Limonene (/ˈlɪmənˌiːn/) is a colorless liquidaliphatichydrocarbon classified as a cyclicmonoterpene, and is the major component in theessential oil ofcitrus fruitpeels.^[1] The (+)-isomer, occurring more commonly in nature as the fragrance of oranges, is a flavoring agent in food manufacturing.^[1][2] It is also used in chemical synthesis as aprecursor tocarvone and as a renewables-basedsolvent incleaning products.^[1] The less common (-)-isomer has apiny,turpentine-like odor, and is found in the edible parts of such plants ascaraway,dill, andbergamot orange plants.^[3]

Limonene

Limonene (R)-isomer
Limonene extracted from orange peel
Names
Preferred IUPAC name 1-Methyl-4-(prop-1-en-2-yl)cyclohex-1-ene
Other names 1-Methyl-4-(1-methylethenyl)cyclohexene 4-Isopropenyl-1-methylcyclohexene p-Menth-1,8-diene Racemic:DL-Limonene; Dipentene
Identifiers
CAS Number	138-86-3 (R/S) Y 5989-27-5 (R) Y 5989-54-8 (S) Y
3D model (JSmol)	Interactive image
ChEBI	CHEBI:15384 Y
ChEMBL	ChEMBL449062 (R) Y
ChemSpider	20939 (R/S) Y 388386 (S) Y 389747 (R) Y
ECHA InfoCard	100.004.856
KEGG	D00194 Y
PubChemCID	22311 (R/S) 439250 (S)
UNII	9MC3I34447 (R/S) Y GFD7C86Q1W (R) Y 47MAJ1Y2NE (S) Y
CompTox Dashboard(EPA)	DTXSID2029612
InChI InChI=1S/C10H16/c1-8(2)10-6-4-9(3)5-7-10/h4,10H,1,5-7H2,2-3H3 Y Key: XMGQYMWWDOXHJM-UHFFFAOYSA-N Y
SMILES CC1=CCC(CC1)C(=C)C
Properties
Chemical formula	C10H16
Molar mass	136.238 g·mol−1
Appearance	colorless liquid
Odor	Orange
Density	0.8411 g/cm3
Melting point	−74.35 °C (−101.83 °F; 198.80 K)
Boiling point	176 °C (349 °F; 449 K)
Solubility in water	Insoluble
Solubility	Miscible withbenzene,chloroform,ether,CS2, and oils soluble inCCl4
Chiral rotation ([α]D)	87–102°
Refractive index (nD)	1.4727
Thermochemistry
Std enthalpy of combustion(ΔcH⦵298)	−6.128 MJ mol−1
Hazards
Occupational safety and health (OHS/OSH):
Main hazards	Skin sensitizer /Contact dermatitis – After aspiration,pulmonary oedema,pneumonitis, anddeath[1]
GHS labelling:
Pictograms
Signal word	Danger
Hazard statements	H226,H304,H315,H317,H410
Precautionary statements	P210,P233,P235,P240,P241,P242,P243,P261,P264,P272,P273,P280,P301+P330+P331,P302+P352,P303+P361+P353,P304+P340,P312,P333+P313,P362,P370+P378,P391,P403+P233,P405,P501
NFPA 704 (fire diamond)	2 2 0
Flash point	50 °C (122 °F; 323 K)
Autoignition temperature	237 °C (459 °F; 510 K)
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). Infobox references

Limonene takes its name fromItalianlimone ("lemon").^[4] Limonene is achiral molecule, and biological sources produce oneenantiomer: the principal industrial source,citrus fruit, contains (+)-limonene (d-limonene), which is the(R)-enantiomer.^[1] (+)-Limonene is obtained commercially from citrus fruits through two primary methods:centrifugal separation orsteam distillation.

(+)-Limonene is a major component of the aromatic scents andresins characteristic of numerous coniferous and broadleaved trees: red and silver maple (Acer rubrum,Acer saccharinum), cottonwoods (Populus angustifolia), aspens (Populus grandidentata,Populus tremuloides) sumac (Rhus glabra), spruce (Picea spp.), various pines (e.g.,Pinus echinata,Pinus ponderosa),Pinus leucodermis,^[5] Douglas fir (Pseudotsuga menziesii), larches (Larix spp.), true firs (Abies spp.), hemlocks (Tsuga spp.), cedars (Cedrus spp.), variousCupressaceae, and juniper bush (Juniperus spp.).^{[1][failed verification]} It contributes to the characteristic odor oforange peel,orange juice and othercitrus fruits.^[1][6] To optimize recovery of valued components from citrus peel waste, (+)-limonene is typically removed.^[7]

Limonene is a relatively stablemonoterpene and can be distilled without decomposition, although at elevated temperatures itcracks to formisoprene.^[8] It oxidizes easily in moist air to producecarveol,carvone, and limonene oxide.^[1][9] With sulfur, it undergoes dehydrogenation top-cymene.^[10]

Limonene occurs commonly as the (R)-enantiomer, butracemizes at 300 °C. When warmed withmineral acid, limonene isomerizes to theconjugated diene α-terpinene (which can also easily be converted top-cymene). Evidence for this isomerization includes the formation ofDiels–Alderadducts between α-terpinene adducts andmaleic anhydride.

It is possible to effect reaction at one of the double bonds selectively. Anhydroushydrogen chloride reacts preferentially at the disubstituted alkene, whereasepoxidation withmCPBA occurs at the trisubstituted alkene.

In another synthetic methodMarkovnikov addition oftrifluoroacetic acid followed byhydrolysis of the acetate givesterpineol.

The most widely practiced conversion of limonene is tocarvone. The three-step reaction begins with theregioselective addition ofnitrosyl chloride across the trisubstituted double bond. This species is then converted to theoxime with abase, and thehydroxylamine is removed to give theketone-containing carvone.^[2]

In nature, limonene is formed fromgeranyl pyrophosphate, via cyclization of anerylcarbocation or its equivalent as shown.^[11] The final step involves loss of a proton from the cation to form thealkene.

As the main fragrance of citrus peels,D-limonene is used in food manufacturing and some medicines, such as aflavoring agent to mask the bitter taste ofalkaloids, and as a fragrance inperfumery, aftershave lotions, bath products, and otherpersonal care products.^[1] (+)-Limonene is also used as abotanicalinsecticide.^[1][12] (+)-Limonene is used in the organic herbicides.^[13] It is added to cleaning products, such as hand cleansers, to give a lemon or orange fragrance (seeorange oil) and for its ability to dissolve oils.^[1] In contrast, (-)-limonene has apiny,turpentine-like odor.

Limonene is used as asolvent for cleaning purposes, such asadhesive remover, or the removal of oil from machine parts, as it is produced from a renewable source (citrus essential oil, as a byproduct oforange juice manufacture).^[7] It is used as apaint stripper and is also useful as a fragrant alternative to turpentine. Limonene is also used as a solvent in somemodel airplane glues and as a constituent in some paints. Commercial air fresheners, with air propellants, containing limonene are used bystamp collectors to remove self-adhesive postage stamps from envelope paper.^[14]

Limonene is also used as a solvent for fused filament fabrication based3D printing.^[15] Printers can print the plastic of choice for the model, but erect supports and binders fromhigh impact polystyrene (HIPS), apolystyrene plastic that is easily soluble in limonene.

In preparingtissues forhistology or histopathology,D-limonene is often used as a less toxic substitute forxylene when clearing dehydrated specimens. Clearing agents are liquids miscible with alcohols (such asethanol orisopropanol) and with meltedparaffin wax, in which specimens are embedded to facilitate cutting of thin sections formicroscopy.^[16][17][18]

Limonene, from orange peel oil, is also combustible and has been considered as abiofuel.^[19]

Applied to skin, limonene may cause irritation fromcontact dermatitis, but otherwise appears to be safe for human use.^[20][21] Limonene isflammable as a liquid or vapor and it is toxic toaquatic life.^[1]

There is no evidence that the limonene in peel oils of citrus fruits affects the onset or progress of cancer, with one national agency stating, "There is no consistent evidence that people with cancer who consume limonene—either in supplement form or by eating citrus fruits—get better or are more likely to be cured".^[22]

• Essential oil
• Monoterpenes
• Resin

• Mass spectrum of limonene
• Description ofD-limonene on the International Chemical Safety Cards
• D-Limonene information from the United StatesEnvironmental Protection Agency