THE REDUCTION-MECHANISM OF AROMATIC NITRO-COMPOUNDS IN AQUEOUS-MEDIUM.4. THE REDUCTION OF P-NITROBENZOPHENONE BETWEEN H(0)=-5 AND PH-14

The electrochemical reduction of 4-nitrobenzophenone RNO2 has been exa mined in aqueous medium on a Hg electrode between H0 = -5 and pH 14, b y polarography and cyclic voltammetry. It occurs in three main steps: 2e- reduction to dihydroxylamine, which dehydrates to give the nitroso compound, itself reducible (2e-) to the hydroxylamino form. The resul ts were analyzed using the theory of the nine-member square scheme wit h fast protonations [E. Laviron, J. Electroanal. Chem., 146 (1983) 15] . The phenomena observed constitute an epitome of those occurring duri ng the reduction of organic compounds in aqueous medium, as far as two main factors, adsorption and protonations, are concerned. In acidic m edium (pH <1), the first 2e- reduction appears as heterogeneous, altho ugh it occurs via the adsorbed species. The apparent heterogeneous ele ctrochemical rate constant is about 10(5) larger than the elementary r ate constants. For pH > 1, adsorption is much stronger, and the reacti ons are of a purely surface nature, in polarography and a fortiori for any value of the sweep rate in cyclic voltammetry. The calculated val ues of the elementary surface electrochemical constants, 7.8 x 10(8) s -1, are of the order of magnitude predicted by Brown and Anson's formu la. The first 2e- stage is controlled by the electrochemical reaction up to H0 almost-equal-to -2, and by dehydration for H0(pH) larger than about 0. The order of addition of the electrons and protons at each p H is determined. The global 4e- reduction is of the ECE type up to pH almost-equal-to 10. A novel feature of the reduction of aromatic nitro compounds in alkaline medium has been brought out; basically a le- re versible wave, followed by a 3e- irreversible wave, should be observed . However, the influence of the dehydration causes the two waves to me rge, producing a single 4e- wave (EECE process).