1^4 Plant Pigments [March, 



f ere with color f ormation. Thus, diazomethane, CH2\ II is yel- 

 low, but tolazone, ^ 



CH3 CH3 



<::>-<":> 



\ / 



N N 



is but slightly colored. 



0£ special interest is the influence that the position of the double 



bonds may have. Benzene, as has been pointed out, though color- 



less, shows absorption bands in the ultra-violet part of the spec- 



trum; fulvene, CH = CH. 



I >C = CH2 



CH = CH' 



an isomer of benzene, with the same nuniber of double bonds, is 

 yellow. 



The most important " auxochromes " are the amino and hy- 

 droxyl groups, of which the former is the stronger. The color is 

 usually intensified by increasing the number of auxochromes, or, 

 in the case of the amino group, by substituting aklyl and aryl rad- 

 icles for the hydrogen atoms. On the other band, acetylating a 

 hydroxyl group inhibits auxochromic action: 



O O 



Fluoran (chromo- Fluorescein, Diacetyl fluorescein, 



gen), colorless brown colorless 



That the position of the " auxochrome " group may be of impor- 

 tance is shown by contrasting quinolphthalein, 



O 



HO\A/\/OH 



which is colorless, with its isomer, fluorescein.^ 



c Hewitt : Chromophores and chromogens, Thorpe's Dict, of Applied Chem., 

 1912, ii, p. 59- 



