Drugs Containing Volatile Oils

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Chapter: Pharmacognosy and Phytochemistry : Drugs Containing Volatile Oils

Volatile oils are odorous volatile principles of plant and animal source, evaporate when exposed to air at ordinary temperature, and hence known as volatile or etheral oils. These represent essence of active constituents of the plant and hence also known as essential oils.


Drugs Containing Volatile Oils

 

INDRODUCTION

 

Volatile oils are odorous volatile principles of plant and animal source, evaporate when exposed to air at ordinary temperature, and hence known as volatile or etheral oils. These represent essence of active constituents of the plant and hence also known as essential oils. In most instances the volatile oil preexists in the plant and is usually contained in some special secretory tissues, for example, the oil ducts of umbelliferous fruits, the oil cells, or oil glands occurring in the sub-epidermal tissue of the lemon and orange, mesophyll of eucalyptus leaves, trichomes of several plants, etc.

 

In few cases the volatile oil does not preexist, but is formed by the decomposition of a glycoside. For example, whole black mustard seeds are odourless, but upon crushing the seeds and adding water to it a strong odour is evolved. This is due to allyl isothiocyanate (the main constituent of essential oil of mustard) formed by decomposition of a glycoside, sinigrin, by an enzyme, myrosin. Glycoside and enzyme are contained in different cells of the seed tissue and are unable to react until the seeds are crushed with water present, so that the cell contents can intermingle.

 

Volatile oils are freely soluble in ether and in chloroform and fairly soluble in alcohol; they are insoluble in water. The volatile oils dissolve many of the proximate principles of plant and animal tissues, such as the fixed oils and fats, resins, camphor, and many of the alkaloids when in the free state.

 

These are chemically derived from terpenes (mainly mono and sesqui terpenes) and their oxygenated derivatives. These are soluble in alcohol and other organic solvents, practically insoluble in water, lighter than water (Clove oil heavier), possess characteristic odour, have high refraction index, and most of them are optically active. Volatile oils are colourless liquids, but when exposed to air and direct sunlight these become darker due to oxidation. Unlike fixed oils, volatile oils neither leave permanent grease spot on filter paper nor saponified with alkalis.

 

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