RATIONALE OF DYES SYNTHESIS PROGRAM 355 In his day-to-day laboratory work, the dye synthesis chemist em- ploys reasoning based on the general principles that have just been briefly presented. With experience, he will have accumulated a fairly extensive, fingertip knowledge of the general effect of various structural modifications on the color of a given chromophoric system. His may be described as a "shifty" occupation for, in essence, he begins with a particular colorless or colored molecule and, by synthesizing appropriate derivatives, imparts bathochromic or hypsochromic shifts to its absorp- tion spectrum to obtain the desired range of colors. Usually super- imposed on the color itself is the additional requirement of end use, for the dyes must have affinity for the substrates for which they are being designed, be it cotton, wool, nylon, hair, or what have you. This latter requirement often complicates the solely color aspect since substantivity to a particular substrate may also require structural modification of the chromophore, and the effect of such variations on absorption must also be taken into account in deciding what structures will give the desired color. Finally, the dyes which the chemist makes must meet certain practical requirements such as washfastness, lightfastness, perspiration stability, stability toward bleach, and so on. To illustrate the general approach taken by the organic chemist, it may be useful to take a hypo- thetical, specific problem of the type which is often presented and follow its development. DYE SYNTHESIS PROGRAM The aromatic chemicals department of Company XYZ, which also manufactures a range of dyes, has put into large-scale, economical pro- duction a colorless aromatic amine not previously commercially avail- able, viz., sulfanilic acid. Having a strong position in aromatic (benzene and naphthalene) intermediates plus a good position in acid dyes for wool, the management feels that it would be worthwhile to prepare a preliminary series of acid azo dyes for wool using the newly available amine to determine possible commercial utility. The dye synthesis man has had, therefore, the characteristic chromophore defined for him as the azo group. In addition, the added requirement of sulfonic acid groups has been imposed so that the dye will go on wool. In general, he knows that the color and dyeing properties of this class of dyes will be a function of the number and position of the azo groups, the type of aro- matic nuclei employed (benzene, naphthalene, pyridine, etc.), the number and position of additional chromophores and auxochromes and, finally,

356 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS H03S N•N , II CH0 (a) • (b) C! (d) (c) .0CH• CH:• Figure 7. Yellow to orange monoazo dyes the number and position of the sulfonic acid groups. The latter point, however, generally has only a minor effect on the color of azo dyes. The starting point of the proposed family of dyes, sulfanilic acid, is colorless and absorbs in the ultraviolet at 248 mt• (7). Knowing that simple monoazo dyes containing only benzene rings with no additional chromophores are yellow, the chemist might couple the colorless sul- fanilic acid with aniline and obtain the reddish-yellow dye shown in Fig. 7a. If a greener shade of yellow is also desired, two possibilities can be exploited. One is to use a coupling component other than aniline (e.g., a pyrazolone) to obtain a hypsochromic shift, and the other is to in- troduce an appropriate group into the aniline coupler. These products are shown in Fig. 7b and c. The large-sized chlorine group in a position ortho to the azo linkage prevents the chromophoric system from achiev- ing maximum planarity and thereby decreases the effect of orbital over- lap. This is an example of an auxochrome group causing a hypso- chromic effect instead of the usually expected bathochromic one. The next logical shade in the sequence is an orange and several ap- proaches are possible. If an auxochrome stronger than the original amino group, viz., a dimethylamino group, is introduced into the parent yellow dye, a bathochromic shift will be obtained and a yellowish-orange dye will result (Fig. 7d). A redder shade of orange could be obtained by using additional auxochromes such as the methyl and methoxyl