Talk:Mod:leoboon
I apologize immensely, but when I moved onto module 2 I completely forgot about the Optical Rotation. only now I realize I had two files waiting to be assessed. Unfortunately I cannot gather any information on Optical Rotation by them. Please assume that I skipped that part. Sorry again. Pa07 19:52, 8 December 2009 (UTC)
---
Q1: Your energy values are spot on for the two isomers of the dimer and the hydrogenated endo product. The dimerisation is indeed under kinetic control, the difference must therefore be in the energies of the transition states so the attempt to calculate them was the right approach to analyse the problem. Unfortunately the method you have (MM2) is not capable of transition state calculations; in order to minimise to a transition state you need to use a higher level of theory and identify an imaginary vibration as you will find out in the last module. Bending strain is indeed the most important factor in differentiating the energies of the hydrogenated products. Bending strain relates to deviation from the ideal bond-angle so the differences lie in the ability of the double bond to achieve its ideal sp2 bond angles. Again, an attempt to calculate the transition states is the correct idea for fully analysing the reaction, but MM2 is inappropriate. For the hydrogenation of an alkene, typically a transition metal catalyst will be used (along with hydrogen gas), so it is necessary to consider how accessible double bonds are and whether the conversion of metal hydrides into alkyl metal compounds is a reversible reaction.
Q2. Since there is already a defined stereocentre in this molecule before the addition of the nucleophile, the relationship between the two possible products is diastereomeric, not enantiomeric. For compounds to be enantiomeric, all stereocentres present must bear the opposite configuration. The calculated structures appear to be correct and the rationalisation for stereoselectivity is also correct. The reason MM2 cannot be used to minimise a structure including Mg is that the set of parameters used to carry out MM2 calculations doesn’t include Mg; it could be modified with the input of some experimental data in order to include elements that aren’t included in the MM2 parameters. The steric clash between the carbonyl group and aniline nucleophile is strong because both of the groups involved are electron rich.
Q3: Your calculations appear to be fine and the lowest energy conformer has been identified. You have allude to the fact that you found some different energy conformations in carrying out the calculations but it would have been nice to see some of the structures and discussion about the differences between them. Hyperstable olefins are indeed found in the bridgehead position (anti-Bredt)of medium rings, but the full definition is that the strain felt by the analogous alkane – e.g. if you hydrogenated the double bond – is greater than that felt by the olefin itself.
Q4: The most nucleophilic double bond is actually the one situated syn to the chlorine atom, but your choice of the other double bond was at least logical given your calculated HOMO; the bond stretches you have reported appear to be fine. You should have found that the C-Cl bond is weakened by the presence of an anti double bond in the ring system. This arises due to a partial donation of double bond electron density into the proximal C-Cl sigma* orbital.
MINI PROJECT: The introduction is nice and concise, although perhaps would have benefitted from having the reaction scheme shown. The literature labelling is no doubt a set numbering system for this kind of molecule, or maybe it has been previously defined for this exact product, whereas the GaussView labelling is pretty much arbitrary. Of course, you can make the assignment of the spectrum easily, because GaussView tells you which atom corresponds to which peak. The differences between the calculated and experimental NMR data don’t seem to great, but there is no discussion about this here. Often this kind of comparison is handled graphically, e.g. in a bar chart to give a quick visual guide to which atoms have been accurately calculated etc. Also, when differentiating between two slightly different molecules it is often the case that the calculated data for both molecules is compared to the experimental data of one molecule directly; the hope is that the calculated molecule that is actually the same as the synthesised molecule will have a closer match to its data than the wrong molecule would; this would hence show that the molecules can be distinguished in this way. It is not the case that the two products would necessarily have equal, opposite optical rotations because their relationship is diastereomeric not enantiomeric; there are five stereocentres and to get from 2 to 3 you only need to invert one of them, whereas to convert to an enantiomer you need to invert every stereocentre.