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

204 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Table X Specific Second-Order Rate Constants for the Reaction of Monoimines with m-Diamines p-Aminophenol m-Diarnine k (oe rnol -• rnin -•) Parent Parent 6.6 x 10 2 2-Me Parent 3.2 x 10 3-Me Parent 3.6 x 10 Parent 2-Me 3.2 x 10 • Parent 4-Me 2.4 x 10 3 Parent 4-Me0 1.1 x 10 4 rate-controlling step involves reaction of neutral monoimine with p-aminophenolate ion (k a = 48.6 l mol -• min -•) and of the conjugate acid of monoimine with p-aminophenolate ion (k 6 = 1.5 X 10 8 l mol -• min-•). As can be seen from Table XI, the rate of self-coupling of p-aminophenol is two to four orders of magnitude slower than the rate of coupling with commonly used couplers. A similar observation was made in the case of self-coupling of p- phenylenediamine (1). Competitive Reactions In oxidative dyeing a mixture of primary intermediates and couplers is normally employed. The color development in such a system will involve a complex series of competing reactions, the outcome of which will be dependent on the relative rates of diffusion of the various dye precursors, the concentration and relative rates of oxidation of the various primary intermediates, and the relative reactivity and concentration of the various couplers. To date, there is little information available on diffusion rates, so we must restrict our consideration of the situation to the reactions occurring in aqueous solution. While the rates of oxidation of p-aminophenol and p-phenylenediamine by hydrogen peroxide have not been measured, it is known that oxidation of p-aminophenol by molecular oxygen is considerably faster than that of oxidation of p- phenylenediamine. Furthermore, consideration of the redox potentials (2) indicates that the rapidly established equilibrium. PAP + p-Benzoquinone di-imine • Monoimine q- PPD lies essentially to the right-hand side at all pH values. It is thus apparent that, under the Table XI Rate Constants for the Reaction of Various Couplers with p-Benzoquinone Monoimine at 30øC and Various pH Values kto• (oe mol • min -•) Monoimine pH 11 pH 10 pH 9 p-Arninophenol 4.8 x 10 3.2 x 10 3.2 x 10 m-Aminophenol 2.4 x 10 4 1.6 x 10 4 2.4 x 10 3 2,4-Diaminoanisole 1.1 x 10 4 1.1 x 10 4 1.2 x 10 4 Resorcinol 1.5 x 10 6 5.7 x 10 5 2.4 x 10 5 4-Arnino-2-hydroxy-toluene 1.5 x 10 5 8.0 x 10 4 1.4 x 10 4