View source for Isoprene

{{Short description|Bio-organic compound that serves as the building unit of terpenes}}
{{Redirect-distinguish|Isoprenyl|isopropenyl}}
{{chembox
| Watchedfields  = changed
| verifiedrevid  = 477495308
| Name = Isoprene
| ImageFileL1 = Isoprene-Structure.svg
| ImageNameL1 = Full structural formula of isoprene
| ImageFileR1 = Isoprene.svg
| ImageNameR1 = Skeletal formula of isoprene
| ImageFileL2 = Isoprene-3D-balls-B.png
| ImageNameL2 = Ball-and-stick model of isoprene
| ImageFileR2 = Isoprene-3d.png
| ImageNameR2 = Space-filling model of isoprene
| IUPACName = Isoprene
| PIN = 2-Methylbuta-1,3-diene
| OtherNames = 2-Methyl-1,3-butadiene
| Section1 = {{Chembox Identifiers
| CASNo = 78-79-5
| CASNo_Ref = {{cascite|correct|CAS}}
| KEGG_Ref = {{keggcite|correct|kegg}}
| KEGG = C16521
| UNII_Ref = {{fdacite|correct|FDA}}
| UNII = 0A62964IBU
| ChEBI_Ref = {{ebicite|correct|EBI}}
| ChEBI = 35194
| PubChem = 6557
| ChemSpiderID_Ref = {{chemspidercite|correct|chemspider}}
| ChemSpiderID = 6309
| SMILES = CC(=C)C=C
| InChI = 1/C5H8/c1-4-5(2)3/h4H,1-2H2,3H3
| InChIKey = RRHGJUQNOFWUDK-UHFFFAOYAS
| StdInChI_Ref = {{stdinchicite|correct|chemspider}}
| StdInChI = 1S/C5H8/c1-4-5(2)3/h4H,1-2H2,3H3
| StdInChIKey_Ref = {{stdinchicite|correct|chemspider}}
| StdInChIKey = RRHGJUQNOFWUDK-UHFFFAOYSA-N
}}
| Section2 = {{Chembox Properties
| Formula = C<sub>5</sub>H<sub>8</sub>
| MolarMass = 68.12 g/mol
| Density = 0.681 g/cm<sup>3</sup>
| MeltingPtC = −143.95
| BoilingPtC = 34.067
}}
}}

'''Isoprene''', or '''2-methyl-1,3-butadiene''', is a common [[volatile organic compound]] with the formula CH<sub>2</sub>=C(CH<sub>3</sub>)−CH=CH<sub>2</sub>. In its pure form it is a colorless volatile liquid. It is produced by many plants and animals<ref>{{cite journal | vauthors = Sharkey TD | title = Isoprene synthesis by plants and animals | journal = Endeavour | volume = 20 | issue = 2 | pages = 74–8 | year = 1996 | pmid = 8690002 | doi = 10.1016/0160-9327(96)10014-4 }}</ref> (including humans) and its polymers are the main component of [[natural rubber]].

== History and etymology ==
[[Charles Greville Williams]] named the compound in 1860 after obtaining it from the [[pyrolysis]] of natural rubber. He correctly deduced the mass shares of carbon and hydrogen<ref>{{cite journal | vauthors = Williams CG |title=On isoprene and caoutchine |journal=Proceedings of the Royal Society of London |date=1860 |volume=10 |pages=516–519 |url=https://babel.hathitrust.org/cgi/pt?id=hvd.32044092762079;view=1up;seq=550|doi=10.1098/rspl.1859.0101 |s2cid=104233421 |url-access=subscription }}</ref> (but arrived at an incorrect formula C<sub>10</sub>H<sub>8</sub> because the modern atomic weight of carbon was not adopted until the [[Karlsruhe Congress]] held later that year). He did not specify the reasons for the name, but it is hypothesized that it came from "[[propylene]]" with which isoprene shares some physical and chemical properties. The first one to observe recombination of isoprene into rubber-like substance was {{Ill|Gustave Bouchardat|de}} in 1879, and [[William A. Tilden]] identified its structure five years later.<ref>{{cite book |url=https://books.google.com/books?id=rjD7CAAAQBAJ&pg=PA10|title=Analysis of Rubber and Rubber-like Polymers| vauthors = Loadman MJ |page=10|date=2012-12-06|publisher=Springer|isbn=978-94-011-4435-3}}</ref>

==Natural occurrences==
[[Image:Dimethylallyl diphosphate.svg|thumb|[[Dimethylallyl pyrophosphate]], not isoprene itself, is the source of most terpenes.|left]]
Isoprene is produced and emitted by many species of trees (major producers are [[oak]]s, [[poplars]], [[eucalyptus]], [[phytoplankton]], and some legumes). Yearly production of isoprene emissions by vegetation is around 600 million [[metric ton]]s, half from tropical broadleaf trees and the remainder primarily from [[shrub]]s.<ref>{{cite journal | vauthors = Guenther A, Karl T, Harley P, Wiedinmyer C, Palmer PI, Geron C  |title=Estimates of global terrestrial isoprene emissions using MEGAN (Model of Emissions of Gases and Aerosols from Nature) |journal=Atmospheric Chemistry and Physics |volume=6 |issue=11 |pages=3181–3210 |year=2006 |doi=10.5194/acp-6-3181-2006 |bibcode=2006ACP.....6.3181G |doi-access=free |hdl=20.500.11820/429435d3-e131-45e2-8bba-42a3d552cc59 |hdl-access=free}}</ref> This is about equivalent to [[methane emissions]] and accounts for around one-third of all [[hydrocarbons]] released into the atmosphere. In [[deciduous]] forests, isoprene makes up approximately 80% of hydrocarbon emissions. While their contribution is small compared to trees, microscopic and macroscopic [[algae]] also produce isoprene.<ref>{{cite journal | vauthors = Johnston A, Crombie AT, El Khawand M, Sims L, Whited GM, McGenity TJ, Colin Murrell J | title = Identification and characterisation of isoprene-degrading bacteria in an estuarine environment | journal = Environmental Microbiology | volume = 19 | issue = 9 | pages = 3526–3537 | date = September 2017 | pmid = 28654185 | pmc = 6849523 | doi = 10.1111/1462-2920.13842}}</ref>

===Plants===
Isoprene is made through the [[Non-mevalonate pathway|methyl-erythritol 4-phosphate pathway]] (MEP pathway, also called the non-[[mevalonate pathway]]) in the [[chloroplast]]s of plants. One of the two end-products of MEP pathway, [[dimethylallyl pyrophosphate]] (DMAPP), is cleaved by the enzyme [[isoprene synthase]] to form isoprene and diphosphate. Therefore, inhibitors that block the MEP pathway, such as [[fosmidomycin]], also block isoprene formation. Isoprene emission increases dramatically with temperature and maximizes at around 40&nbsp;°C. This has led to the hypothesis that isoprene may protect plants against heat stress (thermotolerance hypothesis, see below). Emission of isoprene is also observed in some bacteria and this is thought to come from non-enzymatic degradations from DMAPP. Global emission of isoprene by plants is estimated at around 350 million tons per year.<ref>{{Cite web |last= |first= |title=Isoprene emissions version 2021 |url=https://emissions.aeronomie.be/index.php/bottom-up/isoprenev2021 |access-date=2022-09-26 |website=emissions.aeronomie.be |language=en}}</ref>

====Regulation====
Isoprene emission in plants is controlled both by the availability of the substrate (DMAPP) and by [[enzyme]] (isoprene synthase) activity. In particular, light, CO<sub>2</sub> and O<sub>2</sub> dependencies of isoprene emission are controlled by substrate availability, whereas temperature dependency of isoprene emission is regulated both by substrate level and enzyme activity.

=== In humans and other organisms ===
Isoprene is the most abundant hydrocarbon measurable in the breath of humans.<ref name="Gelmont">{{cite journal | vauthors = Gelmont D, Stein RA, Mead JF | title = Isoprene-the main hydrocarbon in human breath | journal = Biochemical and Biophysical Research Communications | volume = 99 | issue = 4 | pages = 1456–60 | date = April 1981 | pmid = 7259787 | doi = 10.1016/0006-291X(81)90782-8}}</ref><ref name="King">{{cite journal | vauthors = King J, Koc H, Unterkofler K, Mochalski P, Kupferthaler A, Teschl G, Teschl S, Hinterhuber H, Amann A| title = Physiological modeling of isoprene dynamics in exhaled breath | journal = Journal of Theoretical Biology | volume = 267 | issue = 4 | pages = 626–37 | date = December 2010 | pmid = 20869370 | doi = 10.1016/j.jtbi.2010.09.028 | arxiv = 1010.2145 | bibcode = 2010JThBi.267..626K | s2cid = 10267120 | author7-link = Susanne Teschl}}</ref><ref>{{cite journal | vauthors = Williams J, Stönner C, Wicker J, Krauter N, Derstroff B, Bourtsoukidis E, Klüpfel T, Kramer S | display-authors = 6 | title = Cinema audiences reproducibly vary the chemical composition of air during films, by broadcasting scene specific emissions on breath | journal = Scientific Reports | volume = 6 | article-number = 25464 | date = May 2016 | pmid = 27160439 | doi = 10.1038/srep25464 | pmc = 4862009 | bibcode = 2016NatSR...625464W}}</ref> The estimated production rate of isoprene in the human body is 0.15 [[mole (unit)|μmol]]/(kg·h), equivalent to approximately 17&nbsp;mg/day for a person weighing 70&nbsp;kg. Human breath isoprene originates from lipolytic cholesterol metabolism within the skeletal muscular peroxisomes and ''IDI2'' gene acts as the production determinant.<ref>{{Cite journal |last=Sukul |first=Pritam |last2=Richter |first2=Anna |last3=Junghanss |first3=Christian |last4=Schubert |first4=Jochen K. |last5=Miekisch |first5=Wolfram |date=2023-09-30 |title=Origin of breath isoprene in humans is revealed via multi-omic investigations |url=https://www.nature.com/articles/s42003-023-05384-y |journal=Communications Biology |language=en |volume=6 |issue=1 |pages=1–12 |doi=10.1038/s42003-023-05384-y |issn=2399-3642|pmc=10542801}}</ref> Due to the absence of ''IDI2'' gene, animals such as pigs and bottle-nose dolphins do not exhale isoprene.

Isoprene is common in low concentrations in many foods. Many species of soil and marine bacteria, such as [[Actinomycetota]], are capable of degrading isoprene and using it as a fuel source. 
[[File:PolyIsopreneCorrected.svg|thumb|350px|left|Chemical structure of ''cis''-polyisoprene, the main constituent of natural rubber]]

==Biological roles==
Isoprene emission appears to be a mechanism that trees use to combat [[abiotic stress]]es.<ref name="Sharkey">{{cite journal | vauthors = Sharkey TD, Wiberley AE, Donohue AR | title = Isoprene emission from plants: why and how | journal = Annals of Botany | volume = 101 | issue = 1 | pages = 5–18 | date = January 2008 | pmid = 17921528 | pmc = 2701830 | doi = 10.1093/aob/mcm240 }}</ref> In particular, isoprene has been shown to protect against moderate heat stress (around 40&nbsp;°C). It may also protect plants against large fluctuations in leaf temperature. Isoprene is incorporated into and helps stabilize cell membranes in response to heat stress.

Isoprene also confers resistance to reactive oxygen species.<ref name="Vickers1">{{cite journal | vauthors = Vickers CE, Possell M, Cojocariu CI, Velikova VB, Laothawornkitkul J, Ryan A, Mullineaux PM, Nicholas Hewitt C| title = Isoprene synthesis protects transgenic tobacco plants from oxidative stress | journal = Plant, Cell & Environment | volume = 32 | issue = 5 | pages = 520–31 | date = May 2009 | pmid = 19183288 | doi = 10.1111/j.1365-3040.2009.01946.x}}</ref> The amount of isoprene released from isoprene-emitting vegetation depends on leaf mass, leaf area, light (particularly photosynthetic photon flux density, or PPFD) and leaf temperature. Thus, during the night, little isoprene is emitted from tree leaves, whereas daytime emissions are expected to be substantial during hot and sunny days, up to 25 μg/(g dry-leaf-weight)/hour in many oak species.<ref>{{cite journal | vauthors = Benjamin MT, Sudol M, Bloch L, Winer AM |title=Low-emitting urban forests: A taxonomic methodology for assigning isoprene and monoterpene emission rates |journal=Atmospheric Environment |volume=30 |issue=9 |pages=1437–1452 |year=1996 |doi=10.1016/1352-2310(95)00439-4 |bibcode=1996AtmEn..30.1437B}}</ref>

===Isoprenoids===
The isoprene skeleton can be found in naturally occurring compounds called [[terpenes]] and [[terpenoid]] (oxygenated terpenes), collectively called isoprenoids. These compounds do not arise from isoprene itself. Instead, the precursor to isoprene units in biological systems is [[dimethylallyl pyrophosphate]] (DMAPP) and its isomer [[isopentenyl pyrophosphate]] (IPP). The plural 'isoprenes' is sometimes used to refer to [[terpene]]s in general.

Examples of isoprenoids include [[carotene]], [[phytol]], [[retinol]] ([[vitamin A]]), [[tocopherol]] ([[vitamin E]]), [[dolichol]]s, and [[squalene]]. [[Heme]] A has an isoprenoid tail, and [[lanosterol]], the sterol precursor in animals, is derived from squalene and hence from isoprene. The functional isoprene units in biological systems are [[dimethylallyl pyrophosphate]] (DMAPP) and its isomer [[isopentenyl pyrophosphate]] (IPP), which are used in the biosynthesis of naturally occurring isoprenoids such as [[carotenoid]]s, [[quinone]]s, lanosterol derivatives (e.g. steroids) and the [[prenyl]] chains of certain compounds (e.g. phytol chain of chlorophyll). Isoprenes are used in the cell membrane monolayer of many [[Archaea]], filling the space between the diglycerol tetraether head groups. This is thought to add structural resistance to harsh environments in which many Archaea are found.

Similarly, [[natural rubber]] is composed of linear [[polyisoprene]] chains of very high [[molecular weight]] and other natural molecules.<ref name=Ullmann>{{Cite book |doi=10.1002/14356007.a23_225 |chapter=Rubber, 2. Natural |title=Ullmann's Encyclopedia of Industrial Chemistry |year=2000 | vauthors = Greve HH |isbn=978-3-527-30673-2 }}</ref>

[[Image:Sterol synthesis.svg|thumb|left|350px|Simplified version of the steroid synthesis pathway with the intermediates [[isopentenyl pyrophosphate]] (IPP), [[dimethylallyl pyrophosphate]] (DMAPP), [[geranyl pyrophosphate]] (GPP) and squalene shown. Some intermediates are omitted.]]
{{clear left}}

==Industrial production==
Isoprene is most readily available industrially as a byproduct of the thermal [[cracking (chemistry)|cracking]] of [[petroleum naphtha]] or oil, as a side product in the production of [[ethylene]]. Where thermal cracking of oil is less common, isoprene can be produced by dehydrogenation of [[isopentane]].  Isoprene can be synthesized in two steps from [[isobutylene]], starting with a [[Prins reaction]] with [[formaldehyde]] to give [[isoprenol]], which is then dehydrated to isoprene:<ref>{{cite book |doi=10.1002/14356007.a14_627 |chapter=Isoprene |title=Ullmann's Encyclopedia of Industrial Chemistry |date=2000 |last1=Weitz |first1=Hans Martin |last2=Loser |first2=Eckhard |isbn=3-527-30673-0}}</ref> 
[[File:EneIsoprene.svg|thumb|left|400px|Production of isoprene from isobutene via [[ene reaction]]]]

Where cheap [[acetylene]] is produced from coal-derived [[calcium carbide]], it may be combined with [[acetone]] to make 3-methylbutynol which is then hydrogenated and dehydrated to isoprene.<ref>{{Cite web |date=2024-03-25 |title=Isoprene: Properties, Production And Uses |url=https://chemcess.com/isoprene-properties-production-and-uses/ |access-date=2024-11-03 |language=en-US}}</ref>

About 800,000 metric tons are produced annually. About 95% of isoprene production is used to produce cis-1,4-polyisoprene—a [[synthetic rubber|synthetic]] version of [[natural rubber]].<ref name=Ullmann/>

Natural rubber consists mainly of poly-cis-isoprene with a molecular mass of 100,000 to 1,000,000 g/mol. Typically natural rubber contains a few percent of other materials, such as proteins, fatty acids, resins, and inorganic materials. Some natural rubber sources, called [[gutta percha]], are composed of trans-1,4-polyisoprene, a structural [[isomer]] that has similar, but not identical, properties.<ref name=Ullmann/>

== See also ==
* [[Natural rubber]]
* [[Neoprene]]

==Further reading==
*{{cite journal |first=C. |last=Greville Williams |title=On Isoprene and Caoutchine |journal=Proceedings of the Royal Society of London |volume=10 |year=1860 |pages=516–519 |jstor=111688 }}

== References ==
{{Reflist}}

== Further reading ==
{{refbegin|30em}}
* {{cite book | veditors = Budavari S, O'Neil MJ, Smith A, Heckelaman PE | title = The Merck Index | edition = 11th | location = Rahway NJ. USA | publisher = Merck & Co Inc. | date = 1989 | isbn = 978-0-911910-28-5}}
* {{cite journal | vauthors = Bekkedahl N, Wood LA, Wojciechowski M |title=Some physical properties of isoprene |journal=Journal of Research of the National Bureau of Standards |volume=17 |issue=6 |page=883 |year=1936 |doi=10.6028/jres.017.052 |doi-access=free}}
* {{cite journal |doi=10.1023/A:1006300616544 |title=Impact of Non-Methane Hydrocarbons on Tropospheric Chemistry and the Oxidizing Power of the Global Troposphere: 3-Dimensional Modelling Results |year=2000 | vauthors = Poisson N, Kanakidou M, Crutzen PJ |journal=Journal of Atmospheric Chemistry |volume=36 |issue=2 |pages=157–230  |bibcode=2000JAtC...36..157P |s2cid=94217044}}
* {{cite journal | vauthors = Claeys M, Graham B, Vas G, Wang W, Vermeylen R, Pashynska V, Cafmeyer J, Guyon P, Andreae MO, Artaxo P, Maenhaut W | display-authors = 6 | title = Formation of secondary organic aerosols through photooxidation of isoprene | journal = Science | volume = 303 | issue = 5661 | pages = 1173–6 | date = February 2004 | pmid = 14976309 | doi = 10.1126/science.1092805 | bibcode = 2004Sci...303.1173C | s2cid = 19268599 }}
* {{cite journal | vauthors = Pier PA, McDuffie C |title=Seasonal isoprene emission rates and model comparisons using whole-tree emissions from white oak |journal=Journal of Geophysical Research: Atmospheres |volume=102 |pages=23963–23971 |year=1997 |issue=D20 |doi=10.1029/96JD03786  |bibcode=1997JGR...10223963P |doi-access= }}
* {{cite journal | vauthors = Pöschl U, Von Kuhlmann R, Poisson N, Crutzen PJ |doi=10.1023/A:1006391009798 |title=Development and Intercomparison of Condensed Isoprene Oxidation Mechanisms for Global Atmospheric Modeling |year=2000 |journal=Journal of Atmospheric Chemistry |volume=37 |issue=1 |pages=29–52 |bibcode=2000JAtC...37...29P |s2cid=93419825 }}
* {{cite journal |doi=10.1146/annurev.ecolsys.32.081501.114136 |title=Biospheric Trace Gas Fluxes and Their Control over Tropospheric Chemistry |journal=Annual Review of Ecology and Systematics |volume=32 |pages=547–576 |year=2001 | vauthors = Monson RK, Holland EA }}
{{refend}}

== External links ==
{{Commons category|Isoprene}}
* [https://web.archive.org/web/20161115062414/http://ntp.niehs.nih.gov/pubhealth/roc/index-1.html Report on Carcinogens, Fourteenth Edition; U.S. Department of Health and Human Services, Public Health Service, National Toxicology Program]
* [http://www.sciencenews.org/view/generic/id/46200/title/A_source_of_haze Science News article describing how isoprene released by plants is converted to light-scattering aerosols] {{Webarchive|url=https://web.archive.org/web/20110604222657/http://www.sciencenews.org/view/generic/id/46200/title/A_source_of_haze |date=2011-06-04 }}

{{Authority control}}

[[Category:Alkadienes]]
[[Category:Hemiterpenes]]
[[Category:IARC Group 2B carcinogens]]
[[Category:Monomers]]
[[Category:Conjugated dienes]]
[[Category:Substances discovered in the 19th century]]