Diethyl selenide (C2H5)2Se, is a yellowish heavy oil of unpleasant odor. It combines readily with chlorine to form a chloride (C2H5)2SeCl2, and the latter is oxidized by nitric acid to form an oxide (C2H5)2SeO,[(39)]. Diethyl sulphide is a colorless syrupy liquid, as well as diethyl amine and diethyl ether.
The gradation of color is quite pronounced in the case of selenonaphthene quinone[(40)].
It would be most natural to conclude that the chromophore :CS is more powerful than :CO, and that :CSe is most powerful of all, as shown in our study of quinazoline compounds. It would equally follow that :S is a more powerful color-forming radical in a cyclic compound than that of :O; and :NH than that of :S; and :Se again most powerful of the whole series.
Lesser and Weiss[(41)] in their research on selenoindigo stated that the selenium dyestuff, on account of its greater molecular weight than sulphur, shows a deeper blue. This hypothesis meets a difficulty in the case of coumorandione, thionaphthenequinone and isatin series, where the -NH- radical has an atomic weight of 15, and -S- 32, and showed the reversed order of color. This seems to be the case in the selenophene series also. Therefore this theory is not without exceptions.
The diselenides present a very interesting study also. Methyl disulphide is colorless, but methyldiselenide[(42)] is a reddish yellow liquid. Methyl disulphide only becomes yellow when it is treated with chlorine, and in such cases (CH3)2S2Cl2 is formed[(43)], in yellow rhombic crystals. Ethyldisulphide is colorless: ethyldisulphidedichloride is a faint yellow oil[(44)]. But the corresponding ethyldiselenide is a red liquid[(45)]. Phenyl disulphide is colorless, and phenyldisulphide dibromide is of mother-of-pearl appearance, and practically colorless[(46)], while phenyl diselenide forms pure yellow needles[(47)], and phenyldiselenide dibromide orange red ones.