Competition between S(N)2 and single electron transfer reactions as a function of steric hindrance illustrated by the model system alkylCl + NO-

The S(N)2 reaction is a good example of the dichotomy and connection betwee n electron-pair transfer chemistry and single electron transfer (ET) chemis try. Based on experimental stereochemical and kinetic data and on theoretic al considerations, the dichotomy may be envisioned in two ways. One is comp etition between two distinct pathways, implying the existence of two distin ct transition states on the potential energy hypersurface representing the reacting system, each connected to the S(N)2 and ET products, respectively. The other considers a single transition state which could competitively gi ve rise to both products. In both cases, steric hindrance is expected to fa vor the formation of the ET over the S(N)2 products. An ab initio quantum c hemical analysis of the model systems RCl + NO-, with R = methyl, ethyl, is opropyl, and tert-butyl, taking account of electron correlation (at the MP2 level) and of solvation, shows the existence of distinct S(N)2 and ET tran sition States. As steric hindrance increases, the S(N)2 activation free ene rgy increases while the ET activation free energy does not vary much. The r esult is that the balance favors more and more the ET reaction, which becom es predominant in the tert-butyl case. The geometries of the two transition states are drastically different, being characterized by a N, C, Cl atom s equence in the S(N)2 transition state and a C, Cl, N sequence in the ET tra nsition state. The looseness of its transition state and the lesser directi onality of attack as compared to the S(N)2 reaction are factors favorable t o the ET reaction. An indirect ET pathway may follow the S(N)2 transition s tate. Its importance increases with steric hindrance. The tert-butyl case r epresents an extreme situation where the S(N)2 transition state is connecte d with the ET products rather than with the S(N)2 products, while the direc t ET pathway becomes more facile than the S(N)2 pathway. All directions of attack lead to single electron transfer. with similar activation energies, with similar reacting distances and negligible bonded interaction in the tr ansition state. This reaction thus offers a good illustration of an outer-s phere process, as conceived in previous models of dissociative electron tra nsfer.