COLOUR AND CONSTITUTION 651 



carbons are known as Fidvencs, and the mother-substance 

 Fulvene, which is isomeric with benzene, has the formula — 



CH =CH. 

 I >C = CH„. 



CH = CJT 



Especially highly coloured are the methyl phenyl and dimethyl 

 derivatives, substituted in the CH 2 group. The presence of this 

 five-membered ring and the third double bond seem to be 

 essential to the production of colour, for when the latter is 

 transformed into simple bonds by reduction or addition {e.g. 

 of halogen), colourless substances ensue. Accepting the Kekule 

 formula for the benzene ring also in the polycyclic compounds, 

 e.g. in the gold-yellow acenaphthylene, 1 



CH = CH 

 we find that all the coloured hydrocarbons contain the group 



= c> c =- 



That the reverse of this is not true is witnessed by the colour- 

 less diphenyl. 



Chromophore-Auxochrome Theory 



Much earlier than the discovery of these coloured hydro- 

 carbons, similar appearances with other groups than C = C led 

 Witt 2 to the formulation of his Chromophore-Auxochrome 

 Theory. 



Certain groups exist which, when bound up with hydro- 

 carbon radicles in sufficient number and in a suitable manner, 

 tend to produce coloured compounds. These groups are 

 termed Chromophores. As a rule the Chromophores by them- 

 selves do not cause colour, but require the presence of other 

 groups, called by Witt A itxockromes, before their action becomes 

 noticeable. The auxochromes alone have no colour-producing 

 properties. 



1 Ber. d. d. Chem. Ges. 26, 2354 (1893). 



2 Ibid. 9, 522 (1876). 



