OXIDATIVE ttAIR DYEING 125 6 7 8 9 10 11 pH Figure ii. Effect of pH on rate of reaction of p-benzoquinone diimine with m-amino- phenol at 300C, showing experimental points and contribution (broken lines) of the reactions between the various ionic species to the total rate (solid line) calculated from the theoretical equation H N•"...• / H•N •'"'•.-f", O - I-I•.N OH end/or • / xxx) /+ di-irn inc H•N•'OH ( rest)/ or •2 [o] •i)p-diamine ••••• p-diamineorHzOz (XlV) (ii) Ox •N O + (XXX) Figure 12. Mechanism of reaction of p-benzoquinone diimine wifi• m-aminopheno1 as a side reaction. However, any (XXXI) •at is {ormed is ultimately con- verted into •e normal aminoindoaniline (XII). Our data indicate that 6- methyl-3-aminophenoI is about nine times more teacfive than m-aminopheno]. Kinetic studies of the reaction o{ diimine with 4-methyI-3-aminophenoI in- dicate •at it {ollows an analogous mechanism to that proposed above {or the reaction o{ diimine wi• 6-methyl-3-aminophenoI, except that coupling lead- ing directly to dye formation occurs para to the amino group.

126 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Studies have shown that C-methylation of the diimine decreases the rate of reaction with the m-aminophenolate ion by a factor of 4, while C-chlorination of the diimine increases the rate by a factor of 1.5. Summary of Mechanisms The mechanism studies of the reaction of diimines with various m-disubsti- tuted benzenes have shown that, in the pH range normally employed in oxi- dative dyeing, the rate-controlling step is the nucleophilic attack of the di- irainJure ion on the most electron-rich form of the coupler, i.e., that form bear- ing neutral amino groups and/or ionized hydroxy groups. This reaction gives a leuco dye (diphenylamine derivative) which undergoes rapid oxidation to the indo dye. At a particular pH, the rate of reaction of various diimines with a particular coupler is in the order C-chlorodiimine diimine C-methyldiimine. The reactivity of couplers is increased by alkyl or alkoxy substituents. Stability o[ Indo Dyes and Dyeings The stability of shades produced in oxidative dyeing is not as good as one could wish for. The only documented data on the stability of dyeings pro- duced by individual reactant pairs are those reported by Tucker (13), who made measurements on dyed tresses in a standard fadeometer. No study of the stability of individual dye species in aqueous solution has been previously reported. Examination of Tucker's results shows that, in a fadeometer, perceptible fading generally occurs within one day. For dyeings with p-phenylenediamine and meta disubstituted couplers he reports the following order of stability: 2,4-diaminoanisole 2,4-diaminotoluene m-phenylenediamine 6-meth- yl-3-aminophenol resorcinol m-aminophenol. However, since Tucker does not specify whether the fading he considered was off-shade or on-shade, the order given may not correspond to the relative practical importance of the reactions of individual dye species. We have now made a study of the fading of individual dye species on hair tresses stored in clear glass and dark glass bottles at 0, 35, and 88% relative humidity and room temperature. The results, for tresses stored in dark bot- tles, are given in Table XII while we found that those stored in a north •vin- dow, in clear glass bottles, showed only a very slightly accelerated ageing effect. From Table XII, it is quite clear that the relative humidity has a marked effect on the rate of off-shade fading of all the dyeings examined. This is particularly marked in the case of dyeings with 2-amino-5-methylin- daminc where almost complete retention of the original color was observed after 3 months at 0% RH, while complete loss (by conversion to the diamino- phenazine) was observed after 3 months at 88% RH. The dyeings with p-phenylenediamine and 2,4-diaminoanisole (giving 2- amino-5-methoxyindamine) are markedly more stable than those with 2-