Benzoquinone imines. Part VIII. Mechanism and kinetics of the reaction of p-benzoquinone monoimines with monohydric phenols

The reaction of p-benzoquinone monoimines with monohydric phenols gives indophenol dyes by a process involving a rate-controlling electrophilic coupling of the monoimine to the para-position of the phenol, followed by rapid oxidation of the resulting leuco-dye by a second molecule of the monoimine. At pH > 10, the neutral monoimine and the phenolate ion are the major reactive species, while below pH 10 the reaction between the conjugate acid of the monoimine and the phenolate ion becomes significant and, at pH 6–8 is the major contributor to the reaction. At pH < 5, the reaction between the conjugate acid of the monoimine and the neutral phenol makes a significant contribution to the rate. C-Methylation decreases the reactivity of the neutral monoimine by a factor of ca. 20 but increases the reactivity of the phenolate ion by factors of 27 (ortho) and 10 (meta). C-Chlorination increases the reactivity of the monoimine by a factor of 4·5–8. In polysubstituted compounds the effect of methylation is cumulative.The visible spectra of the indophenol dyes and their anions indicate that in the major tautomer the more highly methylated ring is the quinonoid ring and the more highly chlorinated ring is the benzenoid ring.