Author: O. J. Cope

Publisher:

Published: 1962

Total Pages: 0

ISBN-13:

As a prerequisite to the study of the reduction of nitrobenzene by sulphide and hydrosulphide ions, the composition of sodium polysulphide solutions has been examined. The spectro- photometric methods used indicated that the composition of these solutions is dependent on pH, A method was developed, using spectrophotometrie techniques, which allowed the polysulphide produced during the course of nitrobenzene reduction by sulphide or hydrosulphide to be determined. However attempts to adapt this method to the determination of the degree of hydrolysis of these polysulphide solutions, was unsuccessful. The reduction of nitrobenzene by sodium sulphide in aqueous ethanol has been examined. 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, idiich forms poly (probably di-) sulphide, 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 S -f H^O ^ HS OH and S 4 H 0^ HS 4- OH , but this is off- set by greater loss of the active elemental sulphur as thiosulphat thus decreasing the autocatalytic acceleration of rate. The reduction of nitrobenzene by sodium hydrosulphide and hydrodisulphide in aqueous media has also been examined. Nitrobenzene is reduced by the HS ion initially as far as phenyl- hydroxylamine v>hich is then reduced by HS much more slowly than is nitrobenzene. As the reaction progresses HS^ is formed which, along with the small concentration of S^ present, is responsible for the observed autocatalysis. However HS^ reduces phenylhy- droxylamine faster than it does nitrobenzene and thus the phenyl- hydroxylamine concentration reaches a maximum^thereafter declining, Thus only aniline and some unreacted nitrobenzene are present towards the end of the reaction. The reduction of phenylhydroxylamine by HS to aniline produces SO but no detectable amounts of HS whereas reduction by HS involves the conversion of yellow HS_ only to the colourless S^O ion. On the other hand the reduction of nitrobenzene by HS and HS^ is accompanied by an increase in the HS concentration besides the formation of S 0 . A comparison of the rate constants shows that under the reaction conditions, HS (with the trace of S^ presumably present) reduces nitrobenzene about seven times faster than the HS ion. Phenylhydroxylamine is reduced two to three times more rapidly by HS- (and the trace of S present) than is nitrobenzene. Using the appearance of polysulphide as a measure of the rate of reduction, a comparison of the relative reduction rates of p-alkylnitrobenzenes by hydrosulphide ion has been carried out, vdiere the alkyl substituents were methyl, ethyl, i-propyl and t-butyl. A Baker-Nathan order was observed and the results are discussed in terms of hyperconjugation and solvent effects.