Talk:Organic:luke4912

Q1. All of the energies you have calculated are good, but it is not necessary to quote the results to so many decimal places. Your reasoning about the kinetically controlled dimerisation is good – the reason for the kinetic control (secondary orbital interaction stabilising the endo-forming transition state) could have been discussed. For the hydrogenated endo isomer, the increased stability of compound 4 is indeed due to difference in bending strain. This is technically defined as the energy contribution due to deviation from ideal bond angles, so the difference in energy is more due to one of the alkenes having bond angles further from the ideal (120°). Perhaps you would have benefitted from briefly describing the difference between thermodynamic and kinetic control. If you find that you form the least stable isomer in a reaction, it means that the reaction is under kinetic control; if you find that you form the most stable isomer, it means the reaction could be under thermodynamic OR kinetic control. The thermodynamic product could also be the kinetic product.

Q2. Your calculation for compound 5 is correct, 7 is a bit high, is it possible the positive charge on N is missing. In any case, the pictures seem to show that you made the wrong isomer for 7: the methyl group on the 7-ring has the wrong configuration ((S) when it should be (R)). It is important to take care when manipulating these structures in Chem3D and GuassView not to change the absolute configuration. Your reasoning for selectivity of the Grignard attack is correct, for aniline, the lone pair of the nucleophile will be repelled by the lone pairs on the carbonyl oxygen and that is why it attacks from the opposite face.

Q3. Your results are good and I think you approached the problem in exactly the right way: identifying the most changeable parts of the molecule (in this case the cyclohexane part). Although cyclohexane prefers the chair conformation, this may not always be the most favourably conformation when substituents are added – as you found with the twist-boat containing isomer.

Q4. The calculations appear to be good, judging by the appearance of the MO diagrams and the change in C-Cl bond strength, but you should report your calculated energies. NB it is better to refer to the alkene groups as being syn the C-Cl bond rather than underneath. You correctly identify the double bond syn to the C-Cl as being more nucleophilic as shown by the MO distribution in the HOMO.

MINI PROJECT. The calculations appear to be good and you have a wide range of data. You are correct to suggest that the IR spectrum would change as the ketone is converted to the alcohol product, although there would also be changes in the 1H NMR and this is the more usual diagnostic tool. The idea of comparing J coupling s good, as you could indeed use this to tell which isomer you have formed. The calculated optical rotation does appear to be a good way to determine which isomer is which in the context of this project, it would have been interesting to see how well the calculated NMR values for both isomers compare to the experimental data in terms of the error for each resonance. This is the way structural assignments are usually assessed in published computational chemistry.