Variable transition state theory elimination reactions

There is another useiiil way of depicting the ideas embodied in the variable transition state theory of elimination reactions. This is to construct a three-dimensional potential energy diagram. Suppose that we consider the case of an ethyl halide. The two stepwise reaction paths both require the formation of high-energy intermediates. The El mechanism requires formation of a carbocation whereas the Elcb mechanism proceeds via a caibanion intermediate. 

Fig. 5.11. Variable transition state theory of eUmination reactions. J. F. Bunnett, Angew. Chem. Int. Ed. Engl., 1, 225 (1962) J. F. Bunnett, Survey Prog. Chem., 5, 53 (1969) W. H. Saunders, Jr., and A. F. Cockerill, Mechanisms of Elimination Reactions, John Wiley Sons, New York, 1973, pp. 48-55 W. H. Saunders, Jr., Acc. Chem. Res., 9, 19 (1976).

Variable transition state theory of elimination reactions... 

Generally, however, bond breaking might lead to bond making or vice versa since a completely symmetrical case is almost certainly idealized. This is accommodated by the creation of the variable transition state theory of elimination reactions (see Bunnett, 1969, p. 53 ff.). 

PhaCHBr, and 2-bronio-2-methyladamantane reveals dispersion which has been attributed to negative effects of nucleophilic solvation in the protic solvents. Arguments in support of formation and deprotonation of solvent-separated ion pairs are consistent with application of the generalized elimination rule to explain solvolytic dehydrobromination reactions of vicinyl dibromides, 4-bromo-4-bromomethyltetrahydropyran and 3,4-dibromo-4-methyltetrahydropyran, by the action of AcOH-AcONa and AcOH-NEts the regiochemistry reported is contrary to that predicted by the variable transition-state theory. ... 

Three-dimensional potential energy diagrams of the type discussed in connection with the variable E2 transition state theory for elimination reactions can be used to consider structural effects on the reactivity of carbonyl compounds and the tetrahedral intermediates involved in carbonyl-group reactions. Many of these reactions involve the formation or breaking of two separate bonds. This is the case in the first stage of acetal hydrolysis, which involves both a proton transfer and breaking of a C—O bond. The overall reaction might take place in several ways. There are two mechanistic extremes ... 

The correlation of many features of 6-elimination reactions is facilitated by recognition that these three mechanisms represent variants of a continuum of mechanistic possibilities. Many 6-elimination reactions occur via mechanisms that are intermediate between the limiting mechanistic types. This idea, called the variable E2 transition state theory, is outlined in Figure 5.11. 

The theory of the variable transition state for base-catalysed E2 reactions has been reviewed and has been used to explain the Hammett reaction con-stants , kinetic isotope effects " , and orientational features " of some base-catalysed elimination reactions (c/. Section 2.2). 

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