Talk:Mod:YJY1109

FEEDBACK

Q1: Your values are all good and the factors that contribute to strain and differentiate the different isomers are correctly identified and explained. The discussion of kinetic vs thermodynamic control is correct, although it is not really possible to comment much on the selectivity of the hydrogenation reaction without further information. If the reaction is under thermodynamic control the lowest energy isomer you show would be the predicted product. Hydrogenations of this type are typically, metal catalysed, irreversible, and therefore under kinetic control. To predict the outcome of this type of reaction, the transition states would need to be calculated using a DFT method in order to find the kinetically favoured product.

Q2. The lowest energy isomer is correctly identified and the energy values are good. Although, the chair conformation is the lowest energy form in both cases here, this is not necessarily true for all 6-rings. In some cases, the specific substituents can mean that the twist-boat or even the boat form is the most stable conformation. The analysis of 6-ring conformation is a rational starting point for this energy minimisation, but it would have been nice to hear a little more about other ways in which you attempted to get the lowest energy form. Hyperstable olefins are correctly defined and it would be possible to get a value for the olefin strain if you had performed DFT calculations. However, with the molecular mechanics calculations you have it is not possible, because strictly you can only compare isomers.

Q3. This was very well answered. The MOs and IR stretches are good, the rationalisation of the reactivity with dichlorocarbene is correct, and the analysis of the IR stretch changes with and without the pi-sigma* interaction is correct.

Q4. Your calculated structures and energies are a fair bit off. It is possible to twist the acetyl group so that its carbonyl oxygen points above or below the oxonium carbon in all cases without employing a ring-flip of the other substituents. Also, for C* and D*, the idea was to perform calculations on the trans-fused [6.5] bicyclic system in comparison to the cis-fused [6.5] bicycles C and D. Do not hesitate to ask a demonstrator if there is any similar confusion about questions in future modules. R=methyl and semi-empirical calculations were correct choices for this problem for the reasons you gave. Diastereoselectivity in the formation of C/D over C*/D* is due to the intermediates A/B being favoured thermodynamically and also being more reactive (better trajectory and distance for nucleophilic attack).

MP. Although you have listed the calculated and lit NMR values, it would be better to show the differences in ppm. This can be done graphically to good effect, showing instantly the error and the overall deviation. In order to assess the use of NMR prediction to distinguish between different isomers, it would be necessary to compare the calculated data of both isomers to the lit data. If the prediction technique is valid, the error in the comparison using the actual isomer should be less than in the comparison of the false isomer.