Product distribution in preparative scale electrolysis: Part VII. Competition at the level of the first-electron intermediate between self-coupling, coupling with the substrate and first-order deactivation followed by further electron transfer

Abstract

The relationship between the product distribution and the intrinsic (rates) and operational (concentration, current density, diffusion-layer thickness) parameters are established for a reaction scheme involving the competition between two modes of dimerization: self-coupling and coupling with the substrate. This dependence depends strongly upon the electrolysis regime. For potentiostatic conditions with the electrode potential being located at the plateau of the wave, the outcome of the competition simply depends upon the ratio of the rate constants of the two coupling reactions without any influence of the initial concentration and the diffusion-layer thickness. In the case of a galvanostatic regime such as the electrode potential remains on the rising portion of the wave, the coupling with the substrate can be regarded as a pseudo-first-order reaction. The yield in self-coupling dimer then increases with the current density and decreases when raising the substrate concentration and the thickness of the diffusion layer. For this type of electrolysis regime the relationships between product distribution and parameters of the system have been extended to a more complex reaction scheme involving the additional interference of a first-order deactivation reaction, itself followed by further electron transfer, which competes with the two coupling reactions.