ESSENTIAL OILS. The terms essential, ethereal or volatile oils are applied to volatile odoriferous bodies of an oily character, obtained almost exclusively from vegetable sources. They must be distinguished from the two other great groups of naturally occurring substances with which they share the general name "oil," or for none but rather superficial reasons. Its minerals differ from them in consisting of hydro-carbons (compounds of carbon and hydrogen without oxygen; see PETROLEUM) ; the ani mal or vegetable oils, on the other hand, are oxygen-containing substances consisting mainly of esters (q.v.) belonging to the class of glycerides (q.v.), that is, compounds of glycerine (q.v.) or fatty acids (q.v.) ; they are discussed in the article OILS, FATS AND WAXES, and the more important of them in individual articles.
The essential oils of plants are present, as a rule, in very small amounts; in cloves there is as much as 16-18% of oil, but rose petals yield as little as o• 2 % of the essential oil, while from jasmine blossoms only A. of this quantity can be obtained. The oils may be found in special cells, glands or ducts, in one particular organ, or distributed over many parts of the plant, such as leaves, bark, roots, flowers or fruits. From the work of Charabot and others it appears that the oils are actually formed in the green (chlorophyll-bearing) parts of the plant, possibly as a result of pathological processes or as by-products of plant metabolism. As the plant matures the oils are trans ported to other tissues, in particular the flowering shoots, where secondary changes (oxidation, esterification, etc.) in the con stitution of the oils may occur. In the case of the plants (lavender, peppermint, etc.) studied by Charabot, the oil content reached a maximum immediately before fertilisation of the flowers. At this stage there was notable consumption, or loss, of oil, the proportion of which further decreased as the fruits ripened. The quantity and composition of the essential oils of plants may vary with the conditions of cultivation, e.g., light, climate, soil, alti tude, etc., as is instanced by the different ester content of oils from lavender grown in England and the south of France.
Occasionally the odoriferous ethereal oils are not present as such in the living plant, but occur as scentless compounds of sugars termed glucosides, e.g., amygdalin in bitter almonds, gaultherin in wintergreen, sinigrin of mustard seeds. When the tissues are macerated the ferment action of the enzymes present in the plant decomposes the glucosides, liberating the character istically odorous ethereal oil.
The function of the oils in the plant is uncertain; they may be merely excretory by-products (e.g., resins, oil of turpentine), or secretions for specific purposes, for example, to attract insects to the flowers : the high consumption of essential oil during the period of fecundation points to a possible nutritive value.
The animal secretions musk, ambergris, civet and castor, are considered in the separate articles under these headings and under PERFUMERY. The crude secretions are used in perfumery and no essential oils are separated. It is doubtful whether the odour of these bodies is due to volatile oils ; recently, however, certain characteristic aromatic ketones have been isolated.
The volatile oils are mostly insoluble in water, but freely soluble in alcohol, ether, fatty and mineral oils. As a rule they are not oleaginous to the touch and leave no per manent grease-spot. They are usually liquid at ordinary tem peratures; some, on standing, deposit solid matter (stearoptenes) which crystallise from the liquid remainder (elaeoptene), e.g., oil of camphor, rose oil. In specific gravity the ethereal oils range from o•85 to 1.142 ; they possess a high refractive power and the majority rotate the plane of polarised light.
The natural volatile oils consist usually of mixtures of several chemical individuals; the properties of the various components are in accordance with their chemical constitution, which de termines the means to be employed for their identification and estimation.
The present state of our knowledge does not permit of strict correlation of odour and chemical constitution. The hydrocarbons have, on the whole, no pronounced odour; scent appears with the introduction of oxygen, nitrogen or sulphur in certain groups such as hydroxyl (OH), aldehyde (—CHO), nitrile (—CN), isothio cyanate (—SCN), etc., to the molecule, but particular groupings cannot be associated with particular types of odour. Different groups may have practically equal effects, for instance, benzalde hyde
have almost identical odours. In the case of the synthetic musks, symmetrical arrange ment in the benzene nucleus of the three nitro- (or substituted nitro- groups) seems to be essential to the production of odour.
The components of the ethereal oils are, as a rule, compounds of hydrogen, carbon and oxygen only; hydro carbons occur to a considerable extent in some oils, e.g., pinene in oil of turpentine ; nitrogen and sulphur compounds are also found (see below). The constituents of the volatile oils may be classed in the following groups according to their chemical nature : (I) Hydrocarbons, e.g., styrolene in storax; the terpenes (q.v.), e.g., pinene in oil of turpentine, limonene in lemon and orange-peel oils. (2) Alcohols (a) straight chain, e.g., linalool from oils of lav ender, neroli and linaloe, geraniol in rose otto, palmarosa and cit ronella oils: (b) terpene alcohols, e.g., borneol in Borneo camphor; terpineol in oils of linaloe, cajeput, etc., also artificially prepared from oil of turpentine for lilac odours : (c) phenols, e.g., menthol in oil of peppermint ; thymol in thyme oil; eugenol in oil of cloves. (3) Esters, e.g., isoamyl acetate, synthetic for pear flavour; methyl salicylate in oil of wintergreen, and synthe sised from salicylic acid; geranyl acetate in oils of palmarosa, lemongrass, pettitgr?in, etc., and synthesised from geraniol for rose and orange flower odours. The acids occasionally occur in the free state in natural oils, but have the greatest importance in combination as esters. (4) Aldehydes, e.g., citral in lemongrass and lemon oils ; citronellal from oil of lemon ; benzaldehyde in oil of bitter almonds, laurel oil; vanillin in vanilla, gumbenzoin, etc., and artificially prepared from eugenol or guaicol; heliotropin (piperonal) in oil of spiraea, synthesised from safrole for helio trope odour. (5) Ketones, e.g., menthone in oil of peppermint ; camphor in oil of camphor, ionone synthesised from citral for violet odours. (6) Oxides and lactones, e.g., safrole from oils of sassafras and camphor; coumarin in tonka bean, woodruff, syn thesised from salicylic aldehyde for "new mown hay" odours; eucalyptol (cineol) in oil of eucalyptus. (7) Nitrogen and sulphur compounds, e.g., prussic (hydrocyanic) acid in oil of bitter almonds, laurel oil; nitrobenzene ("oil of mirbane") synthesised from benzene for almond odour ; trinitrobutylxylene (Baur's xylene musk) synthesised from xylene for substitute musk : ally lisothiocyanate in oil of mustard. Many other compounds exist and most are to be found in a variety of natural oils ; only the principal sources can be quoted here.
Essential oils are obtained from plants by the following methods : (I ) Steam distillation, the most important method, is applicable to the widest range of substances, e.g., peppermint, camphor, cloves, etc. ; it is especially favoured for the production of the more stable oils, such as oils of cedar, cloves, etc. (2) Expression, by hand ("ecuelle" and "eponge") or by machinery, used almost entirely for the production of oils contained in the rinds of fruits such as lemon, bergamot, etc. (See LEMON.) (3) Extraction by (a) volatile solvents such as petroleum ether, alcohol, etc. (a practice which has increased greatly of recent years) ; (b) by cold neutral fats ("enfleurage") or (c) by hot oils or fats ("maceration") . Descriptions of these processes are given in the article PERFUME.
Terpeneless oils are prepared from several natural oils, e.g., oil of lemon, by removal of the terpene hydrocarbons present by fractional distillation. These hydrocarbons have no odour value, and, on account of their liability to atmospheric oxidation, impair the keeping properties of the oil; further they diminish the solu bility in dilute alcohol.
The so-called synthetic oils may be grouped into two classes: (I) Isolates or artificial oils, consisting of chemical individuals isolated in a more or less pure condition from a natural essential oil, e.g., menthol, cineol, isolated by freezing from oils of peppermint and eucalyptus respectively; (2) oils synthesised by chemical means from such isolates (e.g., piperonal from safrole of sassafras oil) or from other raw materials such as coal-tar derivatives (e.g., methyl salicylate from salicylic acid). The synthetic essences may be identical with the odorous bodies present in a natural oil, for example, vanillin, synthesised from eugenol of oil of cloves, or from coal-tar guaiacol, and coumarin produced from salicylic aldehyde ; many, how ever, are bodies which do not occur naturally in the plants. Such are ionone, derived from citral isolated from oil of lemongrass, which is the basis of all violet scents, and perhaps the most im portant of all the synthetic perfumes; nitrobenzene which although chemically distinct from the benzaldehyde of almond oil, never theless possesses a coarse almond odour, and the exceedingly interesting synthetic musks. These latter are structurally very different from the ketones isolated from natural musk which, however, they closely resemble in odour and fixative properties. (Further examples of synthetic perfumes are quoted in the article PERFUME.) Applications.—Essential oils find an extensive range of uses, of which the principal are the various applications in perfumery (q.v.), for perfumes, toilet preparations, soaps, etc. Important also are the uses in connection with foods. The value of flavour ing herbs, condiments, etc., is largely due to the ethereal oils contained in them, while the delicate aroma of tea, coffee, wine. etc., depends on the presence of minute quantities of the same bodies. Consequently essential oils, and in particular the ter peneless oils, are employed in the flavouring of aerated beverages, etc. ; the synthetic fruit essences are largely used in confectionery.
Many of the volatile oils find extensive use in medicine, either on account of their disinfectant properties, e.g., thymol, eucalyp tol, borneol (the principal constituent of Borneo camphor, which, mentioned by Rhazes as early as A.D. 923, was used as a prophy lactic for the plague in the Middle Ages), or for their analgesic value, e.g., camphor, oil of wintergreen (treatment of rheu matism) .
In the arts, oil of turpentine is used on the largest scale in the manufacture of paints and varnishes and as a source of artificial camphor. Oils of lavender and spike are used as paint vehicles, more especially for the painting of pottery and glass.
The cultivation of plants for ethereal oil pro duction has become a large industry; the most important producing area for natural floral products is the Alpes Maritimes district in the south of France, but the industry is now extending to Germany and Italy and the countries bordering on the Mediter ranean, such as Bulgaria, which has long been the centre for rose oil production. The increased consumption of synthetic perfumes has in many cases stimulated the agricultural side of the trade; enormous quantities of sassafras and lemongrass are now culti vated solely for the recovery of the respective isolates safrole and citral. Lavender and peppermint oils of English origin rank as the best qualities; Japan has practically a monopoly of menthol production, while from the Far East come also the spice oils, or their raw materials, such as cinnamon and cloves, with eucalyptus, camphor, etc. America imports the bulk of the essential oils re quired; her production, however, of peppermint, lemon and orange oils is rapidly increasing. Attempts are being made to develop the cultivation of essential oils of orange and lemon in Africa and Australia respectively. See also PERFUME, TER PENES, LEMON, EUCALYPTUS, MUSK, etC.
and Hoffmann, The Volatile Oils Bibliography.-Gildermeister and Hoffmann, The Volatile Oils (1926, trans. by Kremers) ; E. J. Parry, The Chemistry of the Essen tial Oils, and Artificial Perfumes (1922-26) ; Cyclopaedia of Perfumery (192 5) ; E. Gattef osse, Nouveaux Parfums Synthetiques (1927) ; M. P. Otto, L'Industrie des Parfums (1924) ; E. J. Parry, Raw Materials of Perfumery (Pitman's Common Commodities of Commerce, 1924, elementary) ; E. T. Charabot and Gatien, Le parfum chez la Plante (Paris, 1908) ; E. T. Charabot, Les principes odorants des vegetaux (Paris, 1912). (E. L.; G. H. W.)