Carbocation lifetimes and entropy of water addition to carbocations dependent on their stability

Abstract

Iminodiazonium ions (α-azidobenzyl carbocations) were produced in aqueous solution by chemical initiation (solvolysis) of benzyl-gem-diazides. Solvolysis reactions of benzyl-gem-diazides in water proceed by a stepwise mechanism through α-azidobenzyl carbocation intermediates, which are captured by water to give the corresponding carbonyl compounds as the sole detectable products. Rate constant ratio kaz/ks (M-1) for partitioning of the carbocation between reaction with azide ion and reaction with water was determined by analysis of azide common ion inhibition of the solvolysis reaction. Rate constants ks (s-1) for the reaction of the cation with solvent water were determined from the experimental values of kaz/ks and ksolv, for the solvolysis of the benzyl-gem-diazides, using kaz = 5 × 109 M-1 s-1 for diffusion-limited reactions of azide ion with α-azidobenzyl carbocations. Values of 1/ks were thus the lifetimes of the α-azidobenzyl carbocations. From the study of effect of temperature on ksolv and ks the activation parameters are determined and discussed.