BENZENE DERIVATIVES IN OXIDATIVE HAIR DYEING 197 Table V Spectra of N'-(p-Hydroxyphenyl)-2-Aminobenzoquinone Di-imines )kma x (log Compound Monocation pK a Zwitterion pKa Anion Parent 490 (4.12) 8.1 628 (4.46) 11.7 510 (4.14) 5-Methyl 492 (4.07) 8.3 624 (4.36) 11.8 504 (4.07) Table VI shows that the effects of methyl and chloro substitution are similar to those noted above for other types of indo dyes. Data presented now and in our previous paper indicate that, as a general rule, C-methylated p-phenylenediamines and p-aminophenols and C-chlorinated couplers give rise to indo dyes absorbing at longer wavelengths than the parent dyes, while C-chlorinated para components and C-methylated couplers give dyes absorbing at shorter wavelengths. Table VI Spectra of 2-Hydroxyindophenols •kma x (log Monoanion pK• Dianion Parent 464 (3.81) 9.0 546 (4.07) 5-methyl 465 (3.81) 9.2 530 (4.03) 5-chloro 466 (3,82) 9.1 558 (4.08) 2'-methyl 465 (3.74) 9.3 568 (3.87) 3'-methyl 465 (3.74) 9.3 570 (4.01) 3'-chloro 480 (3.62) 7.9 542 (4.09) MECHANISM OF DYE FORMATION It has been established (10) that the reactive species in oxidative coupling with p-aminophenol is p-benzoquinone monoimine (II) which reacts as the conjugate acid (XI) at high pH in the neutral form and at lower pH values (generally 7.5). This is in marked contrast to the reactive species in coupling reactions with p-phenylenediamine where the neutral di.imine is apparently unreactive. The reactivity of neutral monoim- ine is probably due to development of a partial positive charge on the imino group resulting from electron withdrawal by the quinonoid oxygen in the para position. To study the kinetics of oxidative coupling reactions ofp-aminophenols with a variety of couplers it is convenient to generate monoimine by oxidation of p-aminophenol with ferricyanide. It has been shown (2) that, above pH 7, the equilibrium p-aminophenol -t- 2[Fe(CN)6] -3• Monoimine -t- 2[Fe(CN)6) -4 + 2H + is established almost instantaneously and lies completely to the right-hand side. We have also found that dilute (ca. 10-4M) solutions of the monoimine can be generated by injecting the appropriate amount of a concentrated solution of p-aminophenol into a pH 10-11 buffer, whereupon oxidation of air is complete within 2 min.

198 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Oxidative coupling reactions can be followed spectrophotometrically by following the rate of color development upon adding a coupler to a solution of the monoimine. Depending on the reactivity of the coupler, reactions can be followed under second-order kinetic conditions by using 2 mol of monoimine to 1 mol of coupler or under pseudo first-order conditions by employing 10 mol of coupler. Since monoimine undergoes hydrolysis to p-benzoquinone (III) with a half-life, at 30øC, of about 60 min, it is desirable to choose reaction conditions under which color formation is complete within a few minutes. By measuring the rate of reaction as a function of pH, it is possible to deduce the nature of the reactive species. Monoimines with Phenols Reaction of monoimines with monohydric phenols gives rise to indophenol dyes. Studies of the stoichiometry of the reaction indicate that, in the absence of an excess of oxidant, 2 mol of monoimine react with 1 mol of phenol to give 1 mol of indophenol and 1 mol ofp-aminophenol, i.e., Monoimine + Phenol • Leucoindophenol Leucoindophenol + Monoimine --• Indophenol + p-AP The reaction exhibits second-order kinetics and the rate is proportional to phenol concentration, indicating that formation of the leuco-dye is rate-controlling. Figure 3 shows the affect of pH on the rate of reaction ofp-benzoquinone monoimine with 2,6-xylenol. Above pH 11 the rate is independent of pH, indicating neutral monoimine and xylenolate ion are the active species. Below pH 11 the rate decreases with pH due to the decrease in concentration of phenolate ion. In the pH range 6-8 the rate is again pH-independent suggesting that the reactive species are the conjugate acid of monoimine and the phenolate ion or neutral monoimine and neutral phenol. Evaluation of specific rate constants and comparison with reactions of di-imines with phenols (12) suggests that the ionic pair of reactants are the major contributors in this pH range. At pH 6 the increase in rate with decreasing pH is indicative of reaction between the conjugate acid of monoimine and neutral phenol becoming the major contributor. Thus, at any pH, the observed rate constant k0 can be expressed as k 0 = kaO/MO/p- q- kbO/M+O/p- q- kcO/M+O/p... (1) where ka, kb, and kc are the specific second-order rate constants for reactions between the various pairs of reactive species M and P-, M + and P-, and M + and P, respectively, and the a's are the fractions existing as the designated species at the pH to which ko pertains. In the pH range in which oxidative dyeing is performed the reaction of neutral monoimine and phenolate ion is the most significant. From data on 23 pairs of reactants we have shown (8) that the rate constant k a for any pair can be expressed as a function of the rate constant for the reaction of the parent monoimine with the phenolate ion (k a' = 50.7 l mol -• min -• at 30øC) and cumulative substituent factors shown in Table VII. Thus for reaction of 2-methyl-p-benzoquinone