Talk:Mod:cherrybakewell

Q1: All of your energy values are correct and the analysis of strain contribution is good. The discussion of kinetic vs thermodynamic control is all correct – as you say you can’t say much about the outcome of the hydrogenation without further mechanistic information and transition state calculations.

Q2. The structures and energies you have are correct but it would have been nice to see some discussion about how you achieved this minimisation – e.g. showing other conformations found and the key features that needed tweaking. You correctly identified the 6-ring conformation as a key area to look at - the twist boat forms would have been good to look at because the chair is usually the favoured conformation for substituted cyclohexanes but not always. The analysis of the stability of these compounds is correct – the parent alkane experiences a lot of strain due to the type of ring system; it is specifically a “hyperstable olefin”. NB: the reaction you are talking about here is a hydrogenation not a hydration which would be addition of water over the double bond (e.g. one -OH group one -H group).

Q3.Your MOs and IR stretches are correct. The more reactive alkene (towards electrophiles) is correctly identified. The C-Cl bond is indeed weakened by the pi-simga* interaction that you describe. One thing worth noting is that you should include all of the energies that you calculate (e.g. MOPAC as well as MM) because these are used as a judge of the calculations to some extent.

Q4. Your energy values are excellent (only one of the MM2 calculations is a little off (C)) and I can’t really see what’s wrong with the structure. As you said with MOPAC A=C and B=D – this is because the bonding interaction of proximal groups can be determined (unlike with MM2 in which the bonds are fixed at the start). Having obtained some energy values and structures for C/C’/D/D’ you could have commented further on the stereoselectivity of the glycosidation. C, C’, D and D’ all favour one product after nucleophilic attack and the distribution will be heavily in favour of C and D since they are much lower in energy to C’ and D’; additionally the trajectory of attack is better in these lower energy configurations.

MP. Your discussion of the geometry optimisation is good – one aim from the course is to use the methods from the set questions in the more open ended questions. For your NMR data: presenting the information in tables is fine and it is good that you have given the deviations from lit values, but there should be some more discussion in the text that describes the accuracy of the results. There are also more creative ways of performing this analysis - this sort of data is usually depicted graphically (bar charts are typical). Since you have calculated data for both isomers it would also have been possible to determine whether the calculation can distinguish between them. The most useful application of this type of calculation is being able to work out which isomer you have given the NMR of an unknown compound. With the IR data – comparison with the lit is necessary to tell if it accurate (admittedly the IR data is not usually extensive in synthetic reports).