206 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Table XlI Stability of Dyeings on Hair Color of Hair Swatch Couple Initial After 88 wks.• p-Aminophenol light brown light brown p-AP + m-phenylenediamine brown violet black p-AP + 2,4-diaminoanisole red-violet black p-AP + m-aminophenol warm brown dark brown p-AP + 5-amino-0-cresol red red-brown p-AP + resorcinol yellow green medium brown •At 80% relative humidity. noted for dyeings with p-phenylenediamine and couplers (1). In general there was a tendency for the color to darken on ageing, with a loss in distinct overtones even at 0% humidity. It is well established that many 2-aminoindo dyes show a marked tendency to undergo intramolecular cyclization to phenazines. Such reactions, while slow compared with the rate of dye formation in oxidation dyeing, are significant in terms of shade stability on hair. Thus we have shown (1) that the stability of dyeouts decreases as the relative humidity increases, and that cyclization of 2-aminoindamines occurs on the hair. We have examined the stability of some 2-aminoindo-dyes, derived from p-aminophe- nol, in aqueous solution. As shown above, p-aminophenols couple para to the hydroxy group of m- aminophenols to produce orange to brownish red colors on hair. The initial coupling products are 2-aminoindophenols (eg. VII) which, at high pH, have the characteristic blue color of the anion (XIII): On heating, the blue solution slowly becomes yellow due to the formation of the corresponding 2,8-dihydroxyphenazine (XIV). Consider- able effort was involved in seeking a suitable method to study the kinetics of cyclization, since the anion (XIII) is readily attacked by hydrogen peroxide formed in the oxidation of the dihydrophenazine intermediate. Eventually, it was found that an excess of m-aminophenol in the reaction mixture could act as a hydrogen peroxide scavenger and a good spectrophotometric course could be obtained for the reaction. At pH 10, the rate of cyclization was found to be independent of pH. In the pH range 7-8, the rate decreased tenfold for each unit decrease in pH. Since the pKa of the 2-aminoindophenols is ca. 8.6, the observation indicates that the anion (XIII) is the active species in the rate-controlling step. In the pH range 6-7, the 2-aminoindophenols are particularly stable. However, at lower pH, hydrolytic fission occurs to give p-aminophenol and the corresponding 2- amino-/•-benzoquinone (XV). The rate of this reaction, in the pH range 2-6, increases tenfold for each unit decrease in pH and becomes pH independent below pH 1. This suggests that the cationic species (XVI) is the active species in the hydrolytic fission. The decomposition of 2-aminoindophenols in aqueous solution can be represented as shown in Figure 7. Although the 2-aminoindophenols are relatively stable at intermediate pH values, slow decomposition does occur. Spectral examination of the solutions does not, however,

BENZENE DERIVATIVES IN OXIDATIVE HAIR DYEING 207 IH + (I x) + 02 - H20 2 H•N XVI Figure 7. Decomposition of 2-aminoindophenols. xv reveal any characteristic features. Apparently, cyclization and hydrolytic fission occur slowly, but the products decompose as they are formed. Table XlII gives rate data for the cyclization and hydrolytic fission of some 2-aminoindophenols. For the cyclization reaction the specific first-order constant k c is expressed as kobs = kc oz where kob s is the measured rate constant, and oz is the fraction of indophenol present as the anion at the pH to which kob s pertains. From the table it can be seen that chloro Table XlII Rates of Cyclization and Hydrolytic Fission of 2-Aminoin_dophenols Indophenol 104kc(s (79øC) kh (J• mol -• rain -1) (30øC) Parent 3-chloro 3-methyl 3'-chloro 3'-methyl 3'-methyl-3-chloro 3-methyl-3'-chloro 2.57 7.27 0.55 1.50 7.00 15.3 0.54 0.039 -- -- __ --