A rate and product study of the effect of solutes upon the solvolysis of benzyl toluene-p-sulphonate in aqueous trifluoroethanol

Abstract

The effects of several electrolytes, including sodium perchlorate, thiocyanate, acetate, and halides, upon the rate and products of solvolysis of benzyl tosylate in 1 : 1 (v/v) aqueous trifluoroethanol at 25 °C have been investigated. In the absence of any electrolyte, the ratio of benzyl alcohol to benzyl trifluoroethyl ether is only marginally greater than the molar ratio of water to trifluoroethanol in the reaction medium. Sodium perchlorate gives rise to a kinetic salt effect but no detectable perturbation of the product ratio. Thiocyanate, iodide, and bromide lead to second-order rate terms and the formation of substitution products. Thiocyanate, the most effective nucleophile used, does not affect the internal ratio of the solvent-derived products. We find no evidence of nucleophilic capture of carbonium ion intermediates formed in prior rate-determining steps. Chloride, acetate, and hydroxide are very weak nucleophiles in this medium and do not lead to overall rate enhancements. Replacing a small proportion of the trifluoroethanol by ethanol leads to the formation of a low yield of benzyl ethyl ether. There is no evidence of benzyl tosylate reacting other than by S_N2 mechanisms, either uncoupled in reactions with solvent and hard nucleophilic solutes, or coupled with soft nucleophiles such as iodide and thiocyanate.