Contribution to the understanding of the reduction mechanism of nitrobenzene

Abstract

Electrochemical reduction of nitrobenzenes has been studied extensively [1,2]. For the processes involved Haber [3] proposed a scheme which is generally accepted in a modified form [4,5]. Nitrobenzene was the first organic compound for which a current-voltage curve obtained with the dropping mercury electrode was reported [6]. At this electrode, where, because of the continuous surface renewal the short contact period excludes accumulation and side-reaction of products [7], the reduction occurs in a single four-electron step. In acidic media the monoand di-protonated forms of arylhydroxylamine [8-lo] formed in the first step are reduced at more negative potentials in a second, two-electron step. The overall four-electron process involves also a transfer of four protons (reaction 1): ArNO,+4e-+4H++ArNHOH+H,O (1) The sequence of transfer of protons and electrons cannot be deduced in aqueous media from the variation in the limiting current of the first step, as this is pH-independent. Mechanistic studies in non-aqueous media [11,12] do not offer information about the process in aqueous solutions because of the difference in solvation of all the species involved and in their acid-base properties. Similarly for mechanistic studies in the presence of surfactants [13-151 the role of micelles and other specific effects of adsorbed surfactants cannot be excluded. The first type of experimental evidence that can be used in the elucidation of the sequence of electron and proton transfer is the fact that the sequential reduction of the nitro group to a nitroso and a hydroxylamino group has never been reported to occur in two two-electron steps. The presence of a single four-electron wave