Benzoquinone imines. Part V. Mechanism and kinetics of the reaction of p-benzoquinone monoimines with m-phenylenediamines

Abstract

Oxidation of mixtures of p-aminophenol and m-phenylenediamines is shown to give aminoindoaniline dyes by a reaction between p-benzoquinone monoimine and the m-diamine. In the coupling reaction, the rate-controlling step involves, at pH > 8·5, electrophilic attack by the neutral monoimine on the neutral m-diamine and, at pH < 5, by the protonated monoimine on the neutral m-diamine. In the pH region 5–8·5, both reactions make a significant contribution. The resulting leuco-indoaniline then undergoes rapid oxidation, by a second molecule of monoimine, to give the dye. The effect of C-methylation of the reactants on the rate of coupling is consistent with the suggested mechanism. Thus, C-methylation of the monoimine decreases the rate of coupling, while C-methylation of the m-diamine increases the rate.