Lifetimes of a-halo and a-azidobenzyl carbocations in aqueous solution

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 alpha-halobenzyl carbocation and alpha-azidobenzyl carbocation intermediates, which are captured by water to give the corresponding carbonyl compounds as the sole detectable products. Rate constant ratio kx/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 constants ks(s–1) for the reaction of the cation with solvent water were determined from the experimental values of kx/ks and ksolv, for the solvolysis of the benzyl-gem-dihalides and benzyl-gem-diazides respectively, using kx = 5 cross 10^9 M–1 s–1 for diffusion-limited reaction of halide/azide ion with a- substituted benzyl carbocations. The values of 1/ks are thus the lifetimes of the alpha-halobenzyl carbocations and a-azidobenzyl carbocations respectively.