Phellandrene

Phellandrenes are organic compounds 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 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 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 of Eucalyptus dives. 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. 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 allylic carbocation. Finally, an elimination reaction occurs at one of two positions, yielding either α-phellandrene or β-phellandrene.

Safety

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