Limonene

Limonene is a hydrocarbon, classed as a terpene. It is a clear, 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, which is the (R)-enantiomer (CAS number 5989–27–5, EINECS number 227–813–5).

Uses

As the main odour constituent of citrus, d-limonene is used in food manufacture as a flavouring, and added to cleaning products such as washing up liquid to give a lemon fragrance. Limonene is increasingly being used as an environmentally friendly alternative to mineral oils as a solvent for cleaning purposes, such as the removal of oil from machine parts, being more easily degradable than mineral oils, and produced from a renewable source (citrus oil, as a byproduct of orange juice manufacture.)

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.

Safety information

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.⁽⁵⁾

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. [1]

Europe

Limonene is listed in annex 1 of the directive 67/548/EEC under index number 601–029–00–7 [2]. The pure substance is classified as irritant (Xi) and dangerous for the environment (N). The following risk and safety statements are obligatory:

• 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 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.

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.

Canada

Limonene is classified [3] under the categories

• B3: Combustible liquid
• D2B: Toxic material causing other toxic effects

The presence of limonene in a preparation at a concentration greater than 1.0% must be disclosed.

Australia

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

Notes and References

1. IARC Monographs on the evaluation of carcinogenic risks to humans 1999, 73, 307–27.

2. Source: European Chemicals Bureau.

3. Source: CSST Workplace Hazardous Materials Information System.

4. 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.

5. M. Matura et al., J. Am. Acad. Dermatol. 2002, 33, 126–27.