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 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 with organic solvents.

Etymology and occurrence

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 ...

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 oil. The main source of β-phellandrene is terpentine. β- pinene 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 and isopentenyl pyrophosphate condensing in an S_N1 reaction to form geranyl pyrophosphate. 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