118 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Kinetic techniques for studying the reactions of diimines with various cou- plers by following the rate of color development have already been described (14). These have now been applied to the reactions with resorcinols, m-dia- mines, and m-aminophenols. Diimines with Resorcinols While Brody and Burns (39) reported that they could not detect an early oxidation stage in the oxidation of p-phenylenediamine and resorcinol, we have now found that when oxygen is bubbled into a dilute solution of the re- actants, a magenta color develops. The same colored species is formed more rapidly when ferricyanide is used as oxidant or when a fresh solution of pure diimine is added to a solution of resorcinol. As mentioned earlier, the spec- trum of the magenta dye is consistent with its being 2-hydroxyindoaniline (V). Attempts to confirm the structure by unambiguous synthesis have so far been unsuccessful and isolation of the dye has proved impossible due to its marked instability. Studies of the stoichiometry of the reaction indicate that 2 moles of di- i•nine reacted with one mole of resorcinol to give one mole of magenta dye and one mole of p-phenylenediamine (Table IX). Thus, the overall reaction can be represented: or 2 diimine + resorcinol • hydroxyindoaniline + p-diamine dii•nine + resorcinol • lcuco dye followed by leuco dye + diinfine • hydroxyindoaniline + p-diamine The rate of formation of the indoaniline can be followed spectrophotomet- rically at 484 nm. Kinetic runs using stoichiometric amounts of reactants show that the rate of dye formation follows second-order kinetics (Table X). Fur- thermore, it was also found that the rate is proportional to the resorcinol con- centration. Thus, the reaction is first order with respect to the concentration of both diimine and resorcinol, and the rate-controlling step must involve attack of the diimine on resorcinol. The effect of pH on the rate of reaction was examined. As can be seen from Fig. 6, the rate decreases with increasing pH at pH 10.5 and with decreas- ing pH at pH 10. This rate/pH profile is consistent with a rate-controlling step involving electrophilic attack by the coniugate acid of the diimine (DH +) on the mono-(RH-) and/or dianion (R =) of rcsorcinol. Thus, the observed second-order rate constant kobs is given by the equation where kob s = k a OIDii+ OtR= -I- kl• OtDII OtRII- and kb are the specific second-order rate constants for the reactions DH + R = and DH + + RH-, respectively, and the rr values are the fractions of

OXIDATIVE HAIR DYEING 119 Table IX The Stoichiometry of the Reaction of p-Phenylenediamine, Oxidant, and Resorcinol p-Diamine Ferricyanide Resorcinol Optical Density (M) (M) (M) (484 nm.) • 2 X 10 -4 4 X 10 -4 1 X 10 -4 0.97 1 X 10 -4 4 X 10 -4 1 X 10 -4 0.95 I X 10 -• 4 X 10 -• 2 X 10 -4 0.97 1 X 10 -4 4 X 10 -• 1 X 10 -4 0.49 2 X 10 -4• 0 1 X 10 -4 0.96 1-cm cell. Using pure p-benzoquinone diimine. Table X Rate of Formation of 2-Hydroxyindoaniline from the Reaction of p-Benzoquinone Diimine (1.0 X 10-•M)with Resorcinol (5 X 10-•M) at pH 9.94 and 30øC Optical Dye Formed Time k Density (480)' (%) (see) (1 mole -• min-•) 0.435 25 1.1 1.82 X 10 • 0.87 150 3.15 1.71 X 10 • 1.30 75 10.5 1.71 X 10 * 1.52 87.5 22.2 1.87 X 10 • Mean rate constant 1.78 X 10 • For a 4.0-cm path length. the reactants present as the designated species at the pH to which ko,,• per- tains. Both ka and k• are, by definition, independent of pH. Using pKa values of 5.75, 9.20, and 11.27 for DH+, RH2 and 'RH-, respectively, values of k, = 5.75 x 10 'ø i mole-•min -• and k• = 1.21 x 10 s i mole4min -• were obtained by computer. Using these values, the theoretical log k versus pH curve (Fig. 6) was obtained. The good agreement between the experimental points and the theoretical curve supports the mechanism for the formation of the magenta dye shown in Fig. 7. It can be concluded that the first product of the oxidation of p-phenylenediamine resorcinol mixtures is the magenta dye, i.e., 2-hydroxy- indoaniline (V). As reported above, when 2 moles of p-benzoquinone diimine and i mole of resorcinol are allowed to react, i mole of 2-hydroxyindoaniline and I mole of p-phenylenediamine are formed. If the reaction mixture is kept, the gradual disappearance of the indoaniline peak, accompanied by an increase in the ab- sorbance at 325-425 nm and 600-700 nm is observed. As can be seen from Fig. 8, these changes exhibit isosbestic points at 428 and 600 nm furthermore, the final spectrum closely resembles that of the green dye (X). The results in Fig. 8 were obtained for a solution containing oxygen. In a similar experiment per- formed in the absence of oxygen, similar changes are observed but the rate of disappearance of color at 484 nm is doubled and the amount of green dye