191J/-] Organic Compounds 83 



The chi'omophore effect of the ^^=1^ group is illustrated in the 

 numerous azo dyes. It is peculiar to note that in the case of the 

 azo group, ring structure seems to diminish rather than to inr 

 tensif J color. The blue or green effect which most of the nitroso 

 compounds exhibit may be attributed to the N=0 group. The 

 a'bsence of color in certain compounds containing this chromo- 

 phore may be explained by the bimolecular structure and con- 

 sequent saturation of valencies. 



From a study of compounds containing the chromophores it 

 seems that they may be divided into two classes, those such as 

 E^^iN" and N=0 which jjroduce color independent of the na- 

 ture of their environment and are therefore called independent 

 chromophores, and those such as C==0 and ILC^^CIL which 

 act only in conjunction with other groups and hence are called 

 dependent chromophores. 



That a slight difference may have a marked effect on the 

 color of a compound is shown by certain stereoisomers such as 

 the two dibenzoyl ethylenes, CsHg.CO.CHrCH.CO.CsHs, one 

 of which is colorless and the other yellow. 



As has been stated above, a compound containing a chromo- 

 phore may not be colored and the introduction of an auxochrome 

 may be necessary to produce color. The introduction of an auxo- 

 chrome into a compound already colored serves to intensify the 

 color. Of the two chief auxochromes, OH and NH2, the latter 

 seems the stronger as shown by a comparison of para-nitrophenol 

 and para-nitraniline. It is worthy of note that both groups possess 

 residual affinity and like the chromophores are highly reactive. 

 It has been suggested that there may be an interaction of the two 

 kinds of groups to produce a banded spectrum when light is 

 absorbed. 



The color of a compound is deepened or intensified by the 

 replacement of hydrogen in the ISTHa by alkyl or aryl radicals, 

 and as the molecular weight of the radical is increased the ab- 

 sorption is shifted towards the red. If the hydrogen of the OH 

 group is replaced by a radical, it may either intensify or dim- 

 inish color. It might be expected that the relative position of 

 the auxochrome to the chromophore would have marked effect 

 on the color. But the fact is that the closeness of the auxo- 



