Phellandrenes are a pair of

organic compound In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon bonds. Due to carbon's ability to catenate (form chains with other carbon atoms), millions of organic compounds are known. T ...

s that have a similar molecular structure and similar chemical properties. α-Phellandrene and β-phellandrene are cyclic monoterpenes and are double-bond isomers. In α-phellandrene, both double bonds are endocyclic and in β-phellandrene, one of them is exocyclic. Both are insoluble in water, but miscible with

ether In organic chemistry, ethers are a class of compounds that contain an ether group—an oxygen atom connected to two alkyl or aryl groups. They have the general formula , where R and R′ represent the alkyl or aryl groups. Ethers can again be ...

. α-Phellandrene was named after ''Eucalyptus phellandra'', now called ''

Eucalyptus radiata ''Eucalyptus radiata'', commonly known as the narrow-leaved peppermint or Forth River peppermint, is a species of tree that is endemic to south-eastern Australia. It has rough, fibrous to flaky bark on the trunk and larger branches, smooth grey b ...

'', from which it can be isolated. It is also a constituent of the essential oil of ''

Eucalyptus dives ''Eucalyptus dives'', commonly known as the broad-leaved peppermint or blue peppermint, is a species of tree that is endemic to south-eastern Australia. It has rough, finely fibrous bark on the trunk and larger branches, smooth bark above, lance- ...

''. β-Phellandrene has been isolated from the oil of water fennel and Canada balsam oil. The phellandrenes are used in fragrances because of their pleasing aromas. The odor of β-phellandrene has been described as peppery-minty and slightly citrusy. The α-phellandrene isomer can form hazardous and explosive peroxides on contact with air at elevated temperatures.

Biosynthesis

The biosynthesis of phellandrene begins with

dimethylallyl pyrophosphate Dimethylallyl pyrophosphate (DMAPP; or alternatively, dimethylallyl diphosphate (DMADP); also isoprenyl pyrophosphate) is an isoprenoid precursor. It is a product of both the mevalonate pathway and the MEP pathway of isoprenoid precursor biosynt ...

and

isopentenyl pyrophosphate Isopentenyl pyrophosphate (IPP, isopentenyl diphosphate, or IDP) is an isoprenoid precursor. IPP is an intermediate in the classical, HMG-CoA reductase pathway (commonly called the mevalonate pathway) and in the ''non-mevalonate'' MEP pathway of i ...

condensing in an S_N1 reaction to form

geranyl pyrophosphate Geranyl pyrophosphate (GPP), also known as geranyl diphosphate (GDP), is the pyrophosphate ester of the terpenoid geraniol. Its salts are colorless. It is a precursor to many natural products. Occurrence GPP is an intermediate in the isopreno ...

. The resultant monoterpene undergoes cyclization to form a menthyl cationic species. A hydride shift then forms an allylic carbocation. Finally, an

elimination reaction An elimination reaction is a type of organic reaction in which two substituents are removed from a molecule in either a one- or two-step mechanism. The one-step mechanism is known as the E2 reaction, and the two-step mechanism is known as the E1 r ...

occurs at one of two positions, yielding either α-phellandrene or β-phellandrene. Phellandrene_mechanism

References

{{reflist Hydrocarbons Perfume ingredients Monoterpenes Cyclohexenes Cyclohexadienes Conjugated dienes