126 



STAINING 



benzene nucleus. The formula ordinarily assumed for quinone, 

 therefore, is 0=C ,C=0. This formula is more 



commonly written in the simpler form 0^( 



:0. In 



this compound it will be noticed there is a different type of benzene 

 ring from that observed in derivatives like xylene or toluidine, 

 as it has two sets of external double bonds instead of single bonds 

 at each carbon atom ; it is known as the quinonoid ring. Two 



types of quinonoid rings are known, the para 



and 



the ortho 



Ordinarily a compound containing the 



quinonoid ring is coloured, although not necessarily a dye ; and 

 any chemical which, due to reduction, oxidation, or change in 

 reaction, destroys the quinonoid structure, thus returning the 

 normal form of the benzene ring, decolorises the compound. 

 This is important to remember in connection with the colourless 

 forms of such dyes as fuchsin and methylen blue. 



235. Chromophores. Radicals like the quinonoid ring which 

 have the above-mentioned relation to colour in the compounds 

 are known as chromophores. There are quite a number of 

 radicals of this nature known, but only one beside the quinonoid 

 ring is of special significance from the standpoint of the biological 

 stains ; namely, the azo group (— N = N — ), which occurs in 

 the azo dyes. Of all the chromophores the quinonoid ring is by 

 far the most significant and occurs in the greatest variety of dyes. 

 Methylen blue and acid fuchsin are good examples of the 

 importance of this chromophore ; the reactions illustrated below 

 show how reduction in one case and change of reaction in the 

 other break the quinonoid ring and destroy the colour. 



^CH3 



(CH3), • N_/\_S_/^\=N-CH3 + H-> 



^Cl 



Methylen blue 



