Competition between SN2 and single electron transfer reactions as a function of steric hindrance illustrated by the model system alkylCl+NO-

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 activation free energy increases while the ET activation free energy does not vary much. The result is that the balance favors more and more the ET reaction, which becomes predominant in the tert-butyl case. The geometries of the two transition states are drastically different, being characterized by a N, C, Cl atom sequence in the SN2 transition state and a C, Cl, N sequence in the ET transition state. The looseness of its transition state and the lesser directionality of attack as compared to the SN2 reaction are factors favorable to the ET reaction. An indirect ET pathway may follow the SN2 transition state. Its importance increases with steric hindrance. The tert-butyl case represents an extreme situation where the SN2 transition state is connected with the ET products rather than with the SN2 products, while the direct ET pathway becomes more facile than the SN2 pathway. All directions of attack lead to single electron transfer with similar activation energies, with similar reacting distances and negligible bonded interaction in the transition state. This reaction thus offers a good illustration of an outer-sphere process, as conceived in previous models of dissociative electron transfer.