1.Oxygenated & non-oxygenated alkaloids
– Beside C, H & N, most alkaloids contain O
– Few are oxygen-free e.g. nicotine & coniine.
– Beside C, H & N, most alkaloids contain O
– Few are oxygen-free e.g. nicotine & coniine.
2.Salt formation
Due to basic character: alkaloids + acids 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.
Due to basic character: alkaloids + acids 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.
- Exposure to light, heat, oxygen, acids & alkalis should be avoided.
- Alkaloids are less stable in solution than in dry state.
1.Effect of heat: Alkaloids are decomposed by heat, except caffeine which sublimes without decomposition.
2.Effect of heat & light in presence of oxygen: Most 3ary amine alkaloids N-oxides when exposed to light & oxygen at elevated temperature.
3.Effect of acids: Hot dilute acids & conc. mineral acids may
cause:
cause:
- Dehydration - anhydro- or apo-alkaloids, e.g. morphine - apomorphine; atropine - apoatropine.
- O-demethylation - phenolic alkaloids e.g. codeine + HI - morphine.
- O-demethylation - phenolic alkaloids e.g. codeine + HI - morphine.
- Hydrolysis of ester alkaloids (atropine & reserpine) & glucoalkaloids (solanine).
4. 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.
- Hot alkalis - hydrolysis of ester alkaloids e.g. atropine, cocaine & physostigmine or cleavage of lactone ring - corresponding acid e.g. pilocarpine - pilocarpic acid.