Limonene

Limonene is a colourless liquid hydrocarbon classified as a cyclic terpene possessing a strong smell of oranges. It is used in chemical synthesis as a precursor to carvone and as a renewably-based solvent in cleaning products.

Limonene takes its name from the lemon, as the rind of the lemon, like other citrus fruits, contains considerable amounts of this compound, which contributes to their odor. Limonene is a chiral molecule, and 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. D-Limonene is obtained commercially by extraction from orange peel with liquid CO₂.

Chemistry

Reactions

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 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. The final step involves loss of a proton from the cation to form the alkene.

Uses

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 and is also useful as a fragrant alternative to turpentine. Limonene is also used as a solvent in some model airplane glues. All-natural commercial air fresheners, with air propellants, containing limonene are used by philatelists to remove self-adhesive postage stamps from envelope paper.

As it is combustible, limonene has also been considered as a biofuel.

Limonene can be used to dissolve polystyrene, and is a more ecologically friendly substitute for acetone.

Safety

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.

Compendial status

See also

Notes