116 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Table VIII Visible Spectra of Indo Dyes From Various p-Diamines and m-Aminophenols p-Diamine 3-Aminophenol Xm.. (log e) Parent Parent 511 (3.98)" Parent 6-Me 492 (3.96)" 2-Me Parent 520 (3.90)" 2-CI Parent 500 (3.86)" 2,5-Cla Parent 500 (3.86) a 2,6-Cla Parent 485 (3.68)" Parent 6-MeO 489 (3.97)" Parent 4-Me 504 (3.99) * 2,6-C1, 4-Me 493 (3.64) • Parent 4-MeO 492 (4.00) * "2-Aminoindoanilines. * 2-Hydroxyindamines. 1.5 [-- 8-9 0.,5 500 600 Wove[engfh, nm Figure 4. Effect of pH on the spectrum of 2-amino-5-methylindamine (1.0x 10-•M in 1.0-cm cell) neutral solution. These results indicate the existence of the ionic species shown in Fig. 5, and that the dye exists in hair as its conjugate acid (XVIII). The spectra of 2-aminoindoaniline and its derivatives are m•affected by changes in pH, over the range 4-14, and exist in hair as the neutral species.

OXIDATIVE HAIR DYEING 117 H•I • H •N •'•--.,,/•IH• H•N •"k'• H•N •"t"'•.,/"•H• H• •N'"t'"• H •N•NH red (XVIII) blue red pK•.4.o pKa* 10'4 Figure 5. Ionic forms of 2-amino-5-mefi•ylindamine Detailed data on the effect of pH on the spectra of 2-hydroxyindoanilines and 2-hydroxyindamines are not available but comparison of their spectra with those of the analogous 2-methoxy compounds (Table V) suggest that the indoanilines exist as anionic species (XIX) and the indamines as the Zwitter ions (XX). Mechanism o[ Dye Formation It has been established (2) that the reactive species in oxidalive dyeing with p-phenylenediamine is p-benzoquinone diimine (XXI) which usually reacts ( 14 ) as the diiminium ion (XXII). NH• NH z NH NH (XX1) (XX11) It has also been shown (46) that the diimine is generated, essentially in- stantaneously, by reaction of p-phenylenediamine with ferricyanide. Above pH 8, this equilibrium p-phenylenediamine + 2 [Fe (CN)o] -a • diimine + 2 [Fe (CN)o] -4 + 2H + lies completely to the right hand side.