General Description
Colorless to pale yellow oily liquid with a sweet rose odor.
Reactivity Profile
An unsaturated aliphatic hydrocarbon and an alcohol. Flammable and/or toxic gases are generated by the combination of alcohols with alkali metals, nitrides, and strong reducing agents. They react with oxoacids and carboxylic acids to form esters plus water. Oxidizing agents convert them to aldehydes or ketones. Alcohols exhibit both weak acid and weak base behavior. They may initiate the polymerization of isocyanates and epoxides.
Chemical Properties
colourless to pale yellow liquid with an odour of roses
Chemical Properties
Geraniol has a characteristic rose-like odor The physical constants vary for the various commercial products, depending on the total geraniol content; specifc gravity and refractive index may be indicative of the purity of the product Commercial geraniol cannot be classifed according to its alcohol content, as most of the recurring impurities are alcoholic in nature (nerol, citronellol, tetrahydrogeraniol) Gas chromatography techniques may be usefully employed to determine the gera- niol content in a product.
Chemical Properties
Geraniol occurs in nearly
all terpene-containing essential oils, frequently as an ester. Palmarosa oil contains
70–85% geraniol; geranium oils and rose oils also contain large quantities. Geraniol
is a colorless liquid, with a floral, rose-like odor.
Since geraniol is an acyclic, doubly unsaturated alcohol, it can undergo a number
of reactions, such as rearrangement and cyclization. Rearrangement in the
presence of copper catalysts yields citronellal. In the presence of mineral acids, it
cyclizes to form monocyclic terpene hydrocarbons, cyclogeraniol being obtained
if the hydroxy function is protected. Partial hydrogenation leads to citronellol,
and complete hydrogenation of the double bonds yields 3,7-dimethyloctan-l-ol
(tetrahydrogeraniol). Citral may be obtained from geraniol by oxidation or by
catalytic dehydrogenation. Geranyl esters are prepared by esterification.
Geraniol is one of the most frequently used terpenoid fragrance materials.
It can be used in all floral, rose-like compositions and does not discolor soaps. In
flavor compositions, geraniol is used in small quantities to accentuate citrus notes.
It is an important intermediate in the manufacture of geranyl esters, citronellol,
and citral.
Occurrence
The presence of geraniol in nature has been reported in more than 160 essential oils: ginger grass, lemongrass, Ceylon and Java citronella, tuberose, oak musk, orris, champaca, ylang-ylang, mace, nutmeg, sassafras, Cayenne Bois-de-Rose, Acacia farnesiana, geramium clary sage, spike, lavandin, lavender, jasmine, coriander, carrot, myrrh, eucalyptus, lime, mandarin petitgrain, bergamot petitgrain, bergamot, lemon, orange and others The essential oils of palmarosa and Cymbopogon winterianus contain the highest levels of geraniol (approx 80 to 95%) Also reported in numerous other sources including apple juice, citrus peel oils and juices, bilberry, cranberry, other berries, guava, papaya, cinnamon, ginger, corn mint oil, mustard, nutmeg, mace, milk, coffee, tea, whiskey, honey, passion fruit, plums, mushrooms, mango, starfruit, cardamom, coriander leaf and seeds, litchi, Ocimum basilicum, myrtle leaf, rosemary, clary sage, Spanish sage and chamomile oil
Definition
ChEBI: A monoterpenoid consisting of two prenyl units linked head-to-tail and functionalised with a hydroxy group at its tail end.
Preparation
A convenient route for the production of geraniol and nerol consists
of the hydrogenation of citral, which is used in large quantities as an intermediate
in the synthesis of vitamin A. Large-scale processes have, therefore, been developed
for producing geraniol. Currently, these are far more important than isolation
from essential oils. Nevertheless, some geraniol is still isolated from essential
oils for perfumery purposes.
1) Isolation from essential oils: Geraniol is isolated from citronella oils and from
palmarosa oil. Fractional distillation of, for example, Java citronella oil (if necessary,
after saponification of the esters present) yields a fraction containing
about 60% geraniol, as well as citronellol and sesquiterpenes.
Aproductwith a higher geraniol content and slightly different odor quality for
use in fine fragrances is obtained by fractionating palmarosa oil after saponification
of the geranyl esters.
2) Synthesis from β-pinene: Pyrolysis of β-pinene yields myrcene, which is converted
into a mixture of predominantly geranyl, neryl, and linalyl chloride by
addition of hydrogen chloride in the presence of small amounts of catalyst, for
example, copper(I) chloride and an organic quaternary ammonium salt.
After removal of the catalyst, the mixture is reacted with sodium acetate in
the presence of a nitrogen base (e.g., triethylamine) and converted to geranyl
acetate, neryl acetate, and a small amount of linalyl acetate.
Geraniol is obtained after saponification and fractional distillation of the
resulting alcohols.
3) Synthesis from linalool: A 96% pure synthetic geraniol prepared by isomerization
of linalool has become commercially available. Orthovanadates are used
as catalysts, to give a >90% yield of a geraniol–nerol mixture. Geraniol
of high purity is finally obtained by fractional distillation.
A considerable portion of commercially available geraniol is produced by a
modified process: linalool obtained in a purity of about 65% from α-pinene is
converted into linalyl borates, which rearrange in the presence of vanadates
as catalysts to give geranyl and neryl borates. The alcohols are obtained by
hydrolysis of the esters.
4) Synthesis from citral: Citral has very recently come to be produced petrochemically
in very large quantities, so partial hydrogenation of citral has
become a very economical route for the production of geraniol. A high
selectivity for this reaction can be achieved by the use of special catalysts
[106] or by special reaction techniques.
Aroma threshold values
Detection: 4 to 75 ppb.
Taste threshold values
Taste characteristics at 10 ppm: sweet foral rose, citrus with fruity, waxy nuances.
Flammability and Explosibility
Nonflammable
Anticancer Research
Starting from antitumor activity against several cell lines by an arrest occurring atthe G0/G1 cell cycle and ultimately with an increase of apoptosis, this molecule wasfound to interfere with the mevalonic cycle enzyme. Suppression of prenylation ofproteins leads to the inhibition of DNA synthesis, and the suppression of 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) leads to a reduction of the mevalonate pool andthus limits protein isoprenylation. In the same way, a reduction of cholesterol biodisponibilitywas controlled (Pattanayak et al. 2009; Ni et al. 2012; Dahham et al.2016).
Synthesis
By fractional distillation from those essential oils rich in geraniol, or synthetically from myrcene.
Metabolism
Geraniol is metabolized in the rabbit by ω-oxidation and by reduction of an α β-unsaturated bond (Parke, 1968). The products of geraniol metabolism are 'Hildebrandt acid' and 7-carboxy-3-methylocta-6-enoic acid. The latter acid is optically active (Williams, 1959).
storage
4°C, protect from light
Purification Methods
Purify geraniol by ascending chromatography or by thin layer chromatography on plates of kieselguhr G with acetone/water/liquid paraffin (130:70:1) as solvent system. Hexane/ethyl acetate (1:4) is also suitable. Also purify it by GLC on a silicone-treated column of Carbowax 20M (10%) on Chromosorb W (60-80 mesh). [Porter Pure Appl Chem 20 499 1969.] Store it in full, tightly sealed containers in the cool and protect from light. It has a pleasant odour. [cf p 681, Beilstein 1 IV 2277.]
