Benzoquinone imines. Part 16. Oxidation of p-aminophenol in aqueous solution

Abstract

Oxidation of p-aminophenol gives NN′-bis-(p-hydroxyphenyl)-2-hydroxy-5-amino-1,4-benzoquinone di-imine. Data from reactions using potassium ferricyanide as oxidant indicate that this product is formed from 3 molecules of p-aminophenol and that rate-controlling steps depend on solution pH. Thus, rate-controlling steps are coupling of protonated monoimine and neutral p-aminophenol at pH <8, coupling of protonated monoimine and p-aminophenolate at pH 8–10, and above pH 10, coupling of neutral monoimine and p-aminophenolate. The resulting aminohydroxydiphenylamine is rapidly attacked at C-4 by a second molecule of monoimine to give the reduced form of the above di-imine, to which it is then oxidized by a third molecule of monoimine.