Nancy Grant
and Renee G. Naves Newton College of the Sacred Heart Newton, Massachusetts 02159
I
I
Perfumes and the Art of Perfumery
The world of perfumes1 is vast and complex. I t includes all areas of chemistry: inorganic, organic, physical, analytical, plus many areas of biology and physics; yet it is finally dependent not on scientific accuracy but on pleasing the people who may or may not purchase a certain scent. The term perfume is generally considered to designate only volatile emanations of bodies which are usually considered pleasant. The word odor, by contrast, has a more general meaning and applies to any smell. It is the essence of a perfume which gives it its characteristic scent. The essence can be defined as the peculiar quality of a perfume which evaporates at normal temperatures and affects the olfactory organs, strongly or lightly, for some length of time (1). The vegetable world produces by far the greatest number of perfumes. The number produced by animals is quite limited, although their importance is great, especially that of musk. Perfume is generally a normal product or a by-product of a plant's natural physiological mechanisms. The fragrance of a flower can be a specific attractant to the bees and the birds which serve to pollinate it. The earliest perfumes were natural aromatics. They constituted one of the earliest media of exchange because they were rare, and small in volume, also mere used extensively in religious cults. Modern perfumes, even among the most elaborate and costly are chiefly synthetic. Perfumes are associated with the sense of smell which results from the chemical stimulation of the olfactory nerve by airborn particles, which are volatile. The nasal passages comprise several interconnected cavities, separated by bony layers called turbinates. The upper turbinate contains branched endings of the olfactory nerve. Stimulation of these endings by odors results in the sensation of smelling. Sniffinga given odor several times forces the odor containing air up into the third tnrbinate; repeated sniffing results in increased sensation. However, receptors exposed to a particular scent over a long period become insensitive to that odor, although these same receptors can still distinguish other odors (8). This is a bothersome fact to perfumers and to wearers of perfumes; even the most delightful and effectivefragrance loses its effect after a while. For this reason, most perfumes, are a combination of several fragrances which have effectsbeginning and ending a t different times with different intensities. Some additives such as musk, when added to perfumes increase their longevity. What is a beautiful fragrance to one person might be
' For information on the chemical basis of olfaction see RODERICK, W. R., THIS JOURNAL, 43,510 (1966). 526 / Journal of Chemical Education
displeasing to another. Some odors are pleasing to most people; however like or dislike of an odor is a subjective matter. For the general market perfumers try to use the most popular scents. Also because of reactions with the skin a perfume may change its note depending on the wearer. I t is evident that all the structural factors of a given molecule which alter its reactivity may affect its odor. The atom or group of atoms in the molecule which is responsible for the fragrance is called osmophore. Similarities and differences of odor appear to be stereospecificity dependent. Enantiomers of a molecule have different odors as to note, amount of strength. The racemic mixtures have a different scent. The ethylenic stereoisomers geraniol (I) and nerol (11) are considered a classical case of this theory. The two n isomers, suitably purified, develop distinct notes.
I1
I
The cis (111) and trans (IV) jasmones, the first of which exists in jasmine are different. The odor of the trans jasmone is relatively dry. As for cyclanic stereoisomers a very interesting case is that the of menthols. Menthol (V) has a pleasant odor of peppermint, isomenthol has a slightly musty, somewhat disagreeable smell, isomenthol more so, and neoisomenthol is frankly unpleasant. CH3-CH2--CH
m
H-C-CH,-
IV
CH,
I
v
Steric hindrance plays a role, although small, in the olfactory differentiation by interfering in the conformation of equilibrium. In unsaturated substances, it inhibits reasonance, which in turn would alter or influence the smell. An example of this would be that of 2'-methy1,p-iouone (VI) which is closer to the odor of of 2'-methy1,n-ionone, than the odor of p ionone is to that of n ionone both of which are unsaturated.
Molecular associations of weak energies, such as multipoles, van der Waals energies, and hydrogen bonds must be regarded as determinants of characteristic odors. The stericity of molecules by hindering or not those associations, determines the odor, especially in the case of the more complex molecules (3). Terpenes artificial or natural, which constitute the major part of the essential oils, or perfumes, possess a distinct structural and chemical relation to the mole cule of isoprene CHe
They are classified according to the following divisions
(4)
atom resulting in geranyl pyrophosphate (VIII), which is the immediate precursor of linalol in plants (7).
VII
WI
Industrial methods of production are of interest. In one method, beta-pinene (IX) is pyrolyzed to 7-methyl,3-methylene1,6-octadiene, or myrcene (X) by passage through a tube heated a t 600°C for a very short time. AIyrceue is obtained in high yield. Dry hydrogen chloride at temperature below - 10°C is then added, and if conditions are controlled carefully, a high percentage of linalyl chloride is obtained (XI). The chloride can he converted to the acetate under suitable conditions.
If cyclic, they are mono-, bi-, or tricyclic, if they form an open structure they aresaid to be acyclic. Examples
.
The use of a Grignard reagent provides a more direct method. Vinyl magnesium bromide (XII) is treated with methylheptenone (XIII) yielding linalol
.
acyclic allo-ocimene
monocyclic: terpinene
A bicyclic: thujene
tricyclic: tricyclene
The chemical properties of terpenes vary tremendously, due to the fact they may contain any number of different functional groups, the most important being alcohols, aldehydes, and ketones. The chemical properties and natural and synthetic methods of production of linalol, geraniol, and nerol, all three of which are closely related, will be considered as examples. Linalol, 3,7-dimethyl-1,6-octadien-3-01is one of the most widely occuring terpenes and its esters have been found in the essential oils of Brazilian rosewood, coriander, bergamot, petitgrain, and numerous other plants (6).
Linalol is soluble in most solvents and can he regarded as a basic material for the synthesis of a large number of terpenoids. It is used in perfumes, natural as well as synthetic, soaps, and detergents, and as a flavoring agent in foods. Having an asymetric carbon atom linalol exists in optically active forms (6). The biogenesis of linalol seems to be the same in all plants, and even in the animal tissues in which it occurs. It is synthetized from mevalonic acid (VII) through a complex series of reactions on the fifth carbon
Linalol is easily reduced to the saturated alcohol. It can also be oxidized. Many oxides have been found to occur in nature. Linalol can be isomerized to geraniol (XIV), by prolonged boiling with acetic anhydride, or by treating it with acetic anhydride containing 1% phosphoric acid in the cold for 24 hr. Thc reaction is reversible in the presence of water a t high temperatures. Treatment of linalol with acids, especially sulfonic acids and formic acid yield small amounts of geraniol and nerol (XV), and high amounts of terpineol (XVI).
A strong acid or a strong catalyst causes complex transformations to take place, yielding a range of hydrocarbons. For example, treatment of linalol with 30% HISOagave myrcene, dipentane, terpenolene, p-cymene, alpha-terpineol, and 1: 4 and 1: 8-cineole (8). Although geraniol and nerol are closely related to linalol their properties differ in many respects. The name geraniol was originally given to the alcohol obtained from geranium oil, a very pleasant smelling liquid, possessing a sweet rose scent. One of the most widely used in perfumes, soaps, detergents, and cosmetics, it is valuable when fresh rose effects are desired Volume 49, Number 8, August 1972
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527
in fragrant formulations, as it is stable and non-discoloring. Nerol, of much lesser importance, is the alcohol from neroli oil. It has a fresher, more lemonlike sweetness. Of the two, geraniol occurs far more often in nature (in approximately 250 plants, while nerol occurs only in about 40) (9) whereit is synthetized from mevalonic acid. The chemical reactivity of geraniol is due to the presence of an alnha-beta unsaturated linkaee and a orimarv hydroxyl group. If the alpha-beta bond is saturated, citronellol is formed
As might be expected, geraniol is convertible to linalol. However, the reaction, run in an autoclave a t 20S°C, is not as easy as the linalol to geraniol conversion and results in small yields. Treated with hydrogen chloride in toluene solution, geraniol yields a mixture of geranyl and linalyl chlorides. Linalyl chloride reacts with silver nitrate to give linalol. Methods of analysis, which are divided into physical and chemical, are those most commonly used for alcohols, aldehydes, and ketones. Since our readers are familiar with such groupings and their qualitative and quantitative determinations we will not mention them here. Although the formula for a given perfume is a closely guarded secret, in most companies only a small elite knows the formulations, there are some compounds which are known to serve as basic constituents of most' perfumery products. Variations in proportions and added scents will create variety. Some toilet waters are known to contain more than thirty different terpenes, the exact proportions constituting a trade secret, a few have been disclosed however in a work published by Maison G. De Navarre (10). For instance the foundation or body of a rose scented perfume will always contain geraniol, citronellol, phenylethyl alcohol, and linalol. The geraniol constitutes the petal like smell of the flower u-hile citronellol imparts the flower freshness. The sweetness one may attribute
528
/
Journal of Chemical Education
to dried roses is provided by the pbenyl ethyl alcohol. Depending on the degree of sweetness, dry-leafy, more woody, or floral note desired other snbstances in infinitesimal quantities will he added. They constitute the modifiers providing the final touch and the fixers, whose role is to add longevity, often by retarding evaporation of the "body," or by acting as carriers to the organ of smell. Sometimes practically odorless, the fixers stabilize a scent, sometimes because of their own fragrance they exalt the odor of the foundation or on the contrary can mask minor mistakes: musk, patchouly oil, isoeugenol, and vanillin are a few of the fixatives used with rose scented perfumery products. A foundation for jasmin will be benzyl acetate, linalol, a l p h ~ a m ycinnamic l aldehyde, cinnamic alcohol and in smaller proportion phenylethyl alcohol. The chief difference between a perfume, a toilet water, a soap, or other cosmetic will be in the amount of scented material added. A perfume will be more concentrated than a toilet water. Creams and lotions tend to be underperfumed since they remain in contact vith the skin or hair for longer periods of time. A good quality soap if perfumed will contain an even smaller amount of scented material since a soft rather than a strong fragrance x~illbe preferred. Besides the question of strength linked to usage, strength influences the quality of a scent. Some of the ionones have been used to flavor commercial celery soups; the same compounds a t a different dilution are used to flavor raspberry candies. The solvent used in connection with an essential oil may also alter the scent. Literature Cited
P..
(1) J e m c ~ n o . "Lea Parfums. Chimie and Industrie," Paris, 1927, p. 16. (21 MOON.T. J., et al., "Modern Biology:' New York. 1960, DD. 50610. (3) NAVES,Y. R.. "Sur le Role du Chimiste dam I'Evolution des Industries des Mstieres premieres Odorantea, p p . 39-49. (4) American Chemical Sooiety, "System of Nomenclsture for T e r p e n e Hydrocarbons." Washinpton. D. 1955, p. 3. "Perfumery and Flavoring Synthetics," New York. (51 BEooonnN,
P..
C..
-~
(6) Ref. (6).p. 233.
( 7 ) B m s r m o . P.. ed.. "Riogenasis of Natural
1967, p p . 84G46. ( 8 ) REoonurAX. P., Ref. (61, p. 239-40. (9) Ref.(61. p. 166.
Compaunda." New
York.
M A I S O N G . . '-The Chemistry and Manufacture of Coameties." Princeton, 1962, Vol 1. p. 173 and follouing.
(10) DE N*V*RRE.
