Pharmacognosy: Crude Drugs, Phytoconstituents and Evaluation
Pharmacognosy in GPAT examines crude drugs, their botanical sources, phytoconstituents, extraction, identification and adulteration. The exam favours distinguishing closely related drugs and linking a chemical test to a constituent class.
Phytochemistry and tests
- Alkaloids: basic nitrogen compounds; Dragendorff, Mayer and Wagner tests. Examples: morphine (Papaver somniferum), quinine (Cinchona), reserpine (Rauwolfia serpentina), atropine (Atropa belladonna).
- Glycosides: Molisch (carbohydrate), Keller–Kiliani (cardiac glycosides), Borntrager (anthraquinones).
- Tannins: ferric chloride blue-black; resins: precipitate on adding water.
Worked example
To isolate alkaloids from powdered bark, moisten with dilute NH4OH to convert alkaloid salts to free bases, extract with chloroform, then back-extract the organic layer with dilute HCl. The alkaloids return to the aqueous phase as salts, while neutral lipids stay in chloroform. Basify again and extract into chloroform for the final alkaloid fraction. The reasoning is pH-dependent solubility: salts are water-soluble, free bases are organic-soluble.
Traps
Mismatching source and drug (e.g., ephedrine from Ephedra, not belladonna); using Keller–Kiliani for alkaloids; forgetting that adulteration can be intentional or unintentional; confusing morphological and chemical evaluation.