3. THE OXYQUINONE GROUP 

 The oxyquinone dyes include derivatives of anthracene, 



thru its oxidation product anthraquinone 



O 



O 



These dyes are the first to be considered here in which the quinoid 

 structure occurs. The quinoid ring, which is the most important 

 chromophore in nearly all the dyes to be discussed in the three fol- 

 lowing chapters, forms very strong chromogens, which require 

 only the addition of auxochrome groups to be converted into strong 

 dyes, either basic or acid. The chromogen anthraquinone is con- 

 verted into a dye by the addition of hydroxyl groups, its best 

 known derivatives among the dyes being: 1:2 dihydroxy-anthra- 

 quinone (alizarin) and 1 :2 :4 trihydroxy-anthraquinone (purpurin). 

 Both of these compounds occur in nature in the root of madder, 

 being the colored principles of madder extract. They have the prop- 

 erty of combining with metalic oxides to form so-called "lakes", 

 insoluble compounds of different color from the dye entering into 

 them. This makes them valuable ones to use after mordanting 

 with aluminium, iron or chromium compounds. 



ALIZARIN 



O OH 



II / OH 



C. I. NO. 1027 



II, 



O 



{An acid dye; absorption maxima about [610.S], 566.5, [5S7.6] 



in alkaline solution.) 



Alizarin stains tissues a feeble yellowish red if used on them 

 directly. In the presence of aluminium compounds intense red 

 colors are formed; bluish violet in the presence of iron; and brown- 

 ish violet in the presence of chromium. It has been used as a stain 

 for nervous tissue. The chief present use of alizarin, however, is 

 as an indicator. 



42 



