Limonene

Limonene (/ˈlɪmənˌiːn/) is a colorless liquid aliphatic hydrocarbon classified as a cyclic monoterpene, and is the major component in the fragrance and essential oil of citrus fruit peels, taking its name from Italian limone ("lemon").

Limonene is a chiral molecule, and most biological sources produce just one enantiomer (isomer). The (+)-isomer, d-limonene, which is the (R)-enantiomer, occurs more commonly in nature in citrus fruit peels, the principal commercial source, from which it is obtained commercially by two primary methods: centrifugal separation and steam distillation. D-limonene is used as a flavoring agent in food manufacturing, in chemical synthesis as a precursor to carvone, and as a renewables-based solvent in cleaning products.

The less common (−)-isomer, l-limonene, which is the (S)-enantiomer, has a piny, turpentine-like odor, and is found in the edible parts of such plants as caraway, dill, and bergamot orange plants.

Limonene is a major component of the aromatic scents and resins characteristic of numerous coniferous and broadleaved trees: red and silver maple (Acer rubrum, Acer saccharinum), cottonwoods (Populus angustifolia), aspens (Populus grandidentata, Populus tremuloides) sumac (Rhus glabra), spruce (Picea spp.), various pines (e.g., Pinus echinata, Pinus ponderosa), Pinus leucodermis, Douglas fir (Pseudotsuga menziesii), larches (Larix spp.), true firs (Abies spp.), hemlocks (Tsuga spp.), cedars (Cedrus spp.), various Cupressaceae, and juniper bush (Juniperus spp.).^{[failed verification]} It contributes to the characteristic odor of orange peel, orange juice and other citrus fruits. To optimize recovery of valued components from citrus peel waste, (+)-limonene is typically removed.

Limonene is a relatively stable monoterpene and can be distilled without decomposition, although at elevated temperatures it cracks to form isoprene. It oxidizes easily in moist air to produce carveol, carvone, and limonene oxide. With sulfur, it undergoes dehydrogenation to p-cymene.

Limonene occurs commonly as the (R)-enantiomer, but racemizes at 300 °C. When warmed with mineral acid, limonene isomerizes to the conjugated diene α-terpinene (which can also easily be converted to p-cymene). Evidence for this isomerization includes the formation of Diels–Alder adducts between α-terpinene adducts and maleic anhydride.

It is possible to effect reaction at one of the double bonds selectively. Anhydrous hydrogen chloride reacts preferentially at the disubstituted alkene, whereas epoxidation with mCPBA occurs at the trisubstituted alkene.

In another synthetic method Markovnikov addition of trifluoroacetic acid followed by hydrolysis of the acetate gives terpineol.