158 DYEING 



contains an auxochrome. We may start with two colourless sub- 

 stances and produce a dye in a single process. A very easy one to 

 make is the dye known commercially 2is par arosaniline. (The par a- 

 in this name has no connexion with the chemical usage of the 



NH, NH, 



Aniline Aniline 



CH3 



NH2 



Paratoluidine 



prefix. It means that the dye is a modified form of another dye 

 with the commercial name of rosaniline.) Practical instructions 

 are given on p. 321. It is only necessary to bring together some 

 aniline and paratoluidine and to heat them in the presence of 

 chloride and a suitable oxidizing agent. The quinonoid chromo- 

 phore then appears, and the -NHg groups are already present to 

 act as auxochromes. 



NH, NH, 



NH2 cr 



+ 

 Pararosaniline 



Pararosaniline is a solid, magenta substance, soluble in water 

 and alcohol. 



In this book attention will be directed to quinonoid chromo- 

 phores by the drawing in of double bonds in quinonoid rings, but 

 aryl rings will be represented by a simple hexagons. 



It will be noticed that one of the three rings — the quinonoid one 

 — has been shown with a positive charge on the nitrogen atom, a 

 charge balanced by the negative charge on the chloride ion. It 

 must not be supposed, however, that the position of this charge is 

 constant. On the contrary, it changes continually. It may be 



