Addition–elimination paths in electron-transfer reactions between radicals and molecules. Oxidation of organic molecules by the OH radical

Abstract

Reactions between the ˙OH radical and molecules Y that ultimately lead to electron transfer from Y to ˙OH have been studied by in-situ radiolysis or photolysis, electron spin resonance and pulse radiolysis techniques with optical and conductance detection. These radical–molecule reactions proceed in aqueous solution via the intermediate formation of covalently bound adducts HO—Y˙. These radicals are able to undergo heterolysis, which may proceed by spontaneous or by catalysed paths. The heterolysis results in a one-electron oxidation of Y, and the overall reaction thus consists in a one-electron transfer from the molecule to ˙OH.[graphic omitted]

In the addition step (I), a reducing radical is formed by reaction of the oxidizing ˙OH with Y, which is usually neither an oxidant nor a reductant. In the heterolysis step (II), however, the reducing HOY˙ is converted into the oxidizing Y^˙+. This phenomenon, termed redox inversion, is the consequence of the change in oxidation state of Y by two units in going from HOY˙ to Y^˙+. Examples of redox processes of this kind are given from the class of substituted benzenes and of N-heterocyclics, and structure–reactivity relations governing the heterolysis of HO—Y˙ are discussed.