Limonene is a colorless liquid

aliphatic In organic chemistry, hydrocarbons ( compounds composed solely of carbon and hydrogen) are divided into two classes: aromatic compounds and aliphatic compounds (; G. ''aleiphar'', fat, oil). Aliphatic compounds can be saturated, like hexane, ...

hydrocarbon In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic, and their odors are usually weak or ...

classified as a cyclic

monoterpene 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 func ...

, and is the major component in the

oil An oil is any nonpolar chemical substance that is composed primarily of hydrocarbons and is hydrophobic (does not mix with water) & lipophilic (mixes with other oils). Oils are usually flammable and surface active. Most oils are unsaturated ...

citrus fruit ''Citrus'' is a genus of flowering trees and shrubs in the rue family, Rutaceae. Plants in the genus produce citrus fruits, including important crops such as oranges, lemons, grapefruits, pomelos, and limes. The genus ''Citrus'' is native to ...

peels. The - isomer, occurring more commonly in nature as the fragrance of oranges, is a flavoring agent in food manufacturing. It is also used in chemical synthesis as a

precursor Precursor or Precursors may refer to: * Precursor (religion), a forerunner, predecessor ** The Precursor, John the Baptist Science and technology * Precursor (bird), a hypothesized genus of fossil birds that was composed of fossilized parts of u ...

carvone 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 (''Carum carvi''), spearmint (''Mentha spicata''), and dill. Uses Both c ...

and as a renewables-based

solvent A solvent (s) (from the Latin '' solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a solution. A solvent is usually a liquid but can also be a solid, a gas, or a supercritical fluid. Water is a solvent for ...

cleaning product Cleaning agents or hard-surface cleaners are substances (usually liquids, powders, sprays, or granules) used to remove dirt, including dust, stains, bad smells, and clutter on surfaces. Purposes of cleaning agents include health, beauty, removing ...

s. The less common - isomer has a piny,

turpentine Turpentine (which is also called spirit of turpentine, oil of turpentine, terebenthene, terebinthine and (colloquially) turps) is a fluid obtained by the distillation of resin harvested from living trees, mainly pines. Mainly used as a spec ...

-like odor, and is found in the edible parts of such plants as

caraway Caraway, also known as meridian fennel and Persian cumin (''Carum carvi''), is a biennial plant in the family Apiaceae, native to western Asia, Europe, and North Africa. Etymology The etymology of "caraway" is unclear. Caraway has been ...

dill Dill (''Anethum graveolens'') is an annual herb in the celery family Apiaceae. It is the only species in the genus ''Anethum''. Dill is grown widely in Eurasia, where its leaves and seeds are used as a herb or spice for flavouring food. Growth ...

, and

bergamot orange ''Citrus bergamia'', the bergamot orange (pronounced ), is a fragrant citrus fruit the size of an orange, with a yellow or green color similar to a lime, depending on ripeness. Genetic research into the ancestral origins of extant citrus cult ...

plants. Limonene takes its name from

Italian Italian(s) may refer to: * Anything of, from, or related to the people of Italy over the centuries ** Italians, an ethnic group or simply a citizen of the Italian Republic or Italian Kingdom ** Italian language, a Romance language *** Regional Ita ...

''limone'' ("

lemon The lemon (''Citrus limon'') is a species of small evergreen trees in the flowering plant family Rutaceae, native to Asia, primarily Northeast India (Assam), Northern Myanmar or China. The tree's ellipsoidal yellow fruit is used for culin ...

"). Limonene is a

chiral Chirality is a property of asymmetry important in several branches of science. The word ''chirality'' is derived from the Greek (''kheir''), "hand", a familiar chiral object. An object or a system is ''chiral'' if it is distinguishable from i ...

molecule, and biological sources produce one enantiomer: the principal industrial source,

, contains -limonene ((+)-limonene), which is the (''R'')-enantiomer.

Racemic In chemistry, a racemic mixture, or racemate (), is one that has equal amounts of left- and right-handed enantiomers of a chiral molecule or salt. Racemic mixtures are rare in nature, but many compounds are produced industrially as racemates. ...

limonene is known as dipentene. -Limonene is obtained commercially from citrus fruits through two primary methods: centrifugal separation or steam distillation.

Chemical reactions

Limonene is a relatively stable

and can be distilled without decomposition, although at elevated temperatures it cracks to form isoprene. It oxidizes easily in moist air to produce carveol,

, and limonene oxide. With sulfur, it undergoes dehydrogenation to ''p''-

cymene ''p''-Cymene is a naturally occurring aromatic organic compound. It is classified as an alkylbenzene related to a monoterpene. Its structure consists of a benzene ring ''para''-substituted with a methyl group and an isopropyl group. ''p''-Cy ...

. Limonene occurs commonly as the (''R'')-enantiomer, but racemizes to dipentene at 300 °C. When warmed with

mineral acid A mineral acid (or inorganic acid) is an acid derived from one or more inorganic compounds, as opposed to organic acids which are acidic, organic compounds. All mineral acids form hydrogen ions and the conjugate base when dissolved in water. Cha ...

, limonene isomerizes to the

conjugated diene In organic chemistry a diene ( ) (diolefin ( ) or alkadiene) is a covalent compound that contains two double bonds, usually among carbon atoms. They thus contain two alk''ene'' units, with the standard prefix ''di'' of systematic nomenclatur ...

α-

terpinene 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 isolate ...

(which can also easily be converted to ''p''-cymene). Evidence for this isomerization includes the formation of Diels–Alder

adduct An adduct (from the Latin ''adductus'', "drawn toward" alternatively, a contraction of "addition product") is a product of a direct addition of two or more distinct molecules, resulting in a single reaction product containing all atoms of all co ...

s between α-terpinene adducts and

maleic anhydride Maleic anhydride is an organic compound with the formula C2H2(CO)2O. It is the acid anhydride of maleic acid. It is a colorless or white solid with an acrid odor. It is produced industrially on a large scale for applications in coatings and pol ...

. It is possible to effect reaction at one of the double bonds selectively. Anhydrous

hydrogen chloride The compound hydrogen chloride has the chemical formula and as such is a hydrogen halide. At room temperature, it is a colourless gas, which forms white fumes of hydrochloric acid upon contact with atmospheric water vapor. Hydrogen chloride ga ...

reacts preferentially at the disubstituted alkene, whereas

epoxidation In organic chemistry, an epoxide is a cyclic ether () with a three-atom ring. This ring approximates an equilateral triangle, which makes it strained, and hence highly reactive, more so than other ethers. They are produced on a large scale ...

with mCPBA occurs at the trisubstituted alkene. In another synthetic method

Markovnikov addition In organic chemistry, Markovnikov's rule or Markownikoff's rule describes the outcome of some addition reactions. The rule was formulated by Russian chemist Vladimir Markovnikov in 1870. Explanation The rule states that with the addition of a p ...

trifluoroacetic acid Trifluoroacetic acid (TFA) is an organofluorine compound with the chemical formula CF3CO2H. It is a structural analogue of acetic acid with all three of the acetyl group's hydrogen atoms replaced by fluorine atoms and is a colorless liquid with ...

followed by

hydrolysis Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution, elimination, and solvation reactions in which water is the nucleophile. Biological hydrolys ...

of the acetate gives

terpineol 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 ...

Biosynthesis

In nature, limonene is formed from

geranyl pyrophosphate Geranyl pyrophosphate (GPP), also known as geranyl diphosphate (GDP), is the pyrophosphate ester of the terpenoid geraniol. Its salts are colorless. It is a precursor to many natural products. Occurrence GPP is an intermediate in the isopreno ...

, 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 In organic chemistry, an alkene is a hydrocarbon containing a carbon–carbon double bond. Alkene is often used as synonym of olefin, that is, any hydrocarbon containing one or more double bonds.H. Stephen Stoker (2015): General, Organic, an ...

The most widely practiced conversion of limonene is to

. The three-step reaction begins with the

regioselective In chemistry, regioselectivity is the preference of chemical bonding or breaking in one direction over all other possible directions. It can often apply to which of many possible positions a reagent will affect, such as which proton a strong Base ( ...

addition of

nitrosyl chloride Nitrosyl chloride is the chemical compound with the formula NOCl. It is a yellow gas that is commonly encountered as a component of aqua regia, a mixture of 3 parts concentrated hydrochloric acid and 1 part of concentrated nitric acid. It is a st ...

across the trisubstituted double bond. This species is then converted to the

oxime In organic chemistry, an oxime is a organic compound belonging to the imines, with the general formula , where R is an organic side-chain and R’ may be hydrogen, forming an aldoxime, or another organic group, forming a ketoxime. O-substituted ...

with a base, and the

hydroxylamine Hydroxylamine is an inorganic compound with the formula . The material is a white crystalline, hygroscopic compound.Greenwood and Earnshaw. ''Chemistry of the Elements.'' 2nd Edition. Reed Educational and Professional Publishing Ltd. pp. 431–43 ...

is removed to give the ketone-containing carvone.

In plants

-Limonene is a major component of the aromatic scents and

resins In polymer chemistry and materials science, resin is a solid or highly viscous substance of plant or synthetic origin that is typically convertible into polymers. Resins are usually mixtures of organic compounds. This article focuses on natu ...

characteristic of numerous coniferous and broadleaved trees: red and silver maple (''

Acer rubrum ''Acer rubrum'', the red maple, also known as swamp maple, water maple, or soft maple, is one of the most common and widespread deciduous trees of eastern and central North America. The U.S. Forest Service recognizes it as the most abundant nati ...

'', ''

Acer saccharinum ''Acer saccharinum'', commonly known as silver maple, creek maple, silverleaf maple, soft maple, large maple, water maple, swamp maple, or white maple, is a species of maple native to the eastern and central United States and southeastern Can ...

''), cottonwoods (''

Populus angustifolia ''Populus angustifolia'', commonly known as the narrowleaf cottonwood, is a species of tree in the willow family (Salicaceae). It is native to western North America, where it is a characteristic species of the Rocky Mountains and the surrounding ...

''), aspens (''

Populus grandidentata ''Populus grandidentata'', commonly called large-tooth aspen, big-tooth aspen, American aspen, Canadian poplar, or white poplar, is a deciduous tree native to eastern North America. Name The name ''Populus'' is from the Latin for poplar, and ' ...

'', ''

Populus tremuloides ''Populus tremuloides'' is a deciduous tree native to cooler areas of North America, one of several species referred to by the common name aspen. It is commonly called quaking aspen, trembling aspen, American aspen, mountain or golden aspen, tr ...

'') sumac (''

Rhus glabra ''Rhus glabra'', the smooth sumac, (also known as white sumac, upland sumac, or scarlet sumac) is a species of sumac in the family Anacardiaceae, native to North America, from southern Quebec west to southern British Columbia in Canada, and so ...

''), spruce ('' Picea'' spp.), various pines (e.g., ''

Pinus echinata ''Pinus echinata'', the shortleaf pine, is a species of pine native to the southeastern United States. Description The tree is variable in form, sometimes straight, sometimes crooked, with an irregular crown. The tree reaches heights of with a ...

'', '' Pinus ponderosa''), Douglas fir (''

Pseudotsuga menziesii The Douglas fir (''Pseudotsuga menziesii'') is an evergreen conifer species in the pine family, Pinaceae. It is native to western North America and is also known as Douglas-fir, Douglas spruce, Oregon pine, and Columbian pine. There are three va ...

''), larches (''

Larix Larches are deciduous conifers in the genus ''Larix'', of the family Pinaceae (subfamily Laricoideae). Growing from tall, they are native to much of the cooler temperate northern hemisphere, on lowlands in the north and high on mountains furt ...

'' spp.), true firs (''

Abies Firs (''Abies'') are a genus of 48–56 species of evergreen coniferous trees in the family Pinaceae. They are found on mountains throughout much of North and Central America, Europe, Asia, and North Africa. The genus is most closely related t ...

'' spp.), hemlocks (''

Tsuga ''Tsuga'' (, from Japanese (), the name of ''Tsuga sieboldii'') is a genus of conifers in the subfamily Abietoideae of Pinaceae, the pine family. The common name hemlock is derived from a perceived similarity in the smell of its crushed foli ...

'' spp.), cannabis (''

Cannabis sativa ''Cannabis sativa'' is an annual herbaceous flowering plant indigenous to Eastern Asia, but now of cosmopolitan distribution due to widespread cultivation. It has been cultivated throughout recorded history, used as a source of industrial fibe ...

'' spp.), cedars (''

Cedrus ''Cedrus'', common English name cedar, is a genus of coniferous trees in the plant family Pinaceae (subfamily Abietoideae). They are native to the mountains of the western Himalayas and the Mediterranean region, occurring at altitudes of 1,500 ...

'' spp.), various Cupressaceae, and juniper bush (''

Juniperus Junipers are coniferous trees and shrubs in the genus ''Juniperus'' () of the cypress family Cupressaceae. Depending on the taxonomy, between 50 and 67 species of junipers are widely distributed throughout the Northern Hemisphere, from the Arcti ...

'' spp.). It contributes to the characteristic odor of

orange peel An orange is a fruit of various citrus species in the family Rutaceae (see list of plants known as orange); it primarily refers to ''Citrus'' × ''sinensis'', which is also called sweet orange, to distinguish it from the related ''Citrus × ...

orange juice Orange juice is a liquid extract of the orange tree fruit, produced by squeezing or reaming oranges. It comes in several different varieties, including blood orange, navel oranges, valencia orange, clementine, and tangerine. As well as vari ...

and other

citrus ''Citrus'' is a genus of flowering trees and shrubs in the rue family, Rutaceae. Plants in the genus produce citrus fruits, including important crops such as oranges, lemons, grapefruits, pomelos, and limes. The genus ''Citrus'' is native to ...

fruits. To optimize recovery of valued components from citrus peel waste, d-limonene is typically removed.

Safety and research

-Limonene applied to skin may cause irritation from

contact dermatitis Contact dermatitis is a type of acute or chronic inflammation of the skin caused by exposure to chemical or physical agents. Symptoms of contact dermatitis can include itchy or dry skin, a red rash, bumps, blisters, or swelling. These rashes are ...

, but otherwise appears to be safe for human uses. Limonene is flammable as a liquid or vapor and it is toxic to

aquatic life An aquatic ecosystem is an ecosystem formed by surrounding a body of water, in contrast to land-based terrestrial ecosystems. Aquatic ecosystems contain communities of organisms that are dependent on each other and on their environment. The tw ...

Uses

Limonene is common as a dietary supplement and as a

fragrance 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 ...

ingredient for

cosmetics Cosmetics are constituted mixtures of chemical compounds derived from either natural sources, or synthetically created ones. Cosmetics have various purposes. Those designed for personal care and skin care can be used to cleanse or protect ...

products. As the main fragrance of citrus peels, -limonene is used in food manufacturing and some medicines, such as a

flavoring A flavoring (or flavouring), also known as flavor (or flavour) or flavorant, is a food additive used to improve the taste or smell of food. It changes the perceptual impression of food as determined primarily by the chemoreceptors of the gus ...

to mask the bitter taste of alkaloids, and as a fragrance in

perfumery Perfume (, ; french: parfum) is a mixture of fragrant essential oils or aroma compounds (fragrances), fixatives and solvents, usually in liquid form, used to give the human body, animals, food, objects, and living-spaces an agreeable scent. Th ...

, aftershave lotions, bath products, and other personal care products. -Limonene is also used as a

botanical Botany, also called , plant biology or phytology, is the science of plant life and a branch of biology. A botanist, plant scientist or phytologist is a scientist who specialises in this field. The term "botany" comes from the Ancient Greek wo ...

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 (see

orange oil Orange oil is an essential oil produced by cells within the rind of an orange fruit (''Citrus sinensis'' fruit). In contrast to most essential oils, it is extracted as a by-product of orange juice production by centrifugation, producing a co ...

) and for its ability to dissolve oils. In contrast, -limonene has a piny,

-like odor. Limonene is used as a

for cleaning purposes, such as

adhesive remover Adhesive remover is a substance intended to break down and remove glue and its remnants from surfaces. Description Adhesive removers are intended to break down glue so that it can be removed from surfaces easily. Formulations may be designed t ...

, or the removal of oil from machine parts, as it is produced from a renewable source (citrus essential oil, as a byproduct of

manufacture). It is used as a

paint stripper Paint stripper, or paint remover, is a chemical product designed to remove paint, wood finishing, finishes, and coatings, while also cleaning the underlying surface. The product's material safety data sheet provides more safety information than ...

and is also useful as a fragrant alternative to turpentine. Limonene is also used as a solvent in some

model airplane A model aircraft is a small unmanned aircraft. Many are replicas of real aircraft. Model aircraft are divided into two basic groups: flying and non-flying. Non-flying models are also termed static, display, or shelf models. Aircraft manufactur ...

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 3D printing or additive manufacturing is the construction of a three-dimensional object from a CAD model or a digital 3D model. It can be done in a variety of processes in which material is deposited, joined or solidified under computer co ...

. Printers can print the plastic of choice for the model, but erect supports and binders from

High Impact Polystyrene Polystyrene (PS) is a synthetic polymer made from monomers of the aromatic hydrocarbon styrene. Polystyrene can be solid or foamed. General-purpose polystyrene is clear, hard, and brittle. It is an inexpensive resin per unit weight. It is a ...

(HIPS), a polystyrene plastic that is easily soluble in limonene. In preparing tissues for

histology Histology, also known as microscopic anatomy or microanatomy, is the branch of biology which studies the microscopic anatomy of biological tissues. Histology is the microscopic counterpart to gross anatomy, which looks at larger structures vi ...

or histopathology, -limonene is often used as a less toxic substitute for

xylene In organic chemistry, xylene or xylol (; IUPAC name: dimethylbenzene) are any of three organic compounds with the formula . They are derived from the substitution of two hydrogen atoms with methyl groups in a benzene ring; which hydrogens are s ...

when clearing dehydrated specimens. Clearing agents are liquids miscible with alcohols (such as

ethanol Ethanol (abbr. EtOH; also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic compound. It is an alcohol with the chemical formula . Its formula can be also written as or (an ethyl group linked to a ...

or isopropanol) and with melted

paraffin wax Paraffin wax (or petroleum wax) is a soft colorless solid derived from petroleum, coal, or oil shale that consists of a mixture of hydrocarbon molecules containing between 20 and 40 carbon atoms. It is solid at room temperature and begins to ...

, in which specimens are embedded to facilitate cutting of thin sections for microscopy. It is also combustible and has been considered as a biofuel.

References

External links

Mass spectrum of limonene

* ttps://www3.epa.gov/pesticides/chem_search/reg_actions/reregistration/fs_PC-079701_1-Sep-94.pdf -Limonene information from the United States Environmental Protection Agency] {{Terpenoids Flavors Histotechnology Hydrocarbon solvents IARC Group 3 carcinogens Insecticides Monoterpenes Solvents Cyclohexenes