Benzoquinone imines. Part XI. Mechanism and kinetics of the reaction of p-benzoquinone di-imines with aniline and its derivatives

Abstract

Indamine [Phenylene Blue, N-(4-aminophenyl)-p-benzoquinone di-imine] and some C-alkyl derivatives have been characterised, for the first time, as the products of the oxidative coupling of p-phenylenediamines with anilines. The coupling reaction involves electrophilic attack of the conjugate acid of the p-benzoquinone di-imine at the 4-position of the neutral aniline, to give 4,4′-diaminodiphenylamine, which undergoes rapid oxidation to the indamine. The reactivity of anilines is increased by electron-donating substituents, while that of the di-imines is increased by electron-withdrawing substituents. The effect of substituents on the rate of the coupling reaction is cumulative.

The visible spectra of the indamine dyes indicate that in unsymmetrically substituted derivatives, the most alkylated ring is quinonoid, while the most chlorinated ring is benzenoid.

Hydrolysis of indamines gives indoanilines, and the pH dependence of the rate of hydrolysis indicates that hydrolysis of the neutral species is more rapid than is that of the conjugate acid. Data is given for the oxidation potential of indamine over the pH range 3·9–5·9.