of these groups displace three hydrogen atoms in a benzene mole- 

 cule, we have the compound trinitrobenzene, 



0,X NO, 



NO. 



which is yellow. It is not a dye, however, but is a chromogen. It 

 is insoluble in water, and is neither an acid nor a base; that is, it 

 does not dissociate electrolvticallv and consequentlv cannot form 

 salts with either alkalies or acids. If, however, one more hydrogen 

 atom is replaced, this time with the hydroxyl group (-0H), which 

 is an auxochrome, the resulting compound, 



OH 



O.X I NO, 



\/ 



NO. 



is an acid, capable of electrolytic dissociation and of forming salts 

 with alkalies. It is the familiar substance picric acid, and is a 

 yellow dye. 



It will thus be seen that the color of picric acid is due to the 

 chromophoric nitro groups, and that its dyeing properties are due 

 to the auxochromic hydroxyl group. If the nitro groups be reduced 

 to amino groups (-XII2), which are not chromophores, the resulting 

 compound is colorless and hence is not a dye. 



Summing up, we arrive at the definition of a dye as an organic 

 compound which contains chromophoric and auxochromic groups 

 attached to benzene rings, the color being attributable to the chro- 

 mophores and the dyeing property to the salt-forming auxochromes. 



Some auxochromes are basic, e.g., the amino group (-XH2), 

 while others are acidic, e.g., the hydroxyl group (-0H). The 

 amino group owes its basic character (which it transmits to the 

 whole molecule) to the ability of its nitrogen atom to become pen- 

 tavalent by the addition of the elements of water (or of an acid), 

 just as in the case of ammonia; thus: 



H H H H H H H 



/ \ \ / / \ / 



H— N + O = H— N and H— N +H— CI = H— N 



\ / / \ \ / \ 



H H H OH H H CI 



ammonia "water ammonium ammonia Hydro- ammonium 



hydroxide chloric chloride" 



acid gas 



15 



