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Alkaloids are a chemically heterogeneous group of natural substances and comprise more than 6000 basic nitrogen containing organic compounds which occur in about 15 per cent of all vascular terrestrial plants and in more than 150 different plant families. The alkaloids exhibit diversity of structures and also show an extraordinary spectrum of pharmacological activities. Because of these characters, they are important for chemical, physiological, taxonomic and biogenetic studies.
Alkaloid Meaning
The term is derived from the word ‘alkali-like’ and hence, they resemble some of the characters of naturally occurring complex amines.
Definition of Alkaloid
These are the organic products of natural or synthetic origin which are basic in nature and contain one or more nitrogen atoms, normally of heterocyclic nature, and possess specific physiological actions on human or animal body, when used in small quantities. The true alkaloids are toxic in nature, contain heterocyclic nitrogen which is derived from amino acids and always basic in nature.
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The ‘proto alkaloids’ or amino alkaloids’ are simple amines in which the nitrogen is not in a heterocyclic ring. Some times, they are considered as biological amines. They are basic in nature and prepared in plants from amino acids. Some of the examples of these alkaloids are mescaline, N N-dimethyl tryptamine, colchicine, and ephedrine. The term ‘pseudoalkaloids’ includes mainly steroidal and terpenoid alkaloids and purines. They are not derived from amino acids. They do not show many of the typical characters of alkaloids, but give the standard qualitative tests for alkaloids. The examples of pseudo alkaloids are conessine and caffeine.
Properties
Physical Properties of alkaloids
- It is Colorless
- Crystalline Solid
- Sharp Melting Point And decomposition range
- Some Alkaloids are amorphous gums
In general, the free bases of alkaloids are soluble in organic non-polar, immiscible solvents. The salts of most alkaloids are soluble in water. In contrast, free bases are insoluble in water and their salts are also very sparingly soluble in organic solvents. The alkaloids containing quaternary bases are only water soluble. Some of the pseudoalkaloids and protoalkaloids show higher solubility in water. For example, colchicine is soluble in alkaline water, acid or water and caffeine (free base) is freely soluble in water. Quinine hydrochloride is highly soluble in water i.e. 1 part of quinine hydrochloride is soluble in less than 1 part of water, while only 1 part of quinine sulphate is soluble in 1000 parts of water.
The solubility of alkaloids and their salts is useful in pharmaceutical industry for the extraction and formulation of final pharmaceutical preparations.
Chemical Properties od Alkaloid
Most of the alkaloids are basic in reaction, due to the availability of lone pair of electrons on nitrogen. The basic character of the alkaloidal compound is enhanced if the adjacent functional groups are electron releasing. The alkaloid turns to be neutral or acidic when the adjacent functional groups are electron withdrawing like amide group which reduces the availability of the lone pair of electrons. But, alkaloids exhibiting basic character are very much sensitive to decomposition and cause a problem during their storage. Their salt formation with an inorganic acid prevents many a time their decomposition.
The alkaloids may contain one or more number of nitrogen and it may exist in the form as primary (R-NH₂), e.g. mescaline; secondary amine (R₂ – NH), e.g. ephedrine; tertiary amine (R3N) e.g. atropine; and quaternary ammonium compounds [R4N+X] e.g. tubocurarine chloride. In the last type, their properties vary from other alkaloids, owing to quaternary nature of nitrogen.
In the natural form, the alkaloids exist either in free state, as amine or as salt with acid or alkaloid N-oxides.
Chemical Tests For Alkaloids
The qualitative chemical tests used for detection of alkaloids are dependent on the characters of alkaloids to give precipitates as salts of organic acids or with compounds of heavy metals, like mercury, gold, platinum, etc.
The different reagents used are Mayer’s reagent (potassium mercuric iodide solution) giving cream coloured precipitate; Dragendorff’s reagent (potassium bismuth iodide solution) giving reddish brown precipitate; and Wagner’s reagent (iodine-potassium iodide solution) yielding reddish brown precipitate. Some alkaloids also give yellow coloured precipitates with picric acid called as Hanger’s reagent and picrolonic acid. Indivadual alkaloid gives color or precipitate with certain specific reagent.
The chemical tests with heavy metals are not solely limited to alkaloids. Proteins, coumarins and a- pyrones also give precipitates with these reagents. It may be also noted that some alkaloids do not give such tests, like caffeine which is highly water soluble. Hence, the tests with heavy metals are in some cases false positive reactions or false negative reactions. For this purpose, the specific tests for individual alkaloids are more important for qualitative evaluation of crude drugs. These tests are covered under individual drugs.
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