J. Soc. Cosmet. Chem., 30, 191-211 (July/August 1979) The role of meta difunctional benzene derivatives in oxidative hair dyeing. II. Reactions with p-aminophenols KEITH C. BROWN and JOHN F. CORBETT Clairol Research Laboratories, 2 Blachley Road,, Stamford, CT 06902. Received.January 5, 1979. Presented at IOth IFSCC Conference, October 1978, Sydney, Australia. Synopsis A study of the REACTIONS OF P-AMINOPHENOLS with m-diamines, m-aminophenols, and resorcinol, under OXIDATIVE conditions, is described. Oxidation of p-aminophenol to p-benzoquinone monoimine is followed by coupling to the meta compound and oxidation to give N-(p-hydroxyphenyl) aminobenzoquinone diimines, aminoindophenols and hydroxyindophenols, respectively. Visible spectra of the dyes are related to DYE COLOR ON HAIR and stability in solution is related to color stability on hair. INTRODUCTION Oxidative hair dyeing involves the in-fiber formation of dyes from colorless interme- diates. The essential chemical process involves primary intermediates, generally p-diamines and p-aminophenols, couplers, generally meta-difunctional benzene deriva- tives such as m-diamines, m-aminophenols, and resorcinol, and hydrogen peroxide. In the first paper in this series (1), the reactions ofp-phenylenediamine with a variety of couplers were described and mechanisms were proposed. In general, dye formation was shown to involve slow oxidation ofp-phenylenediamine by hydrogen peroxide to give p-benzoquinone diimine which, in the form of its highly reactive conjugate acid, reacts with the coupler by electrophilic attack on the most nucleophilic site on the benzene ring of the coupler. The resulting leuco dye is then oxidized to the indamine, indoaniline, or indophenol. In certain cases, further reaction of diimine with the indo dye resulted in the formation of tri-nuclear or polynuclear dyes. Some indo dyes undergo rapid intramolecular cyclization to phenoxazines others undergo slow cyc!ization to phenazines, which is partly responsible for the instability of the color on hair. The present paper describes the reaction of p-aminophenols with couplers and discusses properties of the resulting dyes. 191

192 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS EXPERIMENTAL Spectrophotometric and kinetic techniques employed in this work have been described previously (2,3). As pointed out earlier, oxidation ofp-aminophenol to p-benzoquinone monoimine by hydrogen peroxide or molecular oxygen is slow relative to the rate of reaction of monoimine with couplers. Thus the use of these oxidants is unsuited to studies of the kinetics of coupling reactions. However we have found that, above pH 7, the reaction p-aminophenol + 2 ferricyanide • monoimine + 2 ferrocyanide is quantitative and virtually instantaneous (2). This system was used as far as possible, therefore, for the generation of the monoimine. In kinetic experiments employing 4-equivalent couplers, stoichiometric reaction mixtures can be prepared using the p-aminophenol:ferricyanide:coupler molar ratios of 1:4:1 or 2:4:1. In the first case, the leuco dye is oxidized by ferricyanide while in the second case it is oxidized by a mole of monoimine with consequent formation of a mole ofp-aminophenol. Below pH 7, the above equilibrium lies partly to the left-hand side it is therefore necessary to use freshly prepared aqueous solutions of monoimine for kinetic work at pH 7. With 4-equivalent couplers the required monoimine:coupler ratio is 2:1. In experiments with 2-equivalent couplers, the requisite p-aminophenol:ferricyanide:cou- pier ratio is 1:2:1 and monoimine:coupler ratio is 1:1. All the above systems lead to second-order kinetic forms. With less reactive couplers an excess of coupler must be used and first-order kinetic forms result. Relevant kinetic equations have been given elsewhere as has a consideration of the reaction conditions required to avoid interference of side reactions such as hydrolysis of the monoimine. RESULTS AND DISCUSSION COLORS PRODUCED ON HAIR Tucker (4) has listed the colors given by p-aminophenols alone, and with a variety of couplers, when solutions mixed with hydrogen peroxide are applied to hair. p- Aminophenol alone gives a golden brown shade, while its 2-methyl derivative gives a reddish brown. The color given by the 3-methyl derivative is, by contrast, devoid of warm tones, possibly indicating the formation of a different type of oxidation product. Mixtures ofp-aminophenols and resorcinol give greyish blonde colors of low intensity. With m-aminophenol as the coupler, reddish browns are obtained. With 4-amino- 2-hydroxytoluene as the coupler, an intense reddish orange of excellent stability is produced. Mixtures of p-aminophenols with meta diamine couplers produce reddish brown colors of moderate stability and with a marked tendency to change color with pH. REACTION PRODUCTS Oxidation ofp-aminophenol (I) with two molar equivalent of ferricyanide, at pH 7, gives rise to p-benzoquinone monoimine (II). In the absence of any other reactive species the monoimine undergoes slow hydrolysis to p-benzoquinone (III), as shown in Figure 1. If less than two molar equivalents of ferricyanide are used, monoimine reacts with un-oxidized p-aminophenol to give a brown dye. It seems likely that this dye is an