Volatile Oils 2024-2025 - Part 1 PDF

VOLATILE OILS Introduction: Volatile oils are the odorous principles found in various plant parts. They are called volatile oils or ethereal oils as they evaporate when exposed to air at ordinary temperatures. They are also known as essential oils as they represent the “essences” or the odor...

VOLATILE OILS Introduction: Volatile oils are the odorous principles found in various plant parts. They are called volatile oils or ethereal oils as they evaporate when exposed to air at ordinary temperatures. They are also known as essential oils as they represent the “essences” or the odorous constituents of plants. Volatile oils are colorless as a rule, particularly when they are fresh, but on long standing they may oxidize and resinify , thus darkening in color. To prevent this darkening, they should be stored in a cool, dry place in tightly stoppered, preferably full (not half- emptied), amber glass containers. Depending on the plant family, volatile oils may occur in specialized secretory structures such as glandular hairs (Labiatae), modified parenchymal cells (Piperaceae), oil-tubes called vittae (Umbeiliferae), in lysigenous or schizogenous passages (Pinaceae, Rutaceae). volatile oils may occur in all tissues; in the rose they appear in appreciable quantities only in the petals; in cinnamon, only in the bark and the leaves; in the umbelliferous fruits, only in the pericarp; in the mint, only in the glandular hairs of the stems and leaves; and in the orange, one kind of oil occurs only in the flower petals and another kind only in the rind. chemical constituents of volatile oils may be divided into 2 broad classes based on their biosynthetic origin: (1) terpene derivatives formed via the acetate-mevalonic acid pathway. (2) aromatic compounds formed via the shikimic acid-phenyl propanoid route. physical properties of volatile oils they possess characteristic odors, they are characterize by high refractive index, most of them are optically active and their specific rotation is often a valuable diagnostic property also vol. oils are immiscible with water but soluble in ether, alcohol and most organic solvents. Several points of differentiation exist between volatile oils and fixed oils : Volatile oils can be distilled from their natural sources; they 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 alkalis. Volatile oils do not become rancid as do the fixed oils but instead, on exposure to light and air they oxidize and resinify. uses of volatile oils 1- It is used as flavoring agents 2- Local irritant ex : pinene and camphor 3- Anesthetic ex : clove oil which contains eugenol 4- Prophylactic against insects ex : citronella oil 5- Antibacterial and antiseptic ex : ginger, lemon, rosemary, peppermint 6- Against asthma (expectorant action) 7- Carminatives ex : peppermint oil 8- Urinary antiseptic ex : Buchu leaves 9- Anthelmintic ex : ascaridol from chenpodium oil. Evaluation of volatile oils Several physical & chemical properties are used to evaluate the v. oils. Physical properties 1- solubility 2- specific gravity 3- refractive index : when a source of light pass from less dense to more dense medium it reflect or bent towards the normal. It is measured by refractometer. 4- optical rotation : most essential oils when placed in a beam of polarized light possess the property of rotating the plane of polarization to the right or to the left. The optical activity of volatile oils is determined by polarimeter. Chemical properties 1- determination of acid 2- determination of ester 3- determination of alcohol 4- determination of ketone & aldehyde 5- determination of phenol Methods of obtaining volatile oils 1- Distillation : Volatile oils are usually obtained by distillation of the plant parts containing the oil. The method of distillation depends on the condition of the plant material. Some important types of distillation include: Simple distillation.  Fractional distillation.  Steam distillation.  Vacuum distillation.  Air-sensitive vacuum distillation.  Short path distillation.  Zone distillation.  2- Expression method: Cold expression : It is for obtaining volatile oils from citrus fruits ex: lemon, orange. This method is used to avoid the decomposition of volatile oils, which will necessarily take place by any other distillation method. 3- Extraction : volatile oils are readily soluble in most organic solvents but the disadvantage of this method is that the organic solvent also dissolve the wax & coloring matter present in the crude drugs 4- Enzymatic hydrolysis: Glycosidic volatile oils like bitter almond, mustard oil is obtained by enzymatic hydrolysis of glycosides. In bitter almond seeds amygdalin is acted upon by enzyme emulsin resulting in a mixture of constituents from which the volatile oil may be distilled with steam. In black mustard seeds the glycoside, sinigrin is hydrolyzed by the enzyme myrosin with the product of volatile mustard oil. 5- Enfleurage: This method is especially used for those volatile oils, which are present in such a part which is very small and also liable to decomposition on distillation. In this case odorless and bland fixed oil or fat is spread in a thin layer on glass plates. The part of plant from which volatile oil has to be extracted say for example flower petal is placed in the fat or fixed oil for some time until it’s fragrance is removed as the oil or fat will absorb it. Then the petal is removed from the fixed oil or fat and is subjected to extraction with alcohol. CHEMISTRY OF VOLATILE Oils In only a few cases , volatile oil consists of a single chemical compound , in most cases , they are mixtures of compounds of diverse types. 1- Many volatile oils consist of terpenes. Terpenes are defined as natural products whose structures may be divided into isoprene units (C5H8). These units arise from acetate via mevalonic acid & are branched-chain , 5-carbon units containing two unsaturated bonds. During the formation of terpenes , the isoprene units are linked in a head to tail fashion , the number of units incorporated into a particular terpene make the bases for the classification of the compounds. Monoterpenes are composed of two isoprene units & have the molecular formula C10H16. Sesquiterpenes C15H24 , contain three isoprene units. Diterpenes C20H32 have four isoprene units & triterpenes C30H48 are composed of six isoprene units. The terpenes found most often in volatile oils are monoterpenes. They can occur in acyclic , monocyclic & bicyclic forms as hydrocarbons & as oxygenated derivatives , such as alcohols , aldehydes , ketones , phenols , oxides & esters. 2- Another major group of volatile oil constituents are phenyl propanoids. These compounds contain the C6 phenyl ring with an attached C3 propane side chain. Many of the phenyl propanoids found in volatile oils are phenol or phenol ethers. Biosynthesis of volatile oil constituents The biosynthetic building blocks for terpenes are isoprene units. Isoprene units are isopentenyl pyrophosphate & dimethyl allyl pyrophosphate , compounds that arise from acetate via mevalonic acid. Geranyl pyrophosphate is the C-10 precursor of the terpenes & is believed to play a key role in the formation of monoterpenes. Geranyl pyrophosphate is believed to be the direct precursor to acyclic monoterpenes. However, it must be isomerized to nearly pyrophosphate before the cyclic monoterpenes can be formed because the trans isomer does not have the correct stereo-chemistry for cyclization. Hypothetic mechanism for biosynthetic formation of terpenes. The principal precursors for phenylpropanoid compounds, which are found in volatile oils, are cinnamic acid and p-hydroxycinnamic acid, also known as p-coumaric acid. In plants, these compounds arise from the aromatic amino acids phenylalanine and tyrosine, respectively, which in turn are synthesized via the shikimic acid pathway (Fig. 5-5).