COLOUR AND CONSTITUTION 659 



In this way was built up the " quinone " theory of colour, 

 which commends itself particularly because of its extreme 

 simplicity compared with the intricacies of the auxochrome- 

 chromophore theory, and above all because it gives a chemical 

 explanation of the phenomenon of colour. It is needless to 

 give detailed examples of the method of quinonoid formulation. 

 Some have already been mentioned, and by making assump- 

 tions in some cases it has so far been possible to formulate 

 nearly all the coloured compounds in this way. Instance 

 might be made of the interesting series of compounds known 

 as fulgides. Stobbe : explains the colour of the fulgenic acids 

 and similar compounds by the number and position of the 

 chromophoric phenyl and carboxyl groups, and also by the 

 influence of the conjugated double linkages. The colour can, 

 however, be brought into harmony with the quinone theory, 

 as the substances may be looked on ^ as ortho- and para- 

 " quinonoid " derivatives of tetrahydrofurfuran — 



It is interesting to note that Thiele 2 has succeeded in pre- 

 paring hydrocarbons of undoubtedly " quinonoid " structure. 

 The best known of these compounds is tetrapheny]-/>«ra- 

 xylylene — 



(C S H 5 ) 2 • C = C<£" 2 ch) C = C(C6H ^' 



which forms orange-yellow needles. 



In some cases the generally attributed quinonoid structure 

 has been disputed on chemical grounds. This has been the 

 case with the phthaleins, a class of compounds produced by 

 the condensation of phthalic acid with phenols and which give 

 deeply coloured alkali salts. In the free state these bodies 

 are supposed to contain a lactone 3 ring which by the action 

 of dilute alkali undergoes a desmotropic change into a quinone 

 containing a carboxylic group. 



1 Ann. d. Chem. 349, 333 (1906). 

 1 Ber. d. d. Chem. Ges. 37, 1463 (1904). 



3 The name lactone is given to the type of compounds produced from one 

 molecule of hydroxy acids by loss of water. 



