THE REDUCTION OF NITROBENZENE BY SODIUM SULPHIDE IN AQUEOUS ETHANOL

Abstract

The reduction of nitrobenzene by sodium sulphide in aqueous ethanol has been examined. Hydrosulphide and hydrodisulphide ions give rise to quite similar, though slow, rates of reduction. Sulphide ion appears to be two to three times as effective as the hydrosulphide ion. Both the sulphide and hydrosulphide species are considered to be responsible for the initial, very slow reduction with possibly some contribution from traces of polysulphide ions. The reduction is markedly accelerated by the addition of elemental sulphur to form di- and polysulphide, a much more reactive reducing species. The formation of "active" elemental sulphur during the course of the reduction produces disulphide and thus gives rise to an autocatalytic reaction. Some of this elemental sulphur is lost as thiosulphate by reaction with hydroxyl ions produced by the reduction itself, and by hydrolysis of sulphide and disulphide ions. The addition of base increases the rate of reduction due to a shift in the equilibria [Formula: see text] but this is offset somewhat by greater loss of the active elemental sulphur as thiosulphate, thus decreasing the autocatalytic acceleration of rate.Although, from its absorption at 265 mμ, nitrobenzene concentration could be accurately measured, no satisfactory method was found for estimating the concentration of disulphide ions during the course of the reduction.