How to predict changes in solvolysis mechanisms
Stopped-flow and laser flash techniques have been employed to investigate the individual steps of the solvolysis reactions of benzhydryl (diarylmethyl) halides and carboxylates. In this way, absolute rate constants for the ionization (k1), recombination of the carbocation with the leaving group (k-1), and subsequent reaction with the solvent (kSolvOH) have been determined. As the stabilization of the carbocations increases, the mechanism changes from (a) SN1 reactions with irreversible ionization through (b) SN1 reactions with common-ion return and (c) SN2C+ reactions, where the intermediate carbocations accumulate, to (d) the formation of persistent carbocations which do not undergo subsequent reactions under the selected solvolysis conditions. The correlation equation log k = s(N + E), where the carbocations are characterized by the electrophilicity parameter E, and leaving groups and solvents are characterized by the nucleophile-specific parameters s and N can be employed to predict the changes of mechanism.