Solvolytic studies of organic sulphonates

Abstract

This thesis describes a kinetic study of the hydrolysis of alkyl and aralkyl esters of toluenesulphonic acid in aqueous acetone The information available at the beginning of the present work suggested that these reactions can be regarded as nucleophilic substitution reactions but there was no certainty about the unimolecular mechanism, S(_N)l. The determination of the activation parameters for the hydrolysis of compounds which can be expected to react either by mechanism S(_N)l or by mechanism S(_N)2 showed that mechanistic tests based on the ratio of the heat capacity of activation to the entropy of activation (Bensley and Kohnstam, J.C.S. 1957, 4747.) could be applied to sulphonates as well as to halides. The application of this test to the hydrolysis of p-substltuted benzyl toluenesulphonates showed that only p-substituents which were better electron-donors than the methyl group gave reaction entirely by mechanism S(_N)l. Anions which are more powerful nucleophiles than water gave some bimolecular reaction with the substrate even when hydrolysis occurred unimolecularly and bimoleoularly reaction was observed between hydroxide ions -and benzyl toluenesulphonate, in contradiction to some earlier reports. Changes in the composition of the solvent only have small effect on the reaction mechanism, though the hydrolysis of i-propyl toluenesulphonate was found to occur entirely by mechanism S(_N)l in "50%" aqueous acetone, but not in the "85%" solvent. Comparison of the activation parameters for the hydrolysis of toluenesulphonates and the corresponding halides shows that a significant part of the greater reactivity of the sulphonates results from the more favourable entropy of activation. It is considered that this arises from the fact that sulphonates require less additional solvation by water on activation than the halid.es.