Pharmacognosy-I (Part-8)

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Volatile oils:

Volatile oils Resins and resins combination


VOLATILE OILS Volatile or essential oils, as their name implies, are volatile in steam. They differ entirely in both chemical and physical properties from fixed oils. They are secreted in oil cells, in secretion ducts or cavities or in glandular hairs. They are frequently associated with other substances such as gums and resins and themselves tend to resinify on exposure to air.

Production and uses of volatile oils:

Production and uses of volatile oils The total annual production of volatile oils is estimated to be in the region of 45,000 tones, worth approximately US$700 million. There are about 100 commercially valuable volatile oils directly derived from plants. Volatile oils are used for their therapeutic action , for flavoring (e.g. oil of lemon), in perfumery (e.g. oil of rose) or as starting materials for the synthesis of other compounds (e.g. oil of turpentine).

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For therapeutic purposes they are administered as inhalations (e.g. eucalyptus oil), orally (e.g. peppermint oil), as gargles and mouthwashes (e.g. thymol) and transdermally (many essential oils including those of lavender, rosemary and bergamot are employed in the practice of aromatherapy). Those oils with a high phenol content, e.g. clove and thyme have antiseptic properties, whereas others are used as carminatives. Oils showing antispasmodic activity, and much used in popular medicine.

Composition of volatile oils:

Composition of volatile oils Volatile oils are generally mixtures of hydrocarbons and oxygenated compounds derived from these hydrocarbons. The odour and taste of volatile oils is mainly determined by these oxygenated constituents, which are to some extent soluble in water but more soluble in alcohol.

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Practically all volatile oils consist of chemical mixtures that are often quite complex; they vary widely in chemical composition. Almost any type of organic compound may be found in volatile oils (hydrocarbons, alcohols, ketones, aldehydes, ethers, oxides, esters, and others). “usually volatile oils are classified according to the type of organic compounds”. It is not uncommon for a volatile oil to contain over 200 components, and often the trace constituents are essential to the odor and flavor. The absence of even one component may change the aroma.

Biosynthesis and chemical composition:

Biosynthesis and chemical composition Chemical constituents of volatile oils may be divided into 2 broad classes, based on their biosynthetic origin: Terpene derivatives formed via the acetate-mevalonic acid pathway. Aromatic compounds formed via the shikimic acid-phenylpropanoid route.

Terpene derivatives:

Terpene derivatives

Aromatic compounds:

Aromatic compounds

Physical properties:

Physical properties Although volatile oils differ greatly in their chemical constitution, they have a number of physical properties in common: They possess characteristic odors. They are characterized by high refractive indices. Most of them are optically active. Their density is generally lower than that of water (the essential oils of sassafras, clove, or cinnamon are the exceptions). As a rule, volatile oils are immiscible with water, but they are sufficiently soluble to impart their odor to water. The aromatic waters are dependent on this slight solubility.

Volatile oils  Fixed oils:

Volatile oils  Fixed oils Several points of differentiation exist between volatile oils and fixed oils . Volatile oils can be distilled from their natural sources . Volatile oils do not consist of glyceryl esters of fatty acids . Hence, they do not leave a permanent grease spot on paper and cannot be saponified with alkalies. Volatile oils do not become rancid as do the fixed oils, but instead, on exposure to light and air, they oxidize and resinify.

Preparation of volatile oils:

Preparation of volatile oils The principal methods used in the preparation of volatile oils from plants depend on: Distillation in water or steam. Scarification and expression. Extraction with solvents. Enzymatic hydrolysis ( for glycosidic volatile oils e.g. mustard oil ) . Enfleurage ( extraction of oils used in perfumery ) .

Uses of drugs containing essential oils:

Uses of drugs containing essential oils Pharmacy Perfumery Food technology Miscellaneous industries (as starting materials for the synthesis of the active principles of medicines, vitamins, and fragrances).

Resins and resins combination:

Resins and resins combination

Resins and resins combination:

Resins and resins combination The term ‘resin’ is applied to more or less solid, amorphous products of complex chemical nature. On heating they soften and finally melt. They are insoluble in water and usually insoluble in petroleum ether but dissolve more or less completely in alcohol, chloroform and ether.

Chemical composition:

Chemical composition Chemically , resins are complex mixtures of resin acids, resin alcohols (resinols), resin phenols (resinotannols), esters and chemically inert compounds known as resenes.

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Resins are often associated with volatile oils ( oleoresins ), with gums ( gum-resins ) or with oil and gum ( oleo-gum-resins ). Resins may also be combined in a glycosidal manner with sugars. Resins burn with a characteristic, smoky flame.


Balsams Balsams are resinous mixtures that contain large proportions of cinnamic acid, benzoic acid or both or esters of these acids. The term “balsam” is often wrongly applied to oleoresins and should be reserved for such substances as balsam of Peru , balsam of Tolu and storax , which contain a high proportion of aromatic balsamic acids.

Preparation of resins:

Preparation of resins Two general classes of resinous substances are recognized and this classification is based on the method used in preparing them: Natural resins , occur as exudates from plants, produced normally or as result of pathogenic conditions, as for example by artificial punctures e.g. mastic; or deep cuts in the wood of the plant e.g. turpentine, or by hammering and scorching, e.g. balsam of Peru.

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Prepared resins ; are obtained by different methods. The drug containing resins is powdered and extracted with alcohol till exhaustion. The Concentrated alcoholic extract is either evaporated, or poured into water and the precipitated resin is collected, washed and carefully dried. In the preparation of oleoresins ; ether or acetone having lower boiling point are used. The volatile oil portion is removed through distillation. When the resin occurs associated with gum ( gum-resins ), the resin is extracted with alcohol leaving the gum insoluble.

Classification of resins:

Classification of resins Resins are classified in three different ways: Taxonomical classification , i.e. according to botanical origin, e.g. Berberidaceae resins. Classification according to predominating chemical constituent ; e.g. acid resins, resene resins, glycosidal resins; etc. Resins may be classified according to the portion of the main constituents of the resin or resin combination ; e.g. resins, oleoresins, oleogumresins, balsams.

Examples of resins and resins combination:

Examples of resins and resins combination Resins : colophony, cannabis. Oleoresins : copaiba, ginger. Oleo-gum-resins : asafoetida, myrrh. Balsams : balsam of Tolu, balsam of Peru.

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