Monoterpenes are a class of
terpene
Terpenes () are a class of natural products consisting of compounds with the formula (C5H8)n for n > 1. Comprising more than 30,000 compounds, these unsaturated hydrocarbons are produced predominantly by plants, particularly conifers. Terpenes ...
s that consist of two
isoprene units and have the molecular formula C
10H
16. 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.
Biosynthesis
Monoterpenes are derived biosynthetically from units of
isopentenyl pyrophosphate, which is formed from
acetyl-CoA via the intermediacy of
mevalonic acid in the
HMG-CoA reductase pathway. An alternative, unrelated biosynthesis pathway of IPP is known in some bacterial groups and the plastids of plants, the so-called MEP-(2-methyl-
D-erythritol-4-phosphate) pathway, which is initiated from
C5 sugars. In both pathways, IPP is isomerized to DMAPP by the enzyme isopentenyl pyrophosphate isomerase.
Geranyl pyrophosphate is the precursor to monoterpenes (and hence monoterpenoids).
Elimination of the pyrophosphate group from geranyl pyrophosphate leads to the formation of acyclic monoterpenes such as
ocimene and the
myrcene
Myrcene, or β-myrcene, is a monoterpene. A colorless oil, it occurs widely in essential oils. It is produced mainly semi-synthetically from ''Myrcia'', from which it gets its name. It is an intermediate in the production of several fragrances. � ...
s. Hydrolysis of the phosphate groups leads to the prototypical acyclic monoterpenoid
geraniol. Additional rearrangements and oxidations provide compounds such as
citral,
citronellal
Citronellal or rhodinal ( C10 H18 O) is a mono terpenoid aldehyde, the main component in the mixture of terpenoid chemical compounds that give citronella oil its distinctive lemon scent.
Citronellal is a main isolate in distilled oils from the ...
,
citronellol,
linalool, and many others. Many monoterpenes found in marine organisms are
halogenated, such as
halomon
Halomon is a polyhalogenated monoterpene first isolated from the marine red algae '' Portieria hornemannii''. Halomon has attracted research interest because of its promising profile of selective cytotoxicity that suggests its potential use as ...
.
Main examples
Myrcen.png, Myrcene
Myrcene, or β-myrcene, is a monoterpene. A colorless oil, it occurs widely in essential oils. It is produced mainly semi-synthetically from ''Myrcia'', from which it gets its name. It is an intermediate in the production of several fragrances. � ...
is an acyclic monoterpene, an 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 ...
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 ...
Menthol_skeletal.svg, Menthol is a monocyclic monoterpenoid
Limonene-2D-skeletal.svg, Limonene is a monocyclic monoterpene
Carvone.svg, Carvone is a monoterpenoid
Beta-thujaplicin.png, Hinokitiol is a monoterpenoid, a tropolone derivative
Linalool_skeletal.svg, Linalool is an acyclic monoterpenoid
Bicyclic monoterpenes include
carene,
sabinene,
camphene, and
thujene
Thujene (or α-thujene) is a natural organic compound classified as a monoterpene. It is found in the essential oils of a variety of plants, and contributes pungency to the flavor of some herbs such as Summer savory.''PDR for Herbal Medicines'', ...
.
Camphor,
borneol
Borneol is a bicyclic organic compound and a terpene derivative. The hydroxyl group in this compound is placed in an '' endo'' position. The exo diastereomer is called isoborneol. Being chiral, borneol exists as enantiomers, both of which are foun ...
,
eucalyptol and
ascaridole are examples of bicyclic monoterpenoids containing ketone, alcohol, ether, and bridging
peroxide
In chemistry, peroxides are a group of compounds with the structure , where R = any element. The group in a peroxide is called the peroxide group or peroxo group. The nomenclature is somewhat variable.
The most common peroxide is hydrogen p ...
functional groups, respectively.
Umbellulone is another example of bicyclic monoterpene ketone.
Natural roles
Monoterpenes are found in many parts of different plants, such as
barks,
heartwood, bark and leaves of coniferous trees, in
vegetables,
fruit
In botany, a fruit is the seed-bearing structure in flowering plants that is formed from the ovary after flowering.
Fruits are the means by which flowering plants (also known as angiosperms) disseminate their seeds. Edible fruits in partic ...
s and
herbs.
Essential oils are very rich in monoterpenes. Several monoterpenes produced by trees, such as
linalool,
hinokitiol, and
ocimene have
fungicidal and
antibacterial activities and participate in wound healing. Some of these compounds are produced to protect the trees from
insect
Insects (from Latin ') are pancrustacean hexapod invertebrates of the class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body ( head, thorax and abdomen), three pa ...
attacks.
Monoterpenes are emitted by
forests and form
aerosols that are proposed to serve as
cloud condensation nuclei (CCN). Such aerosols can increase the brightness of clouds and cool the climate.
Many monoterpenes have unique smell and flavor. For example,
sabinene contributes to the
spicy taste of
black pepper,
3-carene
3-Carene is a bicyclic monoterpene consisting of fused cyclohexene and cyclopropane rings. It occurs as a constituent of turpentine, with a content as high as 42% depending on the source. Carene has a sweet and pungent odor, best described as a ...
gives
cannabis
''Cannabis'' () is a genus of flowering plants in the family Cannabaceae. The number of species within the genus is disputed. Three species may be recognized: '' Cannabis sativa'', '' C. indica'', and '' C. ruderalis''. Alternative ...
an earthy taste and smell,
citral has a lemon-like pleasant odor and contributes to the distinctive smell of
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, and
thujene
Thujene (or α-thujene) is a natural organic compound classified as a monoterpene. It is found in the essential oils of a variety of plants, and contributes pungency to the flavor of some herbs such as Summer savory.''PDR for Herbal Medicines'', ...
and
carvacrol
Carvacrol, or cymophenol, C6H3(CH3)(OH)C3H7, is a monoterpenoid phenol. It has a characteristic pungent, warm odor of oregano.
Natural occurrence
Carvacrol is present in the essential oil of ''Origanum vulgare'' (oregano), oil of thyme, oil obt ...
are responsible for the
pungent flavors of
summer savory and
oregano, respectively.
Monoterpenes are considered
allelochemicals
Allelopathy is a biological phenomenon by which an organism produces one or more biochemicals that influence the germination, growth, survival, and reproduction of other organisms. These biochemicals are known as allelochemicals and can have ben ...
.
Uses
Many monoterpenes are
volatile compounds and some of them are well-known
fragrants found in the
essential oils of many plants. For example,
camphor,
citral,
citronellol,
geraniol,
grapefruit mercaptan,
eucalyptol,
ocimene,
myrcene
Myrcene, or β-myrcene, is a monoterpene. A colorless oil, it occurs widely in essential oils. It is produced mainly semi-synthetically from ''Myrcia'', from which it gets its name. It is an intermediate in the production of several fragrances. � ...
,
limonene,
linalool,
menthol,
camphene and
pinenes are used in
perfume
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 ...
s and
cosmetic
Cosmetic may refer to:
*Cosmetics, or make-up, substances to enhance the beauty of the human body, apart from simple cleaning
*Cosmetic, an adjective describing beauty, aesthetics, or appearance, especially concerning the human body
*Cosmetic, a t ...
products.
Limonene and
perillyl alcohol are used in
cleaning
Cleaning is the process of removing unwanted substances, such as dirt, infectious agents, and other impurities, from an object or environment. Cleaning is often performed for aesthetic, hygienic, functional, environmental, or safety purposes. ...
products.
Many monoterpenes are used as food
flavors and
food additives, such as
bornyl acetate
Bornyl acetate is a chemical compound. Its molecular formula is C12H20O2 and its molecular weight is 196.29 g/mol. It is the acetate ester of borneol. It is used as a food additive, flavouring agent, and odour agent.
It is a component of the es ...
,
citral,
eucalyptol,
menthol,
hinokitiol,
camphene and
limonene.
Menthol,
hinokitiol and
thymol
Thymol (also known as 2-isopropyl-5-methylphenol, IPMP), , is a natural monoterpenoid phenol derivative of ''p''-Cymene, isomeric with carvacrol, found in oil of thyme, and extracted from '' Thymus vulgaris'' (common thyme), ajwain, and va ...
are also used in
oral hygiene
Oral hygiene is the practice of keeping one's mouth clean and free of disease and other problems (e.g. bad breath) by regular brushing of the teeth (dental hygiene) and cleaning between the teeth. It is important that oral hygiene be carried ou ...
products.
Thymol
Thymol (also known as 2-isopropyl-5-methylphenol, IPMP), , is a natural monoterpenoid phenol derivative of ''p''-Cymene, isomeric with carvacrol, found in oil of thyme, and extracted from '' Thymus vulgaris'' (common thyme), ajwain, and va ...
also has
antiseptic and
disinfectant properties.
Volatile monoterpenes produced by plants can attract or repel insects, thus some of them are used in
insect repellents, such as
citronellol,
eucalyptol,
limonene,
linalool,
hinokitiol,
menthol and
thymol
Thymol (also known as 2-isopropyl-5-methylphenol, IPMP), , is a natural monoterpenoid phenol derivative of ''p''-Cymene, isomeric with carvacrol, found in oil of thyme, and extracted from '' Thymus vulgaris'' (common thyme), ajwain, and va ...
.
Ascaridole,
camphor and
eucalyptol are monoterpenes that have pharmaceutical use.
Health effects
A study suggests that a range of
floor cleaners with certain monoterpenes may cause indoor
air pollution
Air pollution is the contamination of air due to the presence of substances in the atmosphere that are harmful to the health of humans and other living beings, or cause damage to the climate or to materials. There are many different type ...
equivalent or exceeding the harm to respiratory tracts when the time is spent
near a busy road. This is due to
ozonolysis of monoterpenes like
Limonene leading to the production of atmospheric
SOA. Another study suggests monoterpenes substantially affect ambient organic aerosol with uncertainties regarding environmental impacts. In a review, scientists concluded that they hope that these "substances will be extensively studied and used in more and more in medicine". A 2013 study found that "Based on adverse effects and risk assessments, d-limonene may be regarded as a safe ingredient. However, the potential occurrence of skin irritation necessitates regulation of this chemical as an ingredient in cosmetics." According to a review, several studies showed "that some monoterpenes (e.g., pulegone, menthofuran, camphor, and limonene) and sesquiterpenes (e.g., zederone, germacrone) exhibited liver toxicity" and that i.a. intensive research on terpenes toxicity is needed.
See also
*
Terpene
Terpenes () are a class of natural products consisting of compounds with the formula (C5H8)n for n > 1. Comprising more than 30,000 compounds, these unsaturated hydrocarbons are produced predominantly by plants, particularly conifers. Terpenes ...
*
Terpenoid
*
Diterpene
*
Sesquiterpene
Sesquiterpenes are a class of terpenes that consist of three isoprene units and often have the molecular formula C15H24. Like monoterpenes, sesquiterpenes may be cyclic or contain rings, including many unique combinations. Biochemical modific ...
References
{{Terpenoids
Monoterpenes
Terpenes and terpenoids