Talk:Mod:khlmod1

Q1: Your energy values are correct and the discussion about different strain contributions is good. You should be careful that the key phrases are correctly worded: you state “Therefore, product 3 was found to have the lowest overall energy” when this is clearly the opposite of what you mean. What you say about kinetic vs thermodynamic control is right – indeed there isn’t much you can say about the outcome of the hydrogenation without further information about conditions, mechanism and with some transition state calculations (requiring DFT methods).

Q2. There are some problems with your structures – none of them actually represent the correct configuration of the intermediate. The aspects that are wrong in all or some of the structures are: double bond geometry, cis vs trans ring fusion (supposed to be cis i.e. hydrogens on same side) and relationship between ring fusion hydrogens and the dimethyl bridging group (should be on the same side). When performing these kinds of minimisations it is always important to keep an eye on the structure as it can easily become distorted away from what was intended. That said your approach to this question was exactly what was intended – analysis of different conformations to get a rationalised global minimisation and comparison of the strain contributions as in Q1. The extra material on the hyperstable olefin aspect is good - although accurate energy comparison is best achieved with DFT method, the qualitative consideration of the strain in the parent hydrocarbon is a good approach.

Q3. The MOs look good. The alkene syn to the chlorine is indeed the most reactive towards electrophiles. I think this is what you mean but the wording is a little confused; “HOMO is very susceptible to electrophilic attack” and “LUMO is very susceptible to nucleophilic attack” are tautologies – the important fact is which alkene does what (on the basis of its orbital distribution in the HOMO and LUMO). The IR stretches are good and the differences are well described and explained.

Q4. R=Me is indeed the best choice here to minimise job times. Your PM6 energies and structures are all good, although the MM2 ones are a little higher than expected – it is difficult to comment further on this without seeing the jmols, but it is more difficult to find the lowest energy conformer using MM2 in this task. Although you described the origin of the stereoselectivity at the beginning you didn’t correlate your findings back to this. You have found that there is an overwhelming preference for the cis fused intermediates and this means that nearly all of the reactants go through these intermediates and give the corresponding selectivity. Looking at the structures of the different intermediates, these lower energy conformations also have the best trajectory for attack so they are also more reactive. What you may have noticed is that using PM6 A=C and B=D; the method is capable of recognising the bonding interaction when the acetyl oxygen is close to the oxonium carbon (MM is not capable of this and the bonds are set from the start).

MP. The discussion of geometry minimisation is good – it is hoped that methods used in set questions will be applied to the more open-ended questions as much as possible. Your NMR data is a reasonably good match. Although it is fine to tabulate this data, it can be better presented in graphical form (typically bar charts are used). It is a shame that you did not attempt to compare the calculated data for both isomers with each set of lit data. In this way you could tell to what extent you can differentiate the two isomers and if you could work out the structure from a spectrum of an unknown isomer. With the IR data you need to compare to some experimental data to see if the calculation is accurate (it could be that the difference you see between the isomers is smaller than the error). A note on nomenclature: -Ph = phenyl or benzene ring (not benzyl ring – which is confusing given that a benzyl group is also present).