•,,54 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS of the mechanism of oxidative dyeing, it is worthwhile elaborating on the reasons for believing that, while there is ample indirect evidence for the intermediacy of the diimine, it is unlikely that its presence has, or will be, directly demonstrated under the conditions pertaining in the in vitro dyeing process, i.e. high peroxide (~ 1.0 molar) and diamine (~0.1 molar) concentration and pH 9-10. PROPOSED MECHANISM OF DYEING According to the proposed mechanism of oxidative dyeing (1), the p-diamine is oxidised within the hair shaft by hydrogen peroxide (or, possibly, by oxygen formed by decomposition of peroxide within the hair shaft) to give the diimine. The resulting diimine then undergoes coupling with unoxidised diamine and/or the various couplers, e.g. resorcinol, m-diamines, etc. to give indamine and indoaniline dyes. This scheme is NH• NH Ox. NHz NH I Tr +1 m- D•amJnes /M•½ phenols •"• Resorcinol •j•- D•kefones or • Pyrazolones Ox Bandrowsk•'s base Indamines Indoanilines - Polymenc •ndoamhnes Azomefh•ne dyes • Azo- and nifro-dyes Figure 1. The role of p-phenylenediamine and p-benzoquinonediimine in oxidative dyeing. outlined in Fig. 1. Kinetic studies have shown that while the oxidation of the p-diamine by hydrogen peroxide is slow, the subsequent coupling reactions are very fast. It is important to stress that, contrary to Tucker's statement (2), even the formation of Bandrowski's base is fast, and that it

p--BENZOQUINONEDIIMINE 255 is the prior oxidation of the diamine to the diimine which is rate controlling. Nevertheless, we have found that the commonly employed couplers are even more reactive towards the diimine, than is p-phenylenediamine. Thus, accepting the overall mechanism in Fig. 1, this relative reactivity of p-diamine and other couplers explains the previous observations that Bandrowski's base is formed during the in vitro oxidation of the p-diamine alone, but not when the p-diamine is oxidised in the presence of equimolar amounts of resorcinol (2, 4) or of 2, 4-diaminoanisole (2). In fact, since the dyeing process entails oxidation of only a small proportion of the available diamine, even relatively small amounts of such couplers would be expected to inhibit the formation of the characteristic brown diamine oxidation products. This has been observed by Tucker in tress dyeing (2). These observations, together with those given earlier (1), lend further support to the mechanism outlined in Fig. 1. DETECTION OF THE DIIMINE Since the p-diamine is less reactive than other commonly employed couplers, it can be concluded that if the formation of the diimine is to be demonstrated, it is most likely so to be in the oxidation of p-phenylenedi- amine in the absence of other couplers. Under these circumstances we need consider only three processes:- Oxidation: p-Diamine (P) + oxidant Coupling: k2 3D + P (as "catalyst")* -- Hydrolysis: kl D-- monoimine -NH 3 -NH 3 kl ß • diimine (D) (i) * Bandrowski base (B) (ii) benzoquinone "Humic acid" (iii) Dolinsky et al (3) reported that peroxide oxidation of the diamine gave small amounts of other products, but for the present purpose these can be ignored since they represent 10% of the reaction. According to (i)-(iii) the diimine is formed by oxidation of the diamine:-- d [D•/dt = k' 1 [P] [Oxl *Here "catalyst" implies that while the presence of p-diamine is essential to the formation of Bandrowski's base, it is consumed in the first step (rate controlling) but regenerated in the final step.