Secondary Metabolites – Alkaloids

Secondary Metabolites – Alkaloids

Alkaloids are basic N containing heterocyclic compounds derived from higher plants often having marked physiological activity.

Occurrence Alkaloids:

Occurs in flowers, leaves, stem, bark, roots, fruits as a salt of formic, malic, tartaric, and citric, oxalic and acetic acids.

Chemical Nature of Alkaloids:

They are derivatives of heterocyclic Nitrogenous basic compounds such as pyridine, quinoline, isoquinoline and pyrrole.

Properties of Alkaloids:

Colourless, nonvolatile, crystalline (except nicotine) solids, insoluble in water but soluble in organic solvents, optically active, bitter in taste, poisonous to animals, form salts with acids, form precipitate with phospho-tungstic acid, phospho molybdic acid, picric acid and mercuric iodide.
Physiological Role of Alkaloids:
They are active participants and not the end products of detoxification in plant metabolism. Adenine has important role in nucleic acid metabolism, certain purines act as growth regulating substances. Protect plant against insects. Some function as Coenzyme in oxidative process, act as stimulator, growth regulators, act as reservoir of protein synthesis.

Classification (According to heterocyclic nucleus) :

Sr. No.





Pyridine or Piperdine

Coniine piperine

Hemlock, piper longum




Stachy ys tuber hygrine


Pyrrolidine pyridine





Cocaine Atropine

Coca leaves, Datura, plants of belladona sp.




Cinchona bark



Papavarine Morphine

Opium poppy



Strychine Reserpine

Nux-vomica; Rauwolfia sp




Seeds and corms of colchicum


1) Pyridine:

a) Six membered ring
b) = CH replaced by N

2) Pyrrolidine:  CH is replaced by N e.g. pyrrole and pyrrolidine

3) Pyridine – Pyrrolldine: Nicotine   C10H14N2
1. Combined two rings i.e. pyridine, pyrrolidine and having a side chain of methyl pyrrolidine, liquid at room temp.
2. on oxidation with chromic acid yields nicotinic acid

C10 H14N2 ————–> C5H4NCOOH  
Nicotine Oxidation – Nicotinic acid


Give chemical structure of Nicotine

Properties of Pyridine:

1. Colourless
2. Oily liquid B.P. 247°C
3. Turning brown in air due to oxidation
4. Tobacco like smell.
5. Alkali taste
6. Soluble in water and organic solvents
7. Highly toxic to animals
8. Stimulates the nervous system.

Uses of Pyridine: As an insecticide.

4. Tropane: Cocaine coca leaves (C17H21O4N)

Chemical Nature: On hydrolysis yield ecgonine + methyl alcohol + benzoic acid.

Atropine: C17H23O3N – found in Datura, on hydrolysis yields tropine + Tropic acid

5. Quinoline: Benzene ring fused with pyridine ring and formed quinoline in α, β positions.

Quinine: C20H24N202. It occurs in bark of cinchona.

Chemical Nature: On vigorous oxidation with chromic acid yield quinnic acid.

Properties of Quinoline:

1. White solid crystal
2. Bitter taste
3. Soluble in organic solvent
4. It is a di-acidic base, forming both neutral and acidic salts.

Uses of Quinoline:

1. Used for control of malaria,
2. Used as antipyretic.

6) Isoquinoline: It is made up of a benzene ring fused with a pyridine ring in different position.

Morphine: C17H19O3N It occurs in unripe fruits of opium poppy (10%), upon oxidation yield isoquinoline and acid.

Properties of Isoquinoline:
1. Insoluble in water
2. Bitter taste
3. It affects central nervous system.

Uses of Isoquinoline:

1. It has soothing action
2. It is used as a Narcotic drug, use in medicine.

7. Indole: Union of benzene and pyrrole rings e.g. Strychnine, are obtained from dried seed of nux-vomica. It affects central nervous system.

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