Tannins, Physical and Chemical Nature, Classification

Content

Tannins, Physical, Chemical Nature, and Classification

Objective

At the end of this lecture, student will be able to

• Tannins

• Explain physical and chemical nature of Tannins

• Classify Tannins

• Describe the importance of tannins

Tannins

• Coined by Seguin in 1796

• Tannins are polyhydroxy phenolic – o- dihydroxy or o- trihydroxygroups on phenyl ring

• High molecular weight compounds - non nitrogenous compounds

• Astringent Property – combine with protein – ppt them – used in medicien to treat burn, wounds, diarrhoea

• React with protein of animal hide, prevent their putrefaction and convert hide and skin into leather (Astringent)

• Present in solution form in the cell sap and also in distinct vacuoles

Physical properties of Tannins

State: Non-crystalline

• Form colloidal solutions with water

Solubility: Soluble in water, alcohol, dilute alkalis, glycerol and acetone

Insoluble – organic solvents except acetone

Chemical properties of Tannins

• Tannins have ability to precipitate the solutions of

- Gelatin,

- Alkaloids

- Glycosides

- Heavy metals

- Proteins

• They are precipitated by chromic acid or potassium dichromate and salts of copper, tin and lead

• Colour reactions with iron salts – Ferric chloride – blusih black colour or green colour

• Tannins with potassium ferricyanide and ammonia gives deep red colour

Uses of Tannins

• Astringent property – Healing of wounds, inflammed mucosa, antidiarrhoeal, anti haemmorhagic, antiseptic

• Treatment of varicose ulcers, minor burns, inflammation of gums

• Leather industry – converting hide into leather and also acts as preservative

• Manufacture of inks – with iron salts form deep colours

• Sizing paper, mordant in dyeing, printing fabrics and silk

• Along with gelatin and albumin – manufacture of artificial horns, tortoise shells

• Laboratory reagents – to identify gelatin, proteins and alkaloids – because it precipitates them

• Antidote for heavy metal poisoning – it forms insoluble alk.tannates

Classification of Tannins

• Based on their complex chemical nature and their behavior towards dry distillation

• Hydrolysable tannins

• Condensed tannins

• Pseudo tannins

Hydrolysable Tannins

• Hydrolyzed by enzymes or acids – Gallic acid or ellagic acid Hydrolysable tannins - Dry distillation à Pyrogallols + Phenolic compounds

Hydrolysable tannins + iron salts à Blue colour

Gallotannin – Nutgall, clove, rhubarb

Ellagitannin – Oak, Myrobalan

Condensed Tannins/Non Hydrolysable Tannins

• Non hydrolysable – Resist hydrolysis

• Undergo polymerisation – insoluble red compounds phlobaphens (red colour – cinchona and rhubarb)

• Condensable tannins – Dry distillation à Catechol

• Condensable tannins + iron salts à Brownish Green colour

Examples:

Pale Catechu, Asoka bark, Hamamelis, Cinchona, Cinnamon

Pseudo Tannins

Low molecular weight, do not obey Gold Beaters skin Test

Examples:

Chlorogenic aid - Nuxvomica seed

Ipecacuanhic acid – Ipecac

Summary

• Poly hydroxy phenolic non nitrogenous, astringent

• Precipitate gelatin, alkaloids, glycosides, heavy metals and proteins

• Based on identity of phenolic nuclei – Hydrolysable, condensed and complex tannins

• Hydrolysable tannins on dry distillation gives pyrogallols

• Condensed tannins on dry distillation gives catechol

• Complex tannins are biosynthesized from mixtures of both, hydrolysable and condensed tannins

• Based on tanning test – True and pseudo tannins