Tannins and Resins
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Direct answer
Tannins convert raw hide into leather; resins seal a wounded tree — and both reach the pharmacopoeia as chemically unrelated but practically parallel groups of plant excretory products. Tannins are non-nitrogenous polyphenols of molecular weight roughly 500 to 3000 that precipitate proteins, taste astringent and tan hides, divided into hydrolysable and condensed types; resins are amorphous, combustible, water-insoluble but alcohol-soluble mixtures classified as oleoresins, gum-resins, oleo-gum-resins, balsams and glycoresins. Myrobalan and catechu anchor the tannin questions; asafoetida and the balsams anchor the resin ones.
What you must remember
- Tannins precipitate proteins, alkaloids and heavy metals — the basis of leather tanning, of their astringent action, and of their use as an antidote in alkaloid poisoning.
- Hydrolysable tannins (pyrogallol type) split into a sugar plus gallic or ellagic acid: myrobalan (Terminalia chebula), Turkish or oak galls, sumach; give blue-black colour with ferric chloride.
- Condensed tannins (pyrocatechol type, proanthocyanidins) resist hydrolysis and polymerise to phlobaphenes: catechu (Acacia catechu), gambir, cinnamon, cinchona; give green-black colour with ferric chloride.
- Goldbeater's skin test is the specific identification test for tannins; the phenazone test precipitates them quantitatively.
- Uses of tannins: astringent, antidiarrhoeal, locally in burns (historically tannic acid treatment), antidote to alkaloid poisoning, and ink and dye industries.
- Resins burn with a smoky, sooty flame, are insoluble in water, soluble in alcohol, and usually formed in ducts or schizogenous cavities.
- Oleoresins: ginger, capsicum, turpentine; oleo-gum-resins: asafoetida, myrrh; balsams: Tolu, Peru, storax (benzoic and cinnamic acid esters); glycoresins: jalap, ipomoea; plain resins: colophony, podophyllum.
- Asafoetida comes from Ferula asafoetida (Umbelliferae) as an oleo-gum-resin obtained by incision of the rhizome and root.
Hydrolysable versus condensed, actually reasoned
The two tannin classes are best held apart by what happens to each in the test tube. Boil a hydrolysable tannin with dilute acid and it falls apart cleanly into its sugar and its phenolic acid — gallic or ellagic — because ester bonds join its building blocks. Boil a condensed tannin the same way and nothing useful splits; instead it polymerises towards insoluble red phlobaphenes, because carbon-carbon bonds link its flavan units and hydrolysis cannot cut them. Ferric chloride reads the difference in colour — blue-black for the pyrogallol tannins of myrobalan and nutgall, green-black for the pyrocatechol tannins of catechu. The hide test reads it too: both tan leather, but the condensed tannins bind more firmly, which is why wattle and quebracho dominate the tanning industry while galls dominate pharmacy.
Carry the same reasoning to drugs at the bedside. An astringent that precipitates surface proteins shrinks mucous membranes, checks secretion and controls mild diarrhoea — myrobalan in traditional use. And the protein-precipitating property predicts the classic antidote role: strong tea or tannic acid solution precipitates unabsorbed alkaloids in the stomach, though activated charcoal has largely replaced this in modern practice.
For resins, the reasoning runs through solubility and origin rather than hydrolysis. A resin is what oozes from a cut and hardens; what else it carries names it — oil with it makes an oleoresin (ginger), gum as well makes an oleo-gum-resin (asafoetida), aromatic esters of benzoic or cinnamic acid make a balsam (Tolu, Peru), and sugar-linked resin makes a glycoresin (jalap). Podophyllum resin yields podophyllotoxin, the precursor story that keeps resins current.
Where scripts slip
Two errors dominate. The first is writing tannins off as glycosides — they are not; the phenolic acids bond to sugars through ester links in hydrolysable types only, and condensed tannins contain no sugar at all. The second is the ferric chloride colour swap: blue-black for hydrolysable, green-black for condensed. Attach the colours to galls and catechu respectively and the memory holds.
A quieter trap concerns storage: tannin-containing drugs darken in iron containers because traces of ferric salts form black complexes — which is also why tannin-containing mixtures should never be dispensed with iron-containing tonics. On resins, examiners catch students calling asafoetida a gum-resin instead of an oleo-gum-resin; it contains all three fractions — resin, gum and volatile oil — and the name must say so.
Frequently asked questions
Define tannins and give their two classes with one drug each.
Non-crystalline, non-nitrogenous polyphenolic complexes that astringe and precipitate proteins; hydrolysable (myrobalan, nutgalls) split by acids into sugar plus gallic or ellagic acid, condensed (catechu, cinnamon) resist hydrolysis and polymerise.
What is the Goldbeater's skin test?
Ox skin membrane prepared in dilute hydrochloric acid, washed and placed in the tannin solution, then treated with ferrous sulphate — brown-black staining of the skin confirms tannin.
Classify resins with one example of each class.
Oleoresin — ginger; gum-resin — mastic; oleo-gum-resin — asafoetida; balsam — Tolu; glycoresin — jalap; simple resin — colophony.
Give the source and type of asafoetida.
The oleo-gum-resin obtained by incision of the rhizome and roots of Ferula asafoetida, family Umbelliferae, used as a carminative and antispasmodic.
Why are tannins useful in alkaloid poisoning?
They precipitate alkaloids as insoluble complexes in the gut, limiting absorption — strong tea was the classical household measure.