Lifetimes ofα-halo andα-azidobenzyl carbocations in aqueous solution

Abstract

The title cations were produced in aqueous solution by chemical initiation (solvolysis) of benzyl-gem-dihalides and benzyl-gem-diazides. The solvolysis reactions of benzyl-gem-dihalides and benzyl-gem-diazides in water proceed by a stepwise mechanism through α-halobenzyl carbocation and α-azidobenzyl carbocation intermediates, which are captured by water to give the corresponding carbonyl compounds as the sole detectable products. Rate constant ratiok x/ks(M−1) for partitioning of the carbocation between reaction with halide/azide ion and reaction with water is determined by analysis of halide/azide common ion inhibition of the solvolysis reaction. The rate constantsk s(s-1) for the reaction of the cation with solvent water were determined from the experimental values ofk x/ks andk solv, for the solvolysis of the benzyl-gem-dihalides and benzyl-gem-diazides respectively, usingk x = 5 × 109M−1 s−1 for diffusion-limited reaction of halide/azide ion with α-substituted benzyl carbocations. The values of 1/k s are thus the lifetimes of the α-halobenzyl carbocations and α-azidobenzyl carbocations respectively.