Hammond postulate eliminations

In the following eliminations, the percent Hofmann elimination increases as the leaving group becomes worse. Explain why this occurs using the Hammond postulate and the relative acidities of the hydrogen adjacent to the carbocation center. 

It is now necessary to consider to what extent deviations from smooth correlations of k /kj) with AG or AG in proton transfer reactions reflect departures from the Hammond postulate. The Hammond postulate may be expected to apply to reasonably closely related three-centre reactions of the type considered above, and in particular to their simplified representations by empirical potential-energy surfaces. It does not follow that it applies generally to more complex reactions, and it is now well established that for concerted ionic reactions failure of bond-making and bond-breaking to remain properly in step can lead in the transition state to charge-localizations at atoms that bear no charge in the reactants or products, with the consequence that substituents at these atoms show no correlation between their effects on reaction rates and equilibria [123-126]. This behaviour has been best characterized in E2 j8-eliminations,... 

The origin of this preference has been considered by Richard and co-workers. One aspect of the puzzle can be seen by applying Hammond s postulate. Since the competing reactions have early transition states, it is unlikely that the difference in product stability governs the competition. Instead, the substitution process appears to have a smaller intrinsic barrier (in the context of the Marcus equation see Section 3.2.4). The elimination reaction appears to have a barrier that is 3 kcal higher, at least for ec-benzylic systems. " The structural basis of this difference has not been established, but it may be related to the fact that the elimination process has a bond-breaking component, whereas substitution requires only bond formation. 

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