Chemical analysis of volatile oils
Volatile oils (ethereal oils, or essential oils) are the odourous principals found in various plant parts, because they evaporate when exposed to the air at ordinary temperature. The term is applied because volatile oils represent the “essences” or odouriferous constituents of the plants. Spices are dried, fragrant, aromatic, or pungent plant parts that contain a volatile oil.
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, and others), and only a few possess a single component in a high percentage (clove oil contains not less than 85% of phenohc substances, chiefly eugenol). However, it is not uncommon for a volatile oil to contain over 200 components, and often the trace constituents are essential to the odour and flavour. The absence of even one component may change the aroma. Plants of the same species grown in different parts of the world usually have the same components, but the percentages that are present may differ.
Classification of volatile oils and volatile oils containing MPM is based on the structure of most valuable components: Monoterpenoids are composed of two units and have the molecular formula, СюНіб. Sesquiterpenoids, C15H24, contain three isoprene units. Diterpenoids, C20H32, have four isoprene units. Triterpenoids, С30Н48, are composed of six isoprene units, and tetraterpenoids or carotenoids, C40H6O, have eight units.
The terpenoids are fiirther categorized as acychc (open chain), monocyclic (one ring), bicyclic (two rings), tricyclic (three rings), etc. and can be not only isoprene oligomers but also saturated or partially saturated isomers as well as oxygenated derivatives such as alcohols, aldehydes, ketones, phenols, ethers.
Моnoterpenoids are structurally diverse and have been classified into over 35 different structural types. The structural types that occur most frequently are:
acyclic monoterpenoids of myrcene type: myrcene, ocymene (carbohydrates), geraniol, linalool (alcohols), cytral (aldehyde);
monocyclic monoterpenoids of p-menthane type: limonene, Іеrpіпепе, phelandrene (unsaturated hydrocarbons), menthol, terpineol (alcohols), menthone, carvon (ketones), cineol (oxide), ascaridol (peroxide);
bicyclic monoterpenoids of carane, fenchane, pinane, camphane and thujane types. Compounds of thujane type are thujol, thujon, sabinene, sabinon; of carane type – carane, careen; pinane type – pinane, pinene; camphene – bomeol, camphane, camphor.
Sesquiterpenoids have a wide distribution iature and form the largest class of terpenoids. The main representatives include acyclic sesquiterpenoids (pharnesene, pharnesole), monocyclic (bisabolane, humulane, elemane), bicyclic (cadinane, eudesmane, guajane, and the derivatives of the latter – chamazulene, azulene, guajol).
Aromatic components of volatile oils are derivatives of p-cymene (terpinene), benzene (benzaldehyde, vanillin), phenylpropane (anethol, eugenol). Pleasant odour may be caused by ethers (anethol, piperonal), aldehydes (anise aldehyde, vanillin), alcohols (thymol, carvacrol).
Physicochemical properties. Although volatile oils differ greatly in their chemical constitution, they have a number of physical properties in common. They possess characteristic odours, they are characterized by high refractive indices, most of them are optically active, and their specific rotation is often a valuable diagnostic property. The boiling points vary from 140°C-180°C (monoterpenes) to 240°C (sesquiterpenes). Other physical characteristics are densities from 0.759 to 1.096. As a rule, volatile oils are immiscible with water; however, they are 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 alkalies. Volatile oils do not become rancid, as do the fixed oils, but instead, on exposure to light and air, they oxidize and resinify.
Extraction. Volatile oils are usually obtained by steam distillation of the plant parts containing the oil. The method of distillation depends on the condition of the plant material. Some volatile oils cannot be distilled without decomposition and are usually obtained by expression (lemon oil, orange oil) or possibly by other mechanical means. The process, known as enfleurage was formerly used extensively in the production of perfumes and pomades (After the fat has absorbed as much fragrance as possible, the oil may be removed by extraction with alcohol). In the perftime industry, most of the modern essential oil production is accomplished by extrac tion, using solvent systems based on such volatile solvents as petroleum ether or benzene.
Detection. Certain physical constants of volatile oils are significant: specific gravity, refractive index and sometimes optical rotation, solubility in alcohols.
The EP requires the following tests on quality control of volatile oils: water in essential oils, fatty oils and resinified essential oils in essential oils, odour and taste of essential oils, residue on evaporation of essential oils, solubility in alcohol of essential oils.
A number of quantitative tests are commonly used to evaluate volatile oils: acid value, ether value, ether value after acetylation.
Chromatographical analysis. The volatility of the simple terpenes means that they are ideal subjects for separation by GLC (gas – liquid chromatography). Many have fragrant odours and indeed can often be recognized in plant distillates directly, if present as the major constituent.
TLC can be used with advantage in combination with GLC for the analysis of terpenes, since the two techniques are complementary. The systems toluene-ethyl acetate (95.5,v=v) and toluene-ethyl acetate (93.7, v=v) have been proven to be suitable for the analysis of most important essential oils. General methods of detection include spraying with 0.2% aqueous КМПО4, 5% antimony chloride in chloroform, concentrated H2SO4, or vanillin- or anisaldehyde- H2SO4. The plates are heated after spraying until fiall development of colours has occurred.
Quantitative determination. The determination of essential oils in vegetable drugs is carried out by steam distillation in a special apparatus. The distillate is collected in the graduated tube, using xylene to take up the essential oil; the aqueous phase is automatically returned to the distillation flask.
Biological activity. Antiseptic activity is against various pathogenic bacteria, including strains that are usually resistant to antibiotics. A large number of essential oil drugs (mint) are thought to be efficacious in decreasing or suppressing gastrointestinal spasms. Frequently, they stimulate gastric secretion. When used externally, products such as turpentine cause an increase in capillary blood flow, a sensation of heat, and in some cases, a slight local anesthetic activity. There are many ointments, creams, or gels based on essential oils, and designed to relieve sprains, soreness, strains, and other joint or muscular pains. When administered internally, essential oils are thought to trigger “irritation” processes at different levels. Thus, the oils of eucalyptus and pine are thought to stimulate mucus cells, and to increase the motility of the ciliated epithelium in the bronchia; other oils are thought to enhance the renal excretion of water by a direct local effect (juniper). Other activities are attributed to essential oils are choleretic, healing, spasmolytic and sedative. Essential oils are also the underpinning of a specific therapy: “aromatherapy”.
Theme: Chemical analysis of MPM containing essential 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. VO 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 alkalies. VO do not become rancid as do the fixed oils, but instead, on exposure to light and air, they oxidize and resinify.
Volatile oils (VO) are miscellaneous volatile organic compounds which are produced in plants and cause their specific odour.
Practically all VO 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 VO (hydrocarbons, alcohols, ketones, aldehydes, ethers, oxides, esters, and others), and only a few possess a single component in a high percentage (clove oil contains not less than 85% of phenolic substances, chiefly eugenol). However, it is not uncommon for an essential oil to contain over 200 compounds, and often the trace constituents are essential to the odour and flavour.
VO can be divided into 3 main groups:
– monoterpenoids (acyclic, monocyclic and bicyclic);
– sesquiterpenoids (acyclic, monocyclic, bicyclic, tricyclic);
– aromatic compounds (derivatives of n-cimen, benzol and phenilpropan).
Monoterpenoids are composed of two units and have the molecular formula C10H16. By the number of cycles monoterpenoids are divided into acyclic, monocyclic and bicyclic.
Acyclic monoterpenoids:
Mircen
Monocyclic terpenoids:
Menthol Cineole
Bicyclic monoterpenoids have two condensed non-aromatic rings and due to their structure are divided into 3 groups: group of tuyane, carane, pinane, camphene, phenkhan etc.
Pinane group:
α –pinene β –pinene
Camphan group:
Sesquiterpenoids, C30H48, contain three isoprene units. As monoterpenoids they are also divided into acyclic, monocyclic (most commonly occur in Nature as bisabolane and gumulane types), bicyclic (main types are cadinane, evdesmane and guayane) and tricyclic forms.
A single group of sesquiterpenoids are presented by sesquiterpene lactones:
Aromatic compounds contain derivatives of n-cymene, bezene (bezaldehyde, vanillin) and phenylpropan derivatives (anethol, eugenol etc.):
Thymol Anethol Carvacrol
Occurrence
Essential oils are of wide occurrence in Nature. More than 2500 of plants can accumulate them. Tropical plants are rich in VO. Usually they occur in such families as Lamiaceae, Apiaceae, Asteraceae and Rosaceae. The content may rich up to 20% (Citrus fruits). Ether oils are produced by special exo– and endogenic organs. First are formed from epidermal cells- glandular “spots”, glandular hairs and ether-oil glands, they are situated at petals of rose, heartsease and lily of the valley. To endogenic organs belong secretory cells, secretory cavities and secretory ducts or canals and they are formed in parenchyma.
Physicochemical properties
Ether oils are clear colourless or pale-yellow liquids with specific aromatic odour and bitter, spicy taste. Some of them are blue due to the content of azulene (oil of yarrow, absinthium, matricaria), green (bergamot oil), red (caraway oil). pH is usually neutral or acid. Many oils after cooling off can form crystals (anise oil, spearmint oil, camphor oil).
They are soluble in alcohol, insoluble in water.
Methods of obtaining of volatile oils
VO are usually obtained by steam distillation of the plant parts containing the oil. The method of distillation depends on the condition of the plant material. In the method of direct steam distillation, applicable to fresh plant drugs (peppermint, spearmint), the crop is cut and placed directly into a metal distilling tank on a truck bed.
Some VO caot be distilled without decomposition and are usually obtained by pressing (lemon oil, orange oil) or possibly by other mechanical means.
Often the volatile oil content of fresh plant parts (flower petals) is so small that oil removal is not commercially feasible by the mentioned methods. In such instances, an odourless, bland, fixed oil or fat is spread in a thin layer on glass plates. The flower petals are placed on fat for a few hours; then, repeatedly, the old petals are removed, and a new layer of petals is introduced. After the fat has absorbed as much fragrance as possible, the oil may be removed by extraction with alcohol. This process is known as enfleurage and was formerly used extensively in the production of perfumes and lipsticks.
In the perfume industry, most of the modern essential oil production is accomplished by extraction, using solvent systems based on such volatile solvents as petroleum ether or benzene. The chief advantage of extraction over distillation is that uniform temperature (usually 50°C) can be maintained during most of the process. As a result, extracted oils have a more natural odour that is unmatched by distilled oils, which may have undergone altered chemical constitution by the high temperatures. These features are of considerable importance to the perfume industry; however, the established distillation method is a low-cost operation compared to the cost of the extraction process.
Medicinal and commercial uses
Many crude drugs are used medicinally because of their volatile oil content; however, in numerous cases, the volatile oils separated from the drugs are used as drugs themselves. Similarly, various crude drugs are powdered and are employed as species and condiments (anise, clove, nutmeg). The volatile oil drugs and the separated oils are most commonly used for flavouring purposes. They may possess a carminative action, but a few (eucalyptus oil, winter-green oil) possess additional therapeutic properties. In addition to their pharmaceutical uses, the VO are employed widely as flavours for foods and confections and in the spice, perfume and cosmetic trades. The fabrication of perfumes is a multimillion dollar industry. VO are also used for the manufacturing of soaps, toiletries, and deodorizers and for masking or providing odour to household cleaners, polishes, and insecticides.
Evaluation
Stages of examination of VO include organoleptic analysis (determination of colour, taste, odour, transparency and consistency) and determination of physical and chemical constants.
Physical measurements include specific gravity, optical rotation, refractive index, and solubility in alcohol.
Main chemical measurements are acid value, ester value and saponification value.
Acid number is the amount of potassium hydroxide in milligrams that is needed to neutralize the free acids in 1 gram of volatile oil.
The ester value indicates the number of milligrams of potassium hydroxide required for the saponification of the esters in 1 g of a volatile oil.
The ester number after saponification helps to determine free alcohols in ether oil.
Gas and gas-liquid chromatography are also used for assessment of purity of VO.
