Alkaloids
On this page
Direct answer
Bitter taste, basic reaction and a nitrogen atom inside the ring — that is the alkaloid signature shared by reserpine, morphine, atropine and quinine. Alkaloids are nitrogenous, mostly heterocyclic, physiologically active bases of plant origin, usually bitter, optically active and able to form water-soluble salts with acids while their free bases dissolve in organic solvents. Pharmacognosy examines them along three axes: ring-based classification into pyridine, tropane, quinoline, isoquinoline, indole and other nuclei; the precipitation tests named Dragendorff, Mayer, Wagner and Hager; and the extraction logic of the Stas-Otto method.
What you must remember
- Pyridine-piperidine: lobeline (Lobelia), piperine (black pepper); tropane: atropine and hyoscine (belladonna, datura), cocaine (coca).
- Quinoline: quinine and cinchonine (cinchona bark); isoquinoline: morphine, codeine, papaverine (opium), emetine (ipecac).
- Indole: reserpine (rauwolfia), ergometrine and ergotamine (ergot), strychnine (nux vomica); imidazole: pilocarpine (jaborandi).
- Purine: caffeine (tea, coffee), theophylline; steroidal: conessine (kurchi); tropolone: colchicine (colchicum); phenylalkylamine: ephedrine (ephedra).
- Colour tests: Dragendorff (potassium bismuth iodide, orange), Mayer (potassium mercuric iodide, cream precipitate), Wagner (iodine-potassium iodide, reddish brown), Hager (saturated picric acid, yellow).
- Specific tests: Murexide for purines (caffeine gives purple), Vitali-Morin for tropane alkaloids (violet), thalleioquin (green fluorescence) for quinine, van Urk for ergot.
- Free bases are soluble in organic solvents (chloroform, ether); salts are water-soluble — the basis of extraction and of dosage design.
- Atropine is the racemate obtained by racemising (-)-hyoscyamine during extraction; most alkaloids are laevorotatory.
- Stas-Otto procedure extracts them by acid-base partition: alkaloid salts into acidified water, free bases into organic solvent after basification.
A viva walked from powder to class
Handed a bitter powder in a viva, work in a fixed order. First the screening tests: to an acidic aqueous extract add Dragendorff — an orange precipitate says alkaloid; confirm with Mayer's cream and Wagner's brown. Second, extraction: basify the aqueous phase with ammonia and shake with chloroform; the organic layer now carries the free bases; shaking back with dilute acid re-extracts them as salts. Third, narrow the class with specific chemistry: a positive Vitali-Morin points to tropane and datura or belladonna; a positive Murexide points to purine and caffeine or tea; green fluorescence after thalleioquin points to quinine and cinchona; van Urk's reagent blueing points to ergot. Finally close the loop botanically — each class maps onto classic drug families: tropane to Solanaceae, indole to Apocynaceae and Loganiaceae, isoquinoline to Papaveraceae.
Notice how the chemistry and the botany confirm each other. That double confirmation is what "evaluate a crude drug containing alkaloids" means in a long answer, and pairing test with drug earns far more than listing reagents alone.
Where answers go wrong
The first error is calling all nitrogenous plant compounds alkaloids — caffeine and colchicine are conventionally included, but amino acids, proteins and purines in general are not; alkaloids are defined by basicity, botanical origin and pharmacological potency. The second is the solubility inversion: writing that alkaloids as free bases dissolve in water. The free base is lipophilic and organic-soluble; only the salt is aqueous — this single fact underlies both extraction questions and clinical ones (morphine sulphate is injectable because the salt dissolves).
Third, students present the four precipitation tests as equally diagnostic. They are screening tools only, and some alkaloids behave oddly — caffeine, for instance, is better confirmed by the Murexide test than by precipitation. Name the specific test for the specific alkaloid when the question names the alkaloid; that precision separates a distinction answer from a pass answer.
Frequently asked questions
Define alkaloids and give four general properties.
Basic, nitrogenous, mostly heterocyclic plant constituents with intense physiological action; typically bitter, optically active, soluble as free bases in organic solvents, forming water-soluble salts with acids.
Name the reagents and colours of the standard alkaloid precipitation tests.
Dragendorff — potassium bismuth iodide — orange; Mayer — potassium mercuric iodide — cream; Wagner — iodine in potassium iodide — reddish brown; Hager — saturated picric acid — yellow.
Which test confirms caffeine and what colour appears?
The Murexide test: evaporate with a few drops of hydrochloric acid and a trace of potassium chlorate, then expose to ammonia vapour; a purple-red colour confirms purine alkaloids.
Give the source and use of reserpine.
Reserpine, an indole alkaloid from the roots of Rauwolfia serpentina (Apocynaceae), was used as an antihypertensive and is now mainly of historical and phytochemical importance.
Outline the principle of Stas-Otto extraction.
Repeated acid-base partition: alkaloids are drawn into acidic water as salts, then liberated as free bases with ammonia and transferred to an immiscible organic solvent, separating them from neutral and acidic impurities.