Talk:Mod1:pkm
Q1. The values you have calculated all look good but it is not necessary to report so many decimal places for your answers; you may have in fact noted that the calculations can vary from time to time so there is some error to account for and the last few decimal places are not worth giving. The explanation for the kinetic control in the dimerisation reaction is correct. The comparison of the different contributions to energy is good and the key factor differentiating the two monohydrogenated compounds is the nature of the remaining double bond as you said. The main thing is the bending strain which relates to deviation from ideal bond angles.
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Q2. You are right to say that molecular mechanics can’t be used to analyse reacting molecules (i.e. transitions states) and as you say the magnesium atom can’t be included in MM2 calculations because it is not recognised (the program doesn’t have appropriate parameters for Mg). It is actually possible to manually update the data so Mg can be included and then you could include the coordinated Grignard reagent in the calculation (as an intermediate not transition state). The selectivity is accurately described – the steric clash between the carbonyl group and the aniline is particularly apparent because the atoms interacting are both electron rich and therefore repel each other.
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Q3. Intermediate 10 is indeed lower in energy than 9, but your energy values are a bit off. For Int 10 this is because the double bond geometry is incorrect (gem-dimethyl bridge should be cis to the hydrogen. Your definition of hyperstable alkenes is right, but you then go on to contradict what you said. The slow reactivity is due to the fact that the alkanes produced are unusually strained and high in energy not because the alkene is shielded in any way – this is an extension of the Curtin-Hammett postulate: the transition state (that governs the kinetics) is product-like and therefore also unusually strained.
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Q4. Your MOs and IR stretch values are good and you have correctly identified the syn alkene as being more nucleophilic. For the monoalkene example, you were supposed to remove the double bond anti to the C-Cl bond, from your stretch values it seems as though this is what you have done although your table suggests you removed the other (syn) double bond. This removes the interaction with the C-Cl antibonding orbital which you discussed when talking about the MO diagrams. If you remove this interaction the C-Cl bond is strengthened.
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MP. This is an interesting example for a mini-project since it is a structural revision that was done by means other than calculation of NMR shifts. I am not entirely sure which set of lit. data you are comparing both sets of calculated data to, but it appears it is the NMR of the originally proposed structure (proven to not be that of laurentristich). It is a shame that you didn’t fully analyse the differences between your values and this set of lit. data because it seems as though your calculation much more closely matches one compound over the other. This would suggest you could have used computational chemistry to reassign this molecule by showing that the data more accurately fits the “revised structure” than the “proposed structure”. A good way of showing this error is graphically, with a bar chart giving a good visual sign of the overall accuracy. As you suggest, the IR data is similar in both isomers because there are no major functional differences. Calculated IR stretches often suffer from error because they are done in the gas phase (under experimental conditions the molecule you have studied will not be gases).