Solvolytic studies of benzyl halides

Abstract

The investigation concerns the solvolytic study of P-substituted benzyl halides in aqueous acetone. By a suitable choice of substituents it is possible to obtain a finely graded sequence of mechanisms, by which hydrolysis occurs, ranging from extreme to S(_N)2 to S(_N)1and the work aimed at obtaining information about the mechanistic border-line region, a region where the mechanism is almost, but not quite, S(_N)1. For this a new approach was necessary since criteria which are well established for the investigation of the extreme forms of mechanism could not be used in this region. This approach involved the verification of the hypothesis that the ratio of the heat capacity of activation to the entropy of activation for the S(_N)1 hydrolysis of alkyl chloride in a given solvent at a given temperature has a constant value. Extension of the earlier work carried out by Bensley and Kohnstam(^1) shows that this hypothesis is essentially correct and that the ratio of the heat capacity of activation to the entropy of activation has a lower value for S(_N)2 reactions than for S(_N)1 reactions. Application of these ideas to the present experiments shows that p-methoxy-benzyl chloride in 50% aq.acetone reacts by mechanism S(_N)1, p-methylbenzyl chloride is border-line and the unsubstituted compound and p-nitrobenzyl chloride react by a mechanism which progressively approached the more extreme from of S(_N)2. An additional problem investigated concerns the effect of electrolyte additions on the hydrolysis rate of benzhydryl chloride, a compound which reacts by mechanism (S_N)1. The specific electrolyte effects found are interpreted as arising from changes in the solvent composition caused by the solvation of the electrolyte. This effect would be super-imposed upon the ionic-strength effect of Hughes and Ingold(^2).