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 examined in aqueous medium on a Hg electrode between H0 = -5 and pH 14, by 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 results were analyzed using the theory of the nine-member square scheme with fast protonations [E. Laviron, J. Electroanal. Chem., 146 (1983) 15]. The phenomena observed constitute an epitome of those occurring during the reduction of organic compounds in aqueous medium, as far as two main factors, adsorption and protonations, are concerned. In acidic medium (pH <1), the first 2e- reduction appears as heterogeneous, although it occurs via the adsorbed species. The apparent heterogeneous electrochemical rate constant is about 10(5) larger than the elementary rate constants. For pH > 1, adsorption is much stronger, and the reactions are of a purely surface nature, in polarography and a fortiori for any value of the sweep rate in cyclic voltammetry. The calculated values 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 formula. 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 pH 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- reversible wave, followed by a 3e- irreversible wave, should be observed. However, the influence of the dehydration causes the two waves to merge, producing a single 4e- wave (EECE process).