Limonene

Limonene is a hydrocarbon, classed as a 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 specific 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]

Uses
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, and added to cleaning products such as hand cleansers to give a lemon-orange fragrance. See: orange oil.

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.) Limonene works as paint stripper when applied to painted wood. The (R)-enantiomer is also used as botanical insecticide.

The (S)-enantiomer, also known as l-limonene (CAS number 5989-54-8, EINECS number 227-815-6), is used as a fragrance in some cleaning products. In contrast to the citrus (orange-lemon) scent (see above) of d-limonene, the enantiomer l-limonene has a piney, turpentine-like odor.

Limonene is very common in cosmetic products.

Chemistry
Limonene is a relatively stable terpene, which can be distilled without decomposition, though it forms isoprene when passed over a hot metal filament. It is easily oxidised in moist air to carveol and carvone[4]. Oxidation using sulfur leads to p-cymene and a sulfide.

Limonene occurs naturally as the (R)-enantiomer, but it can be racemised to dipentene simply by heating at 300 °C. When warmed with mineral acid, limonene forms the conjugated diene terpinene, which can itself easily be oxidised to p-cymene, an aromatic hydrocarbon. Evidence for this 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.

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



General
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.

Limonene causes renal cancer in male rats, but not in female rats or in mice of either sex, due to binding of the metabolite limonene-1,2-oxide to α2u-globulin, a protein produced only by male rats. 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. 

Europe
Limonene is listed in annex 1 of the directive 67/548/EEC under index number 601-029-00-7. The pure substance is classified as irritant (Xi) and dangerous for the environment (N). The following risk and safety statements are obligatory: Limonene is not specifically mentioned in directive 1999/45/EC, and so the labelling of preparations containing this compound is at the discretion of the manufacturer.
 * R10: Flammable.
 * R38: Irritating to skin.
 * R43: May cause sensitization by skin contact.
 * R50/53: Very toxic to aquatic organisms, may cause long-term adverse effects in the aquatic environment.
 * S2: Keep out of the reach of children.
 * S24: Avoid contact with skin.
 * S37: Wear suitable non-latex gloves.
 * S60: This material and its container must be disposed of as hazardous waste.
 * S61: Avoid release to the environment. Refer to special instructions/Safety data sheets.

Canada
Limonene is classified under the categories The presence of limonene in a preparation at a concentration greater than 1.0% must be disclosed.
 * B3: Combustible liquid
 * D2B: Toxic material causing other toxic effects

Australia
Limonene is listed in Class 3, flammable liquids, Packing Group III, under the Australian Dangerous Goods Code.

Notes and references

 * 1) J. L. Simonsen, The Terpenes Volume I (2nd edition), Cambridge University Press, 1947.
 * 2) E. E. Turner, M. M. Harris, Organic Chemistry, Longmans, Green & Co., London, 1952.
 * 3) Wallach, Annalen der Chemie, 246, 221 (1888).
 * 4) Blumann & Zeitschel, Berichte, 47,  2623 (1914).
 * 5) J. Mann, R. S. Davidson, J. B. Hobbs, D. V. Banthorpe, J. B. Harborne, Natural Products, pp308-309, Addison Wesley Longman Ltd., Harlow, UK, 1994.  ISBN 0-582-06009-5.
 * 6) IARC Monographs on the evaluation of carcinogenic risks to humans 1999, 73, 307-27.
 * 7) Source: European Chemicals Bureau.
 * 8) Source: CSST Workplace Hazardous Materials Information System.
 * 9) The CAS and EINECS numbers in the table are for the racemic mixture: the relevant numbers for the two enantiomers are given in the text.
 * 10) M. Matura et al., J. Am. Acad. Dermatol. 2002, 33, 126-27.