Competition between SN2 and Single Electron Transfer Reactions as a Function of Steric Hindrance Illustrated by the Model System AlkylCl + NO-

Abstract

The SN2 reaction is a good example of the dichotomy and connection between electron-pair transfer chemistry and single electron transfer (ET) chemistry. Based on experimental stereochemical and kinetic data and on theoretical considerations, the dichotomy may be envisioned in two ways. One is competition between two distinct pathways, implying the existence of two distinct transition states on the potential energy hypersurface representing the reacting system, each connected to the SN2 and ET products, respectively. The other considers a single transition state which could competitively give rise to both products. In both cases, steric hindrance is expected to favor the formation of the ET over the SN2 products. An ab initio quantum chemical analysis of the model systems RCl + NO-, with R = methyl, ethyl, isopropyl, and tert-butyl, taking account of electron correlation (at the MP2 level) and of solvation, shows the existence of distinct SN2 and ET transition states. As steric hindrance increases, the SN2...