Phellandrenes are

organic compound Some chemical authorities define an organic compound as a chemical compound that contains a carbon–hydrogen or carbon–carbon bond; others consider an organic compound to be any chemical compound that contains carbon. For example, carbon-co ...

s with the formula . They 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 formula – that is, the same number of atoms of each element (chemistry), element – but distinct arrangements of atoms in space. ''Isomerism'' refers to the exi ...

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). Such substances are said to be miscible (etymologically ...

with

organic solvents A solvent (from the Latin language, Latin ''wikt:solvo#Latin, solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a Solution (chemistry), solution. A solvent is usually a liquid but can also be a solid, a gas ...

Etymology and occurrence

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 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 ...

of ''

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

''. β-Phellandrene has been isolated from the oil of water fennel and

Canada balsam Canada balsam, also called Canada turpentine or balsam of fir, is the oleoresin of the balsam fir tree (''Abies balsamea'') of boreal North America. The resin, dissolved in essential oils, is a viscous, sticky, colourless or yellowish liquid t ...

oil. The main source of β-phellandrene is terpentine. β-

pinene Pinene is a collection of unsaturated bicyclic monoterpenes. Two geometric isomers of pinene are found in nature, α-pinene and β-pinene. Both are chiral. As the name suggests, pinenes are found in pines. Specifically, pinene is the major comp ...

is a source of β-phellandrene.

Reactions and uses

α-Phellandrene undergoes hydrochlorination to give phellandrene hydrochloride (a cyclohexenyl chloride). Base hydrolysis of this hydrochloride gives piperitol. 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. Like other cyclohexadienes, α-phellandrene reacts with ruthenium trichloride to give (cymene)ruthenium chloride dimer.

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 thousands of natural product, natural products. Occurrence GPP is an int ...

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

allyl In organic chemistry, an allyl group is a substituent with the structural formula . It consists of a methylene bridge () attached to a vinyl group (). The name is derived from the scientific name for garlic, . In 1844, Theodor Wertheim isolated a ...

ic 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

Safety

The α-phellandrene isomer can form hazardous and explosive peroxides on contact with air at elevated temperatures.

References

{{reflist Monoterpenes Cyclohexenes Cyclohexadienes Conjugated dienes Perfume ingredients Vinylidene compounds