Limonene

Limonene



Limonene has lots of uses. Limonene is a colorless liquid hydrocarbon classified as a cyclic terpene. The more common D isomer possesses a strong smell of oranges. It is used in chemical synthesis as a precursor to carvone and as a renewably-based solvent in cleaning products. It is also very useful for cleaning oil from machine parts and it is very renewable. It can also be used as a solvent in some model airplane glues. Another use for Limonene is as a bio fuel since it is combustible.Limonene is normally present in cosmetics. Limonene takes its name from the lemon, as the rind of the lemon, like other citrus fruits, contains considerable amounts of this compound, which contributes to their odor. Limonene is a chiral molecule, and biological sources produce one enantiomer: the principal industrial source, citrus fruit, contains D-limonene ((+)-limonene), which is the (//R//)-enantiomer (CAS number 5989-27-5, EINECS number 227-813-5). Racemic limonene is known as dipentene. home] D-Limonene is obtained commercially from citrus fruits through two primary methods: centrifugal separation or steam distillation. Dangers of Limonene:It is a skin and respiratory irritant although most cases involved long term industrial exposure so it is pretty safe. It has been shown to cause renal cancer in mice but not for humans. In fact, it may have value as an anti-cancer dietary tool for humans. Limonene is found in the essential oils of citrus fruits & many other plant species. Industrial limonene is produced by alkali extraction of citrus residues and steam distillation. this distillate contains more than 90% d-limonene.

Chemical reactions
Limonene is a relatively stable [|terpene] and can be distilled without decomposition, although at elevated temperatures it [|cracks] to form [|isoprene]. [|[3]] It oxidizes easily in moist air to produce [|carveol], [|carvone] , and [|limonene oxide]. [|[4]] With sulfur, it undergoes dehydrogenation to//p//- [|cymene] .[// [|citation needed] //] Limonene occurs naturally as the (//R//)- [|enantiomer], but racemizes to dipentene 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].

Biosynthesis
Limonene is formed from [|geranyl pyrophosphate], via cyclization of a [|neryl] [|carbocation] or its equivalent as shown. [|[5]] The final step involves loss of a proton from the cation to form the [|alkene].

Uses
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 base, and the hydroxylamine is removed to give the ketone-containing carvone. [|[1]] Limonene is common in [|cosmetic] products. As the main odor constituent of [|citrus] (plant family [|Rutaceae] ), D -limonene is used in food manufacturing and some medicines, e.g. as a [|flavoring] to mask the bitter taste of [|alkaloids], and as a [|fragrant] in [|perfumery] ; it is also used as [|botanical] [|insecticide] ., [|[6]] particularly the (//R//)-(+)- [|enantiomer] is most active as an [|insecticide]. It is added to cleaning products such as hand cleansers to give a lemon-orange [|fragrance] (see [|orange oil] ). In contrast, L -limonene has a [|piney], [|turpentine] -like odor. In natural and alternative medicine, D -limonene is marketed to relieve [|gastroesophageal reflux disease] and [|heartburn]. [|[7]] Limonene is increasingly being used as a [|solvent] for cleaning purposes, such as the removal of oil from machine parts, as it is produced from a renewable source (citrus oil, as a byproduct of [|orange juice] manufacture). It also serves as a paint stripper when applied to painted wood and is also useful as a fragrant alternative to [|turpentine]. Limonene is also used as a solvent in some [|model airplane] glues. All-natural commercial air fresheners, with air propellants, containing limonene are used by [|philatelists] to remove self-adhesive postage stamps from envelope paper. [|[8]] As it is combustible, limonene has also been considered as a [|biofuel]. [|[9]] Limonene can be used to dissolve polystyrene, and is a more ecologically friendly substitute for acetone.[// [|citation needed] //] In preparing tissues for [|histology] or histopathology, D -limonene is often used as a less toxic substitute for xylene when clearing dehydrated specimens. Clearing agents are liquids miscible with alcohols (such as ethanol or isopropanol) and with melted paraffin wax, in which specimens are embedded to facilitate cutting of thin sections for microscopy.