Influence of water on the electroreduction of 2-nitrophenylhydrazones in acetonitrile

Abstract

2-Nitrophenylhydrazones of four ketones and two aldehydes were reduced in 0.1 mol/1 Et 4NBF 4 in acetonitrile at the mercury electrode. The mechanism of the electroreduction was studied by means of dc polarography, cyclic voltammetry at the DME, controlled potential electrolysis with coulometry and preparative electrolysis followed by spectral and chromatographic characterization of products. All the substances studied yielded two reduction steps with E I 1 2 ≈ −1.15 and E II 1 2 ≈ −1.55 V vs. aqueous SSCE. The ratio of the limiting currents of the first to those of the second step was approx. 1:4 to 1:5. A detailed analysis of the mechanism was performed with acetophenone 2-nitrophenylhydrazone. The anion A − of the parent substance HA (with dissociated N-H group) is formed in the first reduction step. The electrolysis in the second step yields the dianion with an unreacted nitro group. The study of the influence of water on the peak potentials, peak currents and the formation of the parent substance led to the conclusions that (i) the reaction of the anion A − (resulting in the first step from HA −) with water, reforming the parent substance, is protolytic; (ii) the reduction HA − → HA 2− is accompanied by the catalysed reduction of water at the potentials of the second reduction step; and (iii) the strong base OH −, arising either in protolytic reactions or through the reduction of water, reacts with the parent substance HA and could cause secondary reactions during the electroreduction of nitroaromatic compounds.