two oxygen atoms. Other unsaturated atoms may show similar reactions. Thus the nitro-anilines, H2N—C6H4—NO2, give an absorption-curve similar to that of para-benzoquinone. Here the residual affinities of the nitrogen atoms are disturbed by the motions of the benzol molecule. And, in addition, the unsaturated oxygen atoms of the nitro-group are disturbed by the hydrogen atoms of the amido-group (NH2). These facts, together with the position of the absorption band, point unmistakably to isorropesis, and the two distinct forms thus in equilibrium may be represented by the following figures:
A solution of nitro-aniline in hydrochloric acid gives a colorless solution showing no trace of absorption band to indicate isorropesis. The structure of the hydrochlorate, therefore, is purely benzoïd in type and enters not into the quinoïd form by reason of the saturation of both nitrogen atoms. In the case of the nitrophenols similar reasoning may be followed. The absorption band of para-nitrophenol, O2N—C6H4—OH, in neutral alcoholic solution, is identical with that of para-nitroanisol, O2N—C6H4—OCH3, the methyl ether of this phenol. Consequently their structures may be assumed to be identical. When the phenol, however, is converted into the sodium salt its absorption-curve alters and a band similar to the band of nitro-aniline appears in the visible blue region of the spectrum; in other words isorropesis has been brought about with the natural consequence—the appearance of color in the compound. The hydrogen atom of the free phenolic group (OH) is seen to be but slightly affected by the residual affinities of the oxygen atoms of the nitro-group; the more electro-positive sodium atom, however, shows a greater activity and may be drawn over to one of the oxygen atoms of the nitro-group, and thus a quinoïd type of linking established. In the equilibrium between
these two forms we may unquestionably look for the conditions which underlie the formation of color in the salts of nitro-phenols. The
