VIII] AROMATIC COMPOUNDS 121 



The relationship between these two classes of substances in the plant 

 can only be ascertained by discovering which flavone, flavonol and an- 

 thocyan pigments are present together^ and then to determine whether 

 the relationship is one of oxidation or reduction, a problem which has 

 not yet received adequate attention (Everest, 7). 



A reaction which is of interest in connexion with the relationship 

 between the above two classes of pigments is that which takes place 

 when solutions of some flavone or flavonol pigments are treated with 

 nascent hydrogen. If an acid alcoholic solution of quercetin is treated 

 with zinc dust, magnesium ribbon or sodium amalgam, a brilliant magenta 

 or crimson solution is produced, and this solution gives a green colour 

 with alkalies (Combes, 6). The red substance thus produced has been 

 termed "artificial anthocyanin" or allocyanidin. The product is not a 

 true anthocyan pigment but has, it is suggested, an open formation 

 (Willstatter, 31): 



9-\__ >o" 



OH H 



It is said, however, to contain small quantities of a substance iden- 

 tical with natural cyanidin from the Cornflower (Willstatter, 31). The 

 fact that small quantities of a natural anthocyan pigment can be obtained 

 artificially from a hydroxyflavonol by reduction does not necessarily imply 

 that one class is derived from the other in the living plant. 



From the above reaction of quercetin the result follows that when 

 many plant extracts [most plants (see p. 110) contain flavone or flavonol 

 pigments] are treated with nascent hydrogen, artificial anthocyan pig- 

 ment is produced. Moreover, it seems probable that if the yellow^ 

 pigments acted upon are in the glucosidal state, and if the reduction 

 takes place in the cold, allocyanin (the glucoside of allocyanidin) is 

 formed and the product is not extracted from solution by amyl alcohol. 

 But if the flavone is non-glucosidal, or if the solution is boiled before or 

 after reduction, then allocyanidin (non-glucosidal) is formed and is 

 extracted by amyl alcohol. 



Ea^pt 115. Formation of allocyanidin from quercetin. Make an alcoholic solution 

 of a little of the glucoside of quercetin prepared from %\ih.QY Narcissus or Gheiranthus 



1 The only two satisfactory cases known are Delphinium consolida, which contains 

 kaempferol and delphinidin, and Viola tricolor, which contains quercetin and delphinidin. 

 Neither of these confirms the hypothesis of simple reduction. 



