Activation parameters for benzyl chloride solvolysis in isopropanol-, acetone- and dimethylsulfoxide-water mixtures

Abstract

The first order rate constants for Lenzyl chloriclc solvolys.i.s in aqueous acetone and aqueous dimethylsulfoxide (DMSO) as a function of pressure (1 to 4081 atm.) at so.10°c are reported. From these data volumes of activation, ~v0, and their pressure dependences, (a~V 0/ aP)T' at atmospheric pressure are calculated. It is demonstrated that the variation of both ~VO and (a~V 0/aP)T with solvent composition in each solvent system investigated is complex, with the appearance of one or two extrema over the range of solvent composition studied. The dependence of the partial molal volume of benzyl chloride, 'f/l,, on solvent composition in aqueous acetone and aqueous DMSO has been measured. These data are combined with the volume of activation data to calculate the dependence of the partial molal volume of benzyl chloride transition state, Vl, on solvent composition. It is found that both~ and vt undergo extremum behavior as a function of solvent composition. The relative contributions of the extrema in Vb and -Vt to the behavior of 6V* are discussed. The data outlined above are combined with previously reported data for benzyl chloride solvolysis in aqueous i.-propanol 1G permitting examination of the effect of varying the hydrophilic nature of the cosolvent on the extrema in ~v0, (36V0/ aP)T, V and -Vt . It is found that the magnitudes, but not the positions, of the extrema show a large dependence on the hydrophilic nature of the cosolvent. These phenomena are rationalized, as far as possible, in terms of solvent structural and energetic factors derived from independent non-kinetic evidence. The nature of benzyl chloride transition state in aqueous DMSO is investigated using stable molecular species as 'model' compounds. From these studies it is suggested that the complex variation of yt with solvent composition arises principally from the properties of the incipient benzyl carbonium ion and not from the properties of the incipient chloride anion. It is further shown that in pure water anilinium chloride is a good charge model for benzyl chloride transition state. This is in keeping with the fact that the mechanism of benzyl chloride hydrolysis is SN2 but close to the SNl limit. In the mixed solvent systems, however, tetramethylammonium chloride is found to be a better model than anilinium chloride for benzyl chloride transition state.