304 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Phase 1. 1st. Case• Where [A] • [C] A is oxidized to imine Imine A + Coupler C . Dye AC Phase 2. B is oxidized to imine Imine B Coupler C ) Dye BC Coupler D Dye BD Coupler E • Dye BE 2nd Case• Where [A] • [C] Phase 1o A is oxidized to imine Imine A Coupler C Coupler D Phase 2. B is oxidized to imine Dye AC Dye AD Coupler D • Dye BD Imine B • " , CouPler E • f Dye BE Figure 4. Schematic diagram of the coupling reactions occurring in a composition containing two primary intermediates A and B, where A is the more readily oxidized, and three couplers having relative reactivity [C][D][E].

HAIR COLORANT CHEMISTRY 305 To sum up, the reactions involved in color formation in oxidative dyeing are sequential oxidation of the primary intermediates followed, in each case, by competitive reactions of the resulting imine with the various couplers (Figure 4). The final color produced is a function of the amounts and nature of the individual primary intermediates and couplers in the composition and the pH at which the process is performed. Obviously, the individual dyes are not formed synchronously and the shade does not develop monotonally. Thus we would expect, as found in practice, that the tonal quality of the shade produced will be time-dependent. From the above discussion, it is evident that, while it is not possible to define fully the process of oxidation dyeing in quantitative terms, a stage of understanding exists where formulation of products does not have to proceed entirely by trial and error. SEMIPERMANENT COLORANTS The considerations which go into the formulation of semipermanent hair colorants are quite different from those which have been enumerated for the permanent hair colorants. The semipermanent products make use of preformed direct dyes which are neutral, sparingly soluble, and of relatively low molecular weight. Dyes currently used in the United States are listed in Table II. It can be seen that they are nitrophenylene-diamine and nitroaminophenol derivatives, supplemented with some aminoanthraquinones. Table II Dyes Currently Used in Semipermanent Colorants in the United States Dye Color N-(2-Hydroxyethyl)-2-nitroaniline N•-Tris(hydroxymethyl)methyl-4-nitro-0-phenylenediamine N•-(2-Hydroxyethyl)-2-nitro-p-phenylenediamine N s,N4,N4-Tris(2_hydroxyethyl)_2_nitro_p_phenylenediamine O,N-Bis(2-hydroxyethyl)-2-amino-5-nitrophenol 4-Hydroxy-2'-nitrodiphenylamine 4-Amino-4'-bis(2-hydroxyethyl)-aminoazobenzene 1,4,5,8-Tet raminoanthraquinone 1,4-Diaminoanthraquinone Yellow Yellow Red Blue-Violet Yellow Orange Orange Blue Violet The selection of dyes for use in formulating these products involves consideration of color, molecular size, and stability of the components. Research into the relationship between color or stability and molecular structure has given rise to some interesting correlations which will be discussed below. It has been found that most shades needed to make up a line of products can be achieved through the use of appropriate proportions of yellow, orange, red, and blue dyes. However, it must be remembered that human hair has variable porosity from root to tip. Low molecular weight dyes can penetrate into the cortex even near the root and, obviously, even more readily into the more porous ends. However, since the dyes used have no inherent affinity for the hair keratin, they are readily washed out of the ends so much so, that much of the color can be lost during the rinse step at the end of the process. In order to achieve a uniform shade over the full length of the fiber, it is