Oxidation Reactions of a Series of Benzidines:  Electrochemical Detection of Dimerization of Cation Radicals and Examples of Potential Inversion Caused by Very Small Steric Effects

It has been previously reported that cation radicals of some benzidines undergo dimerization. In the present work dimerization equilibrium constants have been determined by UV−visible absorption spectroscopy for the cation radicals of benzidines 1a−d. This dimerization reaction was shown to produce subtle but significant effects in cyclic voltammograms recorded for oxidation of the benzidines. It is shown that cyclic voltammetry is a simple and rapid technique to detect such reactions, whose quantitative evaluation must, however, be accomplished by other means. The oxidation of certain N-peralkylated benzidines having substituents in the 3 and 3‘ positions, 1e−i, occurs in a single, two-electron oxidation process due to the fact that the two standard potentials for oxidation to the cation radical and dication, respectively, are very similar. In several cases, potential inversion occurs. That is, the potential for removal of the second electron is less positive than that for removal of the first. Causes of this potential inversion are discussed.