View source for Terpene

{{short description|Class of oily organic compounds found in plants}}
{{confuse|Terpin}}
[[File:Résine.jpg|thumb|upright|Many terpenes are derived commercially from conifer resins, such as those made by this [[pine]].]]

'''Terpenes''' ({{IPAc-en|ˈ|t|ɜːr|p|iː|n|z}} {{respell|TUR|-|peenz}}) are a large and diverse class of [[natural product]]s with the general formula {{chem2|(C5H8)<sub>''n''</sub>}}, where ''n''&nbsp;≥&nbsp;2. They serve as crucial biosynthetic building blocks in many organisms, particularly plants. Comprising more than 30,000 compounds, these unsaturated [[hydrocarbons]] are produced predominantly by [[plant]]s, particularly [[Pinophyta|conifers]].<ref name=book>{{cite book|title=Terpenes: Flavors, Fragrances, Pharmaca, Pheromones|author=Eberhard Breitmaier|year= 2006|isbn=9783527609949 |doi=10.1002/9783527609949|publisher=Wiley-VCH}}</ref><ref name=Crot/><ref>{{cite web|url=https://rareterpenes.com/what-are-terpenes-an-overview/|title=What are Terpenes|website=rareterpenes.com|date=13 April 2021}}</ref> In plants, terpenes and terpenoids are important mediators of ecological [[Biological interaction|interaction]]s, 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.

Terpenes are classified by the number of carbons: [[monoterpenes]] (C<sub>10</sub>), [[sesquiterpenes]] (C<sub>15</sub>), [[diterpenes]] (C<sub>20</sub>), as examples. The terpene [[alpha-pinene]] is a major component of [[turpentine]], a common [[solvent]].

One terpene that has major applications is [[natural rubber]] (i.e., [[polyisoprene]]). The possibility that other terpenes could be used as precursors to produce synthetic [[polymer]]s has been investigated. Many terpenes have been shown to have pharmacological effects. Terpenes are also components of some traditional medicines, such as [[aromatherapy]], and as active ingredients of [[pesticide]]s in agriculture.<ref>{{cite book |last=  Stoker|first= H. Stephen|date= 2007|title=General, Organic, and Biological Chemistry, 4th edition |publisher= Houghton Mifflin Company|page= 337|isbn=978-0-618-73063-6}}</ref>

==History and terminology==
The term ''Terpen'' (German) was coined in 1866 by the German chemist [[August Kekulé]] to denote all hydrocarbons having the empirical formula C<sub>10</sub>H<sub>16</sub>, of which [[camphene]] was one. Previously, many hydrocarbons having the empirical formula C<sub>10</sub>H<sub>16</sub> had been called "camphene", but many other hydrocarbons of the same composition had different names. Kekulé coined the term "Terpen" in order to reduce the confusion.<ref>{{cite book |last=Kekulé |first=August |title=Lehrbuch der organischen Chemie |trans-title=Textbook of Organic Chemistry |date=1866 |publisher=Ferdinand Enke |location=Erlangen, (Germany) |volume= 2 |pages=464–465 |language=de |url=https://books.google.com/books?id=IbBmZDznuPsC&pg=PA464 |quote=From pp. 464–465: ''"Mit dem Namen Terpene bezeichnen wir … unter verschiedenen Namen aufgeführt werden."'' (By the name "terpene" we designate in general the hydrocarbons composed according to the [empirical] formula C<sub>10</sub>H<sub>16</sub> (see §. 1540)}}</ref><ref>{{cite book |last=Dev |first=Sukh |editor-last=Rowe |editor-first=John W. |title=Natural Products of Woody Plants: Chemicals Extraneous to the Lignocellulosic Cell Wall |date=1989 |publisher=Springer-Verlag |location=Heidelberg, Germany / Berlin |pages=691–807 |chapter=Chapter 8. Isoprenoids: 8.1. Terpenoids.}} ; [https://link.springer.com/chapter/10.1007/978-3-642-74075-6_19 see p. 691.]</ref> The name is a shortened form of "Terpentin", the german word for "[[turpentine]]".<ref name=Ull/>

Although sometimes used interchangeably with "terpenes", [[terpenoid]]s (or [[isoprenoid]]s) are modified terpenes that contain additional [[functional group]]s, usually oxygen-containing.<ref>{{cite journal |url=https://goldbook.iupac.org/html/T/T06279.html |title=IUPAC Gold Book – terpenoids | doi=10.1351/goldbook.T06279 |doi-access=free |url-access=subscription}}</ref> The terms terpenes and terpenoids are often used interchangeably, however. Furthermore, terpenes are produced from terpenoids and many terpenoids are produced from terpenes. Both have strong and often pleasant odors, which may protect their hosts or attract pollinators. The number of terpenes and terpenoids is estimated at 55,000 chemical entities.<ref>{{cite journal |last1=Chen |first1=Ke |last2=Baran |first2=Phil S. |title=Total synthesis of eudesmane terpenes by site-selective C–H oxidations |journal=Nature |date=June 2009 |volume=459 |issue=7248 |pages=824–828 |doi=10.1038/nature08043|pmid=19440196 |bibcode=2009Natur.459..824C |s2cid=4312428}}</ref>

The 1939 [[Nobel Prize in Chemistry]] was awarded to [[Leopold Ružička]] "for his work on [[polymethylenes]] and higher terpenes",<ref name = Nobel>{{cite book|editor-first = Karl|editor-last = Grandin|chapter = Leopold Ružička|title = Nobel Lectures, Chemistry: 1922-1941|publisher = [[Elsevier Publishing Company]]|location = Amsterdam|date= 1966}}<br />Now available from {{cite web|url = http://nobelprize.org/nobel_prizes/chemistry/laureates/1939/ruzicka-bio.html|title = Leopold Ružička Biography|website = [[nobelprize.org]]|publisher = [[Nobel Foundation]]|date= 1939|access-date = 6 July 2017}}</ref><ref>{{cite web|url=https://www.nobelprize.org/prizes/chemistry/1939/summary/|title=The Nobel Prize in Chemistry 1939}}</ref> "including the first [[chemical synthesis]] of [[male sex hormones]]."<ref name="hillier19">{{cite journal |doi=10.1530/JOE-19-0084|title=Terpenes, hormones and life: Isoprene rule revisited|year=2019|last1=Hillier|first1=Stephen G.|last2=Lathe|first2=Richard|journal=Journal of Endocrinology|volume=242|issue=2|pages=R9–R22|pmid=31051473|doi-access=free}}</ref>

==Biological function==
Terpenes are major biosynthetic building blocks. [[Steroids]], for example, are derivatives of the triterpene [[squalene]]. Terpenes and terpenoids are also the primary constituents of the [[essential oil]]s of many types of plants and flowers.<ref>{{cite journal|last1=Omar|first1=Jone|last2=Olivares|first2=Maitane|last3=Alonso|first3=Ibone|last4=Vallejo|first4=Asier|last5=Aizpurua-Olaizola|first5=Oier|last6=Etxebarria|first6=Nestor|date=April 2016|title=Quantitative Analysis of Bioactive Compounds from Aromatic Plants by Means of Dynamic Headspace Extraction and Multiple Headspace Extraction-Gas Chromatography-Mass Spectrometry: Quantitative analysis of bioactive compounds…|journal=Journal of Food Science|volume=81|issue=4|pages=C867–C873|doi=10.1111/1750-3841.13257|pmid=26925555|s2cid=21443154 |url=https://figshare.com/articles/journal_contribution/5028548}}</ref> In plants, terpenes and terpenoids are important mediators of ecological [[biological interaction|interaction]]s. For example, they play a role in [[plant defense against herbivory]], [[plant disease resistance|disease resistance]], attraction of [[mutualism (biology)|mutualists]] such as [[pollinator]]s, as well as potentially plant-[[plant communication]].<ref>{{cite journal|last1=Martin|first1=D.&nbsp;M.|last2=Gershenzon|first2=J.|last3=Bohlmann|first3=J.|title=Induction of Volatile Terpene Biosynthesis and Diurnal Emission by Methyl Jasmonate in Foliage of Norway Spruce|journal=Plant Physiology|date=July 2003|volume=132|issue=3|pages=1586–1599|doi=10.1104/pp.103.021196|pmid=12857838|pmc=167096}}</ref><ref>{{cite journal|last1=Pichersky|first1=E.|title=Biosynthesis of Plant Volatiles: Nature's Diversity and Ingenuity|journal=Science|date=10 February 2006|volume=311|issue=5762|pages=808–811|doi=10.1126/science.1118510|pmid=16469917|pmc=2861909|bibcode=2006Sci...311..808P}}</ref> They appear to play roles as [[antifeedant]]s.<ref name=Crot/> Other functions of terpenoids include cell growth modulation and plant elongation, light harvesting and photoprotection, and membrane permeability and fluidity control.<ref name="Roberts2007">{{cite journal|last=Roberts|first=Susan C|title=Production and engineering of terpenoids in plant cell culture|journal=Nature Chemical Biology|volume=3|issue=7|year=2007|pages=387–395|issn=1552-4450|doi=10.1038/nchembio.2007.8|pmid=17576426}}</ref>

Higher amounts of terpenes are released by trees in warmer weather,<ref>{{cite web|title=An Introduction to Terpenes|url=https://deliverymeds.ca/an-introduction-to-terpenes-the-hidden-gems-of-the-cannabis-experience/|access-date=2021-10-30|archive-date=2021-10-30|archive-url=https://web.archive.org/web/20211030152435/https://deliverymeds.ca/an-introduction-to-terpenes-the-hidden-gems-of-the-cannabis-experience/|url-status=dead}}</ref> where they may function as a natural mechanism of [[cloud seeding]]. The clouds reflect sunlight, allowing the forest temperature to regulate.<ref>{{cite news| url=https://www.theguardian.com/environment/2008/oct/31/forests-climatechange | work=The Guardian | first=David | last=Adam | title=Scientists discover cloud-thickening chemicals in trees that could offer a new weapon in the fight against global warming | date=October 31, 2008}}</ref>

Some insects use some terpenes as a form of defense. For example, [[termite]]s of the subfamily [[Nasutitermitinae]] [[anti-predator adaptation|ward off predatory insects]] through the use of a specialized mechanism called a [[fontanellar gun]], which ejects a resinous mixture of terpenes.<ref name="Psyche">{{cite journal |first1=W.&nbsp;L. |last1=Nutting |first2=M.&nbsp;S. |last2=Blum |first3=H.&nbsp;M. |last3=Fales |year=1974 |title=Behavior of the North American Termite, ''Tenuirostritermes tenuirostris'', with Special Reference to the Soldier Frontal Gland Secretion, Its Chemical Composition, and Use in Defense |journal=[[Psyche (entomological journal)|Psyche: A Journal of Entomology]] |volume=81 |issue=1 |pages=167–177 |doi=10.1155/1974/13854 |doi-access=free }}</ref>

==Applications==
[[File:Naturkautschuk.svg|thumb|upright=0.5|Structure of natural rubber, exhibiting the characteristic methyl group on the alkene group]]

One terpene that has major applications is [[natural rubber]] (i.e., [[polyisoprene]]). The possibility that other terpenes could be used as precursors to produce synthetic [[polymer]]s has been investigated as an alternative to the use of petroleum-based feedstocks. However, few of these applications have been commercialized.<ref>{{cite book |doi=10.1016/B978-0-08-045316-3.00002-8|chapter=Terpenes: Major Sources, Properties and Applications|title=Monomers, Polymers and Composites from Renewable Resources|year=2008|last1=Silvestre|first1=Armando J.D.|last2=Gandini|first2=Alessandro|pages=17–38|isbn=9780080453163}}</ref> Many other terpenes, however, have smaller scale commercial and industrial applications. For example, [[turpentine]], a mixture of terpenes (e.g., [[pinene]]), obtained from the distillation of pine tree [[resin]], is used as an organic [[solvent]] and as a chemical feedstock (mainly for the production of other terpenoids).<ref name=Ull>{{Ullmann |doi=10.1002/14356007.a26_205 |title=Terpenes |date=2000 |last=Eggersdorfer |first=Manfred }}</ref> [[Rosin]], another by-product of conifer tree resin, is widely used as an ingredient in a variety of industrial products, such as [[ink]]s, [[varnish]]es and [[adhesive]]s. Rosin is also used by violinists (and players of similar [[Bow (music)|bowed]] instruments) to increase friction on the [[Bow (music)|bow]] hair, by [[ballet dancer]]s on the soles of their shoes to maintain traction on the floor, by [[gymnastics|gymnasts]] to keep their grips while performing, and by [[baseball pitcher]]s to improve their control of the baseball.<ref>{{cite web |last=Roberts |first=Maddy Shaw |title=What the heck is rosin – and why do violinists need it? |url=https://www.classicfm.com/discover-music/instruments/violin/what-is-rosin-why-violinists-need-it/ |website=Classic FM |date=22 January 2019 |access-date=22 July 2022}}</ref> Terpenes are widely used as fragrances and flavors in consumer products such as [[perfume]]s, [[cosmetics]] and [[cleaning agent|cleaning products]], as well as food and drink products. For example, the aroma and flavor of [[hops]] comes, in part, from [[sesquiterpene]]s (mainly [[Alpha-humulene|α-humulene]] and [[beta-caryophyllene|β-caryophyllene]]), which affect [[beer]] quality.<ref name="beer">{{cite journal|pmid=25442616|date=2015|last1=Steenackers|first1=B. |last2=De Cooman|first2=L.|last3=De Vos|first3=D. |title=Chemical transformations of characteristic hop secondary metabolites in relation to beer properties and the brewing process: A review|journal=Food Chemistry|volume=172|pages=742–756|doi=10.1016/j.foodchem.2014.09.139}}</ref> Some form hydroperoxides that are valued as catalysts in the production of polymers.

Many terpenes have been shown to have pharmacological effects, although most studies are from laboratory research, and [[clinical research]] in humans is preliminary.<ref>{{cite journal|date=2014|last1=Koziol|first1=Agata|last2=Stryjewska|first2=Agnieszka|last3=Librowski|first3=Tadeusz|last4=Salat|first4=Kinga|last5=Gawel|first5=Magdalena|last6=Moniczewski|first6=Andrzej|last7=Lochynski|first7=Stanislaw|title=An Overview of the Pharmacological Properties and Potential Applications of Natural Monoterpenes|journal=Mini-Reviews in Medicinal Chemistry|volume=14|issue=14|pages=1156–1168|doi=10.2174/1389557514666141127145820|pmid=25429661 |url=https://ruj.uj.edu.pl/xmlui/handle/item/109086 }}</ref> Terpenes are also components of some traditional medicines, such as [[aromatherapy]].<ref>{{cite journal|last1=Koyama|first1=Sachiko|last2=Heinbockel|first2=Thomas|title=The Effects of Essential Oils and Terpenes in Relation to Their Routes of Intake and Application|journal=International Journal of Molecular Sciences|date=2020|volume=21|issue=5|page=1558|doi=10.3390/ijms21051558|pmid=32106479|pmc=7084246|doi-access=free}}</ref>

Reflecting their defensive role in plants, terpenes are used as active ingredients of [[pesticide]]s in agriculture.<ref>{{cite journal|last=Isman|first=M.&nbsp;B.|title=Plant essential oils for pest and disease management|journal=Crop Protection|date=2000|volume=21|issue=8–10|pages=603–608|doi=10.1016/S0261-2194(00)00079-X|bibcode=2000CrPro..19..603I |s2cid=39469817 |ref=9}}</ref>

[[File:Tetrahydrocannabinol.svg|thumb|220px|[[Tetrahydrocannabinol]], a terpenoid, not a terpene, is the active ingredient in marijuana.]]

==Physical and chemical properties==
Terpenes are colorless, although impure samples are often yellow. Boiling points scale with molecular size: terpenes, sesquiterpenes, and diterpenes respectively at 110, 160, and 220&nbsp;°C. Being highly non-polar, they are insoluble in water. Being hydrocarbons, they are highly flammable and have low specific gravity (float on water). They are tactilely light oils considerably less [[Viscosity|viscous]] than familiar vegetable oils like corn oil (28 [[Poise (unit)|cP]]), with viscosity ranging from 1 cP (à la water) to 6 cP. Terpenes are local irritants and can cause gastrointestinal disturbances if ingested.

''Terpenoids'' (mono-, sesqui-, di-, etc.) have similar physical properties but tend to be more polar and hence slightly more soluble in water and somewhat less volatile than their terpene analogues. Highly polar derivatives of terpenoids are the [[glycosides]], which are linked to sugars.  These are water-soluble solids.

{{See also|Triterpene glycoside}}

==Biosynthesis<span class="anchor" id="Biogenetic isoprene rule"></span>==
[[File:TerpeneVterpenoid.svg|thumb|upright=1.2|Biosynthetic conversion of [[geranylpyrophosphate]] to the terpenes [[Alpha-pinene|α-pinene]] and [[Beta-pinene|β-pinene]] and to the terpinoid [[Alpha-terpineol|α-terpineol]].<ref name=Crot>{{cite book|chapter=Cyclization enzymes in the biosynthesis of monoterpenes, sesquiterpenes, and diterpenes|author1=Davis, Edward M. |author2=Croteau, Rodney |title=Biosynthesis|journal=Topics in Current Chemistry|date=2000|volume=209|pages=53–95|doi=10.1007/3-540-48146-X_2|isbn=978-3-540-66573-1}}</ref> ]]

=== Isoprene as the building block ===
Conceptually derived from [[isoprene]]s, the structures and formulas of terpenes follow the '''biogenetic isoprene rule''' or the '''C<sub>5</sub> rule''', as described in 1953 by [[Leopold Ružička]]<ref name="ruzicka53">{{cite journal |doi=10.1007/BF02167631|title=The isoprene rule and the biogenesis of terpenic compounds|year=1953|last1=Ruzicka|first1=L.|journal=Experientia|volume=9|issue=10|pages=357–367|pmid=13116962|s2cid=44195550}}</ref> and colleagues.<ref>{{cite journal |last1=Eschenmoser |first1=Albert |last2=Arigoni |first2=Duilio |title=Revisited after 50 Years: The 'Stereochemical Interpretation of the Biogenetic Isoprene Rule for the Triterpenes' |journal=Helvetica Chimica Acta |date=December 2005 |volume=88 |issue=12 |pages=3011–3050 |doi=10.1002/hlca.200590245 |bibcode=2005HChAc..88.3011E }}</ref> The C<sub>5</sub> isoprene units are provided in the form of [[dimethylallyl pyrophosphate]] (DMAPP) and [[isopentenyl pyrophosphate]] (IPP). DMAPP and IPP are [[structural isomer]]s to each other. This pair of building blocks are produced by two distinct [[metabolic pathway]]s: the [[mevalonate pathway|mevalonate (MVA) pathway]] and the [[non-mevalonate pathway|non-mevalonate (MEP) pathway]]. These two pathways are mutually exclusive in most organisms, except for some bacteria and land plants.{{citation needed|date=March 2021}} In general, most archaea and eukaryotes use the MVA pathway, while bacteria mostly have the MEP pathway. IPP and DMAPP are final products of both MVA and MEP pathways and the relative abundance of these two isoprene units is enzymatically regulated in host organisms.
{| class="wikitable"
|-
! Organism
! Pathways
|-
| [[Bacteria]] || MVA or MEP
|-
| [[Archaea]] || MVA
|-
| Green [[Algae]] || MEP
|-
| [[Plant]]s ||   MVA and MEP
|-
| [[Animal]]s ||  MVA
|-
| [[Fungi]] ||  MVA
|}

==== Mevalonate pathway ====
{{Main|Mevalonate pathway}}
This pathway conjugates three molecules of [[acetyl CoA]].

The mevalonate (MVA) pathway is distributed in all three domains of life; archaea, bacteria and eukaryotes. The MVA pathway is universally distributed in archaea and non-photosynthetic eukaryotes, while the pathway is sparse in bacteria. In photosynthetic eukaryotes, some species possess the MVA pathway, while others have the MEP pathway or both MVA and MEP pathways. This is due to the acquisition of the MEP pathway by a common ancestor of [[Archaeplastida]] (algae + land plants) through the [[Endosymbiont|endosymbiosis]] of ancestral [[cyanobacteria]] that possessed the MEP pathway. The MVA and MEP pathways were selectively lost in individual photosynthetic lineages.

Also, the archaeal MVA pathway is not completely homologous to the eukaryotic MVA pathway.<ref>{{Cite journal|last1=Hayakawa|first1=Hajime|last2=Motoyama|first2=Kento|last3=Sobue|first3=Fumiaki|last4=Ito|first4=Tomokazu|last5=Kawaide|first5=Hiroshi|last6=Yoshimura|first6=Tohru|last7=Hemmi|first7=Hisashi|date=2018-10-02|title=Modified mevalonate pathway of the archaeon Aeropyrum pernix proceeds via trans -anhydromevalonate 5-phosphate|journal=Proceedings of the National Academy of Sciences|language=en|volume=115|issue=40|pages=10034–10039|doi=10.1073/pnas.1809154115|issn=0027-8424|pmc=6176645|pmid=30224495|bibcode=2018PNAS..11510034H |doi-access=free}}</ref> Instead, the eukaryotic MVA pathway is closer to the bacterial MVA pathway.

==== Non-mevalonate pathway ====
{{Main|Non-mevalonate pathway}}

The non-mevalonate pathway or the 2-''C''-methyl-D-erythritol 4-phosphate (MEP) pathway starts with [[pyruvate]] and [[glyceraldehyde 3-phosphate]] (G3P) as the carbon source.

C<sub>5</sub> IPP and C<sub>5</sub> DMAPP are the end-products in either pathway and are the precursors of terpenoids with various carbon numbers (typically C<sub>5</sub> to C<sub>40</sub>), side chains of (bacterio)[[chlorophyll]]s, [[heme]]s and [[quinone]]s. Synthesis of all higher terpenoids proceeds via formation of [[geranyl pyrophosphate]] (GPP), [[farnesyl pyrophosphate]] (FPP), and [[geranylgeranyl pyrophosphate]] (GGPP).

===Geranyl pyrophosphate phase and beyond===
[[File:Synthesis of geranyl pyrophosphate.png|thumb|center|upright=1.2|[[Isopentenyl pyrophosphate]] (IPP) and [[dimethylallyl pyrophosphate]] (DMAPP) condense to produce [[geranyl pyrophosphate]], precursor to all terpenes and terpenoids.]]

In both MVA and MEP pathways, IPP is isomerized to DMAPP by the enzyme isopentenyl pyrophosphate isomerase.  IPP and DMAPP condense to give [[geranyl pyrophosphate]], the precursor to monoterpenes and monoterpenoids.

Geranyl pyrophosphate is also converted to [[farnesyl pyrophosphate]] and [[geranylgeranyl pyrophosphate]], respectively C<sub>15</sub> and C<sub>20</sub> precursors to [[sesquiterpene]]s and [[diterpene]]s (as well as sesequiterpenoids and diterpenoids).<ref name=Crot/> Biosynthesis is mediated by  [[Terpene synthase N terminal domain|terpene synthase]].<ref>{{cite journal|pmid=29017925|year=2017|last1=Kumari|first1=I.|title=Evolution of catalytic microenvironment governs substrate and product diversity in trichodiene synthase and other terpene fold enzymes|journal=Biochimie|volume=144|pages=9–20|last2=Ahmed|first2=M.|last3=Akhter|first3=Y.|doi=10.1016/j.biochi.2017.10.003}}</ref><ref>{{cite journal|pmc=4940680|year=2016|last1=Pazouki|first1=L.|last2=Niinemets|first2=Ü.|title=Multi-Substrate Terpene Synthases: Their Occurrence and Physiological Significance|journal=Frontiers in Plant Science|volume=7|pages=1019|doi=10.3389/fpls.2016.01019|pmid=27462341|bibcode=2016FrPS....7.1019P |doi-access=free}}</ref>

===Terpenes to terpenoids===
The genomes of many plant species contain genes that encode terpenoid synthase enzymes imparting terpenes with their basic structure, and [[cytochrome P450]]s that modify this basic structure.<ref name=Crot/><ref name=boutanaev15>{{cite journal |last1= Boutanaev |first1=A.&nbsp;M. |last2=Moses |first2=T. |last3=Zi |first3=J. |last4=Nelson |first4=D.&nbsp;R. |last5=Mugford |first5=S.&nbsp;T. |last6=Peters |first6=R.&nbsp;J. |last7=Osbourn |first7=A. |date=2015 |title= Investigation of terpene diversification across multiple sequenced plant genomes |journal=Proceedings of the National Academy of Sciences |volume=112 |issue=1 |pages=E81–E88 |doi=10.1073/pnas.1419547112 |pmid=25502595 |pmc=4291660|bibcode=2015PNAS..112E..81B |doi-access=free }}</ref>

==Structure==
Terpenes can be visualized as the result of linking [[isoprene]] (C<sub>5</sub>H<sub>8</sub>) units "head to tail" to form chains and rings.<ref>{{cite journal |title = The isoprene rule and the Biogenesis of terpenic compounds |first = Leopold |last = Ružička|journal = Cellular and Molecular Life Sciences|volume = 9|issue = 10|pages = 357–367 |date= 1953 |doi = 10.1007/BF02167631 |pmid = 13116962 |s2cid = 44195550 }}</ref>  A few terpenes are linked “tail to tail”, and larger branched terpenes may be linked “tail to mid”.

===Formula===
Strictly speaking all monoterpenes have the same chemical formula C<sub>10</sub>H<sub>16</sub>.  Similarly all sesquiterpenes and diterpenes have formulas of C<sub>15</sub>H<sub>24</sub> and C<sub>20</sub>H<sub>32</sub> respectively. The structural diversity of mono-, sesqui-, and diterpenes is a consequence of isomerism.

===Chirality===
Terpenes and terpenoids are usually [[chiral]]. Chiral compounds can exist as non-superposable mirror images, which exhibit distinct [[physical properties]] such as odor or toxicity.

====Unsaturation====
Most terpenes and terpenoids feature C=C groups, i.e. they exhibit unsaturation. Since they carry no functional groups aside from their unsaturation, terpenes are structurally distinctive. The unsaturation is associated with di- and trisubstituted [[alkenes]].  Di- and trisubstituted alkenes resist polymerization (low [[ceiling temperature]]s) but are susceptible to acid-induced [[carbocation]] formation.

===Classification===
<gallery caption="Selected terpenes">
File:Limonene-2D-skeletal.svg|[[Limonene]], a [[monoterpene]].
File:Carvone.svg|[[Carvone]] is a monoterpenoid, a modified monoterpene.
File:Alpha-pinen.svg|[[Pinene]], a monoterpene which exists as two isomers, is a major consistituent of [[turpentine]].
File:Beta-thujaplicin.png|[[Hinokitiol]] is a monoterpenoid, a [[tropolone]] derivative.
File:Humulene.png|[[Humulene]], a [[sesquiterpene]].
File:Taxadiene.svg|[[Taxadiene]], a [[diterpene]], precursor to the diterpenoid [[taxol]], an anticancer agent.
File:Squalene.svg|[[Squalene]], a [[triterpene]] and universal precursor to natural [[steroid]]s.
File:Geosmin Structural Formulae.svg|[[Geosmin]] is a sesquiterpenoid.
</gallery>

Terpenes may be classified by the number of isoprene units in the molecule; a prefix in the name indicates the number of isoprene pairs needed to assemble the molecule. Commonly, terpenes contain 2, 3, 4 or 6 isoprene units; the tetraterpenes (8 isoprene units) form a separate class of compounds called carotenoids; the others are rare.

*The basic unit isoprene itself is a hemiterpene.  It may form oxygen-containing derivatives such as [[prenol]] and [[isovaleric acid]] analogous to terpenoids.
* [[Monoterpenes]] consist of ''two isoprene'' units and have the molecular formula C<sub>10</sub>H<sub>16</sub>. Examples of monoterpenes and monoterpenoids include [[geraniol]], [[terpineol]] (present in [[lilac]]s), [[limonene]] (present in citrus fruits), [[myrcene]] (present in [[hops]]), [[linalool]] (present in [[Lavandula|lavender]]), [[hinokitiol]] (present in [[cypress]] trees) or [[pinene]] (present in [[pine]] trees).<ref>{{cite book|url=https://books.google.com/books?id=9yrbR2WZ8bwC&q=terpenes+pine&pg=PA1|title=Terpenes: Flavors, Fragrances, Pharmaca, Pheromones|first=Eberhard|last=Breitmaier|publisher=John Wiley & Sons|date=2006|isbn=978-3527317868|pages=1–13}}</ref><ref name="Terpenes" /> [[Iridoids]] derive from monoterpenes. Examples of iridoids include [[aucubin]] and [[catalpol]].
* [[Sesquiterpenes]] consist of ''three isoprene'' units and have the molecular formula C<sub>15</sub>H<sub>24</sub>. Examples of sesquiterpenes and sesquiterpenoids include [[humulene]], [[farnesene]]s, [[farnesol]], [[geosmin]].<ref name="Terpenes">{{cite journal |last1=Ludwiczuk |first1=A. |last2=Skalicka-Woźniak |first2=K. |last3=Georgiev |first3=M.I. |title=Terpenoids |journal=Pharmacognosy |date=2017 |pages=233–266 |doi=10.1016/B978-0-12-802104-0.00011-1|isbn=9780128021040 }}</ref> (The ''sesqui-'' prefix means one and a half.)
* [[Diterpenes]] are composed of ''four isoprene'' units and have the molecular formula C<sub>20</sub>H<sub>32</sub>. They derive from [[geranylgeranyl pyrophosphate]]. Examples of diterpenes and diterpenoids are [[cafestol]], [[kahweol]], [[cembrene]] and [[taxadiene]] (precursor of [[taxol]]). Diterpenes also form the basis for biologically important compounds such as [[retinol]], [[retinal]], and [[phytol]].
* Sesterterpenes, terpenes having 25 carbons and ''five isoprene'' units, are rare relative to the other sizes. (The ''sester-'' prefix means two and a half.) An example of a sesterterpenoid is [[geranylfarnesol]].
* [[Triterpene]]s consist of ''six isoprene'' units and have the molecular formula C<sub>30</sub>H<sub>48</sub>. The linear triterpene [[squalene]], the major constituent of [[shark liver oil]], is derived from the reductive coupling of two molecules of [[farnesyl pyrophosphate]]. Squalene is then processed biosynthetically to generate either [[lanosterol]] or [[cycloartenol]], the structural precursors to all the [[steroid]]s.
* Sesquarterpenes are composed of ''seven isoprene'' units and have the molecular formula C<sub>35</sub>H<sub>56</sub>. Sesquarterpenes are typically microbial in their origin. Examples of sesquarterpenoids are ferrugicadiol and tetraprenylcurcumene.
* [[Tetraterpene]]s contain ''eight isoprene'' units and have the molecular formula C<sub>40</sub>H<sub>64</sub>. Biologically important tetraterpenoids include the acyclic [[lycopene]], the monocyclic [[gamma-carotene]], and the bicyclic [[alpha-carotene|alpha-]] and [[beta-carotene]]s.
* Polyterpenes consist of long chains of ''many isoprene'' units. Natural [[rubber]] consists of polyisoprene in which the double bonds are [[Cis–trans isomerism|''cis'']]. Some plants produce a polyisoprene with ''trans'' double bonds, known as [[gutta-percha]].
* Norisoprenoids, characterized by the shortening of a chain or ring by the removal of a methylene group or substitution of one or more methyl side chains by hydrogen atoms. These include the C<sub>13</sub>-norisoprenoid 3-oxo-α-ionol present in [[Muscat of Alexandria]] leaves and 7,8-dihydroionone derivatives, such as {{chem name|megastigmane-3,9-diol}} and {{chem name|3-oxo-7,8-dihydro-α-ionol}} found in [[Shiraz (grape)|Shiraz]] leaves (both grapes in the species ''[[Vitis vinifera]]'')<ref>{{Cite book|doi=10.1021/bk-2002-0802.ch018|title=Carotenoid-Derived Aroma Compounds; chapter 13: Norisoprenoid Aglycon Composition of Leaves and Grape Berries from Muscat of Alexandria and Shiraz Cultivars|series=ACS Symposium Series|year=2001|last1=Günata|first1=Z.|last2=Wirth|first2=J.&nbsp;L.|last3=Guo|first3=W.|last4=Baumes|first4=R.&nbsp;L.|isbn=978-0-8412-3729-2|volume=802|pages=255–261}}</ref> or [[wine]]<ref>{{cite journal | last1 = Winterhalter | first1 = P. | last2 = Sefton | first2 = M.&nbsp;A. | last3 = Williams | first3 = P.&nbsp;J. |date= 1990 | title = Volatile C<sub>13</sub>-Norisoprenoid Compounds in Riesling Wine Are Generated From Multiple Precursors | url = http://www.ajevonline.org/content/41/4/277.abstract | journal = American Journal of Enology and Viticulture | volume = 41 | issue = 4| pages = 277–283 | doi = 10.5344/ajev.1990.41.4.277 | s2cid = 101007887 | url-access = subscription }}</ref><ref>{{Cite journal|doi=10.1016/j.chroma.2009.09.061|title=Rapid tool for assessment of C<sub>13</sub> norisoprenoids in wines|year=2009|last1=Vinholes|first1=J.|last2=Coimbra|first2=M.&nbsp;A.|last3=Rocha|first3=S.&nbsp;M.|journal=Journal of Chromatography A|volume=1216|issue=47|pages=8398–8403|pmid=19828152}}</ref> (responsible for some of the [[Aroma of wine|spice notes]] in [[Chardonnay]]), can be produced by fungal [[peroxidase]]s<ref>{{Cite journal|pmid=19817422|doi=10.1021/jf901438m|title=Generation of Norisoprenoid Flavors from Carotenoids by Fungal Peroxidases|year=2009|last1=Zelena|first1=K.|last2=Hardebusch|first2=B.|last3=Hülsdau|first3=B.|last4=Berger|first4=R.&nbsp;G.|last5=Zorn|first5=H.|journal=Journal of Agricultural and Food Chemistry|volume=57|issue=21|pages=9951–9955 |bibcode=2009JAFC...57.9951Z }}</ref> or [[glycosidase]]s.<ref>{{Cite journal|doi=10.1016/S0141-0229(03)00179-0|title=Wine flavor enhancement through the use of exogenous fungal glycosidases|year=2003|last1=Cabaroğlu|first1=T.|last2=Selli|first2=S.|last3=Canbaş|first3=A.|last4=Lepoutre|first4=J.-P.|last5=Günata|first5=Z.|journal=Enzyme and Microbial Technology|volume=33|issue=5|pages=581–587}}</ref>

[[File:Osmeterium.JPG|thumb|Second- or third-[[instar]] caterpillars of Genus ''[[Papilio]]'' butterflies, like this ''[[Papilio glaucus]]'', emit terpenes from their [[osmeterium]].]]

==Industrial syntheses==
While terpenes and terpenoids occur widely, their extraction from natural sources is often problematic. Consequently, they are produced by chemical synthesis, usually from [[petrochemical]]s. In one route, [[acetone]] and [[acetylene]] are condensed to give [[2-Methylbut-3-yn-2-ol]], which is extended with [[acetoacetic ester]] to give [[geraniol|geranyl alcohol]]. Others are prepared from those terpenes and terpenoids that are readily isolated in quantity, say from the paper and [[tall oil]] industries. For example, [[alpha-Pinene|α-pinene]], which is readily obtainable from natural sources, is converted to [[citronellal]] and [[camphor]]. Citronellal is also converted to [[rose oxide]] and [[menthol]].<ref name=book/>
[[File:IndustrialRouteGeranylAlc.png|thumb|424px|center|Summary of an industrial route to geranyl alcohol from simple reagents (wrong arrow. this is not a retrosynthesis)]]

==References==
{{Reflist}}

==External links==
{{Commons|Terpenes}}
* {{MeshName|Terpenes}}
* {{Cite EB1911|wstitle=Terpenes|volume=26|pages=647–652|first=Frank George|last=Pope}} Survey of terpene chemistry.

{{Terpenoids}}
{{Terpenes}}
{{Authority control}}

[[Category:Terpenes and terpenoids| ]]
[[Category:Plant communication]]