The electrochemical reduction of o-nitrotoluene to o-tolidine—I. Coulometry at controlled potential; production aspects

Abstract

o-Tolidine formed by reduction of o-nitrotoluene is used for production of azo-dyestuffs being important pigments for textiles. At the moment, nitrotoluene is reduced to o-hydrazotoluene by zinc dust after which hydrazotoluene is rearranged into tolidine. Environmental reasons forced us to seek an alternative in the electrochemical route for tolidine production. On the basis of voltammograms, controlled-potential electrolyses were carried out for nitrotoluene in alkaline ethanol-water solutions and in alkaline aqueous solutions containing McKee salt and for azoxytoluene in alkaline and acidic ethanol—water solutions as well as in acidic aqueous solutions containing McKee salt in acid. It has been found that production of tolidine has to take place in at least two steps, namely in alkaline solutions for the reduction of nitrotoluene to azoxytoluene and in acidic solution for the reduction of azoxytoluene to hydrazotoluene which is then directly rearranged into tolidine. To obtain a high chemical yield and current efficiency for azoxytoluene formation in ethanol—water media the reduction has to be carried out at high nitroluene concentrations. For Pt, Ag, Au, Cr, Fe and graphite cathodes a chemical yield of almost 100 per cent was reached at nitrotoluene concentrations higher than about 0.20 M. At concentrations lower than 0.025 M many by-products were formed. The production of tolidine from azoxytoluene can be carried out either in one step in acidic media or in two steps in which case the first occurs in alkaline solution and the second in acidic solution. In the latter case the chemical yield for hydrazotoluene from azoxytoluene is also 100 per cent and a high current efficiency is also attainable. The chemical yield of the rearrangement of hydrazotoluene strongly depends on the nature and pH of the solution and temperature; a chemical yield of about 80 per cent is, however, obtainable in practice. The chemical yield for tolidine formation from azoxytoluene in acidic media containing McKee acid and salt is about 87% with a current efficiency of about 40 per cent. This production route is preferred.