l68 DYEING 



The hydrogens of the amino-groups are often replaced by 

 methyl, ethyl, and phenyl groups. These particular modifiers tend 

 to make a red or reddish dye blueish or blue. There are six such 

 hydrogen atoms in pararosaniline. As more and more of these are 

 replaced by methyl (in the various dyes that together constitute 

 the mixtures called methyl and gentian violet), so the colour 

 becomes bluer and bluer until the hexa-methyl compound, crystal 

 violet, is reached; this is a very blueish violet dye. Ethyl groups 

 have more blueing effect than methyl, and phenyl groups still 

 more, till dyes are reached that are purely blue. 



Crystal violet, an important dye for chromosomes, will serve to 

 show the influence of resonance on colour. When a solution of the 

 dye is acidified, a proton is added to one of the three dimethyl- 



+ 

 HN(CH3)2 N(CH3)2 N(CH3)2 



N(CH3)2 2CI- N(CH3)2 Cr 



+ + 



Acidified crystal violet Malachite green 



amino groups, and this now no longer participates in the resonance 

 of the ion. The resonance is thus limited to that of the related dye, 

 malachite green; and in correspondence with this, the colour 

 changes from blue-violet to green. 



The more central part of a dye ion also affects colour. If two 

 benzene rings are put together to form naphthalene, there is more 

 absorption of electro-magnetic weaves than by two separate 

 benzene rings, and the same principle applies again when a third 

 ring is added to form anthracene. Many dyes have a double-ring 

 component, and a whole group of dyes is based on anthracene 



(P- 175)- 



