COLOUR AND CHEMICAL STRUCTURE 363 



a yellow colour appears. The effect of ring structure in 

 intensifying the colour may be illustrated by comparing 

 colourless benzophenone with red fluorenone, 



The beautiful dyestuff auramine, 



NH 



(CH 3 ) 2 NC 6 H/ N C 6 H 4 N(CH 3 ) 2 . HC1 . 



is a good example of the chromophoric effect of the N = C 

 group. 



The nitro group is somewhat anomalous. It appears to act 

 as a weak chromophore and to be dependent for its effect 

 mainly on its association with other groups. When two nitro 

 groups are present they often weaken one another in their 

 chromophoric action. For instance nitrobenzene is yellow 

 while dinitrobenzene is colourless. 



Coloured substances do not necessarily act as dyes, and 

 Witt showed in 1 894 that the presence of a salt-forming group 

 is required to convert such a substance into a dyestuff. Bodies 

 which contain a chromophore without a salt-forming group 

 have been called " chromogens," and the introduced dye- 

 forming radicle, whose presence often intensifies the colour, 

 is termed an " auxochrome." 



The two chief auxochromes are OH and NH 2 . The effect 

 of the former may be illustrated by comparing yellow anthra- 

 quinone with its dihydroxy derivative, the beautiful red dye 

 alizarin : 



OH 



r"Y C0 Y^ rT C0 Y> H 



XJ UUU 



Anthraquinone. Alizarin. 



Anilin, 



