Monoterpenes are a class of

terpene 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 predomi ...

s 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.

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 myrcenes. Hydrolysis of the phosphate groups leads to the prototypical acyclic monoterpenoid geraniol. Additional rearrangements and oxidations provide compounds such as citral, citronellal, citronellol, linalool, and many others. Many monoterpenes found in marine organisms are halogenated, such as halomon.

Main examples

Myrcen.png, Myrcene is an acyclic monoterpene, an alkene

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 Hydrophobe, hydrophobic; their odor is usually fain ...

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. Camphor, borneol, eucalyptol and ascaridole are examples of bicyclic monoterpenoids containing ketone, alcohol, ether, and bridging peroxide 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

vegetable Vegetables are edible parts of plants that are consumed by humans or other animals as food. This original meaning is still commonly used, and is applied to plants collectively to refer to all edible plant matter, including edible flower, flo ...

fruit In botany, a fruit is the seed-bearing structure in flowering plants (angiosperms) that is formed from the ovary after flowering. Fruits are the means by which angiosperms disseminate their seeds. Edible fruits in particular have long propaga ...

s and herbs.

Essential oil An essential oil is a concentrated hydrophobic liquid containing volatile (easily evaporated at normal temperatures) chemical compounds from plants. Essential oils are also known as volatile oils, ethereal oils, aetheroleum, or simply as the ...

s 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 Hexapoda, hexapod invertebrates of the class (biology), class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body (Insect morphology#Head, head, ...

attacks. Monoterpenes are emitted by

forest A forest is an ecosystem characterized by a dense ecological community, community of trees. Hundreds of definitions of forest are used throughout the world, incorporating factors such as tree density, tree height, land use, legal standing, ...

s and form

aerosol An aerosol is a suspension (chemistry), suspension of fine solid particles or liquid Drop (liquid), droplets in air or another gas. Aerosols can be generated from natural or Human impact on the environment, human causes. The term ''aerosol'' co ...

s 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 Pungency ( ) is the taste of food commonly referred to as spiciness, hotness or heat, found in foods such as chili peppers. Highly pungent tastes may be experienced as unpleasant. The term piquancy ( ) is sometimes applied to foods with a lower ...

taste of

black pepper Black pepper (''Piper nigrum'') is a flowering vine in the family Piperaceae, cultivated for its fruit (the peppercorn), which is usually dried and used as a spice and seasoning. The fruit is a drupe (stonefruit) which is about in diameter ...

, 3-carene gives cannabis 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 family Rutaceae. Plants in the genus produce citrus fruits, including important crops such as oranges, mandarins, lemons, grapefruits, pomelos, and limes. ''Citrus'' is nativ ...

fruits, and thujene and

carvacrol Carvacrol, or cymophenol, C6H3(CH3)(OH)C3H7, is a monoterpene, 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 t ...

are responsible for the pungent flavors of

summer savory Summer savory (''Satureja hortensis'') is among the best known of the Satureja, savory genus. It is an annual, but otherwise is similar in use and flavor to the perennial winter savory. It is used more often than winter savory, which has a sligh ...

and oregano, respectively. Monoterpenes are considered allelochemicals.

Uses

Many monoterpenes are volatile compounds and some of them are well-known fragrants found in the

essential oil An essential oil is a concentrated hydrophobic liquid containing volatile (easily evaporated at normal temperatures) chemical compounds from plants. Essential oils are also known as volatile oils, ethereal oils, aetheroleum, or simply as the ...

s of many plants. For example, camphor, citral, citronellol, geraniol, grapefruit mercaptan, eucalyptol, ocimene, myrcene, limonene, linalool, menthol, camphene and pinenes are used in perfumes and cosmetic products. Limonene and perillyl alcohol are used in cleaning products. Many monoterpenes are used as food flavors and

food additive Food additives are substances added to food to preserve flavor or enhance taste, appearance, or other sensory qualities. Some additives, such as vinegar ( pickling), salt ( salting), smoke ( smoking) and sugar ( crystallization), have been used f ...

s, such as bornyl acetate, citral, eucalyptol, menthol, hinokitiol, camphene and limonene. Menthol, hinokitiol and thymol are also used in oral hygiene products. Thymol 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. 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 presence of substances in the Atmosphere of Earth, air that are harmful to humans, other living beings or the environment. Pollutants can be Gas, gases like Ground-level ozone, ozone or nitrogen oxides or small particles li ...

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.

References