Limonene · aiwiki.page

Limonene is a naturally occurring, cyclic hydrocarbon with the molecular formula $\mathrm{C_{10}H_{16}}$ . It belongs to the monoterpenes, a class of ten-carbon compounds produced by plants, and is a major constituent of citrus peel oils. Limonene is used as a flavoring, fragrance ingredient, and solvent, particularly in cleaning and degreasing products. Its two mirror-image molecular forms occur in different proportions in natural sources. (webbook.nist.gov)

Structure and stereochemistry

The limonene molecule contains a six-membered carbon ring with one carbon–carbon double bond, a methyl group, and an isopropenyl group containing a second double bond. Its systematic name is 1-methyl-4-(prop-1-en-2-yl)cyclohex-1-ene, and its molar mass is approximately $136.23\ \mathrm{g\,mol^{-1}}$ . The two double bonds are not conjugated. (webbook.nist.gov)

Limonene has one stereogenic center and therefore exists as two enantiomers:

$(R)$ -(+)-limonene, commonly called d-limonene, has CAS Registry Number 5989-27-5.
$(S)$ -(−)-limonene, commonly called l-limonene, has CAS Registry Number 5989-54-8. (pubchem.ncbi.nlm.nih.gov)

These forms illustrate chirality: they have the same atomic connectivity but are non-superimposable mirror images. The $R$ and $S$ labels describe absolute configuration, whereas $+$ and $−$ indicate the experimentally observed direction of optical rotation. An equal mixture of the two forms is called racemic limonene or dipentene; CAS Registry Number 138-86-3 is commonly associated with this material and with stereochemically unspecified limonene. (inchem.org)

Physical properties and chemical behavior

Limonene is a colorless, volatile liquid with a characteristic citrus odor. Representative properties reported for d-limonene include a boiling point near $178\,^\circ\mathrm{C}$ , a melting point near $-74\,^\circ\mathrm{C}$ , and a relative density of approximately $0.84$ , with water taken as 1. Its solubility in water is very low, and its closed-cup flash point is approximately $48\,^\circ\mathrm{C}$ . (inchem.org)

Limonene readily oxidizes on exposure to air. This changes the composition of stored material, producing oxygen-containing compounds, including hydroperoxides. The distinction between fresh limonene and its oxidation products is important in both product stability and the assessment of skin sensitization. (inchem.org)

Natural occurrence and biosynthesis

Limonene occurs in the essential oils of numerous plants. The $(R)$ -(+)-form predominates in citrus peel oils, which are its principal commercial source. It is commonly recovered as a by-product of citrus processing through oil separation and distillation. Limonene also occurs in other plant oils, including those of mint species. (inchem.org)

Plants synthesize limonene from geranyl diphosphate, a ten-carbon precursor, through the action of a limonene synthase enzyme. The reaction involves rearrangement and cyclization of the precursor, followed by loss of a proton to form limonene. Different enzymes control the three-dimensional course of the reaction and produce different enantiomers; a (+)-limonene synthase has been structurally characterized from sweet orange, Citrus sinensis. (pmc.ncbi.nlm.nih.gov)

In mint, limonene also serves as an intermediate in the biosynthesis of more extensively modified monoterpenes. Research on peppermint has localized limonene synthase to leucoplasts in the secretory cells of oil glands, connecting its production with specialized structures that manufacture and store essential oils. (pmc.ncbi.nlm.nih.gov)

Uses

The principal uses of limonene depend on its odor and solvent properties. It is incorporated into food flavorings, perfumes, cosmetics, soaps, and household cleaning products. As a solvent, it is used in degreasing and cleaning formulations and has replaced some chlorinated or petroleum-derived solvents in particular applications. (inchem.org)

Limonene has also been used as an active ingredient in insecticidal and animal-repellent products. The United States Environmental Protection Agency has evaluated these pesticide uses separately from its uses as a flavoring or fragrance ingredient. (www3.epa.gov)

Health and environmental considerations

Concentrated d-limonene can irritate the skin and, to a lesser extent, the eyes. Repeated or prolonged contact may cause skin sensitization, particularly after the material has oxidized. Its occurrence in food and plant oils therefore does not imply that concentrated limonene or aged formulations are free of hazards. It is also flammable and very toxic to aquatic organisms. (inchem.org)

Limonene participates in atmospheric chemistry after evaporation. Chamber experiments show that reactions with ozone generate oxygenated products, some of which enter the particle phase and contribute to secondary organic aerosol. Its environmental significance therefore includes both the emitted compound and the products formed during its oxidation. (acp.copernicus.org)

References

Limonene — NIST Chemistry WebBookwebbook.nist.gov
(+)-Limonene — PubChempubchem.ncbi.nlm.nih.gov
Limonene, (-)- — PubChempubchem.ncbi.nlm.nih.gov
Limonene — Concise International Chemical Assessment Document 5inchem.org
ICSC 0918 — D-Limoneneinchem.org
Functional and Structural Characterization of a (+)-Limonene Synthase from Citrus sinensispmc.ncbi.nlm.nih.gov
Direct Evidence of an Enzyme-Generated LPP Intermediate in (+)-Limonene Synthase Using a Fluorinated GPP Substrate Analogpubmed.ncbi.nlm.nih.gov
Limonene Synthase, the Enzyme Responsible for Monoterpene Biosynthesis in Peppermint, Is Localized to Leucoplasts of Oil Gland Secretory Cellspmc.ncbi.nlm.nih.gov
Tolerance Reassessment and Risk Management Decision for Limonenewww3.epa.gov
Multiphase composition changes and reactive oxygen species formation during limonene oxidation in the new Cambridge Atmospheric Simulation Chamber (CASC)acp.copernicus.org
Large differences of highly oxygenated organic molecules (HOMs) and low-volatile species in secondary organic aerosols (SOAs) formed from ozonolysis of β-pinene and limoneneacp.copernicus.org