COLOUR AND CONSTITUTION 655 



Anxochromes 



The present accepted theory of auxochromic action assumes 

 that on the addition of an auxochrome to a chromogen x {i.e. a 

 substance which contains a chromophore group and gives a 

 deeper-coloured compound on the addition of an auxochrome), 

 a strain is induced on the benzene ring, varying in degree 

 according to the auxochrome employed. In this way, according 

 to Kauffmann, 2 the benzene ring is " no dead unalterable struc- 

 ture, but a most delicate picture which on the slightest touch 

 at once alters its properties to a greater or less extent in reply." 



In the strict optical sense all benzene derivatives are coloured 

 in that they all show more or less well-defined absorption 

 spectra. In many of the chromogens of course the absorption 

 bands lie far out in the ultra-violet, but each substituent alters 

 the positions of the lines, some in such a manner that they are 

 moved into the visible portion of the spectrum. To these latter 

 belong the auxochromes. The scope of this article does not 

 admit of the consideration of these theoretically coloured bodies. 

 The relationship between absorption-spectra and constitution 

 is a wider subject, and the term " coloured " is here confined to 

 bodies giving an absorption in the visible spectrum. 



The most important auxochromes are the hydroxyl (— OH) 

 and amino (— NH 2 ) groups, and in virtue of their respective 

 acid and basic properties, many of the coloured substances 

 produced by their action are dyes. In the light of the auxo- 

 chrome theory the — NH 2 group is much more powerful than 



— OH. P<7ra-nitraniline is deep yellow, whereas para-xixXxo- 

 phenol is nearly colourless. Substitution of the hydrogen of 

 the amino group by alkyl or aryl residues produces new auxo- 

 chromes which are more powerful in action than the parent 

 group. The commonest of these new auxochromes are 



— N(CH 3 ) 2 , — N(C 2 H 5 ) 2 , and — NHC fl H 5 , their action increasing 

 with the molecular weight. Similar substitution of the hydroxyl 

 group has no settled effect. Liebermann 3 has shown that 

 alkylation of alizarin and isoanthraflavic acid causes the 

 absorption bands to move towards the violet, whilst similar 

 treatment of anthraflavic acid and quinizarin moves them 



1 Ber. d. d. Chem. Ges. 9, 522 (1876). 

 1 Ibid. 39, 1959 (1906). 

 3 Ibid. 23, 1566 (1890). 



