Tannins for pharmacognosy

1. Tannins
Tannins are complex organic, non nitrogenous derivatives of
polyhydroxy benzoic acid which are widely distributed in the vegetable
kingdom.
-They are the active constituents of materials like oak bark.
-They are present in aerial parts eg. leaves fruits, barks or stems;
Generally occur in immature fruits.
-True tannins have the molecular weight in between 1000 to 5000.
Properties
1. Tannins are very complex substances. Getting in the pure state is very
difficult. Many of them are glycosidal in nature. Glucogallin (sugar+gallic
acid).
2. Tannins are non crystalline substances, occur as mixture of polyphenols
and form colloidal solution in water.

2. Properties
3. Their aqueous solutions are acidic in nature and possess sharp
puckering taste.
4. They are precipitated with solutions of gelatin and alkaloids.
5. They produce greenish black or blue color with ferric chloride and deep
red color with potassium ferricyanide and ammonia.
6. They form precipitate with salts of copper, lead and tin.
7. Tannins precipitate and combine with proteins. This complex is resistant
to proteolytic enzymes. This property is known as astringent action.

3. Classification of Tannin
1. Hydrolysable tannins
2. Condensed tannins
3. Pseudotannins
1. Hydrolysable tannins:
These tannins are hydrolyzed by acids, or enzyme and produce gallic
acid and ellagic acid. Chemically, these are esters of phenolic acid like
gallic acid and ellagic acid. The tannins derived from gallic acid are
known as gallitannins and from that of ellagic acid are known as
ellagitannins. The gallic acid is found in rhubarb, clove and ellagic acid is
found in eucalyptus leave and myrobalans and pomegranate bark. These
tannins treated with ferric chloride to produced blue or black colour.

4. 2. Condensed tannins:
These tannins are resistant to hydrolysis and they derived from the
flavonols, catechins and flavan-3, 4-diols. On treatment with acids or
enzymes they are decomposed into phlobaphenes. On dry distillation
condensed tannin produce catechol. These tannins are called as
catechol tannins. These tannins are found in cinchona bark, male fern,
areca seeds, tea leaves and wild cherry bark, bahera fruits, Amla, etc.
they produce green colour with ferric chlorides.
3. Pseudotannins:
They are phenolic compounds of lower molecular weight and do not
show the goldbeater’s test. They are found in catechu and nux- vomica,
etc.
(i) Catechins: Catechu, cocoa, etc.
(ii) Ipecacuanhic acid: Ipecacuanha
(iii) Chlorogenic acid: Coffee and nux-vomica

5. Solubility of tannins
Tannins are freely soluble in water, alcohol, glycerol, and acetone
and dilute alkalies. They are sparingly soluble in chloroform, ethyl
acetate and other organic solvents. They have an astringent taste.
They combine with alkaloids to form tannates, most of which are
insoluble in water.

6. Chemical tests
1. Gelatin test:
To a solution of tannin, aqueous solution of gelatin and sodium
chloride are added. A white buff coloured precipitate is formed.
2. Goldbeater’s skin test:
A small piece of goldbeater skin (membrane prepared from the
intestine of an ox) is soaked in 20% hydrochloric acid, ringed with
distilled water and placed in a solution of tannin for 5 minutes. The
skin piece is washed with distilled water and kept in a solution of
ferrous sulphate. A brown or black colour is produced on the skin
due presence of tannins.

7. 3. Phenazone test:
A mixture of aqueous extract of a drug and sodium acid phosphate
is heated and cooled and filtered. A solution of phenazone is added
to the filtrate. A bulky coloured precipitate is formed.
4. Match stick test (Catechin test):
A match stick is dipped in aqueous plant extract, dried near burner
and moistened with concentrated hydrochloric acid. On warming
near flame, the matchstick wood turns pink or red due to formation
of phloroglucinol.
5. Chlorogenic acid test:
An extract of chlorogenic acid containing drug is treated with
aqueous ammonia. A green colour is formed on exposure to air.

8. 6. Vanillin-hydrochloric acid test:
Sample solution and added vanillin- hydrochloric acid reagent
(Vanillin 1 gm, alcohol 10 ml, concentrated hydrochloric acid 10 ml).
A pink or red colour is formed due to formation of phloroglucinol.
Phloroglucinol

9. Pale Catechu
Biological Source:
Pale catechu is the dried aqueous extract of leaves and twigs of
Uncaria gambier.
Family: Rubiaceae
Habitat: The Plant is indigenous to Malaya and cultivated in
Indonesia, Malaya and Borneo.

10. Chemical constituents:
• Catechins (7-33%)
• Catechutannic acid (22-50%)
• Catechu red
• Quercetin
• Pyrogallol
• Gambir
• Fluorescein
• Fixed oil and wax

11. Catechin
Pyrogallol

12. Uses:
1. As astringent
2. In Diarrhoea
3. For tanning and dyeing fabrics brown or black color.

13. Black Catechu
Biological Source:
Pale catechu is the dried aqueous extract prepared from the
heartwood of Acacia catechu.
Family: Leguminosae
Habitat: The Plant is a native to India and found to Burma.

14. Chemical constituents:
• Mixture of catechin isomers
• Acacatechin (2-12%)
• Catechutannic acid or phlobatannin (25-35%)
• Catechu red
• Water
Acacatechin contains (-)-epicatechin which is the trans form of
acacatechin.
Structures: Md Ali
Preparation:

15. Uses:
1. Cooling and digestive properties
2. Used in relaxed condition of throat, mouth and gums.
3. In cough and diarrhea
4. As astringent medicament it is applied to ulcers, boils and skin
eruptions.

16. Galls/ Nutgalls
Biological Source:
Galls are vegetable outgrowths formed on the twigs of dyer’s oak.
Quercus infectoria.
Family: Fagaceae
Habitat: The Plant is found in Turkey, Syria, Persia, Cyprus and
Greece.

17. Chemical constituents:
• Galls contain tannin known as gallotannin acid (50-70%)
• Gallic acid (2-4%)
• Ellagic acid
• Sitosterol
• Methyl betulate
• Methyl oleanolate
• Starch
• Calcium oxalate
• Nyctanthic acid
• Roburic acid
• Syringic acid

18. Uses:
• Used as astringent
• For tanning and dyeing
• In the manufacture of inks and tannic acid.