CID 440917

C₁₀H₁₆

Limonene

C 10
H 16

Ball-and-stick Stick Space-filling

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26 atoms, 26 bonds.

IUPAC name: (4R)-1-methyl-4-prop-1-en-2-ylcyclohexene
Molar mass: 136.238(23) g/mol
Atoms: 26
SMILES: CC1=CCC(CC1)C(=C)C
InChIKey: XMGQYMWWDOXHJM-JTQLQIEISA-N
PubChem CID: 440917

Made of

C H

Structural formula of limonene. Each unlabelled corner is a carbon atom, and the eight hydrogens on carbon are not drawn. Wedges come toward you, hashes go back. It shows which atoms are joined to which, not the shape. The model above shows the shape.

Citrus peel oils are more than 90 per cent (+)-limonene, according to the Hazardous Substances Data Bank (HSDB). A hydrocarbon of ten carbon and sixteen hydrogen atoms, C₁₀H₁₆, it has a molar mass of 136.24 g/mol. CAMEO Chemicals describes d-limonene as a clear, colourless, free-flowing liquid with a pleasant, lemon-like smell. It mixes completely with ethanol, yet HSDB gives its solubility in water at 25 °C as a mere 13.8 mg per litre.

The IUPAC name builds it on cyclohexene, a ring of six carbons sharing one double bond. A methyl group, CH₃, sits on carbon 1, and a prop-1-en-2-yl branch, three carbons with a double bond, on carbon 4. The (4R) prefix names the configuration at carbon 4, the arrangement of its groups in space, which the structural formula marks with a wedge. The Chemical Entities of Biological Interest (ChEBI) database records (4R)-limonene as a plant metabolite whose enantiomer is (4S)-limonene, a mirror pairing set out under lactic acid.

Structure and bonding

Six of the ten carbons in the viewer close into a ring; the other four make a methyl on one ring carbon and a three-carbon branch on the carbon opposite. Ball-and-stick draws two bonds as double sticks: one in the ring, at the methyl-bearing carbon, and one in the branch, ending in a CH₂ group. With Labels on, all 26 atoms read C or H.

Those double bonds sort the carbons into two kinds. The four at their ends have three neighbours each, spread flat as in ethylene; PubChem’s computed model gives angles of 116° to 124° around them. The six others have four neighbours, at 106.5° to 113.7°, near the tetrahedral pattern described under molecular shapes. Three single bonds separate the two double bonds, so the model never alternates double and single.

Space-filling swells each atom to full size, and hydrogens cover much of the outside; Stick strips the model back to its bonds. The drawing matches the real bonding, with no charge on any atom. It shows only (4R)-limonene, and none of the products air makes from the liquid.

Where it turns up

The peels and pulp left over from making citrus juice and cold-pressed oils yield d-limonene by steam distillation, according to HSDB. It also records the compound’s isolation from the peel oil of the mandarin, Citrus reticulata. Away from citrus, the LOTUS natural-products database reports (+)-limonene in Camellia sinensis and Magnolia officinalis.

The Consumer Product Information Database lists 1,542 products containing d-limonene, in categories from automotive products and home maintenance to personal care, pet care and pesticides. California’s Safe Cosmetics Program holds 43,730 product reports that name it as a cosmetic ingredient. US regulation permits it in insect-repellent tablecloths and strips for places where food or animal feed is handled. In 1994 the US Environmental Protection Agency (EPA) judged limonene products eligible for pesticide reregistration for all their uses. In the atmosphere, limonene reacts with ozone and with hydroxyl (OH) radicals, according to HSDB.

Why it matters

The US Food and Drug Administration (FDA) lists d-limonene as a flavouring agent. Its list of synthetic flavourings generally recognised as safe includes limonene in the d-, l- and dl- forms. HSDB describes its taste as fresh and citrus. JECFA, the joint food-additives committee of the Food and Agriculture Organization and the World Health Organization, evaluated (+)-limonene as a flavouring in 2004.

As a solvent it replaces chlorinated hydrocarbons and other more volatile, more toxic organic solvents, Haz-Map records. The EPA places it in the solvents class of its Safer Choice programme under a yellow triangle: it meets that class’s criteria but has some hazard issues. According to HSDB, d-limonene is the raw material for l-carvone, and Haz-Map lists resins, insecticides and insect repellents among its uses.

The US National Cancer Institute’s thesaurus lists an oral dietary supplement of limonene with potential cancer-preventive and anti-tumour activity. Taken by mouth, it adds, limonene activates the enzyme aldehyde dehydrogenase 3A1, lowering aldehyde levels. In male rats its 1,2-epoxide binds the protein alpha2u-globulin, the kidney effect described under isooctane.

Properties

Properties of limonene Molar mass summed from IUPAC/CIAAW 2024 standard atomic weights; formula, atom and bond counts and PubChem's molecular weight from PubChem CID 440917.
Property	Value	Unit	Source
Molecular formula (Hill order)	C₁₀H₁₆
Molar mass	136.238(23) g/mol	g/mol	IUPAC/CIAAW 2024
Carbon atoms	10
Hydrogen atoms	16
Bonds	26
Single bonds	24
Double bonds	2
PubChem molecular weight	136.23 g/mol	g/mol	PubChem

Safety

Pure limonene is a flammable liquid that irritates the skin and can cause an allergic skin reaction. The EU’s harmonised classification gives it the signal word Danger; swallowed, it may be fatal if it enters the airways, and it is very toxic to aquatic life. Haz-Map names its air-oxidised form as a skin sensitiser, and IFRA, the International Fragrance Association, singles out the hydroperoxides among its oxidation products. The PubChem Laboratory Chemical Safety Summary for limonene gathers these classifications and the rest of its hazard data.

Questions

What does a limonene molecule look like?

A six-carbon ring carries a methyl group on one carbon and a three-carbon branch on the carbon opposite: 26 atoms in all. One double bond lies in the ring, the other in the branch. The carbons on those bonds hold their neighbours flat; the other six sit at the centres of tetrahedra. No single shape word fits, since a shape name describes the atoms around one central atom and limonene has ten carbon centres.

What is the difference between d-limonene and l-limonene?

d-Limonene is (4R)-limonene and l-limonene is (4S)-limonene, labels for the arrangement of the groups at carbon 4; ChEBI records the two as enantiomers. d-Limonene, the (+) form, is the one in the 3D model; HSDB gives it a specific optical rotation of +123.8° at 19.5 °C. Mixed together, d- and l-limonene make the racemic mixture that Haz-Map calls dipentene. Commercial d-limonene is about 90 to 98 per cent pure, according to HSDB.

Why is limonene used in cleaning products?

Haz-Map records limonene standing in as a solvent for chlorinated hydrocarbons and other organic solvents that are more volatile and more toxic. Companies report its consumer uses to the US Environmental Protection Agency as a solvent, a cleaning agent, a lubricating agent and a fragrance. Haz-Map also lists metal degreasing among the jobs that expose workers to it.

Why can limonene cause a skin allergy?

Limonene oxidises readily in air, Haz-Map notes, and HSDB records that it oxidises to a film, much as rubber and drying oils do. The International Fragrance Association (IFRA) calls the oxidation products, above all hydroperoxides, potent sensitisers, able to set off an allergic skin reaction. Its standard admits limonene to fragrances only with its peroxides kept low, for instance with antioxidants added during manufacture. The EU classification also lists limonene itself as a possible cause of such a reaction.

Made of the same elements

C₈H₁₈ Isooctane 26 atoms, 25 single bonds.
C₈H₁₈ Octane 26 atoms, 25 single bonds.
C₆H₁₄ Hexane 20 atoms, 19 single bonds.
C₆H₁₂ Cyclohexane 18 atoms, 18 single bonds.
C₁₀H₈ Naphthalene 18 atoms, 19 bonds.
C₅H₁₂ Pentane 17 atoms, 16 single bonds.
C₈H₈ Styrene 16 atoms, 16 bonds.
C₇H₈ Toluene 15 atoms, 15 bonds.

All 18 compounds made of carbon and hydrogen

C₄H₁₀ Butane 14 atoms, 13 single bonds.
C₄H₁₀ Isobutane 14 atoms, 13 single bonds.
C₆H₆ Benzene 12 atoms, 12 bonds.
C₃H₈ Propane 11 atoms, 10 single bonds.
C₄H₆ 1,3-Butadiene 10 atoms, 9 bonds.
C₃H₆ Propylene 9 atoms, 8 bonds.
C₂H₆ Ethane 8 atoms, 7 single bonds.
C₂H₄ Ethylene 6 atoms, 5 bonds.
CH₄ Methane 5 atoms, 4 single bonds, tetrahedral.
C₂H₂ Acetylene 4 atoms, 3 bonds.