Cyclic voltammetry of aromatic amines in aqueous and non-aqueous media

Abstract Cyclic voltammetry of some typical aromatic amines has been studied using the tubular graphite electrode in aqueous medium and a plane graphite electrode in non-aqueous medium. All aromatic amines, irrespective of the nature of the medium, undergo anodic oxidation through the formation of a monocation radical. In aqueous medium, the monocation radical undergoes, through its resonance structures, dimerisation involving tail-to-tail, head-to-tail and even head-to-head coupling. The products formed, being more easily oxidisable than the parent substance, undergo further oxidation at the same potential so that the overall oxidation involves a one-step (ie, a single wave), two-electron process. The N-alkyl anilines, however, oxidise in two steps instead of one. This is due to the fact that the electron-releasing alkyl groups attached to nitrogen disperse the positive charge on the cation radical and thus render it stable. The stabilised proportion of the cation radical undergoes further oxidation at a higher potential forming a dication. This accounts for the formation of the second wave. In non-aqueous medium, the monocation radical does not undergo dimerisation through coupling reactions. Retaining its identity, it undergoes further oxidation at a higher potential forming a dication. Thus, in non-aqueous medium, aromatic amines oxidise in two steps involving one electron in each step. The dication radical formed in aqueous or non-aqeous medium combines with a neutral molecule forming hydrazobenzene or its analogue as an intermediate product, the final product being azobenzene or its analogue.