Alkaloids : Physical and Chemical Properties

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Physical Properties of Alkaloids:

  • All the alkaloids are colourless, crystalline solids
  • Some alkaloids are coloured in nature, for example, betanidin is red, berberine is yellow and salts of sanguinarine are copper red in colour.
  • Some alkaloids like coniine, sparteine, nicotine are liquid and volatile in nature.
  • They have a sharp melting point (decomposition range).
  • Some alkaloids are amorphous gums.
  • The free bases of alkaloids are soluble in organic, non-polar, immiscible solvents.
  • The salts of most alkaloids are soluble in water.
  • Free bases of the alkaloids are insoluble in water.
  • Their salts are also very sparingly soluble in organic solvents.
  • The alkaloids which contain quaternary bases are only water soluble.
  • Some of the pseudo alkaloids and proto alkaloids are highly water soluble.
  • Colchicine is soluble in alkaline water, acid or water.
  • Caffeine is freely soluble in water.
  • Quinine hydrochloride is highly soluble in water where as quinine sulphate is sparingly soluble in water.
  • This solubility of alkaloids and their salts is useful in pharmaceutical industry for the extraction and formulation of final pharmaceutical preparations.

 Chemical Properties of Alkaloids:

  • Most of the alkaloids are basic in nature due to the availability of lone pair of electrons on nitrogen.
  • This basic character of alkaloids is enhanced if the adjacent functional groups are electron releasing.
  • When the adjacent functional groups are electron withdrawing like amide group, they reduces the availability of lone pair of electrons and the alkaloids become neutral or acidic in nature.
  • Alkaloids with basic character are very much sensitive to decomposition and creates a problem during the storage.
  • When they form salt with an inorganic acid, they prevent decomposition in many cases.
  • The alkaloids contain one or more number of nitrogen atoms.
  • It may exist in the form of primary amine, secondary amine, tertiary amine and quaternary ammonium compounds.
  • Primary amine – mescaline, secondary amine – ephedrine, tertiary amine – atropine, quaternary ammonium compound – tubocurarine chloride.
  • The properties of the quaternary ammonium compounds vary from the other alkaloids because of the quaternary nature of nitrogen.
  • The alkaloids exist in free state in the natural form as amine or as salt with acid or alkaloid N-oxides.

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