Chemical Properties
The enantiomers (3R)-(+)-citronellol and (3S)-(?)-citronellol
occur in many essential oils.
(?)-Citronellol isolated from natural sources is often named rhodinol. At
present, the name rhodinol is also used for the isopropenyl isomer α-citronellol
or a mixture of the two isomers.
In many natural products, citronellol occurs as a mixture of its two enantiomers;
the pure (+) or (?) formis seldom found. (?)-Citronellol is the predominant enantiomer
in geranium and rose oils, both of whichmay contain up to 50% citronellols.
Citronellol is a colorless liquid with a sweet rose-like odor. The odor of (?)-
citronellol is more delicate than that of (+)-citronellol.
Citronellol undergoes the typical reactions of primary alcohols. Compared with
geraniol, which contains one more double bond, citronellol is relatively stable.
Citronellol is converted into citronellal by dehydrogenation or oxidation; hydrogenation
yields 3,7-dimethyloctan-l-ol. Citronellyl esters are easily prepared by
esterification with acid anhydrides.
Preparation
(?)-Citronellol is still obtained mainly from geranium oil by
saponification followed by fractional distillation (“rhodinol”). Although of high
odor quality, this grade does not possess the true (?)-citronellol odor due to
impurities. Much larger quantities of (+)-citronellol and racemic citronellol are
used and are prepared by partial or total synthesis.
1) Synthesis from citronellal: Citronellal can be hydrogenated to citronellol
by the use of special catalysts and/or special hydrogenation techniques,
for example, [122]. The citronellal that is used as the starting material
may originate from synthetic production or from isolation of essential
oils. Citronellal from citronella oil yields (+)-citronellol; the corresponding
material from citronellal from Eucalyptus citriodora oil is racemic. Pure
(+)-citronellol is also obtained from (+)-citronellal, which is produced as an
intermediate of (?)-menthol. By this asymmetric technology,
pure (?)-citronellal and, therefore, pure (?)-citronellol are also available.
2) Synthesis of racemic or slightly dextrorotatory citronellol from geraniol
fractions of essential oils: This citronellol is produced by catalytic hydrogenation of saponified geraniol fractions (also containing (+)-
citronellol) obtained from Java citronella oil, followed by fractional
distillation. Selective hydrogenation of the double bond in the 2-position
of geraniol in geraniol–citronellol mixtures isolated from essential oils
can be achieved by using Raney cobalt as a catalyst; overhydrogenation to
3,7-dimethyloctan-l-ol can be largely avoided by this method.
3) Synthesis of racemic citronellol from synthetic geraniol, nerol, or citral: A considerable
amount of commercial synthetic racemic citronellol is produced by
partial hydrogenation of synthetic geraniol and/or nerol. Another starting
material is citral, which can be hydrogenated, for example, in the presence
of a catalyst system consisting of transition metals and amines.
4) Preparation of (?)-citronellol from optically active pinenes: (+)-cis-Pinane is
readily synthesized by hydrogenation of (+)-α-pinene or (+)-β-pinene and is
then pyrolyzed to give (+)-3,7-dimethyl-1,6-octadiene. This compound can
be converted into (?)-citronellol (97% purity) by reaction with triisobutylaluminumor
diisobutylaluminumhydride, followed by air oxidation and hydrolysis
of the resulting aluminum alcoholate.
Contact allergens
L-Citronellol is a constituent of rose and geranium
oils. d-Citronellol occurs in Ceylon and Java citronella
oils. As a fragrance allergen, citronellol has to be mentioned
by name in cosmetics within the EU.
