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Dipentene

Limonene
Skeletal structure of the R-isomer
Ball-and-stick model of the R-isomer
Limonene extracted from orange peels.
Names
IUPAC name
1-Methyl-4-(1-methylethenyl)-cyclohexene
Other names
4-Isopropenyl-1-methylcyclohexene
p-Menth-1,8-diene
Racemic: DL-limonene; Dipentene
Identifiers
3D model (Jmol)
ChEBI
ChemSpider
ECHA InfoCard 100.025.284
KEGG
PubChem CID
UNII
Properties
C10H16
Molar mass 136.24 g·mol−1
Appearance colorless to pale-yellow 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)
insoluble
Solubility miscible in alcohol, benzene, chloroform, ether, CS2, and oils
soluble in CCl4
87° - 102°
1.4727
Thermochemistry
−6.128 MJ mol−1
Hazards
R-phrases R10 R38 R43 R50/53
S-phrases (S2) S24 S37 S60 S61
NFPA 704
Flammability code 2: Must be moderately heated or exposed to relatively high ambient temperature before ignition can occur. Flash point between 38 and 93 °C (100 and 200 °F). E.g., diesel fuel Health code 1: Exposure would cause irritation but only minor residual injury. E.g., turpentine Reactivity code 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g., liquid nitrogen Special hazards (white): no codeNFPA 704 four-colored diamond
Flash point 50 °C (122 °F; 323 K)
237 °C (459 °F; 510 K)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Limonene is a colorless liquid hydrocarbon classified as a cyclic terpene. The more common d-isomer possesses a strong smell of oranges. It is used in chemical synthesis as a precursor to carvone and as a renewables-based solvent in cleaning products. The less common l-isomer is found in mint oils and has a piney, turpentine-like odor.

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. Racemic limonene is known as dipentene. d-Limonene is obtained commercially from citrus fruits through two primary methods: centrifugal separation or steam distillation.

Limonene is a relatively stable terpene and can be distilled without decomposition, although at elevated temperatures it cracks to form isoprene. It oxidizes easily in moist air to produce carveol, carvone, and limonene oxide. With sulfur, it undergoes dehydrogenation to p-cymene.

Limonene occurs commonly as the d or (R)-enantiomer, but racemizes to dipentene at 300 °C. When warmed with mineral acid, limonene isomerizes to the conjugated diene α-terpinene (which can also easily be converted to p-cymene). Evidence for this isomerization includes the formation of Diels-Alder adducts between α-terpinene adducts and maleic anhydride.


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