104 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Ox. Ox.• NH• NH OH O Oxidation of the individual primary intermediates gives rise to colored products since the resulting imines are capable of reacting with unoxidized intermediate to produce dye species (Fig. 1). Thus, the reaction of p-benzo- quinone diimine with p-phenylenediamine results in the formation of the brown dye Bandrowski's base (I)(3-5). An analogous product (II) may re- sult from the oxidation of p-aminophenol, but this has not yet been estab- lished. Oxidation of o-aminophenol and of o-phenylenediamine gives rise to 2-aminophenoxazinone (1II) (6,7) and 2,3-diaminophenazine (IV) (8), re- spectively. However, the mechanism, and even the intermediacy of the corre- sponding o-benzoquinone imines, has not yet been established for the latter two oxidations (9). NH• NH z (1•) brown-black N Ox.• HO OH OH N ,•,OH (II) brown N H• Ox, OH (III) orange NH• Ox,) N H•. N H• (IV) red orange Figure 1. Self-coupling products of primary intermediates

OXIDATIVE HAIR DYEINC 105 The products shown in Fig. 1 are the principal products of the oxidative coupling reactions. It is known, however, that other colored species are formed from the peroxide oxidation of p-phenylenediamine (10). In modern oxidative dyeing practice, hydrogen peroxide is the normal oxi- dant, although persalts of various acids or solid organic hydrogen peroxide adducts are employed when product presentation requires the use of a solid oxidant. Nevertheless, it should be noted that the persalts and adducts usually generate hydrogen peroxide on dissolution in water. A particular advantage of hydrogen peroxide is that it is a relatively unreactive oxidant and results in only slow oxidation of the primary intermediates in the dye bath. In fact, we have found that color development is more rapid when dilute solutions of pre- cursors are oxidized with air than when the frequently used 3% hydrogen peroxide is employed. It may well be that peroxide undergoes catalytic de- composition to yield molecular oxygen within the hair fiber, thus resultin• in the desirable situation of a more rapid color development with:n the fiber than pertains in the dyebath. The third component type consists of the couplers, which are important in producing the color nuances necessary to the simulation of natural hair colors. The couplers do not undergo facile oxidation but, by virtue of their bearing strong electron donating groups, produce dyes by reaction with the electro- philie quinone-imines. In practice, any aromatic compound having an amino or hydroxy group and an unblocked para position can react with a quinone- imine to produce an indo dye. However, it has been found that anilines and most monohydric phenols are generally insufficiently reactive to compete with the self-coupling reactions of the primary intermediates, under hair dyeing conditions. For this reason, the most important couplers are phenols or ani- lines bearing a second strong electron donor in the meta position. Thus, al- though the choice has been made pragmatically, we find that the most impor- tant couplers in current practice are resorcinols, rn-phenylenediamines, and rn-aminophenols (11-13). The present paper describes a study of the mechanism of the reactions of the reactive aliimines with the meta difunctional couplers, and of the nature of the major colored species formed in these reactions. It should be noted that chromatographic examination of oxidized reaction mixtures shows the pres- ence of a number of minor species in addition to one or, sometimes, two major products. EXPERIMENTAL The spectrophotometric and kinetic techniques employed in this work have been described previously (1,14). As has been pointed out earlier, the oxidation of p-phenylenediamine to p- benzoquinone diimine by hydrogen peroxide or molecular oxygen is slow rela- tive to the rate of reaction of the diimine with couplers. For this reason, the