A study of aldehyde oxidation at glassy carbon, mercury, copper, silver, gold and nickel anodes

Abstract

The rate, potential and mechanism of the anodic oxidation of aliphatic aldehydes have been found to be highly dependent on solution conditions and electrode material. Aldehyde oxidations in neutral acetonitrile on glassy carbon occur at very positive potentials (ca. +3 V vs. SCE) and the peak potentials correlate with the ionization potentials of the aldehydes. In aqueous base, aldehyde oxidation is assisted by reversible addition of hydroxide to the carbonyl group to form electroactive gem-diolate (II). Oxidations of aldehydes in aqueous base on Hg, Ni, Ag and Au all yield the corresponding carboxylate via two-electron oxidation plus aldol and Cannizzaro byproducts and the oxidations occur at potentials far negative of the unassisted oxidation in neutral acetonitrile. On Ni, Cu and possibly Hg the oxidation involves the formation of a metal oxide which acts as a chemical oxidizing agent. On Ag and Au the oxidations take place on a surface which is not covered by a phase oxide. A mechanism involving a direct electrochemical process with oxidation of gem-diolate adsorbed on an oxide-free metal surface is proposed. A pulsed electrolysis technique was utilized to circumvent deactivation of Ag and Au electrodes during electrolysis and preparation of an “aurized” gold surface with a much slower deactivation rate is described.