Effect of the solvent basicity and additives on the electroreduction of picric acid and tetramethylammonium picrate in aprotic media

The electroreduction of picric acid and tetramethylammonium picrate in acetonitrile, acetone, N,N-dimethylformamide, dimethyl sulfoxide and hexamethylphosphoramide has been studied by normal pulse polarography, cyclic voltammetry and by coulometric analysis. The reduction of undissociated acid starts in all aprotic media with consumption of one electron. The reduction potential is shifted to more negative potentials with rising basicity of the solvent, i.e. with rising dissociation constant of picric acid. The formation of anion radicals is followed by the sequence of acido-basic selfprotonation reactions as well as by the disproportionation leading to a nitroso intermediate. The reduction mechanism does not change by addition of proton acceptor (basic alumina) or by amfiprotic species (anthranilic acid). In the presence of a strong acid; picric acid can mediate the electroreduction of added protons. Proposed mechanisms were supported by comparison with electrochemical behaviour of related nitrophenols in the same aprotic media.