Talk:Mod:CSWmodule1

FEEDBACK

Q1: Your energy values are all spot on and the discussion of contributions to strain is good. The discussion of kinetic vs thermodynamic control is all correct and as you point out, it is not possible to comment further on the outcome of the hydrogenation reaction without more information. The majority of such reactions are irreversible and therefore under kinetic control. It would therefore be necessary to calculate transition state energies to find out which product is preferred.

Q2. Your energies are good as is the discussion of your optimisation approach – this was the best way to answer this question with brief description of your calculations. Analysis of the energy contributions as in Q1 would have been a nice addition here. Hyperstable alkenes are well defined.

Q3. The MOs and IR stretch values are good – you should have included the energies of the main compound obtained with different calculation methods; this isn’t very important in this question, but it is a way in which the calculations are assessed and you may lose marks in the future modules with an omission of this type. The different double bond reactivities is discussed well. The IR stretches are indeed effected by an interaction with the C-Cl sigma* orbital: The pi orbital of the anti double bond is suitably positioned to interact with this antibonding orbital and therefore the C-Cl bond is weaker, and the stretch energy is lower when there is a double bond in that position.

Q4. The combination of R=methyl and semi-empirical methods is indeed the best choice here. Your structures are fine, but better approximations to the problem are found when the carbonyl oxygen of the neighbouring acetyl group is placed in a position where it can attack the oxonium carbon. The jmols for C/C*/D/D* are actually structures for A/A*/B/B*. The energy values you got are nevertheless reasonably close to those expected and you correctly found that the set A/B/C/D is lower in energy than A*/B*/C*/D*. The selectivity in this process is due to a combination of lower energy and better reactivity of the intermediates A and B compared to A* and B*. Using PM6 you should have found that A=C and B=D because the structure calculated incorporates the neighbouring group participation and treats the intermediate as a non-classical carbocation.

MP. You have compared lit and calculated data for both isomers and shown that there is a moderate correlation. It would be good to see whether the calculated spectrum for one isomer is more closely matched to the isomer it is supposed to be than to the other isomer. This would be the key to having an accurate method for assigning the stereochemistry of product of unknown configuration. It is good that you have shown the differences in ppm but there are lots of ways to analyse the differences more quantitatively to give a more general figure to describe the accuracy of the calculation. Something else to consider is how you would distinguish between these isomers experimentally.