“The classical multiple bond indicators – bond lengths and
bond strengths – have no meaning for multiple bonds in which
elements from the higher periods are involved. However,
they are valid for an exceptional element: carbon”

Grutzmacher and Fassler, quotation from Chemical
Communications, 2000, 2175-2181.
</pre>

===Anions===
===Recap: "Carbon Anions"===

We make 'carbon anions' by metallating a haloalkane/ene/whatever, usually with lithium, but sometimes with magnesium (Grignard) or copper. The electropositivity of the metal allows the carbon to behave as though negatively charged. Organolithiums, in particular, cluster to stabilise. The degree of clustering depends on how electron-rich the solvent is. Grignard reagents undergo Schlenk equilibrium, where they dimerise (with bridging halides/carbons), exchange substituents, separate and so on.

===Grignard Compounds===

β-diketoiminate ligands (like a pyridine, but with two Ns and an H in between) can be metallated with lithium, then transmetallated to a Grignard. In the presence of potassium, Grignards dimerise, bonding through the Mg. X-ray crystallography and NMR have proved that there are no hydride bridges.

Instead of magnesium, calcium and other group 2 metals can be used. They are uncommon because the metals are relatively unreactive: CaO will not dissolve in THF or ether, so must be activated. This is done using 1,2-dibromoethane, to brominate, and then sodium, to debrominate and leave the activated Ca (called Reicke-calcium, pyrophoric). This is then reacted with an alkyl halide. Organocalcium compounds have high reactivity. They have a tendency to rearrange, and to cleave ethereal solvents.

===Mixed counterions===

Mixed counterions are when two different metals (for example, lithium and magnesium) are used to make a carbon 'anionic'. An example of this is the Schlosser base, LICKOR, produced by mixing n-BuLi with t-BuOK. Its basicity is between nBuLi and nBuK. It displays higher selectivity and 'suppresses erratic side reactions'. Another example is the so-called 'Turbo Grignard', TMP lithium mixed with MgCl2. This has enhanced reactivity and is regioselective. (TMP is tetramethylpiperidine).

===Boryl anions===

All other main group anions (give or take) satisfy the octet rule. Boryl anions, having only six electrons, do not. They can be synthesised from halo-diaminoborane, metallated, which gives a diaminoboryl anion. This anion can be stabilised using an NHC analog. Diisopropylethylenediamine is treated with magnesium. It is then reacted with BBr3, which is then lithiated, giving a five membered ring with an N-B-N series and a lithiated B. This can then be transmetallated. There is a negative charge on the boron, so it has higher nucleophilicity and no electrophilic reaction are reported.

===Anionic E=E compounds===

Disilenes can be lithiated and that chemistry taken advantage of.

===Weakly co-ordinating anions & cations===

Non-coordinating anions in the condensed phase do not exist, but a similar effect can be obtained by having many weak interactions rather than one strong reaction. This means there must be negative charge over a large area, to give spread out nucleophilicity, so there is no obvious attack point. Tetra(pentafluorophenyl)borate is an example of this.

Cryptands and crown ethers turn contact ion pairs (CIPs) into solvent-separates ion pairs (SSIPs), leaving a naked anionic centre, and increasing nucleophilicity. These become weakly co-ordinating cations.

==Cations==

===Group 14 Cations===

Olah's trimethylcarbenium ion, a positively charged t-butane (stabilised through hyperconjugation) is an example of a main group cation. It ought to be possible, given Si's electronegativity, to make a similar cation from silicon. Such ions are readily observed in mass spectra, but rarely in solutions and solids. Part of this is due to leaving group ability. Many of the leaving groups that work well with carbon do not work well with silicon, as Si-X and Si-O bonds are very strong (due to the very electropositivity that makes it so theoretically promising). This means solvolysis removing a leaving group is not an effective route. Solvolysis of perchlorates (such as Ph3SiOClO3) has been found to work, however, by solvent abstraction of the chlorate portion of the molecule.

The solvent used much have low nucleophilicity: nitriles, ketones, amide, ethers and amines all act as donors. Water and alcohols react to give silanols. Low polarity solvents, such as hexane, cannot dissolve ionic species. Toluene or chlorobenzene are the solvents of choice. The anion used must be very poorly coordinating and unreactive. Halogenated carboranes or tetra(pentafluorophenyl)borate both work. The substituents at Si must be large to protect the electrophilic centre from reaction. There must be no donor groups. The Si-H bond is weaker than the C-H bond, meaning that Ph3C+X- can abstract the hydrogen successfully. The resulting complex, with Si co-ordinated to the now positively charged solvent ring, is like a Wheland intermediate.

===Heavier Group 14 Cations===

The allyl route

Mesyltoe 3I2

2012-12-30T22:07:03Z

Pm1510: /* Grignard Compounds */

=3.I2 Advanced Main Group Chemistry=

A summary of a lecture course given by Dr. Paul Lickiss in Autumn 2012

==Highly Reactive Compounds==
===Radicals===

There are three ways of making radicals: homolytic bond cleavage (by heat or light, particularly of halogen or peroxide bonds), oxidation of anions or reduction of cations. Radicals can abstract hydrogens (eg. from solvent), dimerise (which often has no energetic barrier) and disproportionate, one radical becoming anionic, and one cationic (or near enough). They're implicated in polymerisation (free radical polymerisation) and the destruction of the ozone layer! They're also important in noble gas chemistry.

Radicals can be stabilised either electronically (delocalisation of the radical charge) or sterically (surrounding the reactive centre with steric bulk to prevent reaction).

===Group 14 radicals===

Carbon radicals can be planar, flexible pyramidal (allowing easy inversion about the carbon) or rigid pyramidal, and go in that order in terms of stability. In contrast, Si radicals are more stable as pyramidal structures. This is because the smaller electronegativity of Si results in higher energy MOs, meaning that orbital mixing (or hybridisation) is more favourable. We can tell this using EPR spectroscopy. Higher hyperfine coupling is observed with higher orbital s character, so we can tell that the SOMO has a large quantity of s character if it has large coupling. This shows that for radicals in solution, Si is the only group 14 element that prefers a pyramidal structure.

===Dimer radicals===

Diborane radical anions are examples of radicals stabilised by sterics. When they have mesityl substituents, the repulsion between these bulky substituents keeps the solid-phase bond legnth constant. In dialane radical anions, this steric repulsion can be sufficient to break the bond. This can also occur with phosphorus compounds.

===Interhalogens, noble gas compounds and fluorocarbons===

Radical chemistry is used in the production of CFCs. Light activated chlorine is reacted with methane, giving tetrachloromethane. This is then fluorinated (using HF and SbFCl4 catalyst) and processed further to give CFCs, which as we all know, destroy the ozone layer. This occurs by radical chlorine abstracting oxygen from ozone and trapping it in water and nitrogen dioxide. But CFCs aren't all bad! Chloro-difluoromethane is thermolysed to give tetrafluoroethylene, which when polymerised gives Teflon (PTFE). Hurrah!

Interhalogen compounds are surprising, but exist. Diatomic compounds aren't so surprising, but it's possible to have heptavalent iodine (in IF7). Many of these compounds are powerful fluorinating agents. One particularly powerful is ClF3, which is a better fluorinating agent than fluorine itself. Most of these compounds are prepared from the elements in an appropriate ratio under varying conditions. The bond strengths in these compounds reflect the electronegativity difference between the compounds, so IF is the strongest bond, and BrCl the weakest. As we might expect from this, the weaker the bond, the higher the temperature required for the reaction to occur.

The structure of the complexes conforms to VSEPR. The bonding depends upon the geometry of the complex. In complexes of trigonal bipyramidal overall geometry (regardless of whether vertices are occupied by electron pairs or not) the axial bonds are 3 centre, 4 electron bonds. The equatorial, sp2 hybridised, orbitals hold bonding or lone pair electrons. In octahedral complexes, with fewer than six substituents, the atoms in the equatorial plane are bonded by 3 centre 4 electron interactions. The axial orbitals are sp hybridised, and these orbitals hold either bond or lone pairs. In octahedral complexes with six substituents, all are bonded by 3 centre 4 electron bonds.

[[File:mesyltoe_3I2_interhalogenmo.png]]

As we can see from the MO diagram, this means that along the x axis there are two bonding electrons and two non bonding electrons. This is the origin of the 3 centre 4 electron bond.

Anionic complexes are much more common than cationic complexes, owing to the high electron affinity of the halogens. I3-, I5-, I7- and I9- are all known, and form bi-linear structures (two lines with an angle in between). They are stabilised by large soft cations, like R4N+. The cations are easier to form as the group is descended, and polyatomic iodide cations are the most common, which is as we would expect given this is the least electronegative of the halogens. Strong oxidising agents are required to form these. I5+ is linear with ends 'bent' in opposite directions. I3+ has a bent shape, like water.

===Noble Gas Chemistry===

Xe and O have a similar ionisation potential, so if it's possible to create oxygen based ionic compounds, then it should be possible to create Xe based ionic compounds. Turns out, it is! XeF2 exists and is commercially available, though it is easily hydrolysed to Xe, O2 and HF. KrF2 also exists, but is unstable at rtp.

Obviously, since noble gases already have a full valence orbital, their compounds will be hypervalent. They really bond by a process of hyperconjugation, so:

[[File:mesyltoe_3i2_hyperconjugationxef4.png]]

This means that the average Xe-F bond order is 0.5.

Xenon fluorides react with lewis acid species that can act as fluoride acceptors, to give XeFnMF5 (or similar) products. XeF6 in particular attacks glass, yielding silicon fluoride (the formation of which is the driving force) and XeOF4. Xenon fluorides when hydrolysed produce XeO3, which is highly explosive, and HF. At high pH, XeO3 becomes hydroxenonicacid (?), HXeO4-. XeF2 oxidises and fluorinates simultaneously. It is more reactive in the presence of HF. The 3c, 4e bonds in XeF2 and XeF4 are strongly polarised, and positive at Xe, making Xe susceptible to nucleophilic attack by carbon species. Substitution reactions at Xe are known, but the products tend to be thermally unstable.

Xe can act as a ligand in transition metal complexes, stabilised by weakly co-ordinating anions such as Sb2F10-. KrF2 noted above can act as a strong fluorinating agent for both Xe and Au.

==Heavier Carbene Analogues==

===Carbenes===
In a carbene, the theoretical triplet state (which is the ground state) consists of two singly occupied degenerate p-orbitals, perpendicular to the linear bonding plane. In reality, the bonding is bent, and the carbon is sp2 hybridised. One of the electrons sits in one of these orbitals, whilst the other sits in a p orbital. This means that the two electrons are not degenerate, but the energy separation is smaller than the pairing energy. The bent tripet has diradical reactivity.

The triplet state of a carbene can be stabilised by enforcing linearity, by adding sterically bulky groups, delocalising the spins and by preventing addition at the para positions by using up all the carbon's valence electrons.

===Silylene===

The silylene ground state is a singlet. It is more bent than the carbene. The lone pair has more s character, and the orbitals approach an unhybridised state as the pz orbital is empty. It is both electrophilic and nucleophilic. This means it has a lot of varied rectivity, making it very industrially useful. The singlet state is so stable for SiH2 for the same reason that the Si radicals are pyramidal: the bending and orbital mixing is more favourable due to Si's lower electronegativity. The mixing would not be possible in a linear situation: it is the breaking of symmetry via bending that allows the orbitals to be of the same symmetry, and thus to mix.

Carbene analogues in the other main groups include phosphines and borylenes. Compounds similar to NHCs can be made with selenium.

===Stabilisation and reactivity===

NH substituents can stabilise all carbenes and carbene analogues (silylene, germylene, etc.) by aromatic stabilisation. These have little effect on reactivity, but a profound effect on stabilisation.

==Heavier Multiple Bonding==

===Bonding in Main Group Elements===

Double bonds have less stability the lower down the groups you go. This is because the orbitals are more diffuse, so there is less overlap of electron density, so the bonds are longer. The bond energy includes the energy required to produce a triplet, that is, the gap between a singlet and a triplet state.

The Charles-Goddard-Malrieux-Trinquier model has two different models of double bonding. Where the bond is made up of two singlet fragments, there is a high preparation energy, and this is greater than the approximated π bond energy. This results in a trans-bent double bond, with two sp2 hybridised centres leaning together for overlap between the two p orbitals and two of the sp2 orbitals. There is a smaller HOMO-LUMO gap, and a weaker double bond. Where the bond is made of two triplet fragments, the preparation energy is less than the π bond energy. The double bond is more planar, and the centres interact as we traditionally think of bonds interacting. There can be very large substituents. Any structure distortion tends to be twisting rather than trans-bending.

These different structures affect the HOMO-LUMO gap. The bent structure (where one centre has its substituents 'up', and the other 'down') has a smaller HOMO-LUMO gap than the perfectly planar structure, but a larger HOMO-LUMO gap than the 'twisted' structure. In the twisted structure, where the two substituent planes are at right angles to one another, the HOMO and LUMO become degenerate, and they are both non-bonding, singly occupied MOs.

Disilenes are structurally flexible, and can take up any of these structures depending on circumstances, such as electron density in the solvent. The HOMO-LUMO gap affects the colour of these compounds.

===Si=Si Reactivity===

Disilenes are synthesised by elimination of halide salts: a dilithiated silicon with a dihalogenated silicon, or a dihalogenated silicon with a source of lithium. Oxygen can be added to Si=Si bonds facilely, owing to the low bond enthalpy. It adds to form a dioxygen, disilicon four-membered ring, which then rearranges so that the oxygens are bridging. The Si-Si distance is shorter than a normal bong length in this structure. To create epoxide analogues, N2O is used instead. A similar phenomenon happens with any addition reaction. Because the bond is so much weaker than its C=C equivalent, the additions are much simpler. The diffuse orbitals mean that four membered rings are possible, as there is less ring strain.

Like alkenes, disilenes can also coordinate to transition metals either mono or dihaptically. Dimetallasilanes can undergo transmetallation to the same centre to produce silane-metallacycles, or η2 co-ordinated disilanes.

Diplumbenes are also possible, and take up a trans bent structure.

===Sila- and tristanna-allenes===

Sila-allenes are arene-silicon analogues, like heteroaromatics without the additional electron density. The electropositive nature of silicon allows it to exist in these compounds with only two bonds, like a carbene. Tristanna-allene is a compound made up of a ring of three tin molecules, synthesised from distannenes.

===Group 14===

Potassium graphene can be used as a halogen abstractor in the synthesis of Si-Si triple bonds. A tribromosilane is reacted with potassium graphene. This removes 2 equivalents of bromine, and dimerises the silane. If this is repeated, another four equivalents of bromine will be eliminated, and a Si-Si triple bond results. Unlike alkynes, this has a bent geometry around either silicon. This can then be hydroborated without a catalyst, or aminated. Both methods give novel syntheses of disilenes. Bullky protecting groups are necessary for these compounds to prevent dimerisation.

Digermene and digermyne complexes also exist. It is the buildup of incipient lone pairs on Ge that gives the bending. Distannyne species have also been found. The distannyne has a reversible reaction with ethylene, forming a six membered ring with two tins and four carbons. This has a possible application in ethylene/ethane separation.

Just as the above compounds require bulky substituents for stabilisation, NHCs can also be used for stabilisation. In diborane, NHC stabilisation allows the molecule to be stable at room temperature. Diborane is made from trichloroborane, KC8 and the stabilising NHC.

Digallynes have not been isolated. The bond length of some compounds suggests a triple bond, but this is probably not the case. Complexed Na+ are sandwiched by aryl rings from the bulky substituents, artificially shortening the bond, as the sodium atoms are also complexed to the gallium atoms.

The existence of a ferrogallyne (triple bond between gallium and iron) has also been suggested, due to the short bond distance, but calculations suggest that there is no π overlap. The actual interaction is a dative covalent bond.

Alkyne analogues become increasingly bent as the group is descended due to the inert pair effect, where the electrons in the s orbital are two low in energy to take part in bonding, and so remain non-bonding. The lower down the group an atom is, the higher the likelihood of the lone pair effect.

===Group 15===

Diphosphenes are made by reacting a dichlorophosphene with Mg, causing to dimerise and eliminate magnesium chloride. The bulky groups for stabilisation are, again, necessary, and the structure is bent, owing to those pesky lone pairs. When reacted with an excess of Methyl Triflate, a disphosphenium cation results, with the triflate being the anion.

Dehydrohalogenation is used to make diarsenes. This is achieved using DBU, and involves, you've guessed it, the removal of hydrogen and halogen atoms, which complex with DBU.

A Bi=Bi compound has been made, starting from BiCl3, going via a Bi-Se 6 membered ring, and after addition of phosphine (to remove the Se), the Bi=Bi compound resulted. Like the Si=Si compound, this can be reacted with oxygen to give a compound with bridging oxygens.

===Heteronuclear Double Bonds===

Silenes and germenes are classes of compound that have a main group element double-bonded to carbon. This can be achieved by light activating a silyl-carbonyl compound, causing a 'Brook rearrangement', with one of the R groups from the Si migrating to the oxygen. It can also be achieved by eliminating a salt from a silyl-alkyl (having a halogen and a leaving group, one on C, one on Si/Ge). Heating these with BuLi lithiates the halogen position, and warming again removes the lithium with the other leaving group as a salt. The lithiated position is stabilised by the electropositivity of the α atom. These silenes and germenes can dimerise to form four-membered rings (usually 'head to tail'). ROH can be added to them to give a C-H bond and an ether bond. Oxygenation can cleave the bond to give a carbonyl and a Si=O, the latter of which can form six membered rings. Silenes can be coordinated to transition metals by β-hydride elimination or by reduction. Sila-aromatics are also possible.

Heteronuclear double bonds can be between carbon and phosphorus. A phosphene reacts with an acyl chloride, causing the loss of a TMSCl, and the formation of a single P-C bond. Another R group then migrates (as in the Brook rearrangement) to the O of the carbonyl. All the above reactivity also applies to these bonds.

Silanones are particularly difficult to prepare. Silyl alcohols are reacted with silanes, and the bonding is such that there is a Si-O-Si bond. This is then reacted with either N2O or CO2 to give a silanone.

===Group 14 to Group 16 Double Bonds===

These are more difficult to achieve, as only one end can be sterically protected. It isn't impossible though. Silicon diprotectinggroup dihalide can be lithiated, then reacted with sulfur to form the fifth member of a ring. Phosphine is then used to remove the sulfur (much as it does in the Wittig reaction) and Si=S is the result. Similar reactions are possible with germanium/tin and sulfur/selenium. Lead will react with sulfur in this way. These multiple bonds undergo Diels-Alder reactions readily, unlike carbon analogues.

==='Triple' bonds===

A phosphorus carbon triple bond has been observed, effected by elimination of a silyl ether. The possibility for triple bonding decreases as you descend group 14, due to the inert pair effect. Germanium and tin only double bond to themselves, and lead will only single bond.

===Points to Remember===

* Atoms get larger and bonds weaker on descending group
* Pi interactions get weaker down a group, and are weaker than sigma interactions.
* Oligomerisation maximises strong single bonds, and minimises weak multiple bonds.
* Steric protection is necessary for weak multiple bonds.
* Bulky groups like TMS3E or heavy substituted aryls are good steric protectors.
* Synthetic routes often involve photolysis or salt elim.
* Fewer substituents around a multiple bond means they must be larger.
* Geometry around multiple bonds is influenced by lone pair character.

A final soul-crushing point:

<pre>“The classical multiple bond indicators – bond lengths and
bond strengths – have no meaning for multiple bonds in which
elements from the higher periods are involved. However,
they are valid for an exceptional element: carbon”

Grutzmacher and Fassler, quotation from Chemical
Communications, 2000, 2175-2181.
</pre>

===Anions===
===Recap: "Carbon Anions"===

We make 'carbon anions' by metallating a haloalkane/ene/whatever, usually with lithium, but sometimes with magnesium (Grignard) or copper. The electropositivity of the metal allows the carbon to behave as though negatively charged. Organolithiums, in particular, cluster to stabilise. The degree of clustering depends on how electron-rich the solvent is. Grignard reagents undergo Schlenk equilibrium, where they dimerise (with bridging halides/carbons), exchange substituents, separate and so on.

===Grignard Compounds===

β-diketoiminate ligands (like a pyridine, but with two Ns and an H in between) can be metallated with lithium, then transmetallated to a Grignard. In the presence of potassium, Grignards dimerise, bonding through the Mg. X-ray crystallography and NMR have proved that there are no hydride bridges.

Instead of magnesium, calcium and other group 2 metals can be used. They are uncommon because the metals are relatively unreactive: CaO will not dissolve in THF or ether, so must be activated. This is done using 1,2-dibromoethane, to brominate, and then sodium, to debrominate and leave the activated Ca (called Reicke-calcium, pyrophoric). This is then reacted with an alkyl halide. Organocalcium compounds have high reactivity. They have a tendency to rearrange, and to cleave ethereal solvents.

===Mixed counterions===

Mixed counterions are when two different metals (for example, lithium and magnesium) are used to make a carbon 'anionic'. An example of this is the Schlosser base, LICKOR, produced by mixing n-BuLi with t-BuOK. Its basicity is between nBuLi and nBuK. It displays higher selectivity and 'suppresses erratic side reactions'. Another example is the so-called 'Turbo Grignard', TMP lithium mixed with MgCl2. This has enhanced reactivity and is regioselective. (TMP is tetramethylpiperidine).

===Boryl anions===

All other main group anions (give or take) satisfy the octet rule. Boryl anions, having only six electrons, do not. They can be synthesised from halo-diaminoborane, metallated, which gives a diaminoboryl anion. This anion can be stabilised using an NHC analog. Diisopropylethylenediamine is treated with magnesium. It is then reacted with BBr3, which is then lithiated, giving a five membered ring with an N-B-N series and a lithiated B. This can then be transmetallated. There is a negative charge on the boron, so it has higher nucleophilicity and no electrophilic reaction are reported.

===Anionic E=E compounds===

Disilenes can be lithiated and that chemistry taken advantage of.

===Weakly co-ordinating anions & cations===

Non-coordinating anions in the condensed phase do not exist, but a similar effect can be obtained by having many weak interactions rather than one strong reaction. This means there must be negative charge over a large area, to give spread out nucleophilicity, so there is no obvious attack point. Tetra(pentafluorophenyl)borate is an example of this.

Cryptands and crown ethers turn contact ion pairs (CIPs) into solvent-separates ion pairs (SSIPs), leaving a naked anionic centre, and increasing nucleophilicity. These become weakly co-ordinating cations.

Mesyltoe 3I2

2012-12-30T21:06:56Z

Pm1510: /* Recap: "Carbon Anions" */

=3.I2 Advanced Main Group Chemistry=

A summary of a lecture course given by Dr. Paul Lickiss in Autumn 2012

==Highly Reactive Compounds==
===Radicals===

There are three ways of making radicals: homolytic bond cleavage (by heat or light, particularly of halogen or peroxide bonds), oxidation of anions or reduction of cations. Radicals can abstract hydrogens (eg. from solvent), dimerise (which often has no energetic barrier) and disproportionate, one radical becoming anionic, and one cationic (or near enough). They're implicated in polymerisation (free radical polymerisation) and the destruction of the ozone layer! They're also important in noble gas chemistry.

Radicals can be stabilised either electronically (delocalisation of the radical charge) or sterically (surrounding the reactive centre with steric bulk to prevent reaction).

===Group 14 radicals===

Carbon radicals can be planar, flexible pyramidal (allowing easy inversion about the carbon) or rigid pyramidal, and go in that order in terms of stability. In contrast, Si radicals are more stable as pyramidal structures. This is because the smaller electronegativity of Si results in higher energy MOs, meaning that orbital mixing (or hybridisation) is more favourable. We can tell this using EPR spectroscopy. Higher hyperfine coupling is observed with higher orbital s character, so we can tell that the SOMO has a large quantity of s character if it has large coupling. This shows that for radicals in solution, Si is the only group 14 element that prefers a pyramidal structure.

===Dimer radicals===

Diborane radical anions are examples of radicals stabilised by sterics. When they have mesityl substituents, the repulsion between these bulky substituents keeps the solid-phase bond legnth constant. In dialane radical anions, this steric repulsion can be sufficient to break the bond. This can also occur with phosphorus compounds.

===Interhalogens, noble gas compounds and fluorocarbons===

Radical chemistry is used in the production of CFCs. Light activated chlorine is reacted with methane, giving tetrachloromethane. This is then fluorinated (using HF and SbFCl4 catalyst) and processed further to give CFCs, which as we all know, destroy the ozone layer. This occurs by radical chlorine abstracting oxygen from ozone and trapping it in water and nitrogen dioxide. But CFCs aren't all bad! Chloro-difluoromethane is thermolysed to give tetrafluoroethylene, which when polymerised gives Teflon (PTFE). Hurrah!

Interhalogen compounds are surprising, but exist. Diatomic compounds aren't so surprising, but it's possible to have heptavalent iodine (in IF7). Many of these compounds are powerful fluorinating agents. One particularly powerful is ClF3, which is a better fluorinating agent than fluorine itself. Most of these compounds are prepared from the elements in an appropriate ratio under varying conditions. The bond strengths in these compounds reflect the electronegativity difference between the compounds, so IF is the strongest bond, and BrCl the weakest. As we might expect from this, the weaker the bond, the higher the temperature required for the reaction to occur.

The structure of the complexes conforms to VSEPR. The bonding depends upon the geometry of the complex. In complexes of trigonal bipyramidal overall geometry (regardless of whether vertices are occupied by electron pairs or not) the axial bonds are 3 centre, 4 electron bonds. The equatorial, sp2 hybridised, orbitals hold bonding or lone pair electrons. In octahedral complexes, with fewer than six substituents, the atoms in the equatorial plane are bonded by 3 centre 4 electron interactions. The axial orbitals are sp hybridised, and these orbitals hold either bond or lone pairs. In octahedral complexes with six substituents, all are bonded by 3 centre 4 electron bonds.

[[File:mesyltoe_3I2_interhalogenmo.png]]

As we can see from the MO diagram, this means that along the x axis there are two bonding electrons and two non bonding electrons. This is the origin of the 3 centre 4 electron bond.

Anionic complexes are much more common than cationic complexes, owing to the high electron affinity of the halogens. I3-, I5-, I7- and I9- are all known, and form bi-linear structures (two lines with an angle in between). They are stabilised by large soft cations, like R4N+. The cations are easier to form as the group is descended, and polyatomic iodide cations are the most common, which is as we would expect given this is the least electronegative of the halogens. Strong oxidising agents are required to form these. I5+ is linear with ends 'bent' in opposite directions. I3+ has a bent shape, like water.

===Noble Gas Chemistry===

Xe and O have a similar ionisation potential, so if it's possible to create oxygen based ionic compounds, then it should be possible to create Xe based ionic compounds. Turns out, it is! XeF2 exists and is commercially available, though it is easily hydrolysed to Xe, O2 and HF. KrF2 also exists, but is unstable at rtp.

Obviously, since noble gases already have a full valence orbital, their compounds will be hypervalent. They really bond by a process of hyperconjugation, so:

[[File:mesyltoe_3i2_hyperconjugationxef4.png]]

This means that the average Xe-F bond order is 0.5.

Xenon fluorides react with lewis acid species that can act as fluoride acceptors, to give XeFnMF5 (or similar) products. XeF6 in particular attacks glass, yielding silicon fluoride (the formation of which is the driving force) and XeOF4. Xenon fluorides when hydrolysed produce XeO3, which is highly explosive, and HF. At high pH, XeO3 becomes hydroxenonicacid (?), HXeO4-. XeF2 oxidises and fluorinates simultaneously. It is more reactive in the presence of HF. The 3c, 4e bonds in XeF2 and XeF4 are strongly polarised, and positive at Xe, making Xe susceptible to nucleophilic attack by carbon species. Substitution reactions at Xe are known, but the products tend to be thermally unstable.

Xe can act as a ligand in transition metal complexes, stabilised by weakly co-ordinating anions such as Sb2F10-. KrF2 noted above can act as a strong fluorinating agent for both Xe and Au.

==Heavier Carbene Analogues==

===Carbenes===
In a carbene, the theoretical triplet state (which is the ground state) consists of two singly occupied degenerate p-orbitals, perpendicular to the linear bonding plane. In reality, the bonding is bent, and the carbon is sp2 hybridised. One of the electrons sits in one of these orbitals, whilst the other sits in a p orbital. This means that the two electrons are not degenerate, but the energy separation is smaller than the pairing energy. The bent tripet has diradical reactivity.

The triplet state of a carbene can be stabilised by enforcing linearity, by adding sterically bulky groups, delocalising the spins and by preventing addition at the para positions by using up all the carbon's valence electrons.

===Silylene===

The silylene ground state is a singlet. It is more bent than the carbene. The lone pair has more s character, and the orbitals approach an unhybridised state as the pz orbital is empty. It is both electrophilic and nucleophilic. This means it has a lot of varied rectivity, making it very industrially useful. The singlet state is so stable for SiH2 for the same reason that the Si radicals are pyramidal: the bending and orbital mixing is more favourable due to Si's lower electronegativity. The mixing would not be possible in a linear situation: it is the breaking of symmetry via bending that allows the orbitals to be of the same symmetry, and thus to mix.

Carbene analogues in the other main groups include phosphines and borylenes. Compounds similar to NHCs can be made with selenium.

===Stabilisation and reactivity===

NH substituents can stabilise all carbenes and carbene analogues (silylene, germylene, etc.) by aromatic stabilisation. These have little effect on reactivity, but a profound effect on stabilisation.

==Heavier Multiple Bonding==

===Bonding in Main Group Elements===

Double bonds have less stability the lower down the groups you go. This is because the orbitals are more diffuse, so there is less overlap of electron density, so the bonds are longer. The bond energy includes the energy required to produce a triplet, that is, the gap between a singlet and a triplet state.

The Charles-Goddard-Malrieux-Trinquier model has two different models of double bonding. Where the bond is made up of two singlet fragments, there is a high preparation energy, and this is greater than the approximated π bond energy. This results in a trans-bent double bond, with two sp2 hybridised centres leaning together for overlap between the two p orbitals and two of the sp2 orbitals. There is a smaller HOMO-LUMO gap, and a weaker double bond. Where the bond is made of two triplet fragments, the preparation energy is less than the π bond energy. The double bond is more planar, and the centres interact as we traditionally think of bonds interacting. There can be very large substituents. Any structure distortion tends to be twisting rather than trans-bending.

These different structures affect the HOMO-LUMO gap. The bent structure (where one centre has its substituents 'up', and the other 'down') has a smaller HOMO-LUMO gap than the perfectly planar structure, but a larger HOMO-LUMO gap than the 'twisted' structure. In the twisted structure, where the two substituent planes are at right angles to one another, the HOMO and LUMO become degenerate, and they are both non-bonding, singly occupied MOs.

Disilenes are structurally flexible, and can take up any of these structures depending on circumstances, such as electron density in the solvent. The HOMO-LUMO gap affects the colour of these compounds.

===Si=Si Reactivity===

Disilenes are synthesised by elimination of halide salts: a dilithiated silicon with a dihalogenated silicon, or a dihalogenated silicon with a source of lithium. Oxygen can be added to Si=Si bonds facilely, owing to the low bond enthalpy. It adds to form a dioxygen, disilicon four-membered ring, which then rearranges so that the oxygens are bridging. The Si-Si distance is shorter than a normal bong length in this structure. To create epoxide analogues, N2O is used instead. A similar phenomenon happens with any addition reaction. Because the bond is so much weaker than its C=C equivalent, the additions are much simpler. The diffuse orbitals mean that four membered rings are possible, as there is less ring strain.

Like alkenes, disilenes can also coordinate to transition metals either mono or dihaptically. Dimetallasilanes can undergo transmetallation to the same centre to produce silane-metallacycles, or η2 co-ordinated disilanes.

Diplumbenes are also possible, and take up a trans bent structure.

===Sila- and tristanna-allenes===

Sila-allenes are arene-silicon analogues, like heteroaromatics without the additional electron density. The electropositive nature of silicon allows it to exist in these compounds with only two bonds, like a carbene. Tristanna-allene is a compound made up of a ring of three tin molecules, synthesised from distannenes.

===Group 14===

Potassium graphene can be used as a halogen abstractor in the synthesis of Si-Si triple bonds. A tribromosilane is reacted with potassium graphene. This removes 2 equivalents of bromine, and dimerises the silane. If this is repeated, another four equivalents of bromine will be eliminated, and a Si-Si triple bond results. Unlike alkynes, this has a bent geometry around either silicon. This can then be hydroborated without a catalyst, or aminated. Both methods give novel syntheses of disilenes. Bullky protecting groups are necessary for these compounds to prevent dimerisation.

Digermene and digermyne complexes also exist. It is the buildup of incipient lone pairs on Ge that gives the bending. Distannyne species have also been found. The distannyne has a reversible reaction with ethylene, forming a six membered ring with two tins and four carbons. This has a possible application in ethylene/ethane separation.

Just as the above compounds require bulky substituents for stabilisation, NHCs can also be used for stabilisation. In diborane, NHC stabilisation allows the molecule to be stable at room temperature. Diborane is made from trichloroborane, KC8 and the stabilising NHC.

Digallynes have not been isolated. The bond length of some compounds suggests a triple bond, but this is probably not the case. Complexed Na+ are sandwiched by aryl rings from the bulky substituents, artificially shortening the bond, as the sodium atoms are also complexed to the gallium atoms.

The existence of a ferrogallyne (triple bond between gallium and iron) has also been suggested, due to the short bond distance, but calculations suggest that there is no π overlap. The actual interaction is a dative covalent bond.

Alkyne analogues become increasingly bent as the group is descended due to the inert pair effect, where the electrons in the s orbital are two low in energy to take part in bonding, and so remain non-bonding. The lower down the group an atom is, the higher the likelihood of the lone pair effect.

===Group 15===

Diphosphenes are made by reacting a dichlorophosphene with Mg, causing to dimerise and eliminate magnesium chloride. The bulky groups for stabilisation are, again, necessary, and the structure is bent, owing to those pesky lone pairs. When reacted with an excess of Methyl Triflate, a disphosphenium cation results, with the triflate being the anion.

Dehydrohalogenation is used to make diarsenes. This is achieved using DBU, and involves, you've guessed it, the removal of hydrogen and halogen atoms, which complex with DBU.

A Bi=Bi compound has been made, starting from BiCl3, going via a Bi-Se 6 membered ring, and after addition of phosphine (to remove the Se), the Bi=Bi compound resulted. Like the Si=Si compound, this can be reacted with oxygen to give a compound with bridging oxygens.

===Heteronuclear Double Bonds===

Silenes and germenes are classes of compound that have a main group element double-bonded to carbon. This can be achieved by light activating a silyl-carbonyl compound, causing a 'Brook rearrangement', with one of the R groups from the Si migrating to the oxygen. It can also be achieved by eliminating a salt from a silyl-alkyl (having a halogen and a leaving group, one on C, one on Si/Ge). Heating these with BuLi lithiates the halogen position, and warming again removes the lithium with the other leaving group as a salt. The lithiated position is stabilised by the electropositivity of the α atom. These silenes and germenes can dimerise to form four-membered rings (usually 'head to tail'). ROH can be added to them to give a C-H bond and an ether bond. Oxygenation can cleave the bond to give a carbonyl and a Si=O, the latter of which can form six membered rings. Silenes can be coordinated to transition metals by β-hydride elimination or by reduction. Sila-aromatics are also possible.

Heteronuclear double bonds can be between carbon and phosphorus. A phosphene reacts with an acyl chloride, causing the loss of a TMSCl, and the formation of a single P-C bond. Another R group then migrates (as in the Brook rearrangement) to the O of the carbonyl. All the above reactivity also applies to these bonds.

Silanones are particularly difficult to prepare. Silyl alcohols are reacted with silanes, and the bonding is such that there is a Si-O-Si bond. This is then reacted with either N2O or CO2 to give a silanone.

===Group 14 to Group 16 Double Bonds===

These are more difficult to achieve, as only one end can be sterically protected. It isn't impossible though. Silicon diprotectinggroup dihalide can be lithiated, then reacted with sulfur to form the fifth member of a ring. Phosphine is then used to remove the sulfur (much as it does in the Wittig reaction) and Si=S is the result. Similar reactions are possible with germanium/tin and sulfur/selenium. Lead will react with sulfur in this way. These multiple bonds undergo Diels-Alder reactions readily, unlike carbon analogues.

==='Triple' bonds===

A phosphorus carbon triple bond has been observed, effected by elimination of a silyl ether. The possibility for triple bonding decreases as you descend group 14, due to the inert pair effect. Germanium and tin only double bond to themselves, and lead will only single bond.

===Points to Remember===

* Atoms get larger and bonds weaker on descending group
* Pi interactions get weaker down a group, and are weaker than sigma interactions.
* Oligomerisation maximises strong single bonds, and minimises weak multiple bonds.
* Steric protection is necessary for weak multiple bonds.
* Bulky groups like TMS3E or heavy substituted aryls are good steric protectors.
* Synthetic routes often involve photolysis or salt elim.
* Fewer substituents around a multiple bond means they must be larger.
* Geometry around multiple bonds is influenced by lone pair character.

A final soul-crushing point:

<pre>“The classical multiple bond indicators – bond lengths and
bond strengths – have no meaning for multiple bonds in which
elements from the higher periods are involved. However,
they are valid for an exceptional element: carbon”

Grutzmacher and Fassler, quotation from Chemical
Communications, 2000, 2175-2181.
</pre>

===Anions===
===Recap: "Carbon Anions"===

We make 'carbon anions' by metallating a haloalkane/ene/whatever, usually with lithium, but sometimes with magnesium (Grignard) or copper. The electropositivity of the metal allows the carbon to behave as though negatively charged. Organolithiums, in particular, cluster to stabilise. The degree of clustering depends on how electron-rich the solvent is. Grignard reagents undergo Schlenk equilibrium, where they dimerise (with bridging halides/carbons), exchange substituents, separate and so on.

===Grignard Compounds===

β-diketoiminate ligands (like a pyridine, but with two Ns and an H in between) can be metallated with lithium, then transmetallated to a Grignard. In the presence of potassium, Grignards dimerise, bonding through the Mg. X-ray crystallography and NMR have proved that there are no hydride bridges.

Instead of magnesium, calcium and other group 2 metals can be used. They are uncommon because the metals are relatively unreactive: CaO will not dissolve in THF or ether, so must be activated. This is done using 1,2-dibromoethane, to bromi

Mesyltoe 3I2

2012-12-30T20:52:41Z

Pm1510: /* Anions */

=3.I2 Advanced Main Group Chemistry=

A summary of a lecture course given by Dr. Paul Lickiss in Autumn 2012

==Highly Reactive Compounds==
===Radicals===

There are three ways of making radicals: homolytic bond cleavage (by heat or light, particularly of halogen or peroxide bonds), oxidation of anions or reduction of cations. Radicals can abstract hydrogens (eg. from solvent), dimerise (which often has no energetic barrier) and disproportionate, one radical becoming anionic, and one cationic (or near enough). They're implicated in polymerisation (free radical polymerisation) and the destruction of the ozone layer! They're also important in noble gas chemistry.

Radicals can be stabilised either electronically (delocalisation of the radical charge) or sterically (surrounding the reactive centre with steric bulk to prevent reaction).

===Group 14 radicals===

Carbon radicals can be planar, flexible pyramidal (allowing easy inversion about the carbon) or rigid pyramidal, and go in that order in terms of stability. In contrast, Si radicals are more stable as pyramidal structures. This is because the smaller electronegativity of Si results in higher energy MOs, meaning that orbital mixing (or hybridisation) is more favourable. We can tell this using EPR spectroscopy. Higher hyperfine coupling is observed with higher orbital s character, so we can tell that the SOMO has a large quantity of s character if it has large coupling. This shows that for radicals in solution, Si is the only group 14 element that prefers a pyramidal structure.

===Dimer radicals===

Diborane radical anions are examples of radicals stabilised by sterics. When they have mesityl substituents, the repulsion between these bulky substituents keeps the solid-phase bond legnth constant. In dialane radical anions, this steric repulsion can be sufficient to break the bond. This can also occur with phosphorus compounds.

===Interhalogens, noble gas compounds and fluorocarbons===

Radical chemistry is used in the production of CFCs. Light activated chlorine is reacted with methane, giving tetrachloromethane. This is then fluorinated (using HF and SbFCl4 catalyst) and processed further to give CFCs, which as we all know, destroy the ozone layer. This occurs by radical chlorine abstracting oxygen from ozone and trapping it in water and nitrogen dioxide. But CFCs aren't all bad! Chloro-difluoromethane is thermolysed to give tetrafluoroethylene, which when polymerised gives Teflon (PTFE). Hurrah!

Interhalogen compounds are surprising, but exist. Diatomic compounds aren't so surprising, but it's possible to have heptavalent iodine (in IF7). Many of these compounds are powerful fluorinating agents. One particularly powerful is ClF3, which is a better fluorinating agent than fluorine itself. Most of these compounds are prepared from the elements in an appropriate ratio under varying conditions. The bond strengths in these compounds reflect the electronegativity difference between the compounds, so IF is the strongest bond, and BrCl the weakest. As we might expect from this, the weaker the bond, the higher the temperature required for the reaction to occur.

The structure of the complexes conforms to VSEPR. The bonding depends upon the geometry of the complex. In complexes of trigonal bipyramidal overall geometry (regardless of whether vertices are occupied by electron pairs or not) the axial bonds are 3 centre, 4 electron bonds. The equatorial, sp2 hybridised, orbitals hold bonding or lone pair electrons. In octahedral complexes, with fewer than six substituents, the atoms in the equatorial plane are bonded by 3 centre 4 electron interactions. The axial orbitals are sp hybridised, and these orbitals hold either bond or lone pairs. In octahedral complexes with six substituents, all are bonded by 3 centre 4 electron bonds.

[[File:mesyltoe_3I2_interhalogenmo.png]]

As we can see from the MO diagram, this means that along the x axis there are two bonding electrons and two non bonding electrons. This is the origin of the 3 centre 4 electron bond.

Anionic complexes are much more common than cationic complexes, owing to the high electron affinity of the halogens. I3-, I5-, I7- and I9- are all known, and form bi-linear structures (two lines with an angle in between). They are stabilised by large soft cations, like R4N+. The cations are easier to form as the group is descended, and polyatomic iodide cations are the most common, which is as we would expect given this is the least electronegative of the halogens. Strong oxidising agents are required to form these. I5+ is linear with ends 'bent' in opposite directions. I3+ has a bent shape, like water.

===Noble Gas Chemistry===

Xe and O have a similar ionisation potential, so if it's possible to create oxygen based ionic compounds, then it should be possible to create Xe based ionic compounds. Turns out, it is! XeF2 exists and is commercially available, though it is easily hydrolysed to Xe, O2 and HF. KrF2 also exists, but is unstable at rtp.

Obviously, since noble gases already have a full valence orbital, their compounds will be hypervalent. They really bond by a process of hyperconjugation, so:

[[File:mesyltoe_3i2_hyperconjugationxef4.png]]

This means that the average Xe-F bond order is 0.5.

Xenon fluorides react with lewis acid species that can act as fluoride acceptors, to give XeFnMF5 (or similar) products. XeF6 in particular attacks glass, yielding silicon fluoride (the formation of which is the driving force) and XeOF4. Xenon fluorides when hydrolysed produce XeO3, which is highly explosive, and HF. At high pH, XeO3 becomes hydroxenonicacid (?), HXeO4-. XeF2 oxidises and fluorinates simultaneously. It is more reactive in the presence of HF. The 3c, 4e bonds in XeF2 and XeF4 are strongly polarised, and positive at Xe, making Xe susceptible to nucleophilic attack by carbon species. Substitution reactions at Xe are known, but the products tend to be thermally unstable.

Xe can act as a ligand in transition metal complexes, stabilised by weakly co-ordinating anions such as Sb2F10-. KrF2 noted above can act as a strong fluorinating agent for both Xe and Au.

==Heavier Carbene Analogues==

===Carbenes===
In a carbene, the theoretical triplet state (which is the ground state) consists of two singly occupied degenerate p-orbitals, perpendicular to the linear bonding plane. In reality, the bonding is bent, and the carbon is sp2 hybridised. One of the electrons sits in one of these orbitals, whilst the other sits in a p orbital. This means that the two electrons are not degenerate, but the energy separation is smaller than the pairing energy. The bent tripet has diradical reactivity.

The triplet state of a carbene can be stabilised by enforcing linearity, by adding sterically bulky groups, delocalising the spins and by preventing addition at the para positions by using up all the carbon's valence electrons.

===Silylene===

The silylene ground state is a singlet. It is more bent than the carbene. The lone pair has more s character, and the orbitals approach an unhybridised state as the pz orbital is empty. It is both electrophilic and nucleophilic. This means it has a lot of varied rectivity, making it very industrially useful. The singlet state is so stable for SiH2 for the same reason that the Si radicals are pyramidal: the bending and orbital mixing is more favourable due to Si's lower electronegativity. The mixing would not be possible in a linear situation: it is the breaking of symmetry via bending that allows the orbitals to be of the same symmetry, and thus to mix.

Carbene analogues in the other main groups include phosphines and borylenes. Compounds similar to NHCs can be made with selenium.

===Stabilisation and reactivity===

NH substituents can stabilise all carbenes and carbene analogues (silylene, germylene, etc.) by aromatic stabilisation. These have little effect on reactivity, but a profound effect on stabilisation.

==Heavier Multiple Bonding==

===Bonding in Main Group Elements===

Double bonds have less stability the lower down the groups you go. This is because the orbitals are more diffuse, so there is less overlap of electron density, so the bonds are longer. The bond energy includes the energy required to produce a triplet, that is, the gap between a singlet and a triplet state.

The Charles-Goddard-Malrieux-Trinquier model has two different models of double bonding. Where the bond is made up of two singlet fragments, there is a high preparation energy, and this is greater than the approximated π bond energy. This results in a trans-bent double bond, with two sp2 hybridised centres leaning together for overlap between the two p orbitals and two of the sp2 orbitals. There is a smaller HOMO-LUMO gap, and a weaker double bond. Where the bond is made of two triplet fragments, the preparation energy is less than the π bond energy. The double bond is more planar, and the centres interact as we traditionally think of bonds interacting. There can be very large substituents. Any structure distortion tends to be twisting rather than trans-bending.

These different structures affect the HOMO-LUMO gap. The bent structure (where one centre has its substituents 'up', and the other 'down') has a smaller HOMO-LUMO gap than the perfectly planar structure, but a larger HOMO-LUMO gap than the 'twisted' structure. In the twisted structure, where the two substituent planes are at right angles to one another, the HOMO and LUMO become degenerate, and they are both non-bonding, singly occupied MOs.

Disilenes are structurally flexible, and can take up any of these structures depending on circumstances, such as electron density in the solvent. The HOMO-LUMO gap affects the colour of these compounds.

===Si=Si Reactivity===

Disilenes are synthesised by elimination of halide salts: a dilithiated silicon with a dihalogenated silicon, or a dihalogenated silicon with a source of lithium. Oxygen can be added to Si=Si bonds facilely, owing to the low bond enthalpy. It adds to form a dioxygen, disilicon four-membered ring, which then rearranges so that the oxygens are bridging. The Si-Si distance is shorter than a normal bong length in this structure. To create epoxide analogues, N2O is used instead. A similar phenomenon happens with any addition reaction. Because the bond is so much weaker than its C=C equivalent, the additions are much simpler. The diffuse orbitals mean that four membered rings are possible, as there is less ring strain.

Like alkenes, disilenes can also coordinate to transition metals either mono or dihaptically. Dimetallasilanes can undergo transmetallation to the same centre to produce silane-metallacycles, or η2 co-ordinated disilanes.

Diplumbenes are also possible, and take up a trans bent structure.

===Sila- and tristanna-allenes===

Sila-allenes are arene-silicon analogues, like heteroaromatics without the additional electron density. The electropositive nature of silicon allows it to exist in these compounds with only two bonds, like a carbene. Tristanna-allene is a compound made up of a ring of three tin molecules, synthesised from distannenes.

===Group 14===

Potassium graphene can be used as a halogen abstractor in the synthesis of Si-Si triple bonds. A tribromosilane is reacted with potassium graphene. This removes 2 equivalents of bromine, and dimerises the silane. If this is repeated, another four equivalents of bromine will be eliminated, and a Si-Si triple bond results. Unlike alkynes, this has a bent geometry around either silicon. This can then be hydroborated without a catalyst, or aminated. Both methods give novel syntheses of disilenes. Bullky protecting groups are necessary for these compounds to prevent dimerisation.

Digermene and digermyne complexes also exist. It is the buildup of incipient lone pairs on Ge that gives the bending. Distannyne species have also been found. The distannyne has a reversible reaction with ethylene, forming a six membered ring with two tins and four carbons. This has a possible application in ethylene/ethane separation.

Just as the above compounds require bulky substituents for stabilisation, NHCs can also be used for stabilisation. In diborane, NHC stabilisation allows the molecule to be stable at room temperature. Diborane is made from trichloroborane, KC8 and the stabilising NHC.

Digallynes have not been isolated. The bond length of some compounds suggests a triple bond, but this is probably not the case. Complexed Na+ are sandwiched by aryl rings from the bulky substituents, artificially shortening the bond, as the sodium atoms are also complexed to the gallium atoms.

The existence of a ferrogallyne (triple bond between gallium and iron) has also been suggested, due to the short bond distance, but calculations suggest that there is no π overlap. The actual interaction is a dative covalent bond.

Alkyne analogues become increasingly bent as the group is descended due to the inert pair effect, where the electrons in the s orbital are two low in energy to take part in bonding, and so remain non-bonding. The lower down the group an atom is, the higher the likelihood of the lone pair effect.

===Group 15===

Diphosphenes are made by reacting a dichlorophosphene with Mg, causing to dimerise and eliminate magnesium chloride. The bulky groups for stabilisation are, again, necessary, and the structure is bent, owing to those pesky lone pairs. When reacted with an excess of Methyl Triflate, a disphosphenium cation results, with the triflate being the anion.

Dehydrohalogenation is used to make diarsenes. This is achieved using DBU, and involves, you've guessed it, the removal of hydrogen and halogen atoms, which complex with DBU.

A Bi=Bi compound has been made, starting from BiCl3, going via a Bi-Se 6 membered ring, and after addition of phosphine (to remove the Se), the Bi=Bi compound resulted. Like the Si=Si compound, this can be reacted with oxygen to give a compound with bridging oxygens.

===Heteronuclear Double Bonds===

Silenes and germenes are classes of compound that have a main group element double-bonded to carbon. This can be achieved by light activating a silyl-carbonyl compound, causing a 'Brook rearrangement', with one of the R groups from the Si migrating to the oxygen. It can also be achieved by eliminating a salt from a silyl-alkyl (having a halogen and a leaving group, one on C, one on Si/Ge). Heating these with BuLi lithiates the halogen position, and warming again removes the lithium with the other leaving group as a salt. The lithiated position is stabilised by the electropositivity of the α atom. These silenes and germenes can dimerise to form four-membered rings (usually 'head to tail'). ROH can be added to them to give a C-H bond and an ether bond. Oxygenation can cleave the bond to give a carbonyl and a Si=O, the latter of which can form six membered rings. Silenes can be coordinated to transition metals by β-hydride elimination or by reduction. Sila-aromatics are also possible.

Heteronuclear double bonds can be between carbon and phosphorus. A phosphene reacts with an acyl chloride, causing the loss of a TMSCl, and the formation of a single P-C bond. Another R group then migrates (as in the Brook rearrangement) to the O of the carbonyl. All the above reactivity also applies to these bonds.

Silanones are particularly difficult to prepare. Silyl alcohols are reacted with silanes, and the bonding is such that there is a Si-O-Si bond. This is then reacted with either N2O or CO2 to give a silanone.

===Group 14 to Group 16 Double Bonds===

These are more difficult to achieve, as only one end can be sterically protected. It isn't impossible though. Silicon diprotectinggroup dihalide can be lithiated, then reacted with sulfur to form the fifth member of a ring. Phosphine is then used to remove the sulfur (much as it does in the Wittig reaction) and Si=S is the result. Similar reactions are possible with germanium/tin and sulfur/selenium. Lead will react with sulfur in this way. These multiple bonds undergo Diels-Alder reactions readily, unlike carbon analogues.

==='Triple' bonds===

A phosphorus carbon triple bond has been observed, effected by elimination of a silyl ether. The possibility for triple bonding decreases as you descend group 14, due to the inert pair effect. Germanium and tin only double bond to themselves, and lead will only single bond.

===Points to Remember===

* Atoms get larger and bonds weaker on descending group
* Pi interactions get weaker down a group, and are weaker than sigma interactions.
* Oligomerisation maximises strong single bonds, and minimises weak multiple bonds.
* Steric protection is necessary for weak multiple bonds.
* Bulky groups like TMS3E or heavy substituted aryls are good steric protectors.
* Synthetic routes often involve photolysis or salt elim.
* Fewer substituents around a multiple bond means they must be larger.
* Geometry around multiple bonds is influenced by lone pair character.

A final soul-crushing point:

<pre>“The classical multiple bond indicators – bond lengths and
bond strengths – have no meaning for multiple bonds in which
elements from the higher periods are involved. However,
they are valid for an exceptional element: carbon”

Grutzmacher and Fassler, quotation from Chemical
Communications, 2000, 2175-2181.
</pre>

===Anions===
===Recap: "Carbon Anions"===

We make 'carbon anions' by metallating a haloalkane/ene/whatever, usually with lithium, but sometimes with magnesium (Grignard) or copper. The electropositivity of the metal allows the carbon to behave as though negatively charged. Organolithiums, in particular, cluster to stabilise. The degree of clustering depends on how electron-rich the solvent is. Grignard reagents undergo Schlenk equilibrium, where they dimerise (with bridging halides/carbons), exchange substituents, separate and so on.

Mesyltoe 3I2

2012-12-30T20:15:14Z

Pm1510: /* Points to Remember */

Mesyltoe 3I2

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Pm1510: /* R-Si-Si-R Triple bonds */

Mesyltoe 3I2

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Pm1510: /* R-Si-Si-R Triple bonds */

Mesyltoe 3I2

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Pm1510: /* Section 3 */

Mesyltoe index

2012-12-30T16:31:59Z

Pm1510:

Mesyltoe is a student-made wiki summary of Imperial Chemistry lecture courses. It was made at Christmas, hence the pun, and is designed as a revision tool, both for the writers and the readers. Images, where uncredited, are drawn from the lecture notes distributed with the course, or created by the author.

==IIIA==

* 3.I1 Inorganic Mechanisms & Catalysis, taught by Dr. George Britovsek
* 3.I2 [https://wiki.ch.ic.ac.uk/wiki/index.php?title=Mesyltoe_3I2 Advanced Main Group Chemistry], taught by Dr. Paul Lickiss
* 3.I3 [https://wiki.ch.ic.ac.uk/wiki/index.php?title=Mesyltoe_3I3 Advanced Transition Metal Chemistry], taught by Dr.s Silvia Diez-Gonzalez and James Wilton-Ely

* 3.O3 [https://wiki.ch.ic.ac.uk/wiki/index.php?title=Mesyltoe_3O3 Polymers, The Essential Guide], taught by Dr. Joachim Steinke
* 3.O4 [https://wiki.ch.ic.ac.uk/wiki/index.php?title=Mesyltoe_3O4 Introduction to Organic Physical Chemistry], taught by Prof. Iain McCulloch
* 3.O11 Organic Synthesis: Part 2, taught by Prof. Donald Craig
* 3.O12 An Introduction to Reaction Stereoelectronics, taught by Prof. Alan Spivey

* 3.P3 [https://wiki.ch.ic.ac.uk/wiki/index.php?title=Mesyltoe_3P3 Molecular Reaction Dynamics], taught by Dr. Laura Barter
* 3.P9 [https://wiki.ch.ic.ac.uk/wiki/index.php?title=Mesyltoe_3P9 Photochemistry], taught by Prof. James Durrant and Dr. Saif Haque
* 3.P11 Statistical Thermodynamics, taught by Dr. Fernando Bresme

Mesyltoe 3I2

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Pm1510: /* Silylene */

Mesyltoe 3I2

2012-12-30T16:19:59Z

Pm1510: /* Noble Gas Chemistry */

Mesyltoe 3I2

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Pm1510: /* Radicals */

Mesyltoe 3I2

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Pm1510: /* Radicals */

File:Mesyltoe 3i2 hyperconjugationxef4.png

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Pm1510:

Mesyltoe 3I2

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Pm1510: /* Noble Gas Chemistry */

Mesyltoe 3I2

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Pm1510: /* Interhalogens, noble gas compounds and fluorocarbons */

File:Mesyltoe 3I2 interhalogenmo.png

2012-12-30T12:02:30Z

Pm1510:

Mesyltoe 3I2

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Pm1510: /* Interhalogens, noble gas compounds and fluorocarbons */

Mesyltoe 3I2

2012-12-30T11:20:24Z

Pm1510: /* 3.I2 Advanced Main Group Chemistry */

Mesyltoe 3I2

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Pm1510: Created page with "=3.I2 Advanced Main Group Chemistry= A summary of a lecture course given by Dr. Paul Lickiss in Autumn 2012"

=3.I2 Advanced Main Group Chemistry=

A summary of a lecture course given by Dr. Paul Lickiss in Autumn 2012

File:Mesyltoe 3O3 polymersummary.png

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Pm1510:

Mesyltoe 3O3

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Pm1510: /* Section 5 */

=3.O3 Polymers: The Essential Guide=

This is a summary of a lecture course given by Dr Joachim Steinke in Autumn 2012

==Section 1==
===Polymer Terminology===

Some definitions:
* Radical initiator: a radical species needed to start a polymerisation by reacting with a monomer (usually containing a vinyl group).
* Monomer: a polymer is a series of monomer repeat units.
* Initiation: the first step in a chain reaction.
* Propogation: the second step in a chain reaction where a polymer radical reacts with more monomer to form a longer chain.
* Termination: the final step in a chain reaction, which ends the polymerisation.
* Reactive Chain end: the propogating end of an active polymer chain, which can react with a monomer to extend the chain length.
* Repeat unit: the repeating structural motif in a polymer chain, usually identical to the monomer structure.
* Chain end: what it says on the tin, the end of a chain. If both chain ends have a functional group (distinct from the monomer) the polymer is telechelic. If only one of the ends is modified with a FG it is called a macromonomer.
* Backbone or main chain: the atoms that join the monomers together.
* Sidechain or pendant groups: structural motifs attached to the backbone.
* Random coil: this is a conformation of a polymer chain. If free rotation in the backbone is possible, the polymer adopts a random coil average structure (as all carbon centres can have many different conformations)
* Macromonomer: a macromonomer is a polymer chain containing one (or more) functional groups that can be polymerised further.
* Molar mass distribution: during polymer synthesis a distribution of molecular weights is made (usually).

===Polymer Structures and Properties===

Only highly symmetrical polymers can pack into a crystalline structure. Most polymers have crystalline regions and amorphous (or random coil) regions. Crystalline polymers are opaque and brittle. Semi-crystalline polymers can be either transparent or opaque depending on the size of the crystalline regions, and tend to be mechanically strong providing the amorphous region is above its glass transition temperature. Amorphous polymers with no chromophores will be transparent and colourless (and usually brittle).

Polymers have different properties to small molecules. Polymer chains tangle, and have large intramolecular forces because of their high 'surface area'. They take a very long time to move because they're so much bigger and heavier. The conformation space is vast because of the number of permutations in conformation possible.

Polymers have lots of properties that make them super uesful, including mechanical strength, elasticity, plasticity, thermal stability and very tunable properties.

To tune these properties, we create different structures. There are linear, telechelic and macromonomer polymers, all of which we've already met. There are also branched polymers, brush or comb polymers (long, regular and frequent branching), network polymers (lots of branching and irregular interconnection between backbones), hyperbranched (lots of branching and sub-branching from one backbone), and dendrimer (snowflake-like hyper-regular branching structure).

Most synthetic polymers are produced with a distribution of chain lengths leading to a Gaussian molecular weight distribution. Biopolymers or dendrimers can be monodisperse (have only one molecular weight present), but all others are polydisperse.

Polymers have complex solubility behaviour owing to the size of the molecules and the corresponding size of the the inter and intramolecular reactions. The time taken for them to diffuse also means that polymers take longer to dissolve than smaller molecules, sometimes a lot lot longer.

When polymers are being produced for commercial consideration, the viscosity of the polymer must be balanced with the weight of the polymer. The longer the chain length, the higher the viscosity, and the more energy required to process the polymer. However, the longer the chain length, the better the mechanical properties.

There are four main classes of polymers, defined by their response to stress (for example, sustained weight application). Fibres show a very small change in length under stress as the crystalline regions provide crosslinks to keep the amorphous regions together. Energy can dissipate by converting crystalline regions to amorphous regions. Once there are no more crystalline regions to convert, and the amorphous regions can no longer stretch, the polymer will suddenly break. Rigid plastics are similar, but they have no crystalline regions to dissipate energy into, so the break (failure) is more dramatic. A flexible plastic initially behaves like a rigid plastic, but at a point of increasing strain the amorphous polymer reorganises, dissipating some of the energy. This changes the properties of the polymer irreversibly. An elastomer responds to stress immediately and at low stress values. Flexible (mobile) polymers dissipate energy by switching to higher energy conformations. Once it has reached the highest energy conformer, there is nowhere to dissipate the energy, and the polymer breaks like a rigid plastic. Before it breaks, the elastomer responds reversibly to stress.

Chain entanglement creates viscosity and elasticity in a polymer, but it is time dependent: left to gravity, the polymers will gradually untangle, and elasticity will disappear. Vulcanisation creates crosslinking between polymer chains, meaning that the entanglement is permanent, and that the polymer will hold its shape. The more crosslinking, the less elastic the polymer. In vulcanisation specifically the crosslinks are made of C-S-C bridges. Blood clotting works in a similar way.

===Structure-Property-Temperature Relationship===

When amorphous polymers melt they change from an amorphous solid state to a rubbery state and finally to a liquid (polymer melt). The transition from a glassy state to a rubbery state is called the glass transition temperature (Tg). Amorphous polymers do not have a true melting point. Crystalline polymers don't have a Tg, but do have a true melting point, Tm. Semi-crystalline polymers have both a Tm from the crystalline contribution and a Tg from the amorphous contribution. These temperatures can be measured by specific heat and volume changes. Crystalline polymers have a sharp change in volume and heat capacity. Amorphous polymers show a gradual change. Both kinds of polymer have a lower volume at lower temperatures.

The Tg of a polymer is dependent on the degree of rotational freedom around the backbone and the translational freedom of the chains t slip past one another. This is controlled by intramolecular conformational flexibility and intermolecular packing. In summary, the more flexible a backbone and the smaller and more felxible the sidechain, the lower the Tg. Bulky sidegroups close to the backbone increase Tg, but far from the backbone decrease it. Strong intermolecular interactions increase Tg.

==Section 2==
===Polymer Mechanisms===

Nature has many mechanisms for structurally precise polymers in aqueous solution at room temperature, eg. peptide, carbohydrates, polynucleotides. We can imitate this with automated polypeptide synthesis.

There are two main mechanisms for polymer synthesis: step-growth (growth from both ends) and chain-growth (growth on one end). The implications for molecular weight on each of these mechanisms is different.

We want to control in our syntheses:
* Degree of polymerisation (length of chain)
* Molecular weight distribution
* Polymer chain end groups
* Pattern of co-monomer incorporation
* Degree of branching

How much control there is over these properties is dependent on the synthesis mechanism used. Which mechanism is used depends on the monomer, and which monomer is used depends on the properties desired from the monomer.

===Chain Growth Polymerisation===

The polymer chain grows in order direction with monomer adding only to the reactive polymer chain end. The polymerisation is exothermic and very fast. The reaction is enthalpically driven by the loss of C=C and the formation of C-C bonds. The ratio of recombination (functional groups bonding) to disproportionation (one functional group oxidising the other) depends on the monomer, and affects the molecular weight distribution. Both of these are termination steps, but disproportionation results in a macromonomer. The molecular weight distribution is close to Gaussian.

It is possible to derive the kinetics of a polymerisation from the molecular weight distribution. Steady state kinetics are assumed. The degree of polymerisation depends on Rp (rate of propogation), Rt (rate of termination) and [I] (concentration of initiator). Also important is Rct (rate of chain transfer). At constant [I], <math>\frac{R_p}{R_t+R_{ct}} = DP</math>. The average chain length is thus independent of time, as long as the rates don't vary. We can use all of this to estimate a chain length.

<math>
v = \frac{k_p[M]}{2(fk_tk_d[I])^\frac{1}{2}}
</math>

Where kd is the initiator decomposition rate constant, v is the chain length and f is the initiator efficiency. Efficiency of initiators tend to be in the region of 50%, due to solvent effects and side reactions. This equation means that it's possible to tailor the chain length, sort of.

===Step-Growth Polymerisation===

This is also knows as condensation polymerisation. The chain grows by combination of any two species (monomer or oligomers). This is typically a slow and endothermic reaction, and the reaction is driven by the removal of a volatile condensation product (such as water). The polymer chain grows from both ends, and a high molecular weight of polymer is produced only at high monomer conversions. There is usually no termination step. The limiting factor in polymer size is the solubility of the polymer: when the polymer drops out of solution, it ceases to grow any further.

The kinetics of step-growth have no initiation, activation or termination step. The degree of polymerisation is determined by the stoichiometry of functional groups, viscosity effects on reaction rate and the solubility of the growing polymer chain. No polymer is produced until 95% of monomer has been converted (to oligomer or whatever). Side reactions are less important here than in chain-growth.

===Carothers' Equation===

Crucial for the successful synthesis of high molecular weight polymers is the purity of the starting materials. The correct ratio of starting materials is vital if one or more monomers are being used. The degree of polymerisation can be calculated using Carothers' equation.
<math>
DP = \frac{1}{1-p}
</math>

Degree of polymerisation (DP) is dependent on p, the probability of finding a reacted functional group. If the ratio of comonomers A & B were 1:0.9, for example, p would be 0.9, and the DP would be low. The closer p is to one, the greater the degree of polymerisation.

===Characterising Polymers===
Polymers can be characterised by:
* Light scattering (sensitive to 'particle' size)
* Viscometry (Mark-Houwink equation: η = KMα
* GPC
* MALDI-tof

Most characterisation techniques are the same for small molecules and polymers. Differences appear when it comes to molecular weight characterisation. These methods must be appropriate for polymers. Some, such as viscosity and light scattering, are unique to polymer characterisation, as small molecules don't have the properties necessary for this sort of measurement.

MALDI-tof is a mass spectrometry technique that was developed for proteins. The polymer is dissolved in a solution containing a small molecule matrix which can crystallise in the presence of the polymer, and is able to ionise the polymer chain upon irradiation with a particular wavelength of light. The laser heats the matrix molecules, which then transfer vibrational and rotational to all parts of the polymer along with positive charge. This allows the polymer to vapourise without being heated to temperatures great enough to cause bond cleavage (which would give inaccurate and low mass measurements).

GPC or gel permeation chromatography uses a separation mechanism to determine the molecular weight distribution of a polymer. Longer polymer chains have a larger hydrodynamic radius in solution than shorter ones. The dissolved sample is passed through a porous column material that contains an equal number of different pore sizes. This means that longer polymers travel through the column faster, as there are fewer diffusion paths for the molecules to take.

From either of these methods, we obtain a molecular weight distribution curve. This shows the number average molecular weight, Mn(ΣNiMi/ΣNi), the weight average molecular weight, Mw (ΣNiMi2/ΣNiMi), and the viscosity average molecular weight, Mvisc. Ni is the number of polymers with molecular weight Mi. Another important thing is the width of the distribution, the PDI (polydispersity index).

Chain growth shows a sudden rise in DP, which tails off once a steady concentration in radicals has been reached. Regardless of the percentage conversion, the weight distribution remains the same. Step-growth shows very low DP (oligomers) up to 90% conversion, and then shows a sharp increase up to 100%. The greater the percentage conversion, the higher the average weight distribution.

==Section 3==
===Living Polymerisation===

A living polymerisation is one that has initiation faster than propogation, no chain transfer and no chain termination. This means there is only one possible reaction pathway, and initiation is effectively instant.

Living anionic polymerisation is the best existing chemistry for very narrow molecular weight distribution polymers. Dynamic clusters of lithium ions act as a reversible protecting group. Alkyl lithium is a typical initiator. LiH s eliminated as a termination side reaction.

Dormant chain equilibria are clusters of charged species that do not initiate or propagate (thus are dormant). This prevents them taking part in side reactions. The proportion of the reactants in dormant chains depends on the solvent, concentration and temperature. This means that it can be optimised such that only polymerisation occurs.

Living polymerisation means that molecular weight grows linearly with % conversion (like in nature, hence living). It also means that block co-polymerisation is possible, where a second monomer is introduced partway through the synthesis. This allows immiscible polymers to be made miscible, and also allows creation of patterns on nm scale, for electronic devices.

In theory living polymerisations can be made from any polymerisation. The problem is developing chemistry that can stabilise the reactive chain end to prevent side reactions. This has been made possible.

In Group Transfer Polymerisation, a protecting group is transferred along the chain. The protecting group acts to deactivate the chains to prevent side reactions. In living Cationic polymerisation, like in living anionic polymerisation, clusters of charged species prevent side reactions. Cationic polymerisations, due to their larger active species, have proved harder to control. In living Free Radical Polymerisation, a stable free radical such as TEMPO combines with the C-centred radicals. The TEMPO-activator bond does not form as it is too unstable, even at room temperature. The TEMPO-monomer bond is thermally labile, allowing temperature to control the active proportion of reactant. Atom Transfer Radical Polymerisation using bromide as its protecting group. A copper catalyst that swiftly oxidises and reduces is used to capture and release bromide ions. Reversible Addition Fragmentation Transfer uses a reversible chain transfer to deactivate reactants. This chain transfer is very fast, faster than propogation. The chain transfer agent keeps switching between polymer chains.

Living Ring Opening Polymerisation can circumvent the limitation of high molecular weight not being accessible via step-growth. The release of ring strain is the driving force. This allows the production of polyesters via chain-growth, though the monomer production is more complex. Ring Opening Metathesis Polymerisation produces a metallacycle as an intermediate, and the driving force is relief of ring strain. This process can also be used to produce polymer precursors, which can be purified before the final polymerisation takes place.

For some monomers, no living polymerisation has yet been found. A means of molecular weight control is chain-transfer. Thiols are favoured because of their high CT efficiency, though they aren't as efficient as the CT agents used in RAFT. As a result, there is a maximum chain length they can possibly give, limited by kp/kt, as termination steps still occur. Catalysts can also be used as CT agents, and can act very similar to TEMPO. They react much faster than RAFT reagents. They also result in C=C bonds at the chain end, meaning the products are macromonomers that can, for example, be turned into polymer brushes.

===Controlling Tacticity===

To control the tacticity of our polymers, we can add bulky substituents that can be removed after polymerisation, use a chiral solvent that interacts with the monomer on the chain end in a particular fashion, or by using co-ordinating metal ions to predispose particular monomer configurations. In non-coordinating solvents, reactivity can be governed by coordinating ions, such as Li+, meaning that all groups approach from the same direction. In coordinating solvents, the approach is governed by sterics.

===Summary===

Oh, God, do we need a summary.

{| class="wikitable"
|-
! !! Step !! Chain !! Living
|-
| Polydispersity indices || 2 || 1.5-2 || 1.01
|-
| How MW changes as reaction proceeds || Exponential increase || No change || Linear increase
|-
| Linear block copolymers possible? || Only with difficulty || No || Yes
|-
| Control over chain end functionality || Yes || Yes || Yes
|-
| Control over MW || Full control, lowish MW || Full control || Full control
|-
| Are high MW polymers possible? || No || Yes || Yes
|}

==Section 4==
===Copolymers===

A copolymer is an additional monomer used in polymerisation. They can be in many different structures. A diblock has a block of monomer A and a block of monomer B joined together. A triblock has a block of monomer B between two blocks of monomer A. A graft block has monomer B bonded onto a monomer A backbone, like a brush polymer. Alternating and random structures are self-explanatory. A tapered structure has A and B merging into one another, with a period of alternation in between. A dendro block is a dendrite with one half composed of monomer B. Alternating, random and tapered structures are caused by molecular blending, the two monomers being polymerised at the same time. Block, graft-block and dendro-block polymers have monomer B added onto monomer A after polymerisation.

In many cases, the properties of a homopolymer are insufficient for a particular application. Mixing polymers together is theoretically possible, but in practice entropy means that they tend to phase separate. Copolymers avoid this issue. The extent to which control over structure is possible depends on the mechanism.

In step growth polymerisation, the incorporation of different monomers is statistical. Block copolymers can be made by reacting telechelic homopolymers together. The reversibility of this kind of polymerisation means that there can be backbiting/scrambling between similarly reactive monomers, meaning isomerisation in the polymer. If the monomers have similar reactivity, random distribution is possible. If they have very different reactivites, then it is possible to make block copolymers. It isn't yet possible to control the molecular weight of these processes.

In chain growth (radical) polymerisation, the pattern of monomer incorporation is controlled by the electronic properties of the C=C bond and nucleo/electrophilicity of the the propagating polymer radical. Monomers with an electron rich and electron deficient C=C bond respectively produce alternative sequences. Monomers with C=C bonds of similar electron character give statistical copolymers. Tapered comonomer incorporation is possible with LFRP. The more reactive monomer will polymerise first, and then as it is used up, the less reactive monomer will polymerise. In chain growth (ionic and metal coordination) polymerisation, statistical incorporation of the monomers occurs.

===Reactivity===

The method with which a monomer can best be polymerised depends on the reactivity of the monomer, the nature of the polymerisable group and whether the functional groups are compatible with the chain or step-growth process. Monomers with electron rich C=C bonds work well with cationic and radical approaches. Monomers with electron deficient C=C bonds polymerise via anionic and radical mechanisms.

===How to Choose Your Monomer===

First, it must be identified whether the C=C bond needs to be electron rich or electron deficient. Next, we identify the possibility of radical or ionic chain transfer. Can this species form inductively or mesomerically stabilised radicals? Does the monomer have an acidic or basic site? Then we must consider the steric demands of the polymer. 1,2 disubstituted alkenes polymerise poorly, owing to the steric hindrance being greater than the enthalpic driving force.

It is theoretically possible to work out the co-reactivity of two or more monomers by calculating the frontier orbitals. In short, the more stabilisation energy gained, the more favourable the kinetics. I've never heard that one before...

===Copolymerisation parameters===

To avoid people having to figure out for themselves whether a copolymerisation will work or not, someone came up with the bright idea of inventing copolymerisation parameters, standards that people can measure. There are r1 and r2. The first is the ratio of rate constants of radical A reacting with A to radical A reacting with B. The second is the ratio of rate constants of radical B reacting with A to radical B reacting with B. Both being 1 gives an ideal polymerisation (random distribution). Both being less than one gives a statistical polymerisation. Both being greater than 1 gives a block copolymerisation. Both being equal to zero gives alternating polymerisation (radical B reacts only with A, and radical A only with B). If r1 >1 and r2<1, tapered copolymerisation results (A prefers to polymerise with itself, and B to copolymerise).

===Graft Copolymers===

The random coil conformation has implications on the reactivity in polymer grafting. It has vast steric bulk, and so can cause a lot of hindrance. However, its flexibility means that reactions can still occur. Using macromonomers, the graft is quite easy to control. It is possible, though rare, for the grafting to be so dense that only a few repeat units are possible, as is the case when the side chains are dendrimers.

There are three synthetic strategies for graft copolymers. 'Graft from' synthesised a linear polymer with side chain groups that can be initiating sites. The grafting reaction has the linear polymer dissolved in monomer 2, and polymerisation is initiated. In this synthesis, initiator efficiency is high, steric demand is low, termination reactions are low, reaction times are short, and initiation/propagation is high. The chain length is not well controlled.

The 'graft onto' approach synthesises a linear polymer with side chain groups that can couple to other functional groups. The grafting reaction has a second linear polymer reacting with the active sites of the chains in the first. The conversion rate is high, the steric demand is high, there are no termination reactions, side reactions are low, and the reaction time is dependent on the MW desired. The steric demands of the two polymers means that graft density tends to be low. This can be improve by long reaction times. Because the starting polymers have already been purified, however, it is possible to obtain polymers with low polydispersity indices. This strategy requires a telechelic polymer.

==Section 5==
===Dendrimers===

Dendrimers are unique in that they are perfectly 3-dimensionally branched polymers. They represent an entirely new class of polymers with properties and applications their own. They have monodisperse molecular weight, and are the first polymers to have such. The viscosity of dendrimers, once past the 'starburst' limit, begins to decrease with increasing molecular weight.

The synthesis of dendrimers is bio-inspired. They have a 'snowflake-like' structure: much more rounded and space filling, rather than linear. They are synthesised in a stepwise fashion, and can require protection/deprotection chemistry, much like solid-phase protein synthesis. Most of the linear polymers we've encountered have been synthesised as dendrimers. Dendrimers are synthesised either divergently or convergently.

Divergent synthesis has a centre, eg. ethylene diamine. This undergoes exhaustive (as many as is possible) 1,4 conjugate addition, so it now has four ester 'tails' spread out around it. These 'tails' all have a core molecule added onto them, and so the process repeats. The early steps have the reagents in large excess, to ensure that the reaction has gone to completion. This goes on for between 5 and 10 'generations', each of which consists of one layer of amine, and one of ester. The limit is called the 'starburst limit'. These dendrimers thus have a 'globular' shape, with large cavities and minimal chain entanglements. They are also monodisperse, which make for lots of interest. This method of synthesis is faster, cheaper and less laborious.

In theory hyperbranched polymers should have many of the same properties as dendrimers. The main difference is that some of the chains do not have branching, whereas in a dendrimer every position has branching. This can be a drawback for drug synthesis, for example, but for materials it may not be so important. Hyperbranched polymers can be produced in a single step. Hyperbranched polymers also show the increasing molecular weight-decreasing viscosity phenomenon that dendrimers show, but there is as yet no explanation. The more branching there is, the more they exhibit this effect.

Convergent synthesis has some advantages over divergent synthesis. It offers control of the location of surface functional groups. Chemically different dendrons produced by this method can be combined into one dendrimer. The intermediates are more easily purified, and the core is more easily replaced, meaning that there is more chemical flexibility.

This method starts from what will be the surface, and works inwards. Once the outer segments have been constructed, they are all attached to the core. There is no rule that all segments must be the same, giving greater chemical flexibility. If the wrong core is chosen, either it will not react or the dendrimer will be floppy, with an ill-defined 3D shape. In addition, there is no solid starburst limit, so it is harder to determine the optimum size and shape of the molecule.

===Summary===
[[File:mesyltoe_3O3_polymersummary.png]]

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Pm1510: /* Copolymers */

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Pm1510: /* Section 4 */

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Pm1510: /* Living Polymerisation */

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Pm1510: /* Section 3 */

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Pm1510: /* Characterising Polymers */

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Pm1510: /* Characterising Polymers */

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Pm1510: /* Chain Growth Polymerisation */

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Pm1510: /* Chain Growth Polymerisation */

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Pm1510: /* Polymer Mechanisms */

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Pm1510: /* Structure-Property-Temperature Relationship */

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Pm1510: /* Polymer Structures and Properties */

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Pm1510: /* Polymer Structures and Properties */