Alkaloids : Physical and Chemical Properties

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.