Precipitation reactions :
- Precipitating reagent + neutral or slightly acidic aqueous solution of alkaloidal salts = crystalline precipitates of different colors.
- Reagents that contain heavy metals (Hg, Pt, Bi, Cd) = double salts with most alkaloids.
- Reactions are used for extraction & purification.
Precautions:
– Certain alkaloids do not react e.g. caffeine + Mayer’s reagent = no ppt.
– False positive response may be obtained [most reagents also precipitate proteins, tannins, coumarins & certain flavonoids].
– Certain alkaloids do not react e.g. caffeine + Mayer’s reagent = no ppt.
– False positive response may be obtained [most reagents also precipitate proteins, tannins, coumarins & certain flavonoids].
Color reactions :
- Color reagents + solid free bases [not the salts] - characteristic colored solutions.
- Reagents generally contain conc. H2SO4 + oxidizing agent.
- Reagents - colors with most alkaloids, or may be specific for one alkaloid or a group of related alkaloids.
Examples :
–Van-Urk's test: ergot alkaloids + para-dimethyl amino-benzaldehyde (PDAB) in conc. H2SO4 = blue color.
–Vitalis' test: solanaceous alkaloids + conc. HNO3 + alcoholic KOH = violet color.
–Van-Urk's test: ergot alkaloids + para-dimethyl amino-benzaldehyde (PDAB) in conc. H2SO4 = blue color.
–Vitalis' test: solanaceous alkaloids + conc. HNO3 + alcoholic KOH = violet color.
Isolation of volatile & sublimable alkaloids :
Isolation of volatile alkaloids e.g. nicotine & coniine : By steam distillation
1.Immerse plant material in water.
2.Add caustic soda (NaOH) or Na2CO3 to liberate free bases (NH3 not used due to its volatility).
3.Steam distill the liberated alkaloid.
2.Add caustic soda (NaOH) or Na2CO3 to liberate free bases (NH3 not used due to its volatility).
3.Steam distill the liberated alkaloid.
- Isolation of sublimable alkaloids e.g. caffeine : By direct sublimation from the dried powdered plant.