Esters in essential oils

Essential oils are obtained from fmits and flowers (61,62). Volatile esters of short- and medium-chain carboxyHc acids or aromatic carboxyHc acids with short- and medium-chain alcohols are primary constituents of essential oils, eg, ethyl acetate in wines, brandy, and in fmits such as pineapple ben2yl acetate in jasmine and gardenia methyl saHcylate in oils of wintergreen and sweet birch. Most of these naturally occurring esters in essential oils have pleasant odors, and either they or their synthetic counterparts are used in the confectionery, beverage, perfume, cosmetic, and soap industries (see Oils, essential). 

Perfumes, Flavors, Cosmetics, and Soap. Many naturally occurring esters in essential oils and some synthetic esters are important fragrance and flavor compounds (61,62). They are used in perfumes, flavors, cosmetics, soaps, detergents, and air fresheners. Benzyl, butyl, ethyl, methyl, and phenyl esters of benzoic acid are used as flavors, perfumes, and food preservatives. Glyceryl 4-aminobenzoate [136-44-7] and 2-ethyUiexyl 4-dimethylaminobenzoate [21245-02-3] are used in cosmetic sunscreen preparations. Alkyl esters of 4-hydroxybenzoic acid, called parabens, have been used under various names for fungus infections of the skin, and as preservatives in lotions and creams (101). Soap and cosmetic fragrances use large amounts of amyl and benzyl saHcylate. Benzyl saHcylate [118-58-1] is also used in deodorant sprays. 2-Ethylhexyl saHcylate [118-60-5] and 2-ethylhexyl 4-methoxycinnamate [5466-77-3] are used in sunscreen formulations (102). 

The Detection of Artificial Esters in Essential Oils.—The custom of valuing certain essential oils, such as lavender, bergamot, geranium, petit-grain, etc., by the determination of their ester-content, has led to the use of scientific adulterants in the form of artificial esters which have been deliberately employed for the purpose of misleading the analyst. Of course, the ester determination is not a true criterion of value, as most of this class of oUs owe their perfume value to various other bodies as well. The first compounds of this nature employed for adulteration were ethyl succinate and ethyl oxalate. For the detection of these in lavender oil the foUowing test was proposed by Guildemeister and Hoffman —... 

Occurs in form of esters in essential oils and plant prods, e.g. oil of wintergreen. Used as an antiseptic and antifungal agent, and for various skin conditions. Keratolytic. Simple esters are perfumery and flavouring ingredients. Used as aq. soln. for photometric detn. of Fe, Zr, NH3, Sc indirect photometric-detn. of F extraction separation of Sc. Needles (H2O). Mod. sol. H2O. Mp 159°. BP20 211°. pA ,i 2.98 pK 2 13.6 (25°). Steam volatile. Subl. in vacuo. 

Polyunsaturated fatty acids in vegetable oils, particularly finolenic esters in soybean oil, are especially sensitive to oxidation. Even a slight degree of oxidation, commonly referred to as flavor reversion, results in undesirable flavors, eg, beany, grassy, painty, or fishy. Oxidation is controlled by the exclusion of metal contaminants, eg, iron and copper addition of metal inactivators such as citric acid minimum exposure to air, protection from light, and selective hydrogenation to decrease the finolenate content to ca 3% (74). Careful quality control is essential for the production of acceptable edible soybean oil products (75). 

Bodies of an alcholic nature play a very important part in both natural and synthetic perfumery. They are found to a very large extent in essential oils, both in the free state and also in the form of esters. Some that have not so far been recognised as constituents of essential oils, have been found to be so highly odorous, and so useful as perfume materials, that they are prepared artificially, and enter largely into the composition of the synthetic perfumes which to-day are indispensable to the manufacturer of perfumes. It is obvious that those alcohols which are soluble in water, such as methyl and ethyl alcohols, although they may be original constituents of some essential oils, are removed by the ordinary distillation processes, so that they do not, in fact, appear in the essential oil as found in commerce. 

Linalol is found very widely distributed in essential oils. It forms the principal constituent, in the free state, of oil of linaloe, and the chief odorous constituent, in the form of esters, in bergamot and lavender oils. It is also found in ylang-ylang, rose, champaca leaf, cinnamon, petit-grain, spike, geranium, lemon, spearmint, and numerous. other essential oils. 

There is an isomeric and closely associated alcohol, phenyl-methyl carbinol, CgH5CH OH)CHg, known to chemists. This is a liquid of different odour, but which is not used very much in synthetic perfumery. It is an oil boiling at 203°, and forms an acetate which is found naturally in essential oil of gardenia. This ester is of use in blending perfumes of this type of flower. 

Ethyl Benzoate.—This ester has not been found, so far, to occur naturally in essential oils. It has, however, been prepared by synthetic processes, for example, by condensing ethyl alcohol with benzoic acid by means of dry hydrochloric acid gas. Its odour is very similar to that of methyl benzoate (q.v.), but not quite so strong. It is an oil of specific gravity I OfilO, refractive index 1 5055, and boiling-point 213° at 745 mm. It is soluble in two volumes of 70 per cent, alcohol. 

As the free acids present in essential oils consist in the main of acetic acid, they are, when necessary, calculated in terms of acetic acid in the same way the esters are conventionally calculated from the alkali required for their hydrolysis, in terms of the principal ester present, for example, linalyl acetate in the case of lavender and bergamot oUs, and geranyl tiglate in the case of geranium oil. 

The molecular weights of the esters commonly found in essential oils are as follows —... 

The natural esters present in essential oils are usually those of acetic, butyric, and valerianic acids, and in the case of geranium oil, tiglate acid. 

The determination of alcohols in essential oils depends on the conversion of these compoimds into their acetic esters, and then carrying out an ester determination as described above. 

Such steam heating is even, and avoids the risk of overheating and decomposition that can occur in hot spots when external heating is used. The steam condenses back into water and the droplets coagulate to form liquid oil. Esters and essential oils do not mix with water, so phase separation occurs on cooling, and we see a layer of oil forming above a layer of condensed water. The oil is decanted or skimmed off the surface of the water, dried, and packaged. 

Many short chain fatty acids, aldehydes, alcohols, esters, ketones and hydrocarbons are produced by metabolism of fatty acids (Cie-Cis). These compounds are common in essential oils and are also found in insects. 

Esters of benzenoid and monoterpenic acids and alcohols as well as unsaturated carboxylic acids such as tiglic acid 142 and angelic acid 143 (Structure 4.43) are found in essential oils [60, 61]. 

Benzyl alcohol (1) occurs widely in essential oils both as the free alcohol, and, more importandy from a fragrance standpoint, in the form of various esters. Although benzyl alcohol itself is rather bland in odor, combined with its much more fragrant esters it is an important part of the odor of jasmine, ylang-ylang, gardenia, some rose varieties, narcissus and peony, as well as castoreum, balsams of peru and tolu, and propolis. Benzyl alcohol occurs primarily in flower oils and tree exudates, whereas a large number of essential oils obtained from other parts of a wide variety of plants contain no benzyl alcohol or its esters (1). 

Benzyl alcohol [CAS 100-51-6]. (1) occurs widely in essential oils both as the free alcohol, and. more importantly from a fragrance standpoint, in the form of various esters. 

Acids are important because they react with alcohols to form esters. Many esters present in essential oils are formed as reaction products during the distillation process in extraction. 

Lactone An organic compound that contains an ester group incorporated into a carbon ring structure. Lactones include the coumarins, e.g. umbelliferone or 7-hydroxycoumarin. Lactones and furocoumarins, e.g. bergaptene, exist in small quantities in essential oils and should be used with care. Lactones can be neurotoxic (poisonous to the nervous system) and cause skin allergies. Bergaptene is well known for its phototoxicity on the skin. 

Sucrose Acetate Isobutyrate occurs as a clear, pale yellow, viscous liquid. It consists of a mixture of sucrose esters of acetic and isobutyric acid, with sucrose diacetate hexaisobu-tyrate being the predominant sucrose ester. It is produced through the controlled esterification of sucrose with acetic anhydride and isobutyric anhydride. It is very soluble in essential oils such as orange oil, soluble in ethanol and in ethyl acetate, and very slightly soluble in water. 

Esters.— The simplest class of compounds present in essential oils are the esters or ethereal salts (p. 140). In our early discussion of these compounds in the aliphatic series it was stated that the odor and flavor of common fruits is probably due to ester compounds and that certain empirical mixtures of esters are used as artificial fruit essences. Artificial apple essence, for example, may be prepared by mixing certain proportions of ethyl nitrite, ethyl acetate and amyl valerate with chloroform, aldehyde and alcohol. An example of an essential oil which consists of a single ester is oil of wintergreen. vAdxh is the methyl ester of salicylic acid, ortho-hydroxy benzoic acid (p. 714). 

There has been little scientific evaluation of aromatherapy oils but it is known that the terpenoid and phenylpropane compounds found in essential oils do possess biological activities ranging from antibacterial, antiinflammatory, sedative etc. There is evidence that molecules such as terpenoid hydrocarbons, esters etc. can be absorbed transdermally into the blood-stream. In addition, the massage element involving physical manipulation of strained tissues as well as the psychological effects of touch and of the smell of the oil, is of significance. Some trials of aromatherapy have shown benefits in intensive-care patients, in epilepsy and in endometriosis. The most popular oils are listed below with their claimed uses. 

Acyclic monoterpenes geraniol in essential oils, perfume products and luxury foods, production from other terpene products, e.g. beta-pinene acyclic, double unsaturated alcohol, several possible reactions, occurrence as esters, typicjd rose odor... 

Although the name essential oil and its variation ethereal oil both refer to the volatility of these compounds, this is not always accurate since the strong scent is in fact caused by evaporating molecules even if they are less volatile than e.g. water. Most components in essential oils are Itydrocaibons with 10-15 carbon atoms. Their composition is very different from those of fixed (or fatty) oils (sunflower oil, olive oil), which are glycerol esters of fatty acids. These fatty oils are not volatile and they do not have pleasant (or other) scents, either. 

Esters The esters that naturally occm in essential oils usually provide the fruity and fragrant character to the oil. Some esters are reported to have antifungal... 

The methyl ester of salicylic acid is encountered commonly in essential oils and is especially abundant in the oil of wintergreen (Gaultheria procumbens, Ericaceae). Methyl salicylate occurs as a glycoside but is hydrolyzed upon injury to the plant. This ester is widely used as a flavoring in candies and as a perfumery ingredient. It is a major component of liniments and is reputed to have antirheumatic properties. 

Most acyclic sesquiterpenes are derived from famesol and nerolidol (7) or from the corresponding pyrophosphate esters. Although the number of acyclic sesquiterpenes is relatively small, Z- and -famesol (5 and 6), nerolidol (7), and the corresponding olefins are relatively common in essential oils. The famesenes and a few other compounds, such as 3-sinesal (8), are derived in a straightforward manner, but most acyclic sesquiterpenes are more complex in origin (Fig. 21.4). 

Linalool, coriandrol a doubly unsaturated mono-terpene alcohol found in essential oils. Both optical isomers occur naturally. L. is an oil with a scent resembling that of lily of the valley. M, 154.25, (-h)-L. b.p. 194-198 °C, (o)d-19.4 . The pure alcohol and its esters are used in perfumery. For formula, see Ger-aniol. 

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