COLOUR AND CONSTITUTION 653 



N 

 diazomethane CH/ II and diazoethane, are yellow-coloured, 



N 



but it is in the aromatic compounds that the colour is most 

 noticeable. Azobenzene C 6 H 5 — N = N — C 6 H 6 , the simplest 

 aromatic derivative, forms orange-red crystals. In the light of 

 this theory the azo group is consequently regarded as one 

 of the strongest chromophores, although when bound up in a 

 closed chain, and even in presence of the above-mentioned 

 chromophoric triple ethylene linkage, it is practically inactive. 

 Tolazon, 1 for example, is only slightly coloured. 



CH 3 CH 3 



o-p 



N = N 



Probably no other group is more intimately bound up with 

 the production of colour, or more important in its bearing on 

 the relationship to constitution, than the nitro group, N0 2 . 

 Regarded for the present merely as a chromophore, it does not, 

 for instance, exert so strong an action as the azo group, but it 

 is most important owing to the ease with which it can be intro- 

 duced, and the generality of its action. 



Quinones 



Before proceeding to give an account of the auxochromes, it 

 will be as well to devote a few lines here to a short discussion 

 of that class of compounds known as quinones. 



The quinones may be regarded as derivatives of benzene, in 

 which two hydrogen atoms have been replaced by two atoms of 

 oxygen. The replacement may be either in the ortho- or para- 

 position, when we distinguish between ortho-quinones and para- 

 quinones. Meta-quinones are wholly unknown. All quinones 

 and quinone derivatives so far known are coloured to a greater 

 or less extent. The simplest quinone is />«nz-benzoquinone or 

 ordinary quinone, which is formulated thus : 



CO 



/ \ 



HC CH 



|| or simply 



HC CH 



\ / 



CO o 



1 L. Meyer, Ber. d. d. Chem. Ges. 26, 2230 (1893). 



42 



