Talk:Mod:mod1ii108
Good clear introduction. Q1. All of your energy values are correct and the explanation for kinetic control in the reaction is very good. For the monohydrogenated products, the differences in bending strain are discussed well – the key thing being the deviation from ideal sp2 bond angles. You are right to say that the lowest energy product is the one formed if the hydrogenation happens under thermodynamic control, but whether you have thermodynamic control is dependent on your reaction conditions: Alkene hydrogenation is usually carried out with a metal catalyst (e.g. palladium on charcoal) and hydrogen gas; under these conditions, reactions are usually under kinetic control (i.e. the products are not in equilibrium with the starting materials) and selective hydrogenation of alkenes is due to the steric effects that allow coordination of one alkene more easily than another – in this case the thermodynamic product is also the kinetic product (this is in fact usually true.
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Q2. Energy values are reasonable for your final conformation. A further discussion point you could have raised here is the way in which you attempted to find different conformations and the differences between them. The stereoselectivity in the reaction is well explained. The reason you can’t include the Grignard reagent is that the MM2 method does not have parameters for magnesium.
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Q3. Your energy values are spot on, and you correctly identified the different conformations of the 6-ring (twist-boat for 9 and chair for 10) as being the distinguishing feature – Cyclohexane units are a good starting point for this kind of problem because of the well-known possible conformations. How do the energy terms you have found relate to the two structures? Since bending is an important difference you should consider bond angle differences. Hyperstable alkenes are defined correctly – the key thing being less strain than the parent alkane.
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Q4. The MOs look good and the explanation of the relative reactivity of the double bonds is correct. While it is true to say that the pi-sigma* interaction stabilises the alkene, it is also worth mentioning its effect on the C-Cl bond; donation into the sigma* necessarily weakens this bond. Your IR stretches are about right, but you should have seen for the monohydrogenated system that the C-Cl bond is stronger (higher stretch wavenumber) because the pi-sigma* interaction is no longer possible. You might consider that another reason your double bond stretches don’t match the standard correlation chart for an alkene might be because the alkenes themseleves are atypical – it would be better to compare to other substituted cyclohexenes. The explanation of the various effects with different substituents on the anti-alkene is all correct.
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MP. The reaction is a good choice as the products are conformationally rigid – similar to the bicyclic compounds analysed in the previous questions. The introduction and mechanism are described well. The graphical comparison of experimental and computational 13C NMR shifts is a good way to show the spread of errors. A different approach would be to compare the calculated data to both sets of experimental data to see if your data fits the isomer you expect and hence whether you can differentiate them by simulated NMRs. It seems that your calculations are very accurate, owing to the molecular rigidity, and the key differences e.g. the shifts for atom 11 in each isomer are too accurately calculated. This would suggest you can use computational methods to determine which isomer you have formed. As you have described the isomers can be distinguished by 2D NMR techniques such as NOESY which determines the closeness of protons. Empirical coupling constant predictors such as Janocchio are useful tools for stereochemical analysis as the size of J values are often diagnostic in differentiating different structures (e.g. E vs Z double bonds).