Ab initio calculations combined with an intrinsic reaction coordinate analysis of the title reactions have allowed the identification as well as the geometric and thermodynamic characterization of two types of reaction paths on the same potential energy hypersurface. One involves an outersphere electron transfer from the anion radical which dissociatively reduces the methyl halide acceptor into the methyl radical and the halide ion. The other pathway leads to substitution (two pathways leading to O- and C-substitution respectively in the case of CH2=O-.-). The substitution transition states have a tighter geometry than the outersphere transition state and may be viewed equivalently as S(N)2 reactions or innersphere electron transfers concerted with bond cleavage and bond formation. In the competition between the two types of processes, because of bonded interactions in the transition state, S(N)2 substitution has a lower energy barrier than the outersphere reaction. Owing to the looser geometry of its transition state, entropy favors the outersphere pathway. This factor is not sufficient to reverse the free energy balance with the reactants investigated but this is predicted to occur with sterically hindered systems.