The reduction mechanism of the C=O group .2. The electrochemical reduction of p-diacetylbenzene in aqueous medium

A detailed study of the electrochemical reduction of diacetylbenzene A in aqueous medium between H-o = -5 acid pH 14 is presented. The reactants are strongly adsorbed, so that the reactions are of a surface nature, From H-o = -5 to pH 6, a global 2e(-) reduction yielding an enediol-type intermediate occurs. Analysis using the theory of the square schemes with protonations at equilibrium shows that, up to pH 4, the reaction is controlled by the first electron uptake, the paths being successively H(+)e(-) and e(-)H(+). The elementary electrochemical surface rate constants are 9.6 x 10(7) s(-1) and 1.2 x 10(6) s(-1) for AH(+) and A respectively. From pH 6 to 14, a 1e(-) adsorption wave, corresponding to the formation of(a) monoradical(s), appears and is followed by a 1e(-) wave due to the reduction of the radical(s). A dimerization occurs, due to the coupling A(-.) + AH(.), as in the case of the monocarbonyl compounds. The rate of this surface process, k(d) = 5 x 10(13) cm(2) mol(-1) s(-1), is markedly smaller than the rate of the homogeneous reaction obtained in alkaline ethanol by Saveant et al, for the coupling of the radicals of benzaldehyde, benzophenone and acetophenone.