196 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS the dye to be in the neutral form. The effect of methyl and chloro substituents on the spectrum of indophenol has been studied (8) and it has been shown that an indophenol with a single methyl group exists as the tautomer in which the methyl group is on the quinonoid ring and produces a shift of between +4 and -34 nm. If a second methyl group is introduced on the same ring its effect is similar and additive. However if it is introduced into the unsubstituted ring its effect is to produce a large shift (+12 to + 48 nm) to longer wavelengths. Chloro substituents have an opposite effect to methyl groups. It is noteworthy that the indophenol formed from 1-naphthol absorbs at shorter wavelengths than that formed from phenol. Table IV gives spectral data for a number of 2-aminoindophenols formed from Table IV Spectra of 2-Aminoindophenols X max (log Anion Cation Indophenol 637 (4.50) 496 (3.83) 2-amino 574 (4.23) 470 (3.92) 2-arnino-5-rnethyl 555 (4.21) 462 (3.92) 2-arnino-3'-rnethyl 605 (4.25) 482 (3.90) 2-amino-2'-methyl 600 (4.11) 480 (3.85) 2-amino-3'-chloro 558 (4.19) 468 (3.84) 2-amino-5-chloro 585 (4.27) 475 (3.91) coupling p-aminophenols with m-aminophenols. By comparison with the spectra of indophenol itself it can be seen that the introduction of the 2-amino group results in an hypsochromic shift of 63 nm for the anion and 26 nm for the neutral species. As with indophenols, introduction of a methyl group into the quinonoid ring or a chloro into the benzenoid ring of 2-aminoindophenol results in an hypsochromic shift, while a methyl on the benzenoid ring or a chloro on the quinoid ring produces a bathochromic effect. The products of oxidative coupling of p-aminophenols and m-diamines exist in the tautomeric form (IX) in which the hydroxy bearing ring is benzenoid. There is no unsubstituted analogue of this since simple indoanilines exist in the alternative tautomeric form. In solution the N'-(p-hydroxyphenyl)-2-aminobenzoquinone di- imines exist as a red anion at pH 11.5, a blue zwitterion (X) between pH 8 and pH 11.5 and a red cation below pH 8. The color on hair suggests that the dye is in the cationic form in hair. Spectral data for some derivatives of the di-imine (IX) are given in Table V. N 0 H2 NH 2 At pH 9 the hydroxyindophenols, formed by oxidative coupling ofp-aminophenols and resorcinols, exist as dianions. Below pH 9 the monoanion is the major species.

BENZENE DERIVATIVES IN OXIDATIVE HAIR DYEING 197 Table V Spectra of N'-(p-Hydroxyphenyl)-2-Aminobenzoquinone Di-imines )kma x (log Compound Monocation pK a Zwitterion pKa Anion Parent 490 (4.12) 8.1 628 (4.46) 11.7 510 (4.14) 5-Methyl 492 (4.07) 8.3 624 (4.36) 11.8 504 (4.07) Table VI shows that the effects of methyl and chloro substitution are similar to those noted above for other types of indo dyes. Data presented now and in our previous paper indicate that, as a general rule, C-methylated p-phenylenediamines and p-aminophenols and C-chlorinated couplers give rise to indo dyes absorbing at longer wavelengths than the parent dyes, while C-chlorinated para components and C-methylated couplers give dyes absorbing at shorter wavelengths. Table VI Spectra of 2-Hydroxyindophenols •kma x (log Monoanion pK• Dianion Parent 464 (3.81) 9.0 546 (4.07) 5-methyl 465 (3.81) 9.2 530 (4.03) 5-chloro 466 (3,82) 9.1 558 (4.08) 2'-methyl 465 (3.74) 9.3 568 (3.87) 3'-methyl 465 (3.74) 9.3 570 (4.01) 3'-chloro 480 (3.62) 7.9 542 (4.09) MECHANISM OF DYE FORMATION It has been established (10) that the reactive species in oxidative coupling with p-aminophenol is p-benzoquinone monoimine (II) which reacts as the conjugate acid (XI) at high pH in the neutral form and at lower pH values (generally 7.5). This is in marked contrast to the reactive species in coupling reactions with p-phenylenediamine where the neutral di.imine is apparently unreactive. The reactivity of neutral monoim- ine is probably due to development of a partial positive charge on the imino group resulting from electron withdrawal by the quinonoid oxygen in the para position. To study the kinetics of oxidative coupling reactions ofp-aminophenols with a variety of couplers it is convenient to generate monoimine by oxidation of p-aminophenol with ferricyanide. It has been shown (2) that, above pH 7, the equilibrium p-aminophenol -t- 2[Fe(CN)6] -3• Monoimine -t- 2[Fe(CN)6) -4 + 2H + is established almost instantaneously and lies completely to the right-hand side. We have also found that dilute (ca. 10-4M) solutions of the monoimine can be generated by injecting the appropriate amount of a concentrated solution of p-aminophenol into a pH 10-11 buffer, whereupon oxidation of air is complete within 2 min.