Linalool
Linalool is one of the most interesting acyclic terpene alcohols, both from the
point of view of academic interest and of practical value. Several important essential
oils such as bergamot, lavender, petitgrain and rosewood owe their valuable odor
quality to linalool or its esters. For example, bergamot and lavender oils are rich in
linalyl acetate and may contain this ester to the extent of over 40 per cent. The
delicate and highly pleasant odor of linalool and its esters plays an important part
in the composition of many floral perfumes, colognes, and various other
formulations. Because of the relatively low boiling point, linalool and its esters are
often employed to add a desirable lift to perfumes, and this property becomes
indispensable in compositions where a light and pleasant note is desired.
It is interesting to note that linalool obtained from different sources has
different odor qualities which betray its origin. This can probably be explained by the
presence of small proportions of compounds which cannot be removed by the usual
methods of purification.
Since linalool possesses an asymmetric carbon atom, it is capable of existing in
optically active forms. It is thus a good natural source of a tertiary alcohol for the
study of problems related to optical isomerism. Some of its reactions, such as its
cyclization to an optically active terpineol (discussed below), are not well
understood and offer interesting fields of research to the organic chemist.
In view of its wide occurrence and peculiar structure, it is quite possible that
there is a fundamental relationship between linalool and other terpenes, and that
linalool plays an important role in the biogenesis of essential oils.
Occurrence.—Linalool occurs in nature in both optical modifications. In view of the
fact that its optical activity shows considerable variation
depending upon its source, it is highly probable that linalool occurs in nature as a
mixture of d and 1 forms, with one form predominating.
l-Linalool has been found in a number of sources, some of which are listed below.
Mexican linaloe oil contains 60-75 per cent linalool 1 and is an important source of
commercial linalool.
Oil of linaloe Cayenne 1 (Ocotea caudata M.) contains about 80 per cent linalool.
Bergamot oil (Citrus bergamia R.) 2 contains about 40 per cent linalool, most of it in the
form of its acetate.
Lavender oil (Lavandula vera DC.) 2 contains 35-45 per cent linalool, mostly as its
acetate.
Petitgrain oil (Citrus bigaradia R.) 2 contains 35-45 per cent linalool, mostly as its
acetate.
Japanese Shiu oil (Ho oil) 3 contains 80-90 per cent linalool and is one of its important
commercial sources.
Brazilian bois de rose (Aniba rosedora D.) 4 contains 85-90 per cent
linalool and is by far the most important commercial source of this alcohol. Small amounts
of l-linalool have been reported in the following oils. Cinnamon Ceylon (Cinnamomum
zeylanicum B.). 5
Cinnamon leaf oil (Cinnamomum zeylanicum B.). 6
Sassafras leaf oil (Sassafras officinale N.) .
German rose oi1.8
Clary sage oil (Salvia sclaria L.). 9
Gardenia flower oil (Gardenia florida L.).1 °
Wallflower oil (Chiranthus cheiri L.)."
Acacia flower oil (Robinia pseudoacacia L.). 12
Orange flower oil (Citrus bigaradia R.). 13
Oil obtained from the distillation of peaches."
Morin, Ann. chim. et phys. [5], 25, 427 (1882) ; Barbier, Bull. soc. chim. France [3], 9, 802 (1893).
Semmler and Tiemann, Ber. 25, 1186 (1892). Parfumerie moderne
13, 132 (1920).
4 Guenther, Am. Perfumer 42, No. 6, 31 (1941).
Walbaum and Huthig, J. prakt. Chem. 2, 66, 53 (1907).
6 Schimmel & Co., Report, October (1902), 86.
7 Power and Kleber, Pharmaceutical Review 14, 103 (1896).
8 Walbaum and Stephan, Ber. 33, 2304 (1903).
6 Volmar and Jermstad, Compt. rend. 186, 518 and 783 (1928).
10 Barone, Boll. chim.-farm. 41, 489 (1902).
11 Kummert, Chem. Zt. 35, 667 (1911).
12 Elze, Chem. Zt. 34, 814 (1910).
1
2
d-Linalool has been found in the following oils :
Coriander oil (Coriandrum sativum L.) in which it occurs up to 70 per cent.
Nutmeg oil (Myristica fragrans H.). 1 6
Orange oil (Citrus aurantiurn L.). 1 7
Ginger oil (Cingiber officinale R.). 18
The distillate of cocoa.
Linalool has also been detected in lime oil (Citrus medica L.), geranium oil 21
(Pelargonium odoratissimum) essential oil of green tea, etc.
15
19
20
22
Isolation and Identification.—Linalool was among the first terpene alcohols to be
reported in the literature. Its empirical formula was established some thirty years later
by Grosser 24 as C1011180, and its structure elucidated not long afterward as a result of
thorough investigations by Barbier, Tiemann and Semmler. Bertram and Walbaum 25
found that on chromic oxidation linalool yielded an aldehyde similar to or identical with
citral. This was rather puzzling since it was already known that the primary alcohol,
geraniol, gave citral on similar treatment and that linalool possessed an asymmetric carbon
atom since it exhibited optical activity. Barbier 26 who was also devoting much time to the
study of this compound, discovered that it possesses two ethylenic linkages and noted
that on prolonged heating with acetic anhydride it gave the ester of an alcohol which was
different from linalool. This alcohol was later shown by Bouchardat 27 to be geraniol
and the aldehyde which Barbier obtained by the oxidation of linalool to be citral.
A further advance in the study of the constitution of linalool was made when Barbier
and Bouveault 28 found that oxidation of linalool with chromic acid mixture gave not
only citral but also acetone and methyl heptenone. As a result of these experiments,
numerous structural formulas were put forward which were, however, later shown to be
incorrect. In
23
Grosser, Ber. 14, 2485 (1881).
Power and Saiway, J. Chem. Soc. 91, 2045 (1907).
17 Stephan, J. prakt. Chem. 2, 62, 523 (1900).
18 Brooks, J. Am. Chem. Soc. 38, 430 (1916).
16 Bainbridge and Davis, J. Chem. Soc. 101, 2214 (1912).
20 Guenther and Langenau, J. Am. Chem. Soc. 65, 959 (1943).
21 Schimmel & Co., Report, October (1910), 52.
22 Takei, Sakato and Ono, Bull. Inst. Phys. Chem. Research (Tokyo) 13, 671 (1937).
23 Kawalier, J. prakt. Chem. [1], 58, 226 (1853).
24 Grosser, Ber. 14, 2485 (1881).
25 Bel train and Walbaum, J. prakt. Chem. [2], 45, 599 (1892).
26Barbier, Compt. rend. 114, 674 (1892) ; 116, 883, 993, 1062, 1200, 1459 (1893) ; Bull. Soc. shim.
France [3], 9, 802, 914, 1002 (1893).
27 Bouehardat, Compt. rend. 116, 1253 (1893).
28 Barbier and Bouveault, Compt. rend. 118, 1208 (1894).
15
16
-
1895, Tiemann and Semmler,29 in continuation of their studies, oxidized linalool (I) first
with potassium permanganate, then with chromic acid mixture, and obtained acetone
and levulinic acid (II). This and other evidence led them to conclude that linalool was a
tertiary alcohol and that the formation of geraniol was the result of isomerization under the
influence of acids.
The low boiling point of linalool and other physical constants strongly favored its
being a tertiary alcohol, and the constitution given above was accepted and later
proved correct through its synthesis.
A partial synthesis was accomplished by Barbier and Locquin 3° who hydrogenated
linalool (I) with platinum black and obtained tetrahydrolinalool (III). This on
dehydration and oxidation gave a ketone which was identified as methyl
isohexyl ketone (IV). When this ketone was treated with ethyl magnesium iodide,
it gave an alcohol identical with tetrahydrolinalool (III).
Finally, linalool (I) was synthesized by Ruzicka and Fornasir
very interesting series of reactions. Utilizing a reaction discovTiemann and Semmler, Ber. 28, 2126 (1895).
° Barbier and Locquin, Ann. chim. [9], 2, 400 (1914).
Ruzicka and Fornasir, Heir. Chim. Acta 2, 182 (1919).
29
3
31
31
through a
ered earlier by Nef, they condensed methyl heptenone (V) with sodium acetylide and
obtained dehydrolinalool (VI). The acetylenic linkage was reduced with sodium to the
ethylenic linkage to give dl-linalool (I).
32
Nef, Ann. 308, 264 (1899).
The question of the possible existence of the double bond in 1 and 2 positions has
been raised and as in the case of the other terpene compounds remains to be settled
with certainty.
33
Methods of Preparation.—With the exception of the synthesis of Ruzicka and
Fornasir, other methods of preparation involve the isolation of linalool from natural
sources. Since linalool is a tertiary alcohol, it does not easily form suitable
derivatives from which it may be regenerated.
Its preparation demands careful and efficient fractional distillation of oils rich in
linalool. As the boiling point of linalool is considerably lower than that of other acyclic
terpene alcohols, linalool may be obtained comparatively free from them.
It has also been produced through the isomerization of geraniol. This reaction
which will be discussed later does not, however, constitute a suitable method of
preparation.
Chemical Reactions.—An optically active tertiary alcohol, linalool possesses a
double bond adjacent to the hydroxyl group and presents many properties of
considerable academic interest. In view of these facts, its chemical reactions have
been subjected to many studies.
Its separation in a pure state has been a matter of the greatest difficulty and it is
doubtful that it has been prepared in absolute purity from natural sources. It is
fortunate, however, that a number of essential oils, notably Brazilian rosewood oil and
Mexican linaloe oil, contain a very high percentage of linalool (about 80 per cent),
and a careful fractional distillation of these oils under vacuum through an efficient
fractionating column gives comparatively pure linalool. Linalool prepared in this
manner always contains small quantities of unknown substances, as evidenced by the
fact that on redistillation each fraction possesses a slightly different rotation and
physical constants.
Resolution of linalool was attempted by Paolini and Divisia, 34 who prepared the
strychnine salt of the phthalate of dl-linalool and subjected it to recrystallization in
various solvents. The regenerated linalool had a rotation of +1.7° indicating partial
resolution. Linalool has apparently not been prepared in an optically pure state
either synthetically or from natural sources.
Linalool is easily reduced both to the saturated alcohol and to the
corresponding hydrocarbon. On reducing it with sodium in alcohol, Semmler obtained
dihydromyrcene.35 The same product has been obtained with
33
34
35
129.
Verley, Bull. soc. chim. France [4], 25, 68 (1919).
Paolini and Divisia, Atti. accad. nazi. Lincei [5], 23, II, 175 (1914).
Semmler, Ber. 27, 2520 (1894) ; 34, 3127 (1901); Schimmel & Co., Report, October (1911),
sodamide in liquid ammonia." Reduction with colloidal palladium or platinum 37 gives
both dihydro and tetrahydrolinalool. Similar results may be achieved by using Raney
nickel catalysts.38 Comparatively pure samples of tetrahydrolinalool were obtained by
Stevens and McNiven 39 who carefully hydrogenated purified samples of linalool with
platinum black and then treated the product with permanganate solution to remove
unsaturated products. Using nickel catalyst, Enklaar 40 obtained not only tetrahydrolinalool
but also the corresponding hydrocarbon, 2,6-dimethyloctane. Numerous other
hydrogenation experiments have been reported with results somewhat similar to the
above.
Oxidation reactions of linalool yielding degradation products were discussed above
and need not be repeated here. Linalool monoxide was first prepared by Prileschaev 41
by the action of benzoyl peroxide on linalool. Naves and Bachmann 42 recently subjected
this compound to a thorough study. It is interesting to note that this oxide occurs in oil of
Mexican linaloe, apparently as a result of the oxidation of linalool in vivo in the tree.
The isomerization of linalool to geraniol and vice versa is of general academic
interest and constitutes a classic example of an ionotropic or allylic rearrangement.
This type of rearrangement is common to the allyl type of alcohols. In the case of
linalool, the rearrangement to geraniol may be brought about by prolonged boiling with
acetic anhydride. It is reported 43 that treatment of linalool with acetic anhydride
containing 1 per cent phosphoric acid in the cold for twenty-four hours yields 41 per cent
geraniol.
Conditions for the isomerization of linalool into geraniol and vice versa were studied
in detail by Rivkin and Meerzon.44 They found that the reaction was reversible in the
presence of water at high temperatures and pressures and in the absence of catalysts. Weak
acids caused dehydration, whereas certain salts had catalytic influence on the reaction. In
all cases. the rearrangement took place only to a small extent. A thorough study of the
mechanism of this type of reaction has been made by Prevost.45
Chablay, Ann. dam. [9], 8, 193 (1917).
Paal, Ger. Pat. 298,193.
38 Palfray, Bull. soc. chim. France [5], 7, 404 (1940).
" Stevens and McNiven, J. Am. Chem. Soc. 61, 1295 (1939).
40 Enklaar, Rec. tray. chim. 27, 411 (1909).
41 Prileschaev, Ber. 42, 4813 (1909).
42 Naves and Bachmann, Helv. Chim. Acta 28, 1227 (1945).
43 Nametkin and Fedoseeva, Sintezy Dushistikh Veshchestv, Sbornik Statei, 1939, 257; Chem. Abst.
36, 3777 (1942).
44 Rivkin and Meerzon, J. Gen. Chem. (U.S.S.R.) 5, 274 (1935).
45 Prevost, Compt. rend. 185, 1283 (1927).
36
37
Since the rearrangement is a reversible reaction, it is possible to convert geraniol to
linalool although with greater difficulty and in smaller yields. Heating geraniol with
water in an autoclave at 200° is reported to cause partial conversion to linaloo1.46 A
similar rearrangement is brought about when sodium geranyl phthalate is subjected to
steam distillation.47 Many studies of allylic rearrangements have been carried out through
the halide compound, the transformation taking place with greater ease with the halide
than with the hydroxyl radical. Dupont and Labaune treated linalool (I) with hydrogen
chloride in toluene solution at 100° and obtained a chloride which proved to be
geranyl chloride (VII). The same product was obtained by using phosphorus trichloride
at lower temperatures. This chloride on treatment with silver oxide suspended in benzene
gave dl-linalool (I), whereas treatment with potassium acetate gave dl-lina
48
Schimmel & Co., Report, April (1898), 27.
Stephan, J. prakt. Chem. [2], 60, 252 (1899).
4 8 Dupont and Labaune, Rome-Bertrand fils, Sci. Ind. Bull. [2], 10, 19 (1909) ; [3], 1, 42 (1910) ; [3], 3
(1911).
46
47
Another very interesting property of linalool is the ease with which it
undergoes cyclization. It was discovered in the early stages of the study of linalool
that both dilute sulfuric acid 49 and strong formic acid 50 converted it not only to
geraniol and nerol but also to alpha terpin hydrate. The esters of these alcohols
were obtained by treating linalool with acetic anhydride.
The reaction mechanism involving the formation of alpha terpineol (IX) is not well
understood. It will be noticed that on cyclization, the
asymmetric carbon atom created. Ordinarily one might expect an optically inactive
terpineol from this cyclization reaction. The terpineol formed, however, is optically
active and it always possesses an opposite sign to the linalool used. It appears that
during the cyclization reaction the asymmetric carbon (*) of linalool is exerting an
influence to form a terpineol having a preponderance of one of the isomers. The
reaction offers an interesting case for the study of optical isomerism.51
When linalool is treated with strong acids or strong catalytic agents, complex
transformations take place yielding a range of hydrocarbons. Thus, treatment of
linalool with 30 per cent sulfuric acid at elevated temperatures
gave myrcene,
dipentene, terpinolene, p-cymene, alpha-terpineol, 1:4 and 1 :8-cineole. Similar
transformations are brought about by treatment of linalool with Japanese acid earth."
Heating linalool at high temperatures with oxalic acid has yielded a diterpene and
alpha-camphorene among other products."
52
Tiemann and Schmidt, Ber. 28, 2137 (1895).
Stephan, J. prakt. Chem. [2], 58, 116 (1898) ; Zeitschel, Ber. 39, 1788 (1906).
Chernoyarova, J. Gen. Chem. (U.S.S.R) 7, 885 (1937).
tr' 2 Matsuura, J. Sci. Hiroshima Univ. 8A, 303 (1938).
5 3 Matsuura and Masumoto, J. Sci. Hiroshima Univ. 8A, 121 (1938).
Semmler and Jonas, Ber. 47, 2079 (1914).
49
50
54
51
Vanin,
Gentle dehydration of linalool by distilling under vacuum in the presence of
iodine has yielded myrcene. This is an elegant method of dehydration of tertiary
alcohols and is especially applicable to terpenes.
Linalool reacts with boric acid to form the borate, a property which can be
used to advantage in its separation and purification. The preparation of linalyl
borate is best carried out by heating linalool with a low boiling alcohol borate
such as butyl borate. Butyl alcohol distills off leaving linalyl borate in the
reaction mixture. It may also be carried out by heating linalool and boric acid in
benzene or toluene until the completion of the reaction. The reaction is completed
when no more water distills over along with benzene or toluene.
There is a considerable variation in the physical constants of linalool as
reported in the literature. This variation is due to the fact that no convenient
derivative can be prepared from which pure linalool can be regenerated.
The following constants may be taken for comparatively pure linalool
55
Phenylurethane m. p. ................... 65° C.
a-Naphthylurethane m p. .......... 53° C.
Solubility in alcohol .................... 1 to 2 volumes in 70% alcohol
The optical rotation of linalool varies, depending upon its source. The highest
recorded rotation is —20.7 from oil of lime and +19.8 from orange oil.
Commercial Methods of Preparation.—Commercial linalool is obtaiaed from
several sources, principally from oil of bois de rose, linaloe, shin and coriander.
Purification of linalool is not practicable because of its chemical properties, and its
preparation consequently involves careful fractiona55
Brooks and Humphrey, J. Am. Chem. Soc. 40, 845 (1918). Arbusov and Abramov, Ber. 67B, 1942 (1934).
tion of these oils. Fortunately, linalool occurs to the extent of about 80 per cent in
the first two oils mentioned, and they therefore constitute very convenient sources of
this valuable alcohol. Highly efficient distilling columns are used to effect
separation of linalool from other components of the oil consisting mainly of terpene
hydrocarbons and alcohols other than linalool. Since the boiling point of linalool is
considerably higher than that of the hydrocarbons and somewhat lower than that of
geraniol and citronellol, a good fractionating column yields comparatively pure
linalool. In practice, the first distillation serves to separate the linalool from the
bulk of impurities, and a second fractionation is necessary to obtain a fine grade of
product. Aside from this, it is found that the various fractions of linalool possess
slightly different odors, and these fractions or cuts are graded and employed for
different purposes. As a rule, it is possible to identify the source of linalool merely
by smelling it, since each oil contains traces of substances which are not easily
removed by distillation.
Chemical methods for the purification of linalool have been proposed from time
to time but they have proved to be unnecessary and impractical. It is possible to
form the phthalate or the borate of linalool and then distill off the terpene impurities.
Since these esters have a very high boiling point, a single fractionation is sufficient
to separate the terpene hydrocarbons. Such a procedure is rather expensive,
however, and is not commonly employed in view of the satisfactory results obtained
by the method described in the preceding paragraph.
The most important ester of linalool is its acetate which is prepared in large
quantities. Since there are several cheap sources of linalool and the acetate is in
great demand because of its fresh odor, a considerable amount of work has been
done to devise a satisfactory method of acetylating this alcohol. It is well known that
tertiary alcohols esterify with greater difficulty than do primary and secondary
alcohols. Furthermore, since linalool undergoes dehydration, cyclization, and
isomerization with great ease, ordinary methods of esterification such as heating with
acetic acid in the presence of inorganic acids cannot be used in this case.
A method commonly used involves the gentle heating of linalool with an excess
of acetic anhydride in the presence of powdered anhydrous sodium acetate. The
mixture is usually heated to 100 °-110 ° C. for a period of several hours and then
washed free of acid and fractionated. This method yields approximately 50 per cent
linalyl acetate along with unchanged linalool which can be re-esterified, and small
amounts of terpenes and esters of other alcohols. A low temperature esterification of
linalool has been developed by Isagulyantz and co-workers,56 who treated linalool
with acetic
Tsagulyantz and Smolyaninov, Russ. Pat. 31,430, October 31, 1933;
RiechstoffInd. Kosmetik 8, 194 (1933).
56
anhydride containing less than 1 per cent orthophosphoric acid. They report
almost complete esterification of linalool by holding the mixture at 38-40° for two
hours and then at room temperature for twelve hours. In both of these procedures, a
little geranyl acetate is produced as a result of isomerization of linalool. This can be
largely removed by fractionation since the boiling point of the geraniol ester is
considerably higher.
Evaluation and Analysis.—The finest grades of linalool possess a sweet, lily-like
odor. Commercial samples should be free from woody notes and other by-odors
which indicate incomplete purification. The acetate which is used in the perfumery,
cosmetic and soap industries in large quantities possesses a fresh, highly fragrant,
lavender type odor.
Estimation of the percentage of linalool in commercial products is carried out
conveniently by the Glichitch
method of formylation. Ordinary methods of
esterification with acetic anhydride in presence of anhydrous sodium acetate are not
satisfactory, since linalool is easily dehydrated and isomerized under such
conditions. It was noted by Behal
that a mixture of formic acid and acetic
anhydride converted tertiary alcohols quantitatively to their formates. Based on
this observation, Glichitch developed a suitable method of estimation of tertiary
alcohols (see Determination of Alcohols).
57
58
Another method recently developed by Fiore is the acetyl chloride-dimethyl
aniline method which gives excellent results and has largely superseded Glichitch's
method for the determination of linalool. Fiore's method has been adopted by the
Essential Oil Association of U. S. A. (see Determination of Alcohols).
Trade and Commerce.—The United States Tariff Commission has published the
following statistics for the production and sales of linalool and linalyl acetate.
These figures represent the manufactured product only. Much larger quantities of
linalool and its acetate are used indirectly in the form of such natural oils as
rosewood, lavender, etc.
Linalool and its esters are shipped in glass or tin-lined containers. Good quality
galvanized iron or black iron drums are also used. Poor quality esters of linalool
may undergo a slight hydrolysis on prolonged storage. The unpleasant odor of the
free acid thus produced may be removed by washing with dilute sodium carbonate
solution.