Frances S. Sterren
Hofstra College Hernpsteod, New York
I
I
The Nature of Essential Oils Chemical constituents, analysis
Essential oils are composed of many classes of chemical substances. One oil may consist of as many as 75 individual constituents (as in the case of oil of camphor wood), or may contain largely one chemical compound (for example oil of sweet birch contains 98% methylsalicylate, oil of anise 90% anethole). Even though essential oils usually consist of complex mixtures of acyclic, cyclic, aromatic, and heterocyclic compounds and their oxygenated derivatives, most of the constituentsfall into four main groups: (1) aliphatic, non-terpenic compounds, (2) aromatic compounds, (3) terpenes and terpenoids, and (4) miscellaneous. Aliphatic Compounds Excluding Terpenes
A few aliphatic hydrocarbons have been found and isolated in essential oils, e.g. heptane from pine needle oil. "Stearoptenes, "whichconsist of amixtnreof waxes, contain higher members of the paraffin series. This wax-like material, probably originating from the protective coatings of leaves, flowers, fruits, and seeds, is found in oils of rose and camomille in such large amounts that these oils readily congeal below room temperature. Waxes are also found in rather large percentage in coldpressed citrus oils. Essential oils contain only a few aliphatic alcohols and acids in free form; most of them are esterfied. The lower, water-soluble members of the saturated aliphatic alcohols, such as methyl and ethyl alcohol, and acids such as formic, acetic, propionic, butyric, and valeric acid occur in oils of "cohobation" obtained by redistillation of the distillation water. These alcohols and acids are probably degradation products formed by such processes as hydrolysis during steam distillation or fermentation prior to distillation. Saturated aliphatic alcohols such as butyl, amyl, hexyl, octyl, nonyl, decyl, undecyl in the form of normal or branched isomers have been isolated by fractional distillation from a variety of essential oils. The unsaturated aliphatic alcohol 3-hexen-1-01, the so-called "leaf alcohol," occurs in many green leaves, herbs, and grasses; it has a characteristic grasslike odor and is the main constituent of tea leaf oil. Methyl heptenol isomers are formed in lemon-grass oil. 2,6-Nonadien1-01, called "violet leaf alcohol," has been observed in violet leaf and flower oils (1) and androl (1-nonen-3-01) in the distillation water of fennel oil. The lowest members of aliphatic aldehydes (nonterpenic) do not play an important role in essential oils. For instance formaldehyde and acetaldehyde occur in distillation waters and are probably decomposition or degradation products formed in the course of steam The first pert of this two-part paper dwcrihed the production of essential oils. See J. CHEM.E ~ u c .39, , 203 (1962).
distillation. Propyl, butyl, valeral, and caproaldehyde occur, for example, in the lower fractions of eucalyptus and peppermint oils. The higher aliphatic aldehydes, such as octyl and nonyl, even though contained only in minute quantities, play a more prominent role, due to their characteristic powerful odor and flavor. They are contained in such oils as orris root, coriander seed, rose, lemon, and sweet orange. An unsaturated aliphatic aldehyde, beta hexanal ("leaf aldehyde") contributes to the green leaf odor. 2,6-Nonadien-1-a1 called "violet leaf aldehyde" occurs in violet leaf oil (I). Not many aliphatic ketones occur in essential oils. Again, the lowest members, such as acetone and diacetyl, are formed mainly in distillation waters and are probably decomposition products. Amy1 methyl ketone has been found in the lower boiling fractions of clove oil, and methyl hept,enone in many essential oils. The following fatty acids have been found mostly in essential oils from seeds and roots: alpha and betamethyl butanoic, caproic, enanthic, caprylic, pelargonic, capric, nndecylic, lauric, myristic, hydroxy myristic, palmitic, stearic, methacrylic, isopropylidenacetic, angelic, tiglic, beta-propylacrylic, oleic, and succinic. Some essential oils contain large quantities of fatty acids; thus orris root oil contains up to 85% myristic acid. Esters are among the most important constituents and contribute greatly to the odor and flavor character of the essential oils. Some oils consist almost entirely of esters (oil of wintergreen and oil of sweet birch contain up to 99% methyl salicylate). Many alcohols and acids occur as esters in essential oils. Obviously a great variety of combinations are possible. Aromatic Compounds
This group of compounds comprises a number of important essential oil constituents. The aromatic ring may have substituted one or more functional groups or side chains
For example safrole occurs in sassafras oil, myristicin in nutmeg oil. Some of the aromatic alcohols found in essential oils are benzyl, phenylethyl, phenylpropyl, cinnamyl, and cuminyl. A number of the aromatic aldehydes play an important role in essential oils; for example, cinnamic aldehyde
is the main constituent of cassia oil and cinnamon oil. Some of the more important aromatic aldehydes occurring in essential oils are benzaldehyde, cumaldehyde, phenylacetaldehyde, cinnamaldehyde, dihydrocinnamaldehyde, vanillin, vanillin methyl ether ("methyl vanillin,") piperonal (or heliotropin; 3,4-methylene dioxybenzaldehyde). Aromatic ketonesoccur very rarely in natural essential oils, but some synthetic compounds such as acetophenone and substituted derivatives are widely used in floral-type synthetic perfumes. Phenols and phenol ethers contribute to the flavoring ingredients of many essential oils, for example eugenol in oil of clove, methyl chavicol in oil of estragon, and safrole in oil of sassafras. In other oils the phenols are responsible for the antiseptic and germicidal properties of the oils; for example thymol in oil of thyme and carvacrol in oil of origanum. Thus, the phenols and phenol ethers belong among the most important constituents of essential oils. Some of them are isopropylphenol, thymol, carvacrol, chavicol, methylchavicol, anethol, fenicnlin, p-an01 eugenol (main constituent of oil of clove, cinnamon leaf, and pimenta), eugenol acetate, methyl eugenol, isoeugenol, methylisoeugenol, safrole, isosafrole, myristin, isomyristin, elemicin, and eugenone. Some quinones and hydroqninones have been found to occur in some essential oils, probably as glncosides. Aromatic acids are present in essential oils free and/ or as esters; for example benzoic, phenyl acetic, cinnarnic, salicylic, anisic, piperonylic, veratric, and methoxy cinnamic acid. Some of the aromatic esters found are benzylhenzoate, methyl salicylate (main constituent of oils of wintergreen and sweet birch). Methylethyl-, benzyl and cinnamyl-cinnamate, and isoamyl salicylate are important synthetic aromatic chemicals. Many members of the essential oil constituents are related through simple chemical reactions. For example isomerization followed by oxidation may convert eugenol through isoeugenol to vanillin, a constituent of vanilla oil (3, 3).
OCH. Eugenol
OCHa Vmillin
OCH~ Isoeugenol
Acetaldehyde
The tendency of some essential oil derivatives to resinify is probably due to the number of conjugated double bonds in the structure, which readily form condensation products. Lactones are quite widely distributed in nature; the most important which occur in essential oils are the coumarins, coumarone derivatives, and some furano coumarins. Conmarin (4) can be considered derived from o-hydroxy cinnamic acid:
Coumarins occur in many plants as glucosides and thus
often the plant itself is odorless, but coumarin may be liberated due to enzyme action. Coumarins as such probably contribute to the odor of hay, which develops upon drying of the foliage in the sun. Most coumarins, coumarone, and furano conmarine derivatives have high boiling points; since they are only sparingly volatile with steam, they are found mainly in extracted and expressed oils. For example 5-methoxy-furo(3',Zf,6,7) coumarin in oil of bergamot, angelicin [furo (5',4',7,8) coumarin] in oil of angelica root, limettin (5,7-dimethoxycoumarin) in oils of lemon and lime, umbelliferone (7-hydroxycoumarin) in grapefruit oil, and umbelliferone methyl ether in oils of chamomile and lavender. Different isomers of the same chemical substance often vary greatly in odor and flavor. Six of the monosubstituted methyl coumarins were prepared as pure as possible in order to determine their odor character and their utility in flavor compounds (5); all six are white crystalline solids. Comoorison of Methyl Coumorinr
Position of methvl srour,
Odor Coumarin-like Walnut-like Between 3 and 4 Pleasant coconut odor Very faint coconut-like Almost odorless
3456-
78Terpenes and Terpenoids
The most widely distributed compoimds present in essential oils are the terpenes and sesquiterpenes, and their oxygenated compounds. I n fact, the scient,ific literature on the subject of terpenes and terpenoids has grown very large (6-11). The English word "ter~ene"is derived from the German word "Terpentin" (turpentine, or oil of turpentine, a product subject to much research and known to the ancient Greeks). The term "terpene" refers to hydrocarbons which have a chemical relationship to the simple isoprene (C6Hs) molecule. "Terpenoids" (in analogy with the term steroids) are compounds having features relating them to the terpene structure. The terms terpene and terpenoids usually refer not only to the hydrocarhons but also to their oxygenated compounds. The members of the terpene group can conveniently be classified into compounds containing branched five-carbon chains: hemiterpenes (1 isoprene unit), monoterpenes (2 isoprene units), sesquiterpenes (3 isoprene units), diterpenes (4 isoprene units), and polyterpenes. The most characteristic group present in essential oils are the monoterpenes and their oxygenated compounds with the empirical formulas CloHlr; CIOHEO;CioHi~0. It is purely hypothetical to assume that a Cb chain actually represents the basic unit in the formation of the terpenes in the plants. Many terpene investigators have risked guesses as to the nature of the basic unit. Isoprene is often mentioned as a precursor in biogenesis of terpenic compounds. The mechanism of the biological chemical reaction should be considered with critical reserve. I n vilro it has been possible to synthesize a number of terpenes in accordance with the isoprene rule (13-16). The formulas written here show the principal
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structural details by omitting as many carbon and hydrogen symbols as feasible, showing double bonds, functional groups, and in some cases (where indicated) methyl and methylene groups.
ture, but forms readily by rearrangement from myrcene, for example by application of heat. The same terpenic substance may be reported in various literature references under different names; therefore uniformity of nomenclature could quite often solve rather involved problems (%). Several stages of oxidation and reduction of acyclic terpene hydrocarbons are shown below:
reduction
oridation
Dipentene (Limanene) Isomers
A 3 Isoprenes
bH0reduction
The biogenetic isoprene rule can be considered as a useful working hypothesis, helpful for the elucidation of the structure of the terpene compounds. However, there are also many compounds which do not comply with the isoprene rule. Several other hypotheses were postulated, e.g., condensation of 3-methyl butenal (16) ; formation of terpenes and benzene derivatives from condensations and degradation of sugar derivatives; and synthesis from acetone, acetaldehyde, and acetoacetic acid (17-20). In recent work on the biosynthesis of terpenes (Sf), isopentenyl was found to be the basic unit. 14Glabeled isopentenyl-pyrophosphate, after isomerization and stepwise reaction, yields terpene hydrocarbons of the general formula CHJ
Citml (ClaHx,O)
0
I. , I
0
I/
--O--P-O-P--OH
OH
AH
where n = 1, 2, or 3 depending on the terpene synthesized. Among other terpenes, geranyl pyrophosphate and farnesyl pyrophosphate were prepared. Terpenes occurring in essential oils may be acyclic, monocyclic, bicyclic, and tricyclic. A saturated acyclic hydrocarbon with 10 carbon atoms has the formula C l a n ; a compound CloHlamay be acyclic with three double bonds, or monocyclic with two double bonds, or bicyclic with one double bond, or tricyclic with no double bonds. However, terpenes occur in many stages of oxidation or reduction; therefore, compounds with more or fewer hydrogens are also found. The oxygenated derivatives may contain all kinds of functional groups such as hydroxyl, methoxyl, carbonyl, carhoxyl, etc. Examples of acyclic terpenes CloHlswith three double bonds (formulas are written to indicate possible ring closures) follow:
Myrcene (7-methyl-3-methylene 1,6-octadiene)
QOOH Citronellic acid
Many terpenes are named commonly after the plant from which they have been isolated, e.g. geraniol from oil of geranium, linalool from oil of linaloe, citronellol and citronella1 from oil of citronella, etc. Terpenes are very unstable and readily undergo molecular rearrangement; structural variations of the same empirical formula are found frequently:
Geraniol also exists as cis- and trans-isomer. The cis-isomer is called nerol'after oil neroli bigarade from which it was isolated.
Oeimene (3,Fdimethyl 1,3,7-oetatriene)
Ocimene is not considered a product occurring in:na248 / Journal of Chemical Education
oxldatlon
Nerol (cis-isomer)
Germiol (tms-isomer)
Similar isomerism exists between citronellol and rhodinol, and citronella1and rhodinal. The same substance isolated from various natural sources exhibits difierent odor values, e.g. geraniol derived from oil of geranium smells somewhat different from that derived from oil of rose, palmarosa, or citronella; this is attributed to isomeric mixtures present. Most acyclic terpenes form cyclic derivatives under the influence of acid; for example:
Citral
Teresantalie acid (CLOHXO~)
The sesquiterpenes (CI6HZn)can again be acyclic, monocyclic, bi- or tricyclic. Some of the sesquiterpenes occurring in essential oils are:
Beta cyclocitral
The morioterpene menthadiene, CloHa, has theoretically a total of 32 isomers consideringthe position of the two double bonds, cis- and trans-forms, racernic mixtures, and the possible optical antipodes. I n essential oils, the most frequently encountered cyclic monoter(menthadienes) are the terpinenes, phelpenes, CIOH16, landrenes, terpinolenes, and limonenes.
Alpha-
BetaGammaBisabolene (Ct.Hu) (83, 24)
Cedinene (C.H.) Alpha-
Bets, Tewinenes
Alphs,
Gamma-
Cadinol (CaHzsO)
The most important azulenes (mostly blue-colored compounds) are usually classified as sesquiterpenes and are based upon the general formula ($6):
Bet*
Phellandrenes
Vetivaeulene (CtbHd Terpinolene
(26)
Guaiasulene C H
Vetivone (CdLzO)
Guaiol (CISHXO)
Lirnonene
Many of these structures can be converted ir to each other by relatively simple chemical reactions. Some of the bicyclic monoterpenes are: To the terpenoid group belong the ionones and irones (violet odor) : Alphs, BetaPinene (CmHu)
Alphs,
Bets, Gamma Fenehene (CjoHx8) Alpha-
h
A
Alphs, BetaThujene (ClaHls)
A
A
Thujyaloohol (C,oHuO)
Thujone (C,oH,,O) Alpha-
Tricyclic terpenic compounds occur less frequently as essential oil constituents; an example of tricyclic terpene is:
Ionones (CMHNO)
Bets, Irones (Cz4H7,0)
Beta-
Gamma-
By far the largest groups of essential oil constituents are terpenes and terpenoids and their oxygenated derivatives. Reactions of terpenes, such as isomerization (shifting of double bonds as well as cyclization or opening of ring structure), polymerization, hydrogenation, dehydrogenation, esterification, oxidation, etc., occur Volume
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rather readily, and many variations of terpenes are found in essential oils. This article can merely give an idea of the variety of compounds found as essential oil constituents. Miscdlaneous. A few compounds do not belong to the three groups discussed heretofore. They are nitrogen- or sulfur-containing compounds, such as organic cyanides, indole, skatole, allyl, benzyl, phenylethyl cyanide, methyl authranilate, and methyl-N-methylanthranilate, which are present in certain peel, flower, and leaf oils from citrus fruit; organic disnlfides and sulfides, e.g., allyl sulfides in oil of garlic; organic isothiocyanates, e.g. allyl, butenyl, benzyl, phenylethyl thiocyanate, as constituents of mustard oil. However, there are surprisingly few of these compounds present as essential oil constituents. Analysis
An analysis for the purpose of identifying an essential oil is usually not necessary, since the identity is estahlished by experts from odor alone. Essential oils are analyzed chemically for three main purposes: identification of essential oil constituents; detection of adulteration; or evaluation of the quality of a commercial oil. Identification of the individual constituents is done for research purposes only. For many years the study of these compounds, which are mostly liquids a t room temperature, was based on first separating the individual isomers in as pure a form as possible by fractionation, then preparing crystalline derivatives and identifying them by means of mixed-melting point determination with an authentic sample. This method is based on the general standard qualitative analysis of organic compounds by their functional groups. However, attempts to prepare these derivatives often are complicated by the instability of the essential oil constituent and the fact that even small amounts of isomers, which are usually present, act as impurities and may interfere greatly with the preparation of the solid derivative. A chapter written by the author on "The Preparation of Derivatives of Essential Oil Constituents" gives more detailed information (28). The development of modern instrumentation, such as vapor-phase chromatography, infrared, near infrared, visible, ultraviolet spectrophotometry, mass spectrometry, and even nuclear magnetic resonance have made it possible to separate isomers of high-grade purity seldom achieved before, and have helped to elucidate the structure of terpene isomers (29, SO). The analysis for the detection of adulteration of essential oils is basically the same as the analysis for the evaluation of the quality of an oil. Adulteration frequently occurs in view of the high cost of many of these products. The general procedure of analysis is to give the essential oil first a preliminary and organoleptic examination. Color, clarity, viscosity, presence or absence of sediment, waxes, and water are noted. The study of odor and in some cases flavor can suggest the presence of adulterants, which may then be confirmed by special tests. The trained human nose and taste buds have often a greater minimal sensitivity than the analytical instruments used for identification. Standard procedures for organoleptic tests are carried out (31). Then physical and chemical properties are analyzed, 250
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which include determination of solubility in different fixed strengths of dilute ethyl alcohol (go%, 80% 70% 60%, 50%), specific gravity, optical rotation, refractive index. Depending on the oil under consideration, special tests are carried out, such as acid number, total ester content, total alcohol content, aldehyde content, phenol content, congealing point, evaporation residue, etc. (S2). The analytical figures obtained in as complex a mixture as an essential oil may contain many or only a few diierent individual components and do not always represent the actnal percentage of a single constituent; e.g., in the case of an ester determination all saponifiable material is calculated as a certain individual ester (for example, as linalyl acetate). The analytical figures obtained are then compared with previous analyses and published standard data. These figures are valuable and have great practical signilicance. The relationship between individual chemical and physical properties is very revealing and important. However, the interpretation of all data obtained requires ingenuity and much experience. The use of modern instrumentation such as gas-phase chromatography and spectrophotometry is still mostly restricted to special cases, hut rapid progress is being made along these lines. Much hope lies in the future of these instruments. Vapor-phase chromatography has the unique ability to separate organic compounds, even very similar isomers and homologues, in a purity seldom achieved before. The sensitivity of spectrophotometric determination matches the fine organoleptic concentration requirements. However evident the spectrogram's absorption band may be as a curve, there still remain the problems of interpretation and of identifying the peaks of those items which are vital contributors to the odor. Analytical specification and standardization by instrumentation are being studied by the large essential oil manufacturers, by the National Bureau of Standards, and by Sadtler Research Laboratories using punch-card methods to identify and sort the large number of data available (SO). Even though the senses of odor and taste are the oldest senses, the most primary endowment of prehistoric man, research has not yet produced an analytical classification similar to what we have for white light in the spectrum and for sound in the musical scale. D i e r e n t mixtures of various essential oils and aromatics exhibit basically a similar odor. Literature reports numerous olfactory researches (35-37) and theories about the mechanism of olfactory stimulation based on the chemical structural reactivity, volatility, molecular vibration, etc. Quest,ionssuch as why similar odors are produced by chemically different substances and why concentration changes of the same chemical compound a t times may produce opposite odor effects remain to be answered. Perhaps with new modern instruments and with new research efforts we will be able to solve such problems, as well as duplicate in a test tube the natural essential oils, of which only very few can be successfully imitated today. The study of essential oils and chemical aromatics should prove most fruitful and stimulating in the near future; many interesting problems await solution.
Literature Cited (1) Ruzrc~n,L., ET AL., Helv. Chim. Acta, 21, 1542 (1938); 25, 760 (1942); and 27, 1561 (1944). (2) HUGEN-SMIT,A. J., Themistry Origin and Function in (3)
(4) (5) (6) (7)
E., "The Essential Oils," Vol. 1, Plants," i n GUENTHER, 1948, p. 43. FIEBER,L. F., AND FIESER,M., "Advanced Organic Chemistrv." Reinhold Publishina Cornoration. 1961, D. 828. SETEN*, S. M., A N D SAK,S. N., Chem. Reu:, 36, l(1945). STERRETT, F. S., Dmg &: Co~meticInd., 65, (3), 277 (1949). SIMONSEN, J. L., "The Terpenes," Vols. 1-5, Cambridge University Press, 1931-1957. WALLACE,O., "Terp ne und Campher," 2nd ed., Leipsig,
1914. (8) RUZICKA, L., J. Chem. Sac., 1582 (193:).
( 9 ) PINDER, A. R., "The Chemistry of Terpenes," John Wiley & Sons, Inc., 1960. (10) DEMAYO,P., "The Chemistry of Natural Products," Vols. 2 and 3. Interscience Publishers. Inc.. New York. 1959. (11) ASCHEN,0.. "Naphthenverbindungen, Terpene und Campherarten," Walter de Gruyter & Co., Leipzig, 1929. (12) W u ~ a c n O., , Ann., 246, 225 (1888). F. W., Ber., 43, 1893 (1910). (13) SEMMLER, (14) KER~CHBAUM, M., Ber., 46, 1732 (1913). (15) RuzIcKA; L., Proceed. Chem. Soc., 348, 341 (1959). (16) FmcnER, F. G., A N D LOWENBERG, K., .4nn., 494, 263 (19'32). (17) HALL,M. D., Chem. Ren. 20, 305 (1937). (18) SIMPSON, C., Pe~jumeryEssent. Oil Recard, 14, 113 (1923).
SINGLETON, F., Chem. Ind., 989 (1931). SMEDLEY, I., J . Chem. Soc., 99, 1627 (1911). EGGERER, H., Ber., 94, (11, 174 (1961). "Advances in Chemistry Series of ACS," No. 14, "Nomenclature for Terpene Hydrocarbons," 1955. (23) DE MATO, P., "The Chemistry of Natural Products," Vol. 2., 185.. 192., Interscience Publishers, New York,
(19) (20) (21) (22)
1959.
SORM,F., Znonn~,M., AND HEROUT,V., Coll. czeck. chem. Comm?m, 18, 121 (1953). RUZICKA,L., ET AL., J . Helv. Chim. Acta, 4 , 505 (1921); 5, 926 (1922); and 6, 846 (1923). DE MAYO,P., "The Chemistry of Natural Products," Val. 2. Interscience Publishers, New York, 1959, pp. 183, 261. (27) Zbid., p. 239. (28) STERRETT,F. S., "The Preparation of Derivatives of Essen-
E., "The Essential tial Oil Constituents," i n GUENTEER, Oils," Vol. 2, 1949, p. 771. (29) SMITH,M. D., A N D LEVI, L. J . Agr. Food Chem., 9, 230 (1961). (30) LANGENAU, E. E., AND ROGER,J. A,, JR., Am. P e e Armnat., 75 (3), 38 (1960). (31) LANGENAU, E. E., "Examination and Analysis'' i n GUENTHER. E.. "The Essential Oils." Vol. 1. 1948.. D. - 306. (32) pp. 237-3856, (33) MOORE,D. R., J. CHEM.EDUC.,37,434 (1960). (34) J O H N J. ~ W., ~ NAm. , Perf. Aromat., 75 (a), 51 (1960). (35) THOMPSON, H. W., Sac. Chem. Ind. (London), 1, 103-115 (1957). (36) AMOORE, J. E., Pe7fumery Essent. Oil Record, 43,321 (1952). (37) BEETS,M. G., J . Soc. Chem. I d . (London), l,54-90 (1957).
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