202 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Table VIII Specific Second-Order Rate Constants for the Reaction of Monoimines with Resorcino! Dianions at 30øC p-Aminophenol Resorcinol k (oe mol -•k min -•) 2-Me 4-C1 1.5 x 104 2-Me 4-Me 8.1 x 105 2-Me Parent 1.16 x 105 Parent 4-C1 2.55 x 105 Monoimines with m-Aminophenols The pH dependence for reaction between monoimines and m-aminophenols is similar to that of the reaction with other monohydric phenols. This points to a reaction involving the neutral monoimine and the aminophenolate artion at high pH and the conjugate acid of the monoimine with the phenolate ion at intermediate pH values and with the neutral m-aminophenol at lower pH values. Table IX gives the rate constants, having the same designation as in eq 1, for various Table IX Specific Second-Order Rate Constants for the Reaction of Monoimines with 3-Aminophenolate Ions at 30øC 4-Aminophenol 3-Aminopheno! k a (oe tool -• rain -•) Parent Parent 2.61 ) 10 4 2-Me Parent 1.26 X 10 3 3-Me Parent 1.62 ) 10 3 Parent 2-Me 2.88 x 105 Parent 6-Me 1.60 x 105 2-Me 2-Me 1.45 X 10 4 2-Me 6-Me 1.23 X 10 4 3-Me 2-Me 1.58 x 104 3-Me 6-Me 9.20 x 103 Parent 6-C1 6.90 X 10 3 2-C1 Parent 8.40 x 105 pairs of reactants. C-Methyl groups on the monoimine decrease the reactivity by factors of 0.049 (meta to imino) and 0.056 (ortho to imino) while chloro (meta to imino) increases the reactivity by a factor of 31. On the other hand, methyl groups on the m-aminophenolate ion increase the reactivity by factors of 11 (2-Me) and 5.6 (6-Me) while 6-chloro substitution decreases the reactivity by a factor of 0.24. These changes are in close agreement with those observed for monohydric phenols (Table VII). Further comparison with rates for monohydric phenols shows the amino group to enhance the reactivity by a factor of about 500. Monoimines with m-diam•'nes Under anaerobic conditions, 2 mol of a monoimine and 1 mol of a m-diamine react to give 1 mol each of the N'-(p-hydroxyphenyl)-2-amino-p-benzoquinone di-imine (IX) and of the p-aminophenol. The pH dependence of the reaction is shown in Figure 6. Above pH 8, the rate is independent of pH, indicating the rate-controlling step to be reaction between the two neutral species. The increase in rate with decreasing pH at

BENZENE DERIVATIVES IN OXIDATIVE HAIR DYEING 203 /-'" \ / M+D 0 0 ' I \l I 6 8 10 pH Figure 6. The variation of rate with pH for the reaction of p-benzoquinone monoimine with m-phenylenediamine. The broken lines show the calculated contribution of the reactions of m-diamine with neutral and with protonated monoimine to the total rate (full line). The open circles represent experimental results. pH 8 indicates an increasing contribution from reaction between the conjugate acid of monoimine and neutral m-diamine. At the pH of oxidative dyeing, only the former reaction is important. Rate constants for this reaction are given in Table X. The lowering of the reactivity of the monoimine by C-methyl groups is similar to that observed in coupling with the various types of phenols. C-Methylation increases the reactivity of m-diamine by a factor of about 5, i.e., somewhat less than observed for phenolic couplers. Finally, 2,4-diaminoanisole is about 17 times more reactive than m-phenylenediamine. Monoimine with p- Aminophenol As mentioned above, p-aminophenol undergoes self-coupling, under oxidative condi- tions, to give the trinuclear species (V). It has been shown (13) that at high pH the