Talk:Mod:BMWWiki

FEEDBACK

Q1: Your energy values are correct and the data is well presented. You should make sure you check the compound numbers from the course wiki because your monohydrogenated compounds 3 and 4 are misassigned. It is correct to say that the lower energy compounds are the ones you would expect to obtain under thermodynamic control. The major difference between the monohydrogenated compounds is correctly identified as the bending strain due to deviation from ideal sp2 angle. Just to clarify this corresponds to the bond angle (angle between 3 consecutive atoms) not the dihedral angle (angle between 3 consecutive bonds).

Q2. Your calculations are good. It makes sense to analyse the 6-ring as an obvious site that could have a different conformations. It would have been good to hear about other ways you attempted to optimise the structure (successful or unsuccessful). With the stabilising orbital interaction – I presume you mean an interaction between a C-H sigma bond and the carbonyl pi*, it does look like a better overlap in the structure of isomer 10. (NB: When you draw a CH sigma orbital you should make sure that one lobe is smaller than the other – it is an sp3 orbital not a p orbital). Of course, this contribution cannot be analysed by MM2 calculations which do not incorporate molecular orbital interactions – Perhaps a semi-empirical method would have shown a much larger difference in energy between the isomers due to this. The definition of a hyperstable alkene is spot on.

Q3. The structures look good and the MOs and IR stretches are correct, but you should have stated the energies of the compounds you obtained with the different calculation methods as it is hard to make an overall assessment on the calculation without this detail. The syn double bond is indeed the most reactive towards electrophilic attack. The description of the pi-sigma* orbital interaction and its implications for the IR spectrum of the different compounds is done well.

Q4. Your energy values are good – mostly spot on, others close to expected. I think that the methyl group is the best choice here given a generic alkyl chain and semi-empirical methods are much better for this type of system. Your energies and jmols show that for the MOPAC calculations A=C and B=D. That is the calculation cannot distinguish between them because the cation is treated as a non-classical carbocation and the neighbouring group effect is incorporated into a hybrid structure. This reaction is an example of Curtin-Hammett kinetics, the selectivity is defined by the relative amounts of the isomeric starting materials and the inherent reactivity of each isomer. In this case both factors favour formation of C and D: A and B are lower in energy than A’ and B’ and have a better trajectory for nucleophilic attack.

MP. The task set out at the beginning of this mini-project is ambitious but is exactly the type of question that should be attempted here. Calculation of NMR data for a set of diastereomers and then comparison of that data to a set of genuine data for one of those compounds is a good way to test whether a predicted NMR spectrum can be used to distinguish different isomers. Here you state that the data seems to match the intended compound, but it would have been nice to see some error analysis and a more quantitative approach to making that assessment. The IR assignment is a good extra thing to analyses, however it is unlikely to allow for distinguishing between different isomers because as you stated the calculations aren’t very accurate – for a start they are calculated in the gas phase, whereas in reality they are usually obtained in the liquid or solid state. Did you consider how you could tell these isomers apart experimentally (without calculations); it should be possible to envisage NOESY NMR experiments which can show the proximity of various proton present in the structures.