THE CHEMICAL COMPOSITION OF DYES 



89 



The structure of dyes may be illustrated by the one known 

 commercially 3.spararosaniIine. (The para in this word is not used 

 in its chemical sense.) This is a magenta solid, easily soluble in 

 water. To make it, one needs aniline and paratoluidine. Aniline 



NHo 



Aniline 



NH, 



Aniline 



NH 



NH, 



CH, 



NH, CI- 

 Pararosaniline 



is a colourless fluid (if pure), paratoluidine a colourless, crys- 

 talline solid. Heat them in the presence of a mild oxidizing 

 agent and a source of chloride ions, and a brilliant colour 

 develops. Two molecules of aniline combine with one of para- 

 toluidine to give a substance in which one of the three rings is 

 quinonoid. 



To which of the three rings is colour due: to those derived 

 from aniline, or to that derived from paratoluidine ? The formula 



ci- 



NH2 



Pararosaniline : another 

 resonance position 



