Mechanism of Electrode Reactions Involving Carboxymethyl and Chlorocarboxymethyl Radicals and Ion-Radicals

Abstract

Mechanism of reduction and oxidation of carboxymethyl and chlorocarboxymethyl radicals and ion-radicals adsorbed on a mercury electrode is studied by a laser photoemission method in broad ranges of potentials and solution pH. Versions of reduction and oxidation depend on the solutions" acid–base properties. In acid solutions, a metastable complex ion-radical–(radical)–proton donor undergoes reduction. In neutral and weakly alkaline solutions, the electron transfer onto an ion-radical dominates. In strongly alkaline solutions, a metastable complex ion-radical–proton donor undergoes reduction. The last complex is oxidized in neutral and alkaline solutions at anodic potentials, whereas in acid solutions carboxymethyl radicals and ion-radicals are oxidized. Kinetic parameters of metastable complexes barely depend on the presence of the chlorine atom in the radical, in contradistinction to the reduction overvoltage of ion-radicals and their complexes, which discernibly diminishes following halogenation. The experimental data are interpreted within an earlier model for electrode reactions involving intermediates, which includes two parallel channels for the electron transfer: adsorbed radical (anion-radical) to electrode and metastable complex radical (anion-radical) to donor/acceptor of protons to electrode.