AROMATIC COMPOUNDS AND [CH. 



THE ANTHOCYAN PIGMENTS. 



These pigments are the substances to which practically all the blue, 

 purple and red colours of flowers, fruits, leaves and stems ,are due 

 (Wheldale, 3). They occur in solution in the cell-sap and are very 

 widely distributed, it being the exception to find a plant in which they 

 are not produced. As members of a group, they have similar properties, 

 but differ somewhat among themselves, the relationships between them 

 being much the same as those between the various flavone and flavonol 

 pigments. They occur in solution in the cell-sap but occasionally they 

 crystallize out in the cell. They are present in the plant in the form of 

 glucosides, and in this condition they are known as anthocyanins ; as 

 glucosides they are readily soluble in water and as a rule in alcohol 

 [except blue Columbine (Aquilegia), Cornflower (Centaurea Gyanus) and 

 some others] but are insoluble in ether and chloroform. The glucosides 

 are hydrolyzed by boiling with dilute acids, and the resulting products, 

 which are non-glucosidal, are termed anihocyanidins (Willstatter and 

 Everest, ^5). The latter, in the form of chlorides, are insoluble in ether, 

 but are generally soluble in water and alcohol. The anthocyanins can 

 be distinguished from the anthocyanidins in solution by the addition of 

 amyl alcohol after acidification with sulphuric acid. The anthocyanidins 

 pass over into the amyl alcohol, the anthocyanins do not. The antho- 

 cyanins and anthocyanidins themselves (with one exception) have not 

 yet been crystallized, but of both classes crystalline derivatives with 

 acids have been obtained (Willstatter and Everest, 35). 



In considering the reactions of anthocyan pigments the difference 

 between those given by crude extracts and those of the isolated and 

 purified substances must be borne in mind. With acids the anthocyan 

 pigments give a red colour: with alkalies they give, as a rule, a blue or 

 violet colour when pure, but if flavone or flavonol pigments are present 

 (as may be the case in a crude extract) they give a green colour, due to 

 mixture of blue and yellow. In solution in neutral alcohol and water 

 many anthocyan pigments lose colour, and this is said to be due to the 

 conversion of the pigment into a colourless isomer which also gives a 

 yellow colour with alkalies (Willstatter and Everest, 35) ; hence even a 

 solution of a pure anthocyan pigment may give a green coloration with 

 alkali due to mixture of blue and yellow. The isomerization can be 

 prevented or lessened by addition of acids or of neutral salts which form 

 protective addition compounds with the pigment. With lead acetate 



