Pharmacognosy · Short notes

Alkaloids: Short Notes & Cheat Sheet

Definition and types of alkaloids, classification by ring system with source plants, general precipitating reagents and special colour tests.

Short notes

Read these before you attempt the topic's MCQs.

Classify an alkaloid by its ring, then link it to the source plant; most exam questions pair the two.

Alkaloids are basic, nitrogen-containing plant compounds, usually bitter and pharmacologically active.
True alkaloids have nitrogen in a heterocyclic ring and are made from amino acids (morphine, atropine).
Protoalkaloids have nitrogen outside the ring (ephedrine, mescaline, colchicine).
Pseudoalkaloids are not made from amino acids (caffeine from purines, steroidal alkaloids such as solanine).
Most exist in plants as salts of organic acids. They are extracted by making the material alkaline to free the base, then shaking it into an organic solvent (Stas–Otto principle).

Opium (dried latex of unripe capsules of Papaver somniferum): morphine, codeine, thebaine, papaverine, noscapine.
Cinchona bark: quinine and quinidine. Ipecacuanha root: emetine (amoebicide, emetic).
Solanaceous leaves (Belladonna, Datura, Hyoscyamus): atropine and hyoscine. Coca leaves: cocaine.
Nux vomica seeds: strychnine and brucine. Rauwolfia serpentina root: reserpine.
Catharanthus (Vinca) rosea: vincristine and vinblastine. Ergot (sclerotium of Claviceps purpurea): ergometrine and ergotamine.
Ephedra: ephedrine. Colchicum corm and seed: colchicine. Jaborandi leaves: pilocarpine. Tea, coffee and cocoa: caffeine and theobromine.

General precipitating reagents: Mayer's (potassium mercuric iodide, cream precipitate), Dragendorff's (potassium bismuth iodide, orange-red), Wagner's (iodine in KI, reddish-brown), Hager's (picric acid, yellow).
Vitali–Morin test (fuming nitric acid, then alcoholic KOH gives violet) identifies tropane alkaloids.
Thalleioquin test (bromine water then ammonia gives emerald green) identifies quinine and quinidine.
Murexide test (evaporate with HCl and KClO₃, then ammonia gives purple) identifies caffeine and other purines.
Caffeine does not give a precipitate with Mayer's reagent, unlike most alkaloids.

High-yield facts in one place. Cover the right-hand column and test yourself.

Ring type	Alkaloid (source)
Tropane	Atropine, hyoscine (Belladonna, Datura); cocaine (Coca)
Quinoline	Quinine, quinidine (Cinchona)
Isoquinoline	Morphine, papaverine (Opium); emetine (Ipecac)
Indole	Reserpine (Rauwolfia); vincristine (Vinca); ergotamine (Ergot); strychnine (Nux vomica)
Pyridine / piperidine	Nicotine (Tobacco), arecoline (Areca), lobeline
Imidazole	Pilocarpine (Jaborandi)
Purine	Caffeine (Tea, Coffee), theobromine (Cocoa)
Amino (protoalkaloid)	Ephedrine (Ephedra), colchicine (Colchicum)

Test	For
Vitali–Morin	Tropane alkaloids (violet)
Thalleioquin	Quinine (emerald green)
Murexide	Caffeine, purines (purple)
Dragendorff's	Alkaloids in general (orange)
Mayer's	Most alkaloids (cream); not caffeine

Tropane = Solanaceae + Coca: Belladonna, Datura and Hyoscyamus give atropine and hyoscine; Coca gives cocaine.
VITALI for Tropane, THALLEIOQUIN for Quinine: Each special test is linked to one alkaloid group.

Attempt the Alkaloids MCQs while the notes are fresh. Anything you get wrong is saved to your incorrect list for the next revision round.