Talk:Mod:1992

Q1: Your energy values are all good but it isn’t necessary to give so many decimal places – there is some error to take into account in these approximated quantities. The analysis of strain contributions is well done – the torsional strain is strictly determined by deviation from ideal dihedral angle; this is affected by sterics (as you discussed) but also by the possibility for hyperconjugation and corresponding stabilisation. Also you referred to the alkenic carbons in the hydrogenated compounds as sp3 – I think this was just a typo but be careful in getting important terms correct when editing. All of the discussion of thermodynamic vs kinetic control is correct as are the conclusions you can make about the reactions with the limited information you have.

Q2. There are some problems with your structures for these compounds. In each case the double bond geometry and ring fusion geometry are incorrect – should have the hydrogen of the double bond cis to the gem-dimethyl bridging group and the ring junction should be cis fused (hydrogens on the same side). It is very important to double check the structures you are working on to make sure they still match the compound in question. In answering this question it would have been good to see some different conformations (not necessarily just the low energy ones) and a description of the steps taken in performing the minimisation since this is the bulk of the work required. The definition of hyperstable alkenes is correct and the qualitative analysis of the structure of the hydrogenated derivative is good. You correctly recognised that even if you cannot compare the energy values (using MM2) you can compare the structures generated.

Q3. Some of the MOs look good, although it looks to me that the HOMO has more density on the side anti to Cl. The results of this these surface calculations can be very inconsistent and sometimes it is worth running them a few times with different minimisation attempts (and also symmetrisation) to get them looking more symmetrical and with the appropriate lobes. The IR stretches are all as expected and indeed the C-Cl bond is weakened when the anti-double bond is present due to the pi-sigma* interaction. One side note – you should include all of the energy values you get somewhere (e.g. those from the MOPAC methods) because these are used in part to assess the calculation; this probably applies to the other parts of the course too.

Q4. R=methyl is definitely the right choice for this question. Your energies are all a little off because you have missed an oxygen atom out of the structure the reactive acetyl group should be bound to the ring by this missing oxygen (e.g. OAc not CH2Ac). Aside from this the structures look about as expected. As your results show (both structures and energies) A=C and B=D when MOPAC methods are used. This is because MOPAC can account for the attack of the acetyl group (bond formation); in contrast, MM2 cannot imagine bonds to be any different to the initial input. You are right to say that the C’ and D’ forms are so high in energy they are present in vanishingly small quantities and do not influence the overall stereoselectivity; the approach for nucleophilic attack is also better in C/D.

MP. This NMR data looks reasonably close to the lit values – I think the sets of data would be better presented in a table for this question to clearly see the deviations (or better still graphically in a bar chart for example). You could have analysed this a bit more quantitatively by finding the average error. Working out ratios of peaks in 13C NMR is not normally done because peak intensity does not correspond to relative abundance in the same way as in 1H NMR. This is because the relaxation time of the 13C nucleus is slower and not all nuclei will have been relaxed in time in between scans. Usually the carbon NMR is analysed by counting the number of peaks to ensure that the structure is correct and equivalent nuclei will be taken into account. It is perfectly logical to average out the resonances you get for peaks made non-equivalent in the static form, because that is exactly what is happening experimentally with the rotation of the bond averaging out the chemical environments. You calculated NMR spectra for both forms of the product which is exactly the right approach to see whether you can differentiate between the two forms – to follow up on this you could have shown how close each of the computed resonances match the experimental data to see to what extent the different isomers can be differentiated.