656 SCIENCE PROGRESS 



towards the red. Auxochromes are generally supposed to 

 support each other, and the number of groups present is 

 consequently of importance. In general also it has been found 

 that the auxochromes exert their strongest action when in the 

 position ortho to the chromophor. As pointed out by Kauffmann l 

 this is plainly seen in the case of the fuchsin dyes. 



H,N 



NH 2 C1 



Here we have the quinonoid ring acting as chromophore and 

 the animo groups as auxochromes. When in position 3 (see 

 formula above) the NH 2 produces a violet dye-stuff, when in 

 position 2 a bluish green, and in position 1 a blue-green. 



As a measure of the condition of the benzene ring and 

 consequently of the effect of the auxochromic substituents, 

 Kauffmann 2 claims the relative magneto-optical rotations to be 

 of great importance. He draws the conclusion that the greater 

 the rotation the greater the auxochromic action, i.e. the greater 

 the strain on the benzene ring caused by an auxochrome, the 

 higher will be its colour-producing value. As a consequence 

 Kauffmann places the auxochromes in the following order, 

 based on the magneto-optical constants — 



-O'COCHs OCH3 NHCOCH3 NH, N(CH 3 ) 2 N(C 2 H 5 ) 2 



- 0-260 + 1*459 1*949 3'82i 8-587 8'8i6. 



Salt Formation 



The auxochromes, being as a rule groups strongly acid or 

 basic in character, produce coloured bodies readily capable of 

 forming salts. Chromophors also in some cases cause salt 

 formation, but generally only with strong acids or bases. The 

 effects of salt formation on the colour are far reaching, and 

 vary considerably with the conditions. 



In the case of bases giving salts with colourless acids it has 



1 Zusammenhang z. Farbe u, Konstitution ; Sammlung chem. u. chem.-tech. 

 Vortrcige, IX. 29. 



2 Ber. d. d. Chem. Ges. 39, 1959 (1906). 



