General Nature of Dyes 25 



change of the valency bonds takes place very readily in many 

 dyes, and certain peculiarities of their behavior are explained by 

 it; (see for example p. 199). 



Three mono-substitution products of benzene are of importance 

 in considering the structure of dyes, namely; toluene or methyl- 

 benzene, CeHsCHs; phenol, carbolic acid or phenylic acidjCeHs OH; 

 and anilin or phenyl amine, CeHs NH2. Their constitutional for- 

 mulae are as follows: 



CH3 OH NH2 



I I I 



toluene phenol anilin 



Two important di-substitution products are xylene or dimethyl- 

 benzene C6H4 (CH3)2, and toluidine, C6H4CH3NH2. Both of 

 these occur in the above mentioned three isomeric forms, as shown 

 below for xylene: 



CH3 CH3 ^H3 



I CH, I I 



CH3 1 



CH3 



ortho-xylene meta-xylene para-xylene 



CH3 



I NH. 



ortho-toluidine 



RELATION OF MOLECULAR STRUCTURE TO COLOR 



Although there is still much to learn as to the explanation of 

 color in terms of structural formulae, it is now well known that 

 certain definite atomic groupings (known as chromophores) are 

 associated with color. The basic groupings involved in these 

 chromophores are C:C, C:0, C:S, C:N, N:N, N:0, and NO2; and 

 the more of these that occur in the same compound the more pro- 

 nounced the color. It will be noticed that the quinone ring, as 

 pictured above, contains the following chromophore grouping in 

 duplicate :C-C:C-C: . Quinone, itself, is in fact colored, and 

 the quinone ring is one of the most important chromophores 

 known, all compounds containing it showing intense color. 



Probably if the whole situation were understood, physical chem- 

 istry could give us an explanation of why some compounds are 



