Variable-transition-state theory

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).

The variable transition state theory allows discussion of reactions proceeding through TSs of intermediate character in terms of the limiting mechanistic types. These are called Elcb-like and El-like, as illustrated in Eigure 5.11. The most important factors to be considered are (1) the nature of the leaving group, (2) electronic and steric effect of substituents in the reactant molecule, (3) the nature of the base, and (4) solvent effects. 

The p values for base-catalyzed elimination of HF from a series of aryltrifluoro-, difluoro-, and monofluoroethanes have been measured. The observecj values are given below. Rationalize the trend in p values on the basis of the variable-transition-state theory. 

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. ... 

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