COLOUR AND CHEMICAL STRUCTURE 365 



H 



H 



0:C 



N(CH 3 ) 3 Br 



6. Coumaryl-trimethyl-ammonium bromide (colourless). 



But when the amino group becomes saturated as in 6. coumaryl- 

 trimethyl-ammonium bromide, above, and in the salts of the 

 amino derivatives with mineral acids, the colour disappears. 



That the chromophoric effect of auxochromes is due to 

 their unsaturated character was pointed out by Kaufmann, 

 who showed that the introduction of these groups into the 

 molecule excites not only colour, but a number of other pheno- 

 mena, such as increased luminescence of the vapour under the 

 Tesla discharge and magneto-optical anomaly. He suggested 

 that this may be due to the reaction of their residual affinities 

 on those of the benzene nucleus, which is regarded as the seat 

 of the phenomena, and which tends to assume the quinoid, or 

 D condition. 



The ortho- or para-quinoid structure has so frequently been 

 observed in the strongly coloured dyes that some chemists have 

 assumed that all coloured substances possess this constitution. 

 Armstrong, one of the chief exponents of this theory, proposed 

 the formula 



N N 



H 



H 



for azobenzene in 1882. In 1888 Nietzki suggested the quinone 

 configuration for the triphenylmethane dyes such as : 



NH . HC1 N . CH3HCI 



NH 2 . C 6 H/ X QH 4 . NH 2 



Pararosanilin hydrochloride. 



CH 2 . NH . C 6 H/ X C 6 H 4 . NH . CH $ 



Hofmann's violet. 



and 



