BENZENE DERIVATIVES IN OXIDATIVE HAIR DYEING 201 the phenolate ion increases the reactivity ten to thirtyfold, while C-chlorination decreases it five to tenfold. These effects are in accord with a mechanism involving electrophylic attack of the monoimine on the 4-position of the phenolate ion. Monoimines with Resorcinol As mentioned above, 2-hydroxyindophenols are particularly unstable. Furthermore the rate of reaction of monoimine with resorcinol is extremely fast. In order to deduce the reaction mechanism, it was found necessary to study kinetics of the reaction of the imine from 2-methyl-4-aminophenol with 4-chlororesorcinol. If similar substituent effects pertain as with the monohydric phenols, this pair of reactants should react at a rate 250 times slower than would the parent compounds. Figure 4 shows the effect of pH on the rate of reaction. Above pH 11 the rate is almost independent of pH. Since the first pKa of 4-chlororesorcinol is 10.6, it can be concluded that, at high pH, the reaction is between neutral monoimine and resorcinol dianion. As would be expected, below pH 1! the rate decreases with decreasing pH. However at pH 9 the observed rate constant is higher than would be predicted due, presumably, to contributions of reactions between neutral monoimine and resorcinol monoanion and the conjugate acid of monoimine with the artions. The reaction between monoimines and resorcinols can thus be represented as shown in Figure 5. and/or + HO 0•. NH2 Jr- and/or -- HO H H + II or Xl VI ß I Figure 5. Reaction between monoimine and resorcinol. Table VIII gives rate constants for reactions of certain monoimines with resorcinol dianions. It can be seen that methylation of resorcinol increases the reactivity by a factor of 7, while chlorination reduces it by a factor of 8. Methylation of the imine meta to the imino group decreases the reactivity by a factor of 0.06. Using this data we estimate the rate constant for reaction of neutral monoimine with resorcinol dianion to be 1.9 x 106l mol -• min -•.

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