BENZENE DERIVATIVES IN OXIDATIVE HAIR DYEING 209 2-amino-N'- (p-hydroxyphenyl)-p-benzoquinone di-imines (X). Preliminary experi- ments showed that the products of decomposition of these dyes depend on the pH. At pH 12, the major reaction involves hydrolysis of the terminal imino-group, resulting in formation of the corresponding 2-aminoindophenol (XIII) which then cyclizes to the 2,8-dihydroxyphenazine (XIV), as discussed above. Below pH 5, the major reaction is hydrolytic fission at the azine bridge to give the p-aminophenol and the 2-amino- benzoquinone monoimine (XVII). In the intermediate pH range, the major product is the 2-amino-8-hydroxyphenazine (XVIII) formed by intramolecular cyclization. We have measured the rates of these reactions as a function of pH. A plot of the rate constant for the consumption of the indo-dye is shown in Figure 8. At high pH the rate is almost independent of pH, but falls with decreasing pH in the pH range 11-12.5. This is consistent with the rate-controlling step in the hydrolysis involving attack by the hydroxide ion on the iminium group of the zwitterionic form (X) of the indo dye. At low pH ( 4) the tenfold increase in rate with each unit decrease in pH is consistent with the hydrolytic fission involving attack of a water molecule on the dicationic form (XIX) of the indo dye. This is analogous to the hydrolysis of 2-aminoindamines on which we have reported earlier (1). In the intermediate pH range the rate of reaction shows a minimum at about pH 6. Over the pH range 7-10 the major product is the phenazine (XVIII) and the rate increases with pH in a way that is consistent with cyclization of the zwitterionic form (X) of the indo dye. These reactions are summarized in Figure 9. + HO H2N•['I 2 HO'• XlX XVll -2 H + 02 • ./N -0 H I•IH 2 HO N NH•_ X XVIII OH- XIII ' XIV Figure 9. Decomposition of 2-amino-N'-(p-hydroxyphenyl)-p-benzoquinone diimines.

210 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS The rates of consumption of the indo-dyes can be expressed as kob s = kfo/[H +] + kcoz + kho/[OH- ] where kob s is the experimental first-order rate constant and kf, k o and k h are the specific first- or second-order rate constants for fission, cyclization and hydrolysis, respectively, and oz is the proportion of dye present as the zwitterion at the pH to which kob s pertains. Values of these rate constants for the dyes (XX) are given in Table XIV. .•N RN H,•N x HO H xx It can be seen that methyl substitution, either on the ring or on the 2-amino-group, has only a minor effect on the hydrolysis rate constant. However, cyclization is considera- bly facilitated by methyl substitution on the 2-amino group. This has been previously observed with 2-aminoindamines. Table XIV Rate Date for Decomposition of the Dyes (XX) in Aqueous Solution at 30øC Compound (XX) R X 10 2 kf k h (2mol -•s-') 104kc(s ') (2mol's •) H H 3.14 0.84 1.35 H Me 3.71 0.70 2.45 Me Me 2.53 13.80 0.50 CONCLUSION The present study has shown that, in oxidative dyeing, oxidation of.p-aminophenols to .p-benzoquinone monoimines is the first step in color formation. Unlike the corresponding di-imines, the monoimines react as the free base at pH 8. Coupling of the neutral monoimines with meta difunctional benzene couplers as the free bases or the anions, results in the formation of leucoindo-dyes. The latter undergo rapid oxidation to the highly colored indo-dyes. Oxidation of mixtures containing both .p-aminophenol and .p-phenylenediamine results in preferential oxidation of the .p-aminophenol. In the absence of couplers, the resulting monoimine reacts with both unoxidized .p-aminophenol and with .p-phenyl- enediamine to give analogues of Bandrowski's base. In the presence of couplers, these latter reactions are suppressed and reaction occurs preferentially with the coupler. The reactivity of commonly employed couplers towards monoimines is in the order resorcinol 4-amino-2-hydroxytoluene 2,4-diaminoanisole m-aminophenol.