While phenyldisulphide is colorless, when an auxochrome group is added, such as NH2, the compound is colored. This is the case with o-diaminodiphenyldisulphide[(48)] which is yellow both in solution and in crystalline form. In other words, an auxochrome in addition to the chromophore group transforms a colorless chromogen into a colored one. Therefore groups like -S.S- and -Se.Se- are chromophores in the same sense as -N:N-. This is in agreement with the chromophore ideas of Hugo Kaufmann. The -Se.Se- is a more powerful chromophore than -S.S-.
This brings one directly to the inquiry as to why 2-phenylbenzoselenazole, which contains a :Se radical, should be colorless; and that even 6-nitro-2-phenylselenazole, with the addition of a chromophore NO2, should be only faintly colored. The benzothiazoles, their isomers and derivatives are mostly colorless, and similar causes are probably responsible in the case of the phenylbenzoselenazole, for its lack of color. But when this selenazole is combined with another chromophore, for example an azomethine grouping, the result is a more positively colored compound (in this case benzalaminoselenazole), the crystals being yellow. The corresponding thiazole derivative is light colored.
The tinctorial value of the selenium derivative is further evidenced by the ease with which it forms azo dyes and the deep colors of the latter. This was observed when 6-amino-2-phenylbenzoselenazole was diazotized and coupled with B-naphthol, salicylic acid, etc. The corresponding aminothiazole has been considered difficult to diazotize, on account of its insolubility in hydrochloric acid, cold or hot, but the aminoselenazole dissolves readily and completely, the coupling is almost instantaneous, and the dyes obtained are mostly red and of metallic lustre. In view of the stability of benzoselenazoles toward hot concentrated acids (with the exception of nitric, when nitration ensues) and alkalis, these dyes may prove of some commercial interest.
The azole dyes of the benzoselenazole have been exposed to light for weeks, and also exposed to acids and alkalis, and have been found to be quite fast.
[EXPERIMENTAL]
Preparation of 2-methyl-4-selenoquinazolone
Busch prepared quinazolines by the action of o-amino or o-nitro benzylamine with phosgene, and thioquinazolines with carbon disulphide[(51)]:
