ABSTRACT
Organic chemistry represents an extensive body of facts, from which the contemporary doctrine of reactivity is built. The most important basis of this doctrine is the idea of intermediate species that arise along the way from starting material to final product. Depending on the nature of the chemical transformation, cations, anions, and radicals are created during an intermediate stage. These species form mainly as a result of bond rupture. Bond rupture may proceed heterolytically or homolytically (Scheme 1-1):
Ions or radicals formed from a substrate further react, with other ions or radicals acting as reactants. Such changes in chemical bonds can be accompanied by a one-electron shift. The concept of the one-electron shift (Pross 1985) can be illustrated by Scheme 1-2 for nucleophilic substitution:
The species Nu↑↓R of Scheme 1-2 is not a radical pair; it is a covalent molecule of the product resulting from the SN2 reaction. The process of transfer of the R group to a Nu
reactant proceeds in synchronicity with a one-electron shift and R-Z bond disruption. At that time, two radical particles, Nu· and R· (formed in the course of reaction) remain immediately close and therefore unite rapidly. A one-electron shift may or may not lead to the formation of radical particles. There are many reactions that consist not of a oneelectron shift, but of a one-electron transfer. The initial results of such one-electron transfers involves the formation of ion radicals.
