Limonene
Limonene 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.
Limonene is a chiral molecule, and most biological sources produce just one enantiomer. The -isomer, d-limonene, which is the -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 -enantiomer, has a piny, turpentine-like odor, and is found in the edible parts of such plants as caraway, dill, and bergamot orange plants.
In plants
Limonene is a major component of the aromatic scents and resins characteristic of numerous coniferous and broadleaved trees: red and silver maple, cottonwoods, aspens sumac, spruce, various pines, Pinus leucodermis, Douglas fir, larches, true firs, hemlocks, cedars, various Cupressaceae, and juniper bush. 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.Chemical reactions
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 -enantiomer, but racemizes at 300 °C. When warmed with mineral acid, limonene isomerizes to the conjugated diene α-terpinene. 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.
The most widely practiced conversion of limonene is to carvone. The three-step reaction begins with the regioselective addition of nitrosyl chloride across the trisubstituted double bond. This species is then converted to the oxime with a base, and the hydroxylamine is removed to give the ketone-containing carvone.
Biosynthesis
In nature, limonene is formed from geranyl pyrophosphate, via cyclization of a neryl carbocation or its equivalent as shown. The final step involves loss of a proton from the cation to form the alkene.Uses
As the main fragrance of citrus peels, -limonene is used in food manufacturing and some medicines, such as a flavoring agent to mask the bitter taste of alkaloids, and as a fragrance in perfumery, aftershave lotions, bath products, and other personal care products. -Limonene is also used as a botanical insecticide. -Limonene is used in the organic herbicides. It is added to cleaning products, such as hand cleansers, to give a lemon or orange fragrance and for its ability to dissolve oils. In contrast, -limonene has a piny, turpentine-like odor.Limonene is used as a solvent for cleaning purposes, such as adhesive remover, or the removal of oil from machine parts, as it is produced from a renewable source. It is used as a paint stripper and is also useful as a fragrant alternative to turpentine. Limonene is also used as a solvent in some model airplane glues and as a constituent in some paints. Commercial air fresheners, with air propellants, containing limonene are used by stamp collectors to remove self-adhesive postage stamps from envelope paper.
Limonene is also used as a solvent for fused filament fabrication based 3D printing. Printers can print the plastic of choice for the model, but erect supports and binders from high impact polystyrene, a polystyrene plastic that is easily soluble in limonene.
In preparing tissues for histology or histopathology, -limonene is often used as a less toxic substitute for xylene when clearing dehydrated specimens. Clearing agents are liquids miscible with alcohols and with melted paraffin wax, in which specimens are embedded to facilitate cutting of thin sections for microscopy.
Limonene, from orange peel oil, is also combustible and has been considered as a biofuel.