ARTICLE pubs.acs.org/jchemeduc
The Scent of Roses and Beyond: Molecular Structures, Analysis, and Practical Applications of Odorants Albrecht Mannschreck† and Erwin von Angerer*,# †
Department of Organic Chemistry, and #Department of Pharmacy, University of Regensburg, D-93040 Regensburg, Germany ABSTRACT: A few odorous compounds found in roses are chosen to arouse the reader's interest in their molecular structures. This article differs from some similar reports on odorants mainly by combining the structural description with the presentation of the following types of isomers: constitutional isomers, enantiomers, and diastereomers. The preparation of rose oils by distillation of blossoms with water and the analysis of the oils by gas chromatography mass spectrometry are briefly described. 2-Phenylethanol, β-ionone, β-damascone, β-damascenone, citronellol, rose oxide, geraniol, and nerol are the most important odorous components of many rose species. Beyond these, a few additional isomers of odorants and the practical applications of fragrance materials are briefly presented. The natural or synthetic origin of an odorous compound is economically relevant, but its activity does not depend upon its origin. This article is recommended as a lesson on the relationship between molecular structure and odor for chemistry students in their second year at university. KEYWORDS: Second-Year Undergraduate, Analytical Chemistry, Organic Chemistry, Textbooks/Reference Books, Constitutional Isomers, Diastereomers, Enantiomers, Gas Chromatography, Mass Spectrometry, Stereochemistry
he rose is called “the queen of flowers”. The reasons for this popular honorary name are its shape, color, and fragrance; the latter being the motive for the cultivation of many species of this flower, for example, the one shown in Figure 1. In poetry, the scent of roses has often been highly praised.1 An early example is the famous oriental fairy-tale “Thousand and One Nights”.2 William Shakespeare mentioned the scent of roses;3 in sonnet 54 he wrote
T
structural isomerism. The next sections describe gas chromatography mass spectrometry, types of separable isomers, and the composition of the materials isolated. Besides the constituents of many rose species, examples of other odorant molecules occurring as isomers are briefly described. Finally, applications and origins of fragrance materials are mentioned.
’ ISOLATION OF ODOROUS COMPOUNDS FROM ROSE PETALS
Oh how much more doth beauty beautious seem by that sweet ornament which truth doth give. The rose looks fair, but fairer we it deem for that sweet odor which doth in it live. In this article, some of the major chemical components of rose blossoms and a few additional odorous substances are chosen to engage the reader’s attention and to arouse interest in the molecular structures involved. The content of this article is not new; besides recent research- and application-oriented reviews,4 7 there are other publications8 11 that are helpful for teaching, including a book12 that describes the chemistry of fragrance compounds (6 chapters) and the theoretical background required (9 chapters). The present article, however, combines the topic of some fragrance materials with the isomerism of organic molecules. To benefit from this article, the student should be familiar with the basic structure13a,14a of alkane, alkene, alcohol, ether, and ketone molecules. In addition, the principles8,12,15 of distillation and chromatography should be known. This contribution is recommended as a lesson on the relationship between molecular structure and odor for chemistry students in their second year at university. The first sections of the article are dedicated to the isolation of odorous compounds from rose petals and to the different types of Copyright r 2011 American Chemical Society and Division of Chemical Education, Inc.
Rose Oil
A traditional method for isolating volatile constituents of roses is heating the blossoms with water in the flask of a distillation apparatus or blowing steam through the flask.8,15 Under these conditions, a so-called azeotropic mixture of lipophilic components and water is formed that possesses a boiling point somewhat below 100 °C,12a,16 although the separated chemicals without water show much higher boiling points, for example, about 230 °C in the case of the odorous compounds present in roses. The addition of water generally enables the distillation of high boiling or sensitive lipophilic compounds at a temperature that makes decomposition reactions less likely. Upon cooling the vapor phase, an aqueous liquid and a lipophilic layer are obtained. The aqueous phase from this distillation contains small quantities of partially water-soluble compounds and is called rose water. Most odorous constituents of the petals are found in the lipophilic layer, an essential oil called rose oil.15 The production of rose water and rose oil on an industrial scale uses, for instance, Published: September 13, 2011 1501
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Figure 1. Rosa damascena trigintipetala (Damask rose with 30 petals) is cultivated on large scale mainly for the industrial production of rose fragrances. Figure 3. (Top) Enantiomers of citronellols, 5, and (bottom) the corresponding general structures 6.
’ ODOROUS CONSTITUENTS OF ROSES AND THEIR MOLECULAR STRUCTURES 2-Phenylethanol
Figure 2. Constitutional isomers of odorous compounds in rose petals: phenylethanols (1 and 2), ß-ionone (3), and ß-damascone (4).
500 kg of blossoms of a suitable rose species and 1500 kg of water per load. From 10 kg of blossoms, approximately 2.5 g of clear, colorless to yellow rose oil with an intense rose scent is obtained. Low yield and the expenditure of collecting the material by hand make this oil very expensive, which has led to the expression “king of the essential oils”,17 a term that is sometimes also applied for frankincense. Rose oil is frequently used in the preparation of cosmetics and perfumes. Rose Absolute
Another method for the isolation of fragrance components from roses is based on extraction and affords so-called rose absolutes. In a typical procedure, rose blossoms are extracted with a nonpolar solvent, for example, n-hexane, which is later removed by distillation. The resulting residue is extracted with ethanol; after its removal, the rose absolute remains as a reddish liquid.7,18 It contains essentially the same constituents as rose oil, but in different proportions. An alternative to the use of organic solvents for extraction is the application of supercritical carbon dioxide, but this method requires expensive equipment. Volatile Components by Headspace Technique
An interesting method for separating and identifying volatile constituents is dynamic headspace trapping,19 which avoids both distillation and solvent extraction of the petals. A glass container around the flowers defines the “headspace”. A glass tube, the collector, filled with an adsorbent, for example, charcoal, adsorbs the odorous substances when air is passed via a filter inlet through the container and the collector. In a second step, the volatiles are desorbed, for example, by elution with a solvent.
2-Phenylethanol (1, Figure 2) is abundant in rose petals and contributes strongly to their fragrance.7 The odor of rose water mainly stems from compound 1. The isomeric alcohol 2 (Figure 2) is found in tea leaves but apparently not in roses, although it possesses a rose-like scent.7 Phenylethanols 1 and 2 have the same molecular formula C8H10O, but differ with respect to the bonds connecting the phenyl ring with the ethanol carbon atoms, that is, with respect to their constitutions. Thus, they represent examples of constitutional isomers.13b,14b Another pair of constitutional isomers is β-ionone (3) and β-damascone (4) (Figure 2), which are found in many plants.7 Together with β-damascenone, which differs from 4 by the presence of an additional double bond between C4 and C5, they strongly contribute to the overall scent of roses.7,17,20 Citronellols
Citronellol (5) is a typical example of a natural product with an asymmetric carbon atom, that is, a carbon with four different substituents, and can occur in two different stereoisomeric forms that are assigned by the prefixes (R) and (S) as shown in Figure 3, top. Both materials possess a rose-like scent with the odor of (S)-5 being more delicate than that of (R)-citronellol.6 Citronellol occurs in rose petals and forms a major component of rose oils (up to 50%).7 More than 95% of the citronellol is (S)-5, which is typical for natural products, which are predominantly formed as single stereoisomers. As shown in Figure 3, the two isomeric citronellols do not differ with respect to their constitutions, but possess different orientations of the substituents at C3, which is the stereocenter of this molecule. (R)-5 and (S)-5 cannot be superimposed because they bear four different groups at a tetrahedral carbon atom and represent examples of enantiomers.13c,14c An interchange of two substituents,13d,14d for example, H and CH3 in (R)-6, transforms the latter into the opposite enantiomer (S)-6, the mirror-image molecule of (R)-6 (Figure 3, bottom).13c,14c 1-Phenylethanol (2; Figure 2), sometimes termed styrallyl alcohol, also occurs in two enantiomeric forms because of the asymmetric carbon atom in position 1; the odors of (R)-2 and (S)-2 are different. 1502
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Scheme 1. Different Types of Isomers Described in the Article
Figure 4. Enantiomers and diastereomers of rose oxides. Mixtures of (2R,4R)-7 and (2S,4S)-7 are termed trans-rose oxide, mixtures of (2R,4S)-7 and (2S,4R)-7 are termed cis-rose oxide (see text).
Figure 6. Structural principle of GC MS.
Figure 5. E Z-Isomers as examples for diastereomers geraniol, (E)-8, and nerol, (Z)-8, (top) and the corresponding general structures 9 (bottom).
The prefixes (R) and (S), used to differentiate between the enantiomers, are defined by so-called priorities of the four substituents at the stereocenter. The details of this convention are described elsewhere.13e,14e Rose Oxides
The number of possible isomers increases when a second stereocenter is present in the molecule as exemplified by rose oxide (7), another ingredient in rose petals. This cyclic ether 7 (Figure 4) contains stereocenters in positions 2 and 4 as defined for citronellol.21 Thus, the number of isomers is now four as the result of 2 2 possible combinations. The prefix (R) or (S) is given for both stereocenters (Figure 4). The cyclic structure of rose oxides adds to the complexity of the steric situation because the two alkyl substituents can either be located on the same side or on opposite sides of the sixmembered ring. These different structures are assigned as cisand trans-isomers, both occurring in two mirror image forms and, hence, are enantiomers. Because of the presence of two stereocenters, cis- and trans-rose oxides represent diastereomers. In general, isomeric compounds with two or more stereocenters, that are not enantiomers, are named diastereomers. In contrast to enantiomers, diastereoisomers differ in their physical properties. The odors of the rose oxides are given in Figure 4.21 Though only small quantities of (2R,4R)-7 and (2S,4R)-7 occur in rose petals and rose oil, they introduce a marked floral-green note.22 Geraniol and Nerol
A type of isomerism, different from the previous ones, is displayed by geraniol and nerol. These two isomers of compound 8 differ in the orientation of the substituents at the planar carbon atom 2. Unlike
citronellol (5), they possess different physical and chemical properties that gave rise to two different names geraniol, (E)-8, and nerol, (Z)-8 (Figure 5, top). The prefixes (E) and (Z), based on the priorities of substituents, are used for the unambiguous description of the sterical arrangement at the double bond.13f,14f (E)-8 and (Z)-8 smell rose-like: geraniol more flowery and nerol more fresh.7 Both contribute to the scent of the petals. Because (E)-5 and (Z)-5 cannot be superimposed due of the different groups at each carbon atom of this double bond, they represent stereoisomers. Originally, they were not named diastereomers23 25 but cis trans-isomers; later, the term E Z-isomers was used. In a more general description, stereoisomers that are not enantiomers are termed diastereomers. This definition, which includes all compounds with two or more stereocenters, for example, the rose oxides, has been extended onto alkenes26,27 and is now also used in the literature related to teaching.13g,28,29 Unfortunately, this change of definition by the IUPAC makes the term “diastereomers” less precise than in its original use. An overview over the types of isomers discussed in the preceding sections is given in Scheme 1.
’ ANALYSIS OF VOLATILE COMPOUNDS FROM ROSES Gas Chromatography Mass Spectrometry
A widely used method for the analysis of mixtures of volatile compounds with respect to identity and quantity is gas chromatography mass spectrometry (GC MS), which is briefly described. After injection of a sample consisting of a mixture of volatiles into a capillary column, the components are carried by a stream of nitrogen or helium and separated by moving along a liquid sorbent (Figure 6).8,15 The sorbent is a high-boiling liquid coated on the inside of a long capillary tube made of silica. As a compound leaves the column in the gas stream, it is diverted into a mass spectrometer for detection and identification. In the mass spectrometer, an electron is removed from the 1503
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Table 1. Concentrations of Various Fragrance Constituents in Rose Oil, Obtained by Steam Distillation of R. damascena of Bulgarian Origin Component 2-Phenylethanol (1)
Concentration (%) 1.1a
1-Phenylethanol (2)
-
β-Ionone (3)
1.8
β-Damascone (4)
0.3
β-Damascenone Citronellol (5)
0.1 35.9
cis-Rose oxide (cis-7)
0.5
trans-Rose oxide (trans-7)
0.4
Geraniol ((E)-8) Nerol ((Z)-8) Further constituents
25.7 3.7 30.5
a
The high concentration of 2-phenylethanol (1) in rose petals is considerably reduced upon steam-distillation (see text).
Figure 7. GC MS of a mixture of constituents A, B, and C: (top) simplified gas chromatogram (d = detector response; t = time after injection); (bottom) identification of A by its schematic mass spectrum (a = rel. abundance; m/z = mass-to-charge ratio of the observed ions, F = fragment, M = molecular ion).
molecule M to give the radical cation M+ 3 that breaks into various fragments (F1, F2, ...) characteristic for a certain compound:
These primary fragments may split into further fragments. All ions and radical ions formed from one compound are accelerated in an electric field, then separated by a magnetic field, according to their mass-to-charge ratio, and electronically detected.8,14g,15 The constituents A, B, and C of a mixture leave the capillary at times tA, tB, and tC, which depend on temperature, flow rate of the gas, length of the column, and nature of the stationary phase. The plot of detector response versus retention time gives the gas chromatogram (Figure 7, top). Separate mass spectra of each constituent (e.g., Figure 7, bottom) are matched by a computer with a library of spectra.8,14g,30 Because of the complexity of mass spectra (“fingerprint”), known compounds can be identified. Which Types of Isomers Can Be Separated by Chromatography?
Constitutional isomers such as the phenylethanols 1 and 2, and diastereomers, for example, trans-7 and cis-7 and (E)-8 and (Z)-8 can be separated by standard chromatography because they differ with respect to their physical properties. Successful separation of diastereomers requires certain experimental conditions such as appropriate eluents and stationary phases for liquid chromatography, or suitable flow rate, coating, and temperature for gas chromatography. Enantiomers cannot be separated by standard chromatography31,32 because most of their properties are identical. However, there exist enantioselective methods of chromatography that allow the separation of enantiomers using an asymmetric stationary phase.33,34 This variant of gas chromatography has been applied for the analysis of rose oils.33,35,36 It has been shown that less than 5% of citronellol (5) possess the (R)-configuration, (R)-5. A higher percentage of (R)-5 in a material labeled “Rose Oil” suggests that the latter is adulterated. The enantioselective variant of gas chromatography
also separates the enantiomers of both trans-rose oxide (trans-7) and cis-7 (Figure 4).35
’ COMPOSITION OF ROSE OILS, EXTRACTS, AND HEADSPACE PRODUCTS Rose oil, particularly the one distilled from Rosa damascena, is produced on an industrial scale and used in perfumes and cosmetics. Therefore, an oil of this type has been chosen to provide an idea of the concentrations of the components contributing most importantly to the scent (Table 1).37 The entry “further constituents” in the table comprises more than 100 compounds whose structures have been elucidated. The methods used to obtain the data in Table 1 were standard gas chromatography for the separation and quantification of the components and GC MS for their identification. The concentrations of the rose oil constituents were calculated as %-peak areas of gas chromatography with a flame ionization detector.37 In addition, GC MS was performed to match mass spectra of constituents with library spectra.37 The concentrations of the various components do not reflect the intensities of their odor. For example, β-damascenone, β-damascone, and β-ionone, though only present in minute quantities, are effective notes of rose oil and are today the most important fragrance chemicals in the composition of perfumes. The composition of the volatile constituents in rose petals usually differs from the composition of the corresponding rose oil for the following reasons. Steam distillation requires about 100 °C in the vapor phase and somewhat higher temperatures in the distillation flask causing some decomposition of sensitive compounds. In addition, certain components are partly soluble in the hydrophilic liquid phase and are found in rose water rather than in rose oil. Thus, the exact concentrations of the odorous constituents in rose petals must be determined by other methods. Rose absolute contains essentially the same constituents as rose oils, but in different proportions. The absolute better reflects the relative quantities of odorous chemicals in the rose petals. The content of 2-phenylethanol (1), for instance, is approximately 60 75%7,18 in absolutes and is less than 4%7 in oils (1.1% in Table 1). The dynamic headspace technique is the most sensitive method for the analysis of volatile products from 1504
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essential. Synthetic material is used for this purpose because its preparation is less expensive than the isolation from natural sources.48 Body odor is considered essential in the case of Jean-Baptiste Grenouille, the central figure in the bestselling novel “Perfume. The Story of a Murderer”,49 published by Patrick S€uskind in 1985. Grenouille was born without any odor of his own and is rejected by other people because they are disturbed by his lack of odor. For compensation, he uses the perfect perfume, the body scent of virgins. As a consequence, Grenouille is now loved by people.49 On the other hand, Grenouille’s sense of smell, that is, his ability to perceive fragrances, is unusually developed and helps him to choose his victims, the virgins to be murdered. The sense of smell directly allows the identification of many different chemicals. In this respect, the nose can be considered as a chemical sensor. From an evolutionary point of view, this sense may have been developed to differentiate between rotten and fresh food.
Figure 8. Further odorants and their smell: constitutional isomers 10 and 11, enantiomers (R) and (S)-12, as well as diastereomers (E)- and (Z)-13.
flowers. By this method, compounds 1, 3, 5, 7, (E)-8, and (Z)-8 were found in several rose species.38 40 Solvent extractions and headspace trapping, both followed by gas chromatography and mass spectrometry, allow analytical separation, quantification, and identification of the fragrance components in rose petals. The next step of analysis would be the elucidation of the relationship between a chemical structure and the sensation of a defined odor. Many details of the interaction of odorous chemicals with biological receptors and the cellular transmission of odor signals are now known.9,12b
’ FURTHER ODORANT MOLECULES: MAN AND ODOR Three additional pairs of isomers, which occur in nature, and their odors are shown in Figure 8. The constitutional isomers 10 and 11 are widely found in fruits and berries, but have different odors.41 (E)-2-Octen-1-ol (10) has a citrus-like odor, whereas 1-octen-3-ol (11) is called “mushroom alcohol” because of its odor;42 it is used in certain perfume compositions.7 Transpirol (12) is an example of a human body odor that stems from the bacterial transformation of sweat components. The smells of (R)-12 and (S)-12 differ strongly,43,44 an observation made for several other pairs of enantiomers.5,6,45,46 The odor of (S)-12, the dominant isomer, is characterized by the remark “onion”, whereas the enantiomer (R)-12 exhibits a fruity, grapefruit-like odor. A mixture of 12 is found in minute quantities in human axillary sweat and contributes substantially to its pungent, persistent smell.43 Both diastereomers of 3-hexen-1-ol (13) are isolated from vegetal sources.7 (Z)-13 is called “leaf alcohol”47 because it occurs in green leaves and freshly cut grass. To obtain the “green” note in many perfumes, the addition of (Z)-13 is
’ APPLICATIONS AND ORIGINS OF FRAGRANCE MATERIALS Fragrance chemicals are widely used7,12c in perfumes and as additives to deodorants, cosmetics, soaps, and so forth. Unlike drugs or pesticides, fragrance chemicals are luxury items that are not required for medical or agricultural purposes. However, it would not be agreeable to live without some pleasant scents. As a consequence, odorous materials have become an important class of chemicals and are produced on large scale. In 2006, the global market of essential oils and fragrance chemicals amounted to $6.3 billion and is expected to grow to $7.8 billion in 2011.50 From Figures 2 5, and 8, it might appear that odorants only are found in nature. However, there are many fragrant chemicals synthesized by man that do or do not occur in nature and are demanded by the market. For economic reasons, synthesis has become more and more advantageous6,12d,48 in comparison with the expensive collection and purification of natural materials. Most odorous components can readily be synthesized and are used to substitute compounds originally obtained from nature. Even odorous constituents with defined stereocenters are now accessible by chiral catalysis. Because the cultivation of roses requires the extensive use of herbicides and pesticides and uses valuable farmland, the ecological balance of the natural production has to be seen very critically. Given the high price of rose oils, there is the question why they are still produced from natural sources when most of their components can be synthesized for a fraction of the cost. One reason is that the scent of a perfume derives from complex mixtures of ingredients in the same way that nature uses mixtures of chemicals. With respect to the practical use, there is no difference between a certain fragrance molecule from natural sources and that obtained by synthetic chemistry. The smell of an odorant is only determined by its chemical structure.51,52 Even the scent of roses that has often been praised in poetry is based on something as prosaic as chemistry. ’ SUMMARY The steam-distillation of rose blossoms is performed on an industrial scale and yields precious rose oils containing most of the odorous components of the rose. The structures of the principal odorous chemical constituents of roses are used as examples of different types of isomers comprising constitutional isomers, enantiomers, and diastereomers including E Z-isomers 1505
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Journal of Chemical Education (Figures 2 5). GC MS (Figures 6 and 7) is used successfully for the analysis of mixtures of volatile compounds. 2-Phenylethanol (1), citronellol (5), rose oxide (7), geraniol ((E)-8), and nerol ((Z)-8) are identified as major constituents in rose flowers (Table 1). Minor components such as β-ionone (3), β-damascone (4), and βdamascenone are shown to modify or intensify the scent of rose oil. Odorous materials both from natural sources or chemical synthesis are widely used for the preparation of perfumes but also to add a pleasant odor to many items of daily life.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT The authors are grateful to J. Daub, W. Moll, J. Ruther, and R. Vasold for valuable discussions as well as to Claudia Ott for the information on “Thousand and One Nights”. ’ REFERENCES (1) Ohloff, G. D€ufte. Signale der Gef€uhlswelt; Wiley-VCH: Weinheim, 2004; pp 79 81. (2) Ott, C., Ed.; Tausendundeine Nacht, 10th ed.; C. H. Beck: M€unchen, 2009; pp 335 (128th night), 427 (172nd night). (3) Shakespeare, W. The Sonnets; Cambridge University Press: Cambridge, U.K., 2006; pp 53 (sonnet 54), 74 (sonnet 95). (4) Kraft, P.; Bajgrowicz, J. A.; Denis, C.; Frater, G. Angew. Chem., Int. Ed. 2000, 39, 2980–3010. (5) Abate, A.; Brenna, E.; Fuganti, C.; Gatti, F. G.; Serra, S. Chem. Biodiversity 2004, 1, 1888–1898. (6) Bentley, R. Chem. Rev. 2006, 106, 4099–4112. (7) Surburg, H.; Panten, J. Common Fragrance and Flavor Materials, 5th ed.; Wiley-VCH: Weinheim, 2006. (8) Nimitz, J. S. Experiments in Organic Chemistry. From Microscale to Macroscale; Prentice Hall: Englewood Cliffs, NJ, 1991. (9) Sell, C. Chem. Br. 1997, 33 (3), 39–42. (10) Fortineau, A.-D. J. Chem. Educ. 2004, 81, 45–50. (11) Nicolaou, K. C.; Montagnon, T. Molecules that Changed the World; Wiley-VCH: Weinheim, 2008; pp 29 40. (12) Sell, C. Understanding Fragrance Chemistry; Allured: Carol Stream, IL, 2008; (a) pp 101 103; (b) pp 267 279; (c) pp 231 236; (d) pp 309 311. (13) Vollhardt, K. P. C.; Schore, N. E. Organic Chemistry. Structure and Function, 4th ed.; Freeman: New York, 2003; (a) pp 1 40; (b) pp 37, 163; (c) pp 166 168; (d) pp 179 181; (e) pp 172 175; (f) pp 433 434; (g) pp 164 165, 433 434. (14) Wade, Jr., L. G. Organic Chemistry, 5th ed.; Prentice Hall: Upper Saddle River, NJ, 2003; (a) pp 1 71; (b) p 167; (c) pp 167 169; (d) pp 193 194; (e) pp 174 177; (f) pp 279 280; (g) pp 519 522. (15) Palleros, D. R. Experimental Organic Chemistry; Wiley: New York, 2000. (16) Schwedt, G. Bet€orende D€ufte, sinnliche Aromen; Wiley-VCH: Weinheim, 2008; p 144. (17) Kovats, E. J. Chromatogr. 1987, 406, 185–222. € A.; Tutas, M. Flavour (18) Ayci, F.; Aydinli, M.; Bozdemir, O. Fragrance J. 2005, 20, 481–486. (19) Clery, R. In The Chemistry of Fragrances, 2nd ed.; Sell, C., Ed.; RCS Publishing: Cambridge, U.K., 2006; pp 214 228. (20) Gora, J.; Brud, W. Nahrung 1983, 27 (5), 413–428. (21) Yamamoto, T.; Matsuda, H.; Utsumi, Y.; Hagiwara, T.; Kanisawa, T. Tetrahedron Lett. 2002, 43, 9077–9080. (22) Werkhoff, P.; Brennecke, S.; Bretschneider, W.; Guntert, M.; Hopp, R.; Surburg, H. Z. Lebensm.—Unters. Forsch. 1993, 196, 307–328.
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