364 



SCIENCE PROGRESS 



contains the auxochrome NH 2 but is itself colourless, while 



azobenzene 



,— N : N- 



is an example of the chromophoric effect of the azo group. 

 By the addition of amino groups this substance is converted 

 into dyes such as : 



1— N : N— S~S r^— N : N- 



NH,.HCL 



Anilin Yellow. 



and 



— N :N 



H 2 N , s w x , NH 2 .HCl 



Chrysoidine. 



H,N 



NH 2 . HC1 



NH, 



Triamido-azobenzene. 



a constituent of Manchester brown. 



The effect of substitution in the auxochrome is to shift the 

 absorption bands towards the red, namely to deepen the 

 colour. This is well illustrated by the substance coumarin, 

 the sweet-smelling principle of woodruff and hay. Coumarin 

 is a colourless chromogen containing the compound keto- 

 ethylenic chromophore, — CO.CH = CH — . The addition of 

 unsaturated amino auxochromes such as NH 2 , NH . CH 3 , 

 NH .C 2 H 5 and N(CH 3 ) 2 causes the development of tinctorial 

 properties which deepen with increasing molecular weight of 

 the substituent : 



H H 



H.C 



O :C 



H. C 



O :C 



NH . CH, 



Methyl-6. Amino-coumarin (yellow). 



H 



H . C 



O :C 



^ 



N(CH 3 ) 2 



Ethyl-6. Amino-coumarin (brown). 



Dimethyl-6. Amino-coumarin (brown). 



