Flavonoids

These are mostly yellow pigments in plants. They are phenolic in nature. 

They are derivatives of 2-phenylbenzopyrones. 

A large number of physiological activities have been attributed to them. 

Some flavones a may act as cardiac stimulants, some strengthen week capillary blood vessels, some are good diuretics and of late some have proved extremely good against liver damage.

Chemical tests for flavonoid:

1. Shinoda test to dry powder or extract, add 5 ml of 95% ethanol, few drops of conc. HCl and 0.5 g magnesium turnings. Pink colour observed.

2. To small quantity of residue, add lead acetate solution. Yellow coloured precipitate is formed. Addition of increasing amount of sodium hydroxide to the residue show yellow coloration, which decolouration after addition of acid.

Classification of flavonoids

1. Flavone glycosides:

(a) Parsley- Apin  (b) Buchu- Diosmin. 

2. Flavonol glycosides:

(a) Buck wheat- Rutin. (b) Ring (Crategus oxycantha)- Quereitrin.

3. Flacanone glycosides:

(a) Lemon, sweet orange- Hesperidin  (b) Bitter orange

4. Chalone glycosides:

Safflor red- Carthamin

5. Isoflavonoid glycoside:

(a) Sharapunkha- Tephrosin. (b) Gilas- Prunetrin

6. Anthocyanidin glycosides:

(a) Pelargonium flower- Plargonidin (b) Petunia flower- Petunidin

General properties of flavones and flavonols:

These are crystalline compounds, soluble in water, dilute mineral acids, alkalis, alcohol, etc. 

they are precipitated by mean of lead acetate. 

They give a dull green or a red brown colour with ferric chloride. 

They more highly coloured in the acidic medium than in the bases from which they are derived. 

In acidic medium they form oxonium salts colour.

Isolation of flavones and flavonols:

The plant material containing flavones or flavones is extracted with boiling water and the tannins are removed as lead salts by means of lead acetate the filtrate is diluted with water, acidified with HCl and boiled for some hours when the sugar free flavones or flavonols are precipitated. 

They are extracted with alcohol and may be purified by fractional crystallization of their acetates or by re-crystallization from some organic solvents like benzene, carbon disulphide, alcohol, etc.