The solvolysis of some primary alkyl bromides

Abstract

Recent evidence suggests that the solvolysis of organic halides is associated with Arrhenius activation energies which have negative temperature coefficients. It is considered that this is most probably due to the increased solvation of the transition state of the reaction relative to the initial state of the reactants. Bensley and Kohnstam have suggested that the heat capacity of activation (ΔCP * = dE/dT - R) is a measure of this increase in solvation and also that in S(_N)1 reactions,the entropy of activation, Δ S * , is a measure of the same phenomenon; whereas in S(_N)2 reactions an additional negative contribution to Δ S * due to the covalent binding of the nucleophilic reagent must be considered. The solvolysis of R-Hal i n a given solvent and at a given temperature should, therefore, have a constant value for the ΔCp */ΔS * ratio for SNl reactions, independent of the nature of the group R and the ratio should have a lower value for S(_N)2 reactions (since ΔCp* and ΔS*are both negative in the type of reaction being considered]] At the commencement of this study these tentative suggestions required experimental verification. A parallel study has shown the constancy of ΔCp*/ΔS* for S(_N)l reactions and this thesis describes the determination of this ratio, for the solvolysis of some n-alkyl bromides in 50% aq.acetone when the extreme form of the S(_N)2 mechanism is operative. These compounds all showed negative temperature coefficients of activation energy and the ΔCp*/ΔS* ratio was always lower than for S(_N)l reactions, in agreement with the earlier predictions. It can be concluded that the determination of this ratio provides an additional test of solvolytic mechanism which seems likely to be applicable to border-line reactions in which there is a transition from mechanism S(_N)2 to mechanism S(_N)1. Here, the 'classical' mechanistic tests do not always yield unambiguous results.