662 SCIENCE PROGRESS 



and principally to Hantzsch, to give an experimental proof of 

 the quinonoid structure of the chief dyes and colouring matters. 

 As an example of this may be cited the derivatives of triphenyl- 

 methane which Hantzsch l proved to be derived from triphenyl- 

 carbinol in the colourless leuco-bases, and from a quinonoid 

 body in the coloured salts. The leuco-base of crystal violet, 

 for example, has the formula — 



(CH 3 ) 2 N • C 6 H 4X .C 6 H 4 N(CH 3 ) 2 , 

 > C < 



(ch 3 ) 2 n -c 6 h; n oh 



the hydrochloride, however, being — 



(CH 3 ),N'C 6 H 4 . 



>C= C 6 H 4 N(CH 3 ) 2 -C1. 

 (CH 3 ) 2 N 'C 6 H* 



Similar changes in the acridine series were explained by 



Hantzsch 2 in the same way. 



Nitro Compounds 



Bound up inseparably with the quinone theory and indeed 



serving, to a certain extent, as a test case for its validity we have 



the whole class of nitro compounds and especially the nitro- 



phenols. The nitrophenols themselves are in the solid state 



either colourless or only slightly coloured ; their salts, on the 



other hand, are invariably highly coloured, strongly absorbing 



light in the visible portion of the spectrum. The normal alkyl 



and aryl derivatives again are quite colourless. On the Witt 



theory no explanation of this can be given beyond the statement 



that the auxochromic nature of the hydroxyl group is only 



brought strongly into evidence by salt formation. It was first 



proposed by Armstrong 3 to ascribe to the salts a quinonoid 



structure produced by an intra-molecular change, and in this 



way to explain their colour. 



O 



= NO,Na 



or^o-nitrophenol. Sodium or//zo-nitrophenolate. 



This view was advanced on very slight grounds, and very 

 little experimental evidence was offered in its favour. On the 



1 Ber. d. d. Che?n. Ges. 33, 278 (1900). 



2 Hantzsch and Kalb, Ber. d. d. C/iem. Ges. 32, 3120 (1899). 



3 Loc. cit. 



