FLAVONOIDS AND RELATED COMPOUNDS 197 



anthocyanins purple-»blue 



flavones, flavonols, xanthones yellow 



flavanones colorless, becoming orange-red 



(especially if heated) 



chalcones and aurones immediate red-purple 



flavanonols orange -brown 



The reaction with ferric chloride has been widely used to identify phenolic compounds, 

 but it is of little value in distinguishing different classes. Other things being equal, it 

 gives a greenish color with catechol derivatives and a blue color with pyrogallol deriva- 

 tives; but the "other things" are seldom equal. If a deep blue-black color appears, it is 

 good evidence for the presence of a 3, 4, 5-trihydroxy phenol (e. g. gallocatechin) but the 

 formation of a green color does not necessarily indicate the absence of this group nor the 

 presence of a catechol {urthu dihydroxy) group. 



Reduction with magnesium and concentrated hydrochloric acid produces red colors 

 with flavonols, flavanones, flavanonols and xanthones. The red pigments are not antho- 

 cyanidins but 4, 4' bis anthocyanidin derivatives (55). Chalcones and aurones give imme- 

 diate red colors on adding acid rather than a gradual intensification of color as reduction 

 proceeds. Flavones give some color but much less than flavonols. 



Addition of bromine water has been used to identify catechins and phlobatannins 

 since they give a precipitate while other tannins and other flavonoids do not. At least 

 certain leucoanthocyanidins also give a positive test with bromine water. 



Boiling plant parts with 2N HCl has been used to detect catechins and leucoanthocy- 

 anidins. The former give a yellow-brown color, the latter a red color. For additional 

 confirmation of anthocyanidins the red color may be extracted with amyl alcohol and fur- 

 ther tests for the presence of anthocyanin applied. Pacheco (56) has adapted this method 

 to detect flavanonols. After boiling with acid, he extracted with ether to obtain the fla- 

 vanonols, which were unaffected. The flavanonols were then reduced to flavan-3, 4-diols 

 (leucoanthocyanidins), converted to anthocyanidins with boiling acid, and the anthocy- 

 cyanidins identified by paper chromatography. 



Other color reactions for the flavonoids will be described below under paper chroma- 

 tography since most of the spray reagents described can be equally well applied to solu- 

 tions of the compounds. 



Although extensive degradations for proof of structure are beyond the scope of this 

 book, it should be mentioned that splitting with base has been a most useful technique for 

 determining hydroxylation patterns of unknown flavonoids. Fusion with potassium hydrox- 

 ide (or boiling with concentrated solutions) splits flavonoids to form a phenol from ring 

 A and a phenolic acid from ring B. For example, from luteolin there are obtained phloro- 

 glucinol and protocatechuic acid: 



KOH^ |- II J.HOC<v />0H 



'O' 



