Solvolysis of benzoyl halides in AOT/isooctane/water microemulsions.: Influence of the leaving group

A kinetic study was carried out on the solvolysis of benzoyl bromide, BzBr, benzoyl chloride, BzCl, and benzoyl fluoride, BzF, in microemulsions of AOT/isooetane/water at 25 degreesC, where AOT is sodium bis(2-ethylhexyl)sulfosuccinate. In all cases, the pseudo-first-order rate constant, k(obs), increases together with the [AOT] as a consequence of incorporating the substrates into the interface of the microemulsion. The application of a kinetic model based on the formalism of the pseudophase, which considers the distribution of the reagents between the continuous medium and the interface and that the reaction is taking place only at the interface, has enabled us to obtain the intrinsic values of the rate constant at the interface of the microemulsion for the three benzoyl halides, k(i)(BzX). The values of k(i)(BzX) vary together with the water content of the microemulsion. The study of the variation of k(i)(BzX) with W (W = [H2O]/[AOT]), the correlations with the ET polarity parameter, and the effect of the leaving group have allowed us to conclude that a mechanistic displacement has occurred along with the variation in W. For high water contents, the reaction occurs by an associative and dissociative path simultaneously, the relative weights of which depend on the leaving group, with the dissociative character increasing as the salient group of the benzoyl halide improves. As W diminishes, the associative path is favored as a consequence of an increase in the nucleophilicity of the water, and the dissociative path is disfavored due to the reduction in the capacity of the interfacial water to solvate the leaving group, X-. Consequently, a displacement of the reaction mechanism takes place from a dissociative extreme to an associative one as the water content of the microemulsion is reduced, which causes the sensitivity of the reaction to the nature of the leaving group to vary with W.