Talk:Mod:mw1409module1

FEEDBACK

Q1: Your energy values are all good, but you could have shown at least an image of the structures. The dimerisation is indeed under kinetic control; this is specifically due to stabilising secondary orbital interactions in the Diels-Alder transition state that leads to the endo-product. Bending strain is identified as the key difference between compounds 3 and 4. The reason for this is in fact the double bond in the bridged bicycle deviates further from its ideal sp2 bond angles than the one in the fused 5-ring does.

Q2. The energies are fine and the lowest energy isomer is correctly predicted. You said that other conformations didn’t result in lower energies, but it would have helped to include these results to give a more complete answer that showed your approach to looking for different conformations. Although in this case the lowest energies are obtained with chair conformations for the 6-ring, this is not necessarily true in all cases; some substituents and substitution patterns can lead to the twist-boat or even the boat form being favoured. The definition of a hyperstable alkene is correct.

Q3. The MOs and IR stretching energies are good. Getting symmetrical MOs can be problematic with the methods employed here, but the key thing was to recognise that they weren’t correct! The explanation for the regiocontrol in the reaction with dichlorocarbene is right. The reason that the C-Cl stretch becomes stronger on removal of the anti double bond is that there is an interaction between that pi bond and the C-Cl sigma* which weakens the bond.

Q4. Your energies and structures all look good. I would have recommended that R=methyl is the best choice for a generic methyl group, but your analysis shows that the same results are observed with R=hydrogen. As you found, A/B/C/D have lower energies than A’/B’/C’/D’ and for C’ and D’ the trans-fused bicycles are significantly more energetic. As your numbers and structures indicate when you use PM6 to model these intermediates you find that A=C and B=D. This is because, the semi-empirical method accounts for the neighbouring group effect and the structure calculated is a non-classical carbocation. The origin of the stereoselectivity for the formation of C and D is described in your text but not explicitly referred to as such! The intermediates A and B are both lower in energy and also more reactive (better orientation and angle for nucleophilic attack) than A’ and B’.

MP. You discussed this interesting difference in stereochemical outcome depending on which reducing agent is employed and it would have been nice to see some more discussion of this and how computational chemistry could be employed to elucidate this aspect. Your NMR analysis would have benefitted from a more quantitative approach. Comparison of the lit and calculated values would be best shown as the differences in ppm which could be tabulated, or better displayed graphically. Additionally, in order to show that this method could be used to distinguish between the two isomers it is necessary to show that the calculated data matches the actual data for the isomer better than calculated data for the wrong isomer.