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2. Introduction to Alkaloids- Secondary Metabolites (Part 1) • Class (45) = Alkaloids...
Properties of alkaloids
A. Physical properties
1. Alkaloids are colourless, crystalline, non-volatile, solids; a few such as coniine and nicotine are liquids and a few even coloured, viz. berberine is yellow.
2. The free bases (i.e. alkaloids themselves) are insoluble in water but soluble in most of the organic solvents.
3. Most of the alkaloids are laevorotatory (optically active), although a few are dextrorotatory (e.g. coniine), while a few are even optically inactive, viz. papaverine.
4. Generally, the alkaloids are bitter in taste and have pronounced physiological activity. Although many of the alkaloids possess curative properties and are of great value in medicine, they are powerful poisons. Chemically, the alkaloids are generally basic in nature and precipitated out by the common alkaloid reagents.
B. Chemical Properties of Alkaloid
1. Salt formation
Due to basic character: alkaloids + acids = Alkaloidal salts.
- Strong bases salts with very weak acids.
- Weak bases salts with stronger acids.
- Very weak bases unstable salts (colchicine, piperine, capsaicine, papaverine, narcotine & caffeine).
- Dibasic alkaloids may 2 series of salts.
- Amphoteric alkaloids (containing phenolic or carboxylic groups) - salts with both acids & alkalis.
- Acidic alkaloids do not - salts with acids e.g. ricinine.
2. Exposure to light, heat, oxygen, acids & alkalis should be avoided.
3. Effect of heat: Alkaloids are decomposed by heat, except caffeine which sublimes without decomposition.
4. Effect of heat & light in presence of oxygen: Most 3ary amine alkaloids N-oxides when exposed to light & oxygen at elevated temperature.
5. Effect of acids: Hot dilute acids & conc. mineral acids may
cause:
- Dehydration - anhydro- or apo-alkaloids, e.g. morphine - apomorphine; atropine - apoatropine.
- O-demethylation - phenolic alkaloids e.g. codeine + HI - morphine.
- Hydrolysis of ester alkaloids (atropine & reserpine) & glucoalkaloids (solanine).
6. Effect of alkalis:
- Weak alkalis [e.g. NH3] liberate most alkaloids from their salts
- Weak alkalis [e.g. NaHCO3] + alkaloids containing a -COOH group - salts e.g. narceine.
- Strong alkalis [NaOH & KOH] + phenolic alkaloids - salts e.g. morphine.
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23 сен 2024
