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. The ...
s that have a similar molecular structure and similar chemical properties. α-Phellandrene and β-phellandrene are cyclic
monoterpenes
Monoterpenes are a class of terpenes that consist of two isoprene units and have the molecular formula C10H16. Monoterpenes may be linear (acyclic) or contain rings (monocyclic and bicyclic). Modified terpenes, such as those containing oxygen funct ...
and are double-bond
isomer
In chemistry, isomers are molecules or polyatomic ions with identical molecular formulae – that is, same number of atoms of each element – but distinct arrangements of atoms in space. Isomerism is existence or possibility of isomers.
Iso ...
s. In α-phellandrene, both double bonds are endocyclic and in β-phellandrene, one of them is exocyclic. Both are insoluble in water, but
miscible
Miscibility () is the property of two substances to mix in all proportions (that is, to fully dissolve in each other at any concentration), forming a homogeneous mixture (a solution). The term is most often applied to liquids but also applies ...
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 c ...
.
α-Phellandrene was named after ''Eucalyptus phellandra'', now called ''
Eucalyptus radiata'', from which it can be isolated. It is also a constituent of the
essential oil
An essential oil is a concentrated hydrophobic liquid containing volatile (easily evaporated at normal temperatures) chemical compounds from plants. Essential oils are also known as volatile oils, ethereal oils, aetheroleum, or simply as the o ...
of ''
Eucalyptus dives''. 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 condensing in an
SN1 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 isoprenoid ...
. The resultant monoterpene undergoes cyclization to form a menthyl cationic species. A hydride shift then forms an
allylic
In organic chemistry, an allyl group is a substituent with the structural formula , where R is the rest of the molecule. It consists of a methylene bridge () attached to a vinyl group (). The name is derived from the scientific name for garlic, . ...
carbocation. Finally, an
elimination reaction occurs at one of two positions, yielding either α-phellandrene or β-phellandrene.
References
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Hydrocarbons
Perfume ingredients
Monoterpenes
Cyclohexenes
Cyclohexadienes
Conjugated dienes