The electrochemical oxidation of phenylenediamines

Five p-phenylenediamines were oxidized electrochemically at a glassy carbon rotating ring-disk-electrode assembly. The overall reaction comprises the transfer of two electrons per molecule. At low to intermediate potentials the rate determining step for all the five compounds is the acid dissociation of the semiquinonediimine radical cation formed in a pre-equilibrium involving the transfer of one electron. Only at higher potentials can the radical cation be oxidized in parallel to the neutral semiquinone radical to form the quinonediimines. In contrast, the neutral semiquinonediimine radicals are oxidized at the same or even a lower potential than the parent phenylenediamines. The dissociation rate constants of the radical cations are in the range of 10–300 s−1. The effect of the substituents on the oxidation potentials is as expected by their property to donate electrons to the mesomeric system. Their effect on the dissociation rates, however, seems to rather correlate with the inductive effect and might influence not only the electronic structure of the molecule itself but also the structure of the ions and the solvent around them. The semiquinonediimine radical cations are found to be slightly more acidic than their protonated parent phenylenediamine cations with pKa values somewhat below 6.