Talk:Mod4:Tcg06

Q1

• Structure 3 was wrong, hence the wrong energy.

• The ChemDraw picture of structure 4 is wrong.

• Using compound numbering would have made it much easier to follow your argument.

• “This can be explained since the exo- transition state has a more favourable pi-orbital overlap”. It should be the other way around. The endo dimer 2 is the kinetic product, so its transition state must be lower in energy.

• You correctly indentified the bending energy as the main difference between 3 and 4. But I don’t think your explanation of this difference is very clear/correct. Again, a numbering system of C atoms or visual aid would have helped. Comparison between the bond angles and the ideal values in structure 3 and 4 would have been eye-opening.

Q2

• You seem to find the correct optimised structures of 9 and 10, and your energies are correct.

• Good account of the twisted-boat and chair conformations of the 6-member ring.

• I think you understand the concept of ‘hyperstable alkenes’ although you could have shown the calculation for the hydrogenated product to back it up.

Q3

• Compound 12 isn’t a dichlorocarbene, Cl2C: is and will be a reactant to react with compound 12.

• You should have described how you optimised the structure, how the MOs were calculated in more details.

• The MOs seem to be correct, although I would prefer larger structures which I can see clearly. Translucent MOs would also help.

• Surely compound 12 has higher symmetry than C1?

• Which method was used for the calculation of IR?

• You identified the vibrational frequencies correctly, but some analysis on the changes in C-Cl frequencies and C=C bond lengths which are proofs of the sigma*-pi interaction in structure 12 is required here.

Q4

• Your explanation of neighbouring group effect is rather non-canonical. The calculation will show that the adjacent acetyl group actively participate in the reaction, rather than just blocking one face from nucleophilic attack.

• Correct choice of the Me group, but you didn’t explain why we do not use H.

• Structures A’, B’, C’ and D’ were not included. I found out that your A is actually B. B’ is also B, optimised in PM6. C is actually D, D is actually C. Most of them are not yet the lowest energy conformers.

• Individual optimised structures from MM2 and PM6 are required, especially when they’re different. As it stands, I can’t tell which technique produced the Jmol structure! (I can tell, but you need to tell me, and you need them both).

• Rationalisation of the diastereospecificity is missing.

Mini project

• Your calculated values of 13C NMR and IR for the two structures are significantly out compared to experimental data.

This should have been discussed with us prior to submission.

• One important piece of information, which isn’t included is how you optimised the two structures, on what level of theory, and how the data were calculated. I’m afraid you’re giving me very little to work with other than your calculated results.