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]

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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.

Due to its combustible nature, d-limonene has also seen limited use as an experimental biofuel.[2]

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.^[2] 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.^[3] The final step involves loss of a proton from the cation to form the alkene.

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.^[4]

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 α_2u-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.^[4]

Notes and references

1. ^ J. L. Simonsen, The Terpenes Volume I (2nd edition), Cambridge University Press, 1947.
2. ^ Source: European Chemicals Bureau.
3. ^ 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.
4. ^ ^{a b} IARC Monographs on the evaluation of carcinogenic risks to humans 1999, 73, 307-27 [1]

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.