94 AROMATIC COMPOUNDS AND [CH. 



The flavone and flavonol pigments are yellow crystalline substances, 

 and as members of a class they have similar properties. They occur in 

 the plant most frequently as glucosides, one or more of the hydroxyl 

 groups being replaced by glucose, or, sometimes, by some other hexose, 

 or pentose. In the condition of glucosides, they are much less coloured 

 than in the free state, and, being present in the cell -sap in very dilute 

 solution, they do not produce any colour effect, especially in tissues 

 containing chlorophyll. Occasionally they give a yellow colour to tissues, 

 as in the rather rare case of some yellow flowers (Antirrhinum) where 

 colour is due to soluble yellow pigment. 



In the glucosidal state, the flavone and flavonol pigments are, as a 

 rule, readily soluble in water and alcohol, but not in ether. In the non- 

 glucosidal state they are, as a rule, readily soluble in alcohol, somewhat 

 soluble in ether, but soluble with difficulty in water. 



The flavone and flavonol pigments can be easily detected in any 

 tissue by the fact that they give an intense yellow colour with alkalies 

 ( Wheldale, 29). If plant tissues be held over ammonia vapour, they turn 

 bright yellow, showing the presence of flavone or flavonol pigments: the 

 colour disappears again on neutralization with acids. (The reaction is 

 especially well seen in tissues free from chlorophyll, such as white flowers.) 

 This reaction will be found to be almost universal, showing how wide is 

 their distribution. With iron salts, solutions of the pigments give green or 

 brown colorations. With lead, insoluble salts are formed. Several of 

 the members are powerful yellow dyes, and hence some plants in which 

 they occur, such as Ling (Erica cinerea), Dyer's Weld or Rocket (Reseda 

 luteola), have been used for dyeing purposes. The value of these colour- 

 ing matters as dyes has led to their chemical investigation, and as a 

 result the constitution, etc., of the hydroxy-flavones and flavonols is 

 well established. 



Expt. 95. Demonstration of the presence of flavone or flavonol pigments in tissues 

 without chlorophyll. Take flowers of any of the undermentioned species and put 

 them in a flask with a few drops of ammonia. They will rapidly turn yellow owing 

 to the formation of the intensely yellow salt of the flavone or flavonol pigments 

 present in the cell-sap. If the flowers are next treated with acid the yellow colour 

 will disappear. 



Also make an extract of some of the flowers with a little boiling water. Filter, 

 cool and add the following reagents : 



(a) A little alkali. A yellow colour is produced. 



(6) A little ferric chloride solution. Either a green or brown coloration is 

 produced. 



(c) A little basic lead acetate solution. A yellow precipitate of the lead salt of 

 the flavone or flavonol pigment is formed. 



