View source for Talk:Mod:ap1707

Q1. Your energy values are correct and you have compared the energy contributions well. You are right in pointing out the key structural differences (steric clashes for 3 vs 4 and deviation from ideal bond angles for 5 vs 6). These contributions to strain are represented by torsional and bending strain respectively. For the hydrogenation, although the lowest energy product is formed, this does not necessarily mean that the reaction is under thermodynamic control: If the product formed is the higher energy isomer (as in the dimerization of cyclopentadiene) it can be said for certain that the reaction proceeds under kinetic control (by the process of elimination since it can’t be under thermodynamic control). If the lowest energy isomer is formed, the reaction could be proceeding under thermodynamic OR kinetic control and it is not possible to say for certain without closer consideration of the reaction conditions and transition states. Alkene hydrogenation is usually carried out with a metal catalyst (e.g. palladium on charcoal) and hydrogen gas; under these conditions, reactions are usually under kinetic control (i.e. the products are not in equilibrium with the starting materials) and selective hydrogenation of alkenes is due to the steric effects that allow coordination of one alkene more easily than another.

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Q2. Your structures and dihedral angles look good and the explanation for the selectivity is right. The way you answered the question is precisely what we were looking for – finding different conformations and analysing the structure of the lowest energy conformation. In the mechanism for the Grignard addition you have the magnesium coordination represented well, but you should draw the curly arrow from the Mg-C bond rather than from the Mg atom. The aniline is specifically repelled by the carbonyl group because both species are electron rich and the localised negative charges (i.e. lone pairs) clash.

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Q3. Very well answered overall. The down isomer is indeed the lowest energy conformation – your discussion of the optimisation process is good (the 6-ring is the most obvious place to start to optimisation). The definition of a hyperstable alkene is spot on.

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Q4. As you say, attack at the syn double bond is favoured because this is where most of the HOMO is located, so it is the most nucleophilic of the two double bonds. The fact that you have found the anti double bond to have a lower energy IR stretch does not contradict this finding. The reactivity is due to the energy of the pi orbital on the double bond (or rather the energy of the MO in which it is largely located), whereas the overall bond strength is a factor of both the sigma and pi bond and also stretching a bond impacts on the rest of the molecule which has implications for the energy required. The strengthened C-Cl bond in the monoalkene is indeed due to loss of orbital interactions with the double bond, specifically donation of electron density into the C-Cl antibonding orbital.

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MP. The reaction could indeed be monitored by IR which would be a good technique to use as it is relatively quick and cheap. You could also probably monitor by TLC as alcohols are usually much more polar than (and therefore distinguishable from) ketones from which they are derived. How did you assign the corresponding peaks from calculated and experimental NMR data. The numbers given by chemdraw/Gaussian are totally random and will bear no resemblance to any existing numbering system unless you manually change them. The best way to work out which is which (if the experimental data is not already assigned) is to line them up in increasing order. Also the best way to tell if you can distinguish two possible isomers from a set of experimental data is to calculate for all of the possible isomers and then compare both to the real data and see which fits best (and if the difference is big enough to say definitively which isomer has been produced in the reaction).