Talk:Mod:babes
Q1. Your calculated energies are all correct and you are right to say the reaction is under kinetic control; the reason for this is that the transition state leading to the endo product is stabilised by secondary orbital interactions. You correctly identify torsion as the major difference between the endo and exo isomers and bending strain as the difference between the monohydrogenated compounds and the relation of these facts to the structures you obtained was well done.
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Q2. I think it was a good choice to use MOPAC for this problem given the problems with molecular mechanics. The structures look fine and the angles you have found for the lowest energy conformation are right. The variation in energy for different CO angles is interesting, but in this case, variation of that dihedral angle is not the only structural element that can be varied: so it is possible to have conformers that differ by both CO dihedral and other structural changes – it then becomes difficult to analyse varying dihedral angle as the other changes may be more important overall. The aniline nucleophile reactions on the opposite side to the carbonyl group as due to sterics, there is large repulsion because the two groups are oxygen and nitrogen containing so there will be repulsion between these electron rich atoms.
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Q3. The energy values are close and you have the isomers the right way around in terms of energy. The analysis of the different conformations and variation of the 6-ring shape was well done. In fact one of the isomers has a twist-boat 6-ring in its most stable conformation. The term hyperstable alkene is correctly defined – alkane produced by hydrogenation is more strained than the alkene; I would say this is common for medium bridgehead alkenes but not all bridgehead alkenes (if you have small rings they are not allowed = Bredt’s rule).
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Q4. The MOs look good and you have correctly observed that the location of the HOMO on the syn double bond shows this is the most nucleophilic. Later on you say that this double bond has a more energetic IR stretch and this contradicts the first observation but this is not true: the reactivity relates to the energy of the pi orbital (or rather the amount of the double bond pi character in higher energy molecular orbitals) whereas the bond strength is a combination of the strength of the sigma and pi bond and stretching the bond also impacts on the rest of the molecule. You have described the interaction with the antibonding orbital on C-Cl well and as you say the reason the C-Cl bond is stronger if you take the double bond away is that you remove this interaction. For the differently substituted analogues, the important point is the electronics of the new group (electron donating (OH) or withdrawing (CN)). If the double bond is made more electron rich or electron poor this impacts on its interaction with the C-Cl antibonding orbital.
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MP. This is obviously a viable reaction for study by computational chemistry as it has already been looked at – but for this same reason it perhaps wasn’t ideal because it comes down to a comparison of different computational techniques rather than a confirmation/reassignment of structure. Your disagreement in terms of energies of products is interesting. Is it possible that you haven’t found the lowest energy conformation – normally structures are optimised with increasingly complex levels of theory. It would have been good to compare your NMR data to some experimental data (or Balci’s computational results) to assess the how closely they match – this can be done nicely using a bar chart to give a quick visual representation. The IR data is not assigned or compared to the lit. but you would probably have seen quite a large error in the stretches because the calculation is done in the gas phase, whereas the molecules in question were almost certainly not analysed experimentally as gases.