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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