COLOUR AND CONSTITUTION 663 



other hand, Hantzsch has succeeded in bringing forward a mass 

 of evidence to prove that the nitrophenols are pseudo-acids and 

 that the salts are derived from ««'-nitrophenols — i.e. true acids. 

 In this way they fall into line with the coloured salts of the 

 nitrolic acids. These latter compounds were discovered by 

 V. Meyer, 1 who gave them the formulae of nitroximes, or 

 nitro-isnitroso bodies — 



R . C<^ " . Dissolved in alkalies they undergo a change 



into an isodynamic form with production of red salts, which 

 on standing are gradually converted into more stable colourless 

 salts corresponding to the free acids. 2 Hantzsch expresses the 

 various changes as follows : 



True nitrolic acid derivates. 



Nitro-isnitroso compounds. Enythronitrolic acid salts. Leuconitrolic acid salts. 



// N.OH(R') KOH /,N X ,. N0 2 K 



CH 3 .C< -> CH 3 .C< >0 -> CH 3 .C< 



N0 3 HC1 N x ^NO 



<- -A 



O OK 



Colourless, acid stable. Coloured, labile. Colourless, alkali stable. 



* •■ ' *. - 



V Y Y 



Hantzsch has also shown that for the nitrophenols there 

 exists a series of isomeric labile red quinonoid aci-ethers, 

 corresponding to the colourless stable true nitrophenol ethers. 3 

 This, taken in conjunction with his proof that all constitutionally 

 unalterable nitrobenzene and nitrophenol derivatives are 

 colourless, 4 leads him to the belief that the weak coloured nitro- 

 phenols are solid solutions, and the aqueous solutions equilibria 

 of nitrophenols and <za-nitrophenols (or their ions). 5 



The intensely coloured salts of the nitrophenols are practically 

 completely aa-salts, as their colour and absorption-spectra are 

 almost identical with those of the ethers. Thus the role of the 

 " auxochrome " is transformed from a passive to an active one, 

 and leads us again to a chemical theory of colour. For this 

 Hantzsch wishes to discard the term " quinone " theory, owing 

 to the number of non-benzene derivatives which must be in- 

 cluded, and to replace it with umlagerungs theory, implying 



1 Ann. d. Chem. 175, 88 ; 180, 170. 



* Graul and Hantzsch, Ber. d. d. Chem. Ges. 31, 2854 (1898). 



3 Ber. d. d. Chem. Ges., 39 1084 (1906). 



4 Cf., however, Kauffmann, Ber. d. d. Chem. Ges. 39, 4237 (1906). 



5 Ber. d. d. Chem. Ges. 39, 3072 (1906). 



