D-limonene
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Limonene
IUPAC name	1-Methyl-4-prop-1-en-2-yl-cyclohexene
Other names	4-Isopropenyl-1-methylcyclohexene Racemic: DL-limonene; dipentene
Identifiers
CAS number	[5989-27-5]
SMILES	CC(=C)[C@@H]1CCC(=CC1)C
Properties
Molecular formula	C10H16
Molar mass	136.24 g/mol
Density	0.8411 g/cm3
Melting point	-95.2 °C
Boiling point	176 °C
Hazards
R-phrases	R10 R38 R43 R50/53
S-phrases	(S2) S24 S37 S60 S61
Flash point	50 °C
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox disclaimer and references

Limonene is a hydrocarbon, classified as a cyclic terpene. It is a colourless liquid at room temperatures with an extremely strong smell of oranges. It takes its name from the lemon, as the rind of the lemon, like other citrus fruits, contains considerable amounts of this chemical compound, which is responsible for much of their smell. Limonene is a chiral molecule, and as is common with such forms, biological sources produce one enantiomer: the principal industrial source, citrus fruit, contains D-limonene ((+)-limonene), which is the (R)-enantiomer (CAS number 5989-27-5, EINECS number 227-813-5). Racemic limonene is known as dipentene.^[1]

Biosynthesis

Limonene is formed from geranyl pyrophosphate, via cyclisation of a neryl carbocation or its equivalent as shown.^[2] The final step involves loss of a proton from the cation to form the alkene.

Uses

The major use of D-limonene is as a precursor to carvone.^[3]

Limonene is common in cosmetic products. As the main odour constituent of citrus (plant family Rutaceae), D-limonene is used in food manufacturing and some medicines, e.g., bitter alkaloids, as a flavoring; it is also used as botanical insecticide.^{citation needed} It is added to cleaning products such as hand cleansers to give a lemon-orange fragrance. See: orange oil. In contrast, L-limonene has a piney, turpentine-like odor.

Limonene is increasingly being used as a solvent for cleaning purposes, such as the removal of oil from machine parts, as it is produced from a renewable source (citrus oil, as a byproduct of orange juice manufacture). It also serves as a paint stripper when applied to painted wood.

As it is combustible, limonene has also been considered as a biofuel.^[4]

In perhaps its most exciting application, limonene is useful for recycling polystyrene. In the 1990s, researchers at Sony discovered that limonene dissolves polystyrene, which can be recovered after boiling off the limonene.^[5].

Chemistry

Limonene is a relatively stable terpene, which can be distilled without decomposition, although at elevated temperatures it cracks to form isoprene.^[6] It oxidises easily in moist air to carveol and carvone.^[7] Dehydration with sulfur gives to p-cymene, hydrogen sulfide, as well as some sulfides.

Limonene occurs naturally as the (R)-enantiomer, but racemises to dipentene at 300 °C. When warmed with mineral acid, limonene isomerizes to the conjugated diene α-terpinene, which can itself easily be oxidised to p-cymene, an aromatic hydrocarbon. Evidence for this isomerization includes the formation of Diels-Alder α-terpinene adducts when limonene is heated with maleic anhydride.

It is possible to effect reaction at one of the double bonds selectively. Anhydrous hydrogen chloride reacts preferentially at the disubstituted alkene, whereas epoxidation with MCPBA occurs at the trisubstituted alkene. In both cases the second C=C double bond can be made to react if desired.

In another synthetic method Markovnikov addition of trifluoroacetic acid followed by hydrolysis of the acetate gives terpineol.

The most widely practiced conversion of limonene is to carvone. The three step reaction begins with the regioselective addition of nitrosyl chloride across the trisubstituted double bond. This species is then converted to the oxime with base, and the hydroxylamine is removed to give the ketone-containing carvone.^[3]

Safety

Limonene and its oxidation products are skin irritants, and limonene-1,2-oxide (formed by aerial oxidation) is a known skin sensitizer. Most reported cases of irritation have involved long-term industrial exposure to the pure compound, e.g. during degreasing or the preparation of paints. However a study of patients presenting dermatitis showed that 3% were sensitized to limonene.^[8]

Although once thought to cause renal cancer in rats, limonene now is known as a significant chemopreventive agent^[9] with potential value as a dietary anti-cancer tool in humans.^[10] There is no evidence for carcinogenicity or genotoxicity in humans. The IARC classifies D-limonene under Class 3: not classifiable as to its carcinogenicity to humans.^[8]

Notes

1. ^ J. L. Simonsen, The Terpenes Volume I (2nd edition), Cambridge University Press, 1947.
2. ^ Mann, J. C.; Hobbs, J. B.; Banthorpe, D. V.; Harborne, J. B. (1994). Natural products: their chemistry and biological significance. Harlow, Essex, England: Longman Scientific & Technical, 308-309. ISBN 0-582-06009-5. 
3. ^ ^{a b} Karl-Georg Fahlbusch, Franz-Josef Hammerschmidt, Johannes Panten, Wilhelm Pickenhagen, Dietmar Schatkowski, Kurt Bauer, Dorothea Garbe, Horst Surburg “Flavors and Fragrances“ in Ullmann's Encyclopedia of Industrial Chemistry, 2002, Wiley-VCH, Weinheim. doi:10.1002/14356007.a11_141.
4. ^ Cyclone Power to Showcase External Combustion Engine at SAE Event, Green Car Congress, 20 September 2007
5. ^ Using Oranges for Styrofoam Recycling
6. ^ H. Pakdela, D. Panteaa and C. Roy (2001). "Production of dl-limonene by vacuum pyrolysis of used tires". Journal of Analytical and Applied Pyrolysis 57 (1): 91–107. doi:10.1016/S0165-2370(00)00136-4. 
7. ^ Source: European Chemicals Bureau.
8. ^ ^{a b} IARC Monographs on the evaluation of carcinogenic risks to humans 1999, 73, 307-27 [1]
9. ^ Crowell PL (March 1999). "Prevention and therapy of cancer by dietary monoterpenes". J. Nutr. 129 (3): 775S–778S. PMID 10082788. 
10. ^ Tsuda H, Ohshima Y, Nomoto H, et al (August 2004). "Cancer prevention by natural compounds". Drug Metab. Pharmacokinet. 19 (4): 245–63. PMID 15499193. 

References

1. E. E. Turner, M. M. Harris, Organic Chemistry, Longmans, Green & Co., London, 1952.
2. Wallach, Annalen der Chemie, 246, 221 (1888).
3. Blumann & Zeitschel, Berichte, 47, 2623 (1914).
4. Source: CSST Workplace Hazardous Materials Information System.
5. M. Matura et al., J. Am. Acad. Dermatol. 2002, 33, 126-27.

External links

• Detailed description of d-limonene from Biochem Corp.