They are nitrogenous bases with tertiary amines having an alkaline pH. They occur as salts of nicotinic, quinic, citric, or oxalic acid.
- They are white crystalline substances
- Insoluble in water except for certain salts
- They have a sharp melting point
- They are mostly poisonous, but exert useful physiological effects when used in small quantities.
- They are bitter to taste
Isolation of alkaloids
Alkaloids can be extracted by simple acid-base extraction.
- Step 1
Drug to be tested is powdered and moistured with lime water, so that acids, tannins, phenols are removed
- Step 2
The mixture is extracted with organic solvents like alcohol
- Step 3
The organic lipid now contains alkaloidal salts, which can be isolated easily.
Classification of alkaloids
They are broadly classified into
- Proto or pseudo alkaloids
- True or heterocyclic alkaloids
The examples for pseudo alkaloids are ephedrine, colchicines, aconitine etc
They are again subdivided into
- Indole alkaloids eg. Strychnine, reserpine, ergotamine.
- Steroidal alkaloids e.g. Conicine, withanine.
- Quinoline alkaloids eg.quinine, quinidine.
- Pyridine alkaloids e.g. Trigonelline.
- Pyrrolidine alkaloids e.g. nicotine
- Tropane group e.g. Atropine, cocaine
- Iso-quinoline group e.g. Papaverine, narcine, berberine.
- Phenanthrene group e.g. Morphine, codeine.
- Phenethylamine group eg.ephedrine
- Purine group etc eg caffeine
- Other than these there are quaternary ammonium compounds like choline, neurine, muscharine.
Naming of alkaloids
The alkaloids may be named after,
- Genus name e.g. Withanine
- Species name e.g. Somniferine
- Commercial name eg.ergotamine
- Discoverer name eg.pelletierine
- Action e.g. morphine etc
Test for alkaloids
- Meyer`s test
Whitish yellow or green precipitate
- Wagener’s test
Dirty brown or red precipitate
These alkaloids are associated with prevention or treatment of major diseases like cancer, diabetes, hypertension, AIDS, etc.