136 POPULAR SCIENCE MONTHLY 



two oxygen atoms. Other unsaturated atoms may show similar reac- 

 tions. Thus the nitro-anilines, HJST — C 6 H 4 — N0 2 , give an absorp- 

 tion-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 (NH 2 ). These facts, together with the position of the absorp- 

 tion band, point unmistakably to isorropesis, and the two distinct forms 

 thus in equilibrium may be represented by the following figures : 



HO-N=0 



NH, NH 



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 benzoid in type 

 and enters not into the quino'id 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, 2 N — 

 C 6 H 4 — OH, in neutral alcoholic solution, is identical with that of 

 para-nitroanisol, 2 N — C 6 H 4 — OCH 3 , 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 absorp- 

 tion-curve alters and a band similar to the band of nitro-aniline ap- 

 pears in the visible blue region of the spectrum ;" in other words isor- 

 ropesis 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 resid- 

 ual 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 quinoid type of linking established. In the equilibrium between 



0=N=0 0=N— ONa 



m-*~ 



C 



H n H 



H y H 



OH 



these two forms we may unquestionably look for the conditions which 

 underlie the formation of color in the salts of nitro-phenols. The 



