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Limonene
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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).
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Encyclopedia
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.
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.
Uses
The major use of D-limonene is as a precursor to carvone.
Limonene is common in cosmetic products. As the main odor 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. 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.
Chemistry
Limonene is a relatively stable terpene, which can be distilled without decomposition, although at elevated temperatures it cracks to form isoprene. It oxidises easily in moist air to carveol and carvone. 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 a-terpinene, which can itself easily be oxidised to p-cymene, an aromatic hydrocarbon. Evidence for this isomerization includes the formation of Diels-Alder a-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.
Safety
Limonene and its oxidation products are skin and respiratory 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.
Although once thought to cause renal cancer in rats, limonene now is considered by some researchers to be a significant chemopreventive agent with potential value as a dietary anti-cancer tool in humans. 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.
No information is available on the health effects of inhalation exposure to d-limonene in humans, and no long-term inhalation studies have been conducted in laboratory animals.
D-limonene is biodegradable, but due to its low flash point, it must be treated as hazardous waste for disposal.
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