Aliphatic Substitution and Single Electron Transfer

The discussion so far has considered only heterolytic processes for substitution reactions, but there is increasing evidence that substitutions may take place by a single electron shift synchronous with bond displacement or by a single electron transfer (SET) pathway in which electron transfer precedes bond displacement. To understand the basis of this theory, it is useful to discuss a conceptual model for organic reactions that has been called the valence bond configuration mixing (VBCM) model. As described by 

TABLE 8.17 Values of Nf (Sf) for Selected Leaving Groups and Solvents  

Energy diagrams for homolytic and heterolytic dissociation (a) gas phase and (b) solution. (Reproduced from reference 148.) 

Pross and Shaik, the method is based on an analysis of the energies of all possible valence bond configurations of the reactants and products as a function of the reaction coordinate. 

Electronically excited states are discussed in Chapter 12. Such awided crossings are discussed in Chapter 11. 

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