Chemical Properties
(?)-Citronellal occurs in Java citronella oil at a concentration of 35%. Racemic citronellal is the main constituent of E. citriodora
oil with a content of up to 85%.
Pure citronellal is a colorless liquid with a refreshing odor, reminiscent of
lemon balm. Upon catalytic hydrogenation, citronellal yields dihydrocitronellal,
citronellol, or dihydrocitronellol, depending on the reaction conditions. Protection
of the aldehyde group, followed by addition of water to the double bond in
the presence of mineral acids or ion-exchange resins results in the formation of
3,7-dimethyl-7-hydroxy-octan-l-al (hydroxydihydrocitronellal). Acid-catalyzed
cyclization to isopulegol is an important step in the synthesis of (?)-menthol.
Chemical Properties
Ceylon citronella (C. nardus) and Java citronella (C. winterianus) are both perennial grasses growing more than 1 m
high. The herb is harvested two to three times a year in Ceylon. The freshly cut or partially dried herb is steam distilled. The plant
yields the largest amount of essential oil at about its third year of growth. Citronella is also cultivated and distilled in Java, Guatemala,
Taiwan, Hainan, Argentina and New Guinea. The Java-type essential oil is considered to be of superior quality over the Ceylon type.
Citronella has a characteristic citronella, rose- and lemon-like odor. The Council of Europe (CoE, 2000) has described Ceylon citronella
and Java Citronella separately.
Chemical Properties
Citronellal has an intense, lemon-, citronella-, rose-type odor.
Chemical Properties
clear light yellow liquid
Occurrence
The d-form of citronellal has been reported in the oil of citronella (Ceylon, Jammus, Kaschmis), in the oil from leaves of Barosma pulchella, in the oil from roots of Phebalium nudum and in the oils of Eucalyptus citriodora, Leptospermum citratum and Baeckea citriodora. The /-form is present in the oils of Backhousia citriodora var. A, E. citriodora, Litsea cubeba (fruits) and lemongrass. Citronellal is generally present also in the oils of lemon, mandarin, Lavandula delphinensis, Ocimum canum f. citrata and many others (Fenarolis Handbook of Flavor Ingredients, 1971).
Definition
ChEBI: A monoterpenoid, the main component of citronella oil which gives it its distinctive lemon aroma.
Preparation
Citronellal is still isolated from essential oils in considerable quantities;
it is also produced synthetically.
1) Isolation from essential oils:(+)-Citronellal is obtained from citronella oils
by fractional distillation. Racemic citronellal is isolated from E. citriodora oil;
when necessary, it is purified by using an addition compound, for example,
the bisulfite derivative.
2) Synthesis from geraniol or nerol: Racemic citronellal can be obtained by vaporphase
rearrangement of geraniol or nerol in the presence of, for example, a
barium-containing copper–chromium oxide catalyst.
3) Synthesis from citronellol: Racemic citronellal can also be obtained by dehydrogenation
of citronellol under reduced pressure with a copper chromite
catalyst.
4) Synthesis from citral: Selective hydrogenation of citral to citronellal can be
accomplished in the presence of a palladium catalyst in an alkaline alcoholic
reaction medium. A continuously operating process for the hydrogenation
on a palladium catalyst in the presence of trimethylamine has been developed.
5) Synthesis from myrcene: (+)- and (?)-Citronellal are available from myrcene
via geranyldiethylamine, which is enantioselectively isomerized to (+)- or (?)-citronellalenamine. Hydrolysis yields pure (+)- or (?)-citronellal; see monograph menthol.
Essential oil composition
Citronella oil contains a number of fragrant fractions of which citronellal, geraniol and citronellol are the
major components. Ceylon citronella oil contains 55 to 65% total acetylizable alcohols (calculated as citronellol) and 7 to 15% total
aldehyde (calculated as citronellal). The main constituents are geraniol (18 to 20%), citronellol (6.4 to 8.4%), citronellal (5 to 15%),
geranyl acetate (2%); limonene (9 to 11%) and methyl isoeugenol (7.2 to 11.3%). Other constituents are camphene, caryophyllene, linalool,
citral (neral and geranial), methylheapenone, methyleugenol, l-borneol, nerol, eugenol and farnesol.* Java citronella oil contains
not less than 35% alcohols (calculated as citronellol) and not less than 35% aldehydes (calculated as citronellal). The Java type appears
to have higher concentrations of citronellol (35%) and geraniol (21%) than does the Ceylon type (citronellol 10% and geraniol 18%).
Aroma threshold values
Detection: 31 to 100 ppb
Taste threshold values
Taste characteristics at 10 ppm: floral, green, rosy and citrus-lemon.
General Description
(±)-Citronellal was studied for its fumigant antifungal activity against Pyricularia (Magnaporthe) grisea.
Flammability and Explosibility
Nonflammable
Synthesis
Can be prepared by chemical synthesis or by fractional distillation of natural oils, such as citronella. Industrially
prepared by hydrogenation of β-citronellol or by catalytic hydrogenation of citral; also in the laboratory by dehydration of
hydroxydihydrocitronellal.
Metabolism
Feeding 50 g citronellal to rabbits was followed by the isolation of 13 g of a crystalline glucuronide, which proved to be p-menthane-3.8-diol-D-glucuronide. The citronellal appeared to have been cyclized and the glucuronide obtained was identical with that obtained on feeding p-menthane-3,8-diol (menthoglycol) (Kühn & Low, 1938). However, evidence was produced to show that the cyclization was not, strictly speaking, a biological reaction, but a chemical one which took place in the stomach under the influence of the gastric hydrochloric acid. The conjugation of the menthoglycol with glucuronic acid was, of course, a purely biological reaction. It was found that on shaking 20 g citronellal with 200 ml 0-5% HCl for 48 hr at 37 C, 12 g menthoglycol was formed (Kühn & Low, 1938).
