Compared selectivities of redox-catalyzed and direct electrochemical processes Part 2. Reactions in which product selection involves competition between dimerization and deactivation followed by further reduction (or oxidation)

Possible changes in selectivity between redox-catalyzed processes and direct electrochemical reactions are discussed for a typical reaction scheme where product selection involves competition between a dimerization step and a reduction (or oxidation) pathway in which the reductant (or oxidant) is the catalyst on the one hand and the intermediate resulting from the first electron transfer on the other. This second reduction (or oxidation) pathway may or may not be controlled by the transformation of the first electron transfer intermediate into a second intermediate that undergoes a second electron transfer. In the former case, changes in selectivity between the two types of electrolysis conditions arise solely from the space dependence of the reactant and intermediate concentrations. They usaually favor the reduction (or oxidation) pathway in a redox-catalysed process rather than the direct reaction. In the converse situation, where the transformation of the first electron transfer intermediate is reversible and acts as a pre-equilibrium to the second electron transfer, one source of possible changes in selectivity also results from space depenence of the reactant and intermediate concentrations. However, it tends to play an opposite role, favoring the reduction (or oxidation) pathway in the direct reaction rather than the redox-catalyzed process. This effect is amplified if, as is usually the case, the catalyst is a weaker reductant (or oxidant) than the first electron transfer intermediate of the direct electrochemical reaction.