benzene, CeHo- CH3 ; phenol, carbolic acid or phenylic acid,C6H5- OH ; 

 and anilin or phenyl amine, C6H5NH2. Their constitutional for- 

 mulae are as follows: 



CH, OH NH. 



I ■ I I 



A A 



\/ \/ \/ 



toluene phenol anilin 



Two important di-substitution products are xylene or dimethyl 

 benzene C6H4- (CH3)2, and toluidine, C6H4- CHs- NH2. Both of these 

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

 for xylene : 



CH3 CH3 CH3 



CH, I 1 



\/ \/\ \/ 



CH3 



\ 



CH, 



ortho-xylene meta-xylene para-xylene 



CH3 



NH 



2 



ortho-toluidine 



CHROMOPHORES, CHROMOGENS, AND AUXOCHROMES 



Certain groups of elements are known as chromophores because 

 when they occur in a benzene derivative they impart to the com- 

 pound the property of color. The benzene compounds containing 

 chromophore radicals are known as chromogens. A chromogen, 

 however, altho it is colored, is not a dye, in that it possesses no 

 affinity for fibers or tissues. It may coat them, but only mechan- 

 ically, and it will be easily removed by mechanical processes. 

 That is, it will not "take." (See, however, the discussion of fat 

 stains, p. 33). In order for a substance to be a dye, it must contain 

 in addition to the chromophore group, a group which imparts to 

 the compound the property of electrolytic dissociation. Such 

 auxiliary groups are known as auxochromes. They may slightly 

 alter the shade of the dye, but are not the cause of the color. Their 

 function is to furnish salt-forming properties to the compound. 

 Certain chromophoric groups have also slight auxochromatic 

 properties. 



To illustrate these different types of groups, let us take a typical 

 example. The nitro group (-NO2) is a chromophore. When three 



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