Talk:Mod:BTL091

Q1: Your energy values are all correct, but it is not necessary to give so many decimal places; there is some error in these quantities because they are approximations and the program will show some variation from one calculation to the next. The strain contributions in the hydrogenated compounds is well analysed. Discussion of thermodynamic vs kinetic control is mostly good; in the case of the hydrogenation, you can only say for sure that compound 4 is favoured in the process if it is under thermodynamic control. If the reaction is under kinetic control however, more information (and DFT calculation) would be needed to determine which pathway has the lowest energy activation energy.

Q2. The structures given have the wrong geometry at the ring junction (should be cis-fused). It is very important to regularly check that the structure you are working on matches the compound in question. It would have been good to see some more examples of different conformational isomers (like you did for 10a and 10b) to discuss how you minimised the structure. For example you said you only looked at conformations with the 6-ring in the chair conformation but you could have shown the twist-boat conformations as well. Although cyclohexane prefers a chair-conformation, this is not always the case for substituted 6-rings. The definition of hyperstable alkenes is good.

Q3. Your MOs look good and the explanation for the regioselectivity is correct. The IR stretches are good for the alkenes; the ones for the C-Cl bonds are a little low (possibly C-H vibrations? – it is difficult to tell without an image of the vibration). The C-Cl stretch of the monohydrogenated compound should be higher in value than for the dialkene – your result contradicts this although you say differently! (Wavenumbers are proportional to energy and hence bond strength). If your conclusion (or the lit result) doesn’t add up to your calculated result there has to be some explanation given or maybe it suggests you need to look back at your calculation.

Q4. R=methyl is indeed the best model to use here. Your energy values and structures are pretty much exactly right! You are right to say that the selectivity is largely governed by the huge difference in energy between C/D and C’/D’ giving the higher energy configurations only in vanishingly small quantities. There is also a kinetic factor - the trajectory for nucleophilic attack is better in C/D than C’/D’. The major difference between MM2 calculations and MOPAC calculations here is that MM2 cannot incorporate the neighbouring group participation whereas MOPAC can. You can see from your energy values and your structures that A=C and B=D. This is because with the carbonyl oxygen proximal to the oxonium carbon, the method recognises that bond formation is possible (this isn’t possible in MM2 because the bonds are set at the start).

MP. Wrt regioselectivity: More specifically, the selectivity depends on the ability of the migrating group to stabilise positive charge because this is the group that develops the most charge in the transition state. You’re calculated NMR data looks like a reasonable fit. It makes more sense to discuss the difference in ppm rather than the % difference in this case when analysing the error here and it the deviations are often depicted graphically (in a bar chart for example). The problem with using 15N NMR to attempt to distinguish between the isomers is that you’d have to make the 15N enriched product typically introducing N as ammonia or another cheap feedstock. IR is about as accurate as is expect – not easy to model especially since the calculation is done in the gas phase (experimentally IR is measured in condensed phases). You are right to mention inaccuracy possibly arising from different conformational preferences of the compounds and the difficulty in obtaining a global minimum – to elaborate on this it would be necessary to analyse different conformations available (as in Q2).