Names | |||
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Preferred IUPAC name 1-Methyl-4-(prop-1-en-2-yl)cyclohex-1-ene | |||
Other names 1-Methyl-4-(1-methylethenyl)cyclohexene 4-Isopropenyl-1-methylcyclohexene p-Menth-1,8-diene Racemic:DL-Limonene; Dipentene | |||
Identifiers | |||
3D model (JSmol) | |||
ChEBI | |||
ChEMBL |
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ChemSpider | |||
ECHA InfoCard | 100.004.856 | ||
KEGG | |||
PubChem CID | |||
UNII |
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CompTox Dashboard (EPA) | |||
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Properties | |||
C10H16 | |||
Molar mass | 136.238 g·mol−1 | ||
Appearance | colorless liquid | ||
Odor | Orange | ||
Density | 0.8411 g/cm3 | ||
Melting point | −74.35 °C (−101.83 °F; 198.80 K) | ||
Boiling point | 176 °C (349 °F; 449 K) | ||
Insoluble | |||
Solubility | Miscible with benzene, chloroform, ether, CS2, and oils soluble in CCl4 | ||
Chiral rotation ([α]D) | 87–102° | ||
Refractive index (nD) | 1.4727 | ||
Thermochemistry | |||
Std enthalpy of combustion (ΔcH⦵298) | −6.128 MJ mol−1 | ||
Hazards | |||
Occupational safety and health (OHS/OSH): | |||
Main hazards | Skin sensitizer / Contact dermatitis – After aspiration, pulmonary oedema, pneumonitis, and death [1] | ||
GHS labelling: | |||
Danger | |||
H226, H304, H315, H317, H410 | |||
P210, P233, P235, P240, P241, P242, P243, P261, P264, P272, P273, P280, P301+P330+P331, P302+P352, P303+P361+P353, P304+P340, P312, P333+P313, P362, P370+P378, P391, P403+P233, P405, P501 | |||
NFPA 704 (fire diamond) | |||
Flash point | 50 °C (122 °F; 323 K) | ||
237 °C (459 °F; 510 K) | |||
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). |
Limonene is a colorless liquid aliphatic hydrocarbon classified as a cyclic monoterpene, and is the major component in the volatile oil of citrus fruit peels. [1] The (+)-isomer, occurring more commonly in nature as the fragrance of oranges, is a flavoring agent in food manufacturing. [1] [2] It is also used in chemical synthesis as a precursor to carvone and as a renewables-based solvent in cleaning products. [1] The less common (-)-isomer has a piny, turpentine-like odor, and is found in the edible parts of such plants as caraway, dill, and bergamot orange plants. [3]
Limonene takes its name from Italian limone ("lemon"). [4] Limonene is a chiral molecule, and biological sources produce one enantiomer: the principal industrial source, citrus fruit, contains (+)-limonene (d-limonene), which is the (R)-enantiomer. [1] (+)-Limonene is obtained commercially from citrus fruits through two primary methods: centrifugal separation or steam distillation.
Limonene is a relatively stable monoterpene and can be distilled without decomposition, although at elevated temperatures it cracks to form isoprene. [5] It oxidizes easily in moist air to produce carveol, carvone, and limonene oxide. [1] [6] With sulfur, it undergoes dehydrogenation to p-cymene. [7]
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.
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. [2]
In nature, limonene is formed from geranyl pyrophosphate, via cyclization of a neryl carbocation or its equivalent as shown. [8] The final step involves loss of a proton from the cation to form the alkene.
(+)-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 ), Douglas fir ( Pseudotsuga menziesii ), larches ( Larix spp.), true firs ( Abies spp.), hemlocks ( Tsuga spp.), cannabis ( Cannabis sativa spp.), [9] cedars ( Cedrus spp.), various Cupressaceae, and juniper bush ( Juniperus spp.). [1] [ failed verification ] It contributes to the characteristic odor of orange peel, orange juice and other citrus fruits. [1] [10] To optimize recovery of valued components from citrus peel waste, (+)-limonene is typically removed. [11]
(+)-Limonene applied to skin may cause irritation from contact dermatitis, but otherwise appears to be safe for human uses. [12] [13] Limonene is flammable as a liquid or vapor and it is toxic to aquatic life. [1]
Limonene is common as a dietary supplement and as a fragrance ingredient for cosmetics products. [1] As the main fragrance of citrus peels, D-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. [1] (+)-Limonene is also used as a botanical insecticide. [1] [14] (+)-Limonene is used in the organic herbicides. [15] It is added to cleaning products, such as hand cleansers to give a lemon or orange fragrance (see orange oil) and for its ability to dissolve oils. [1] 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 (citrus essential oil, as a byproduct of orange juice manufacture). [11] 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. [16]
Limonene is also used as a solvent for fused filament fabrication based 3D printing. [17] Printers can print the plastic of choice for the model, but erect supports and binders from High Impact Polystyrene (HIPS), a polystyrene plastic that is easily soluble in limonene.
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. [18] [19] [20]
Limonene, from orange peel oil, is also combustible and has been considered as a biofuel. [21]
Kumquats, or cumquats in Australian English, are a group of small, angiosperm, fruit-bearing trees in the family Rutaceae. Their taxonomy is disputed. They were previously classified as forming the now-historical genus Fortunella or placed within Citrus, sensu lato. Different classifications have alternatively assigned them to anywhere from a single species, Citrus japonica, to numerous species representing each cultivar. Recent genomic analysis defines three pure species, Citrus hindsii, C. margarita and C. crassifolia, with C. × japonica being a hybrid of the last two.
Heptane or n-heptane is the straight-chain alkane with the chemical formula H3C(CH2)5CH3 or C7H16. When used as a test fuel component in anti-knock test engines, a 100% heptane fuel is the zero point of the octane rating scale (the 100 point is 100% iso-octane). Octane number equates to the anti-knock qualities of a comparison mixture of heptane and iso-octane which is expressed as the percentage of iso-octane in heptane, and is listed on pumps for gasoline (petrol) dispensed globally.
Citrus hystrix, called the kaffir lime or makrut lime, is a citrus fruit native to tropical Southeast Asia.
Terpenes are a class of natural products consisting of compounds with the formula (C5H8)n for n ≥ 2. Terpenes are major biosynthetic building blocks. Comprising more than 30,000 compounds, these unsaturated hydrocarbons are produced predominantly by plants, particularly conifers. In plants, terpenes and terpenoids are important mediators of ecological interactions, while some insects use some terpenes as a form of defense. Other functions of terpenoids include cell growth modulation and plant elongation, light harvesting and photoprotection, and membrane permeability and fluidity control.
An essential oil is a concentrated hydrophobic liquid containing volatile chemical compounds from plants. Essential oils are also known as volatile oils, ethereal oils, aetheroleum, or simply as the oil of the plant from which they were extracted, such as oil of clove. An essential oil is essential in the sense that it contains the essence of the plant's fragrance—the characteristic fragrance of the plant from which it is derived. The term "essential" used here does not mean indispensable or usable by the human body, as with the terms essential amino acid or essential fatty acid, which are so called because they are nutritionally required by a living organism.
An aroma compound, also known as an odorant, aroma, fragrance or flavoring, is a chemical compound that has a smell or odor. For an individual chemical or class of chemical compounds to impart a smell or fragrance, it must be sufficiently volatile for transmission via the air to the olfactory system in the upper part of the nose. As examples, various fragrant fruits have diverse aroma compounds, particularly strawberries which are commercially cultivated to have appealing aromas, and contain several hundred aroma compounds.
Linalool refers to two enantiomers of a naturally occurring terpene alcohol found in many flowers and spice plants. Linalool has multiple commercial applications, the majority of which are based on its pleasant scent. A colorless oil, linalool is classified as an acyclic monoterpenoid. In plants, it is a metabolite, a volatile oil component, an antimicrobial agent, and an aroma compound. Linalool has uses in manufacturing of soaps, fragrances, food additives as flavors, household products, and insecticides. Esters of linalool are referred to as linalyl, e.g. linalyl pyrophosphate, an isomer of geranyl pyrophosphate.
Dysphania ambrosioides, formerly Chenopodium ambrosioides, known as Jesuit's tea, Mexican-tea, payqu(paico), epazote, mastruz, or herba sanctæ Mariæ, is an annual or short-lived perennial herb native to Central America, South America, and southern Mexico.
Carvone is a member of a family of chemicals called terpenoids. Carvone is found naturally in many essential oils, but is most abundant in the oils from seeds of caraway, spearmint, and dill.
Neroli oil is an essential oil produced from the blossom of the bitter orange tree. Its scent is sweet, honeyed and somewhat metallic with green and spicy facets. Orange blossom is also extracted from the same blossom and both extracts are extensively used in perfumery. Orange blossom can be described as smelling sweeter, warmer and more floral than neroli. The difference between how neroli and orange blossom smell and why they are referred to with different names, is a result of the process of extraction that is used to obtain the oil from the blooms. Neroli is extracted by steam distillation and orange blossom is extracted via a process of enfleurage or solvent extraction.
Pinene is a collection of unsaturated bicyclic monoterpenes. Two geometric isomers of pinene are found in nature, α-pinene and β-pinene. Both are chiral. As the name suggests, pinenes are found in pines. Specifically, pinene is the major component of the liquid extracts of conifers. Pinenes are also found in many non-coniferous plants such as camphorweed (Heterotheca) and big sagebrush.
Eucalyptol is a monoterpenoid colorless liquid, and a bicyclic ether. It has a fresh camphor-like odor and a spicy, cooling taste. It is insoluble in water, but miscible with organic solvents. Eucalyptol makes up about 70–90% of eucalyptus oil. Eucalyptol forms crystalline adducts with hydrohalic acids, o-cresol, resorcinol, and phosphoric acid. Formation of these adducts is useful for purification.
Orange oil is an essential oil produced by cells within the rind of an orange fruit. In contrast to most essential oils, it is extracted as a by-product of orange juice production by centrifugation, producing a cold-pressed oil. It is composed of mostly d-limonene, and is often used in place of pure d-limonene. D-limonene can be extracted from the oil by distillation.
The terpinenes are a group of isomeric hydrocarbons that are classified as monoterpenes. They each have the same molecular formula and carbon framework, but they differ in the position of carbon-carbon double bonds. α-Terpinene has been isolated from cardamom and marjoram oils, and from other natural sources. β-Terpinene has no known natural source but has been prepared from sabinene. γ-Terpinene and δ-terpinene have been isolated from a variety of plant sources. They are all colorless liquids with a turpentine-like odor.
Monoterpenes are a class of terpenes that consist of two isoprene units and have the molecular formula C10H16. Monoterpenes may be linear (acyclic) or contain rings (monocyclic and bicyclic). Modified terpenes, such as those containing oxygen functionality or missing a methyl group, are called monoterpenoids. Monoterpenes and monoterpenoids are diverse. They have relevance to the pharmaceutical, cosmetic, agricultural, and food industries.
Terpineol is any of four isomeric monoterpenoids. Terpenoids are terpene that are modified by the addition of a functional group, in this case, an alcohol. Terpineols have been isolated from a variety of sources such as cardamom, cajuput oil, pine oil, and petitgrain oil. Four isomers exist: α-, β-, γ-terpineol, and terpinen-4-ol. β- and γ-terpineol differ only by the location of the double bond. Terpineol is usually a mixture of these isomers with α-terpineol as the major constituent.
The enzyme (R)-limonene synthase (EC 4.2.3.20) catalyzes the reversible chemical reaction
Bergamot essential oil is a cold-pressed essential oil produced by cells inside the rind of a bergamot orange fruit. It is a common flavoring and top note in perfumes. The scent of bergamot essential oil is similar to a sweet light orange peel oil with a floral note.
Lavandulol is a monoterpene alcohol found in a variety of essential oils such as lavender oil. The term refers to either of two enantiomers. The (R)-enantiomer is natural and has an aroma described as "weak floral, herbal odor with slightly lemon-like, fresh citrus fruity nuance"; the (S)-enantiomer has only a weak odor.
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