THE CHEMICAL COMPOSITION OF DYES 157 



coloured (yellow), but it is chiefly when a quinonoid ring is intro- 

 duced into more complicated compounds that brilliant colours are 

 produced. Since the paraquinonoid ring confers the property of 

 colour, it is called a chromophore or colour-bearer. It occurs in 

 many dyes that are important in microtechnique. Other dyes owe 

 their colour to other chromophores, which will be mentioned in 



chapter 9. 



O 



Parabenzoqiiinone 



Quinone itself has no tendency to be exhausted from its solvent 

 by attaching itself to a textile or to the substance of a microscopical 

 preparation. In general, substances that have this tendency ionize 

 in aqueous solution, and quinone does not. A chromophore by 

 itself, then, does not confer upon a molecule the capacity of acting 



II 



The paraqiiinojioid ring 



as a dye. An ionizing group is required as well. Those ionizing 

 groups that transform substances that are merely coloured into 

 dyes are called aiixochromes. As their name suggests, they have a 

 tendency to increase the intensity of the colour, often very 

 markedly, and it is sometimes convenient to think of the auxo- 

 chromophoric systems in dyes, in order not to distinguish too sharply 

 between the effects of chromophore and auxochrome. 



One of the commonest auxochromes in dyes is the group -NHg, 



Aniline 



and it is because aniline contains this group that it is of such partic- 

 lar importance in dye-chemistry. 



To make a dye, it is only necessary to combine a molecule that 

 can spring into a chromophoric configuration with another that 



