View source for Alkene

{{Short description|1=Hydrocarbon compound containing one or more C=C bonds}}
{{Distinguish|alkane|alkyne}}
{{Redirect|Olefin|the material|olefin fiber}}
{{Use dmy dates|date=August 2022}}

[[Image:Ethylene-3D-vdW.png|thumb|right|120px|A 3D model of [[ethylene]], the simplest alkene]]

In [[organic chemistry]], an '''alkene''', or '''olefin''', is a [[hydrocarbon]] containing a [[carbon]]–carbon [[double bond]].<ref name="Wade">{{cite book | last = Wade | first = L.G. | title = Organic Chemistry | url = https://archive.org/details/organicchemistry00lgwa_203 | url-access = limited | publisher = Pearson [[Prentice Hall]] | date = 2006 |edition=6th| pages = [https://archive.org/details/organicchemistry00lgwa_203/page/n322 279] | isbn = 978-1-4058-5345-3 }}</ref> The double bond may be internal or at the terminal position. Terminal alkenes are also known as [[Alpha-olefin|α-olefins]].

The [[International Union of Pure and Applied Chemistry]] (IUPAC) [[Preferred IUPAC name|recommends using]] the name "alkene" only for [[Open-chain compound|acyclic]] hydrocarbons with just one double bond; '''alkadiene''', '''alkatriene''', etc., or '''[[polyene]]''' for acyclic hydrocarbons with two or more double bonds; '''cycloalkene''', '''cycloalkadiene''', etc. for [[Cyclic compound|cyclic]] ones; and "olefin" for the general class – cyclic or acyclic, with one or more double bonds.<ref name=PAC1995.alkenes/><ref name=PAC1995.olefins/><ref name="PAC1995">{{cite journal| title=Glossary of Class Names of Organic Compounds and Reactive Intermediates Based on Structure (IUPAC Recommendations 1995)  | last1 = Moss | first1 = G. P.  | journal= [[Pure and Applied Chemistry]] | year=1995| volume=67 | pages=1307–75| doi=10.1351/pac199567081307| last2=Smith| first2=P. A. S.| last3=Tavernier| first3=D.| issue=8–9 | s2cid = 95004254 | doi-access=free}}</ref>

Acyclic alkenes, with only one double bond and no other [[functional group]]s (also known as '''mono-enes''') form a [[homologous series]] of [[hydrocarbon]]s with the general formula {{chem2|C_{''n''}H_{2''n''} }} with ''n'' being a >1 natural number (which is two [[hydrogen]]s less than the corresponding [[alkane]]). When ''n'' is four or more, [[isomer]]s are possible, distinguished by the position and [[cis–trans isomerism|conformation]] of the double bond.

Alkenes are generally colorless [[polarity (chemistry)|non-polar]] compounds, somewhat similar to alkanes but more reactive. The first few members of the series are gases or liquids at room temperature.  The simplest alkene, [[ethylene]] ({{chem2|C2H4}}) (or "ethene" in the [[IUPAC name|IUPAC nomenclature]]) is the [[organic compound]] produced on the largest scale industrially.<ref name="cenews">{{cite journal |title=Production: Growth is the Norm |journal=Chemical and Engineering News |volume=84 |issue=28 |pages=59–236 |date=10 July 2006 |doi=10.1021/cen-v084n034.p059}}</ref>

[[Aromatic]] compounds are often drawn as cyclic alkenes, however their structure and properties are sufficiently distinct that they are not classified as alkenes or olefins.<ref name=PAC1995.olefins/> Hydrocarbons with two overlapping double bonds ({{chem2|C\dC\dC}}) are called [[allenes]]—the simplest such compound is itself called ''[[propadiene|allene]]''—and those with three or more overlapping bonds ({{chem2|C\dC\dC\dC}}, {{chem2|C\dC\dC\dC\dC}}, etc.) are called [[cumulene]]s.

==Structural isomerism==
Alkenes having four or more [[carbon]] atoms can form diverse [[structural isomer]]s. Most alkenes are also isomers of [[cycloalkane]]s. Acyclic alkene structural isomers with only one double bond follow:<ref>{{cite OEIS|A000631|Number of ethylene derivatives with n carbon atoms}}</ref>

* {{chem2|C2H4}}: [[ethylene]] only
* {{chem2|C3H6}}: [[propylene]] only
* {{chem2|C4H8}}: 3 isomers: [[1-Butene|1-butene]], [[2-Butene|2-butene]], and [[isobutylene]]
* {{chem2|C5H10}}: 5 isomers: [[Pentene|1-pentene]], 2-pentene, 2-methyl-1-butene, 3-methyl-1-butene, [[2-Methyl-2-butene|2-methyl-2-butene]]
*  {{chem2|C6H12}}: 13 isomers: 1-hexene, 2-hexene, 3-hexene, 2-methyl-1-pentene, 3-methyl-1-pentene, 4-methyl-1-pentene, 2-methyl-2-pentene, 3-methyl-2-pentene, 4-methyl-2-pentene, 2,3-dimethyl-1-butene, 3,3-dimethyl-1-butene, 2,3-dimethyl-2-butene, 2-ethyl-1-butene

Many of these molecules exhibit [[cis–trans isomerism|''cis''–''trans'' isomerism]]. There may also be [[Chirality (chemistry)|chiral]] carbon atoms particularly within the larger molecules (from {{chem2|C5}}). The number of potential isomers increases rapidly with additional carbon atoms.

==Structure and bonding==
===Bonding===
[[Image:Ethylene 3D.png|200px|thumb|Ethylene (ethene), showing the pi bond in green]]
A carbon–carbon double bond consists of a [[sigma bond]] and a [[pi bond]]. This double bond is stronger than a single [[covalent bond]] (611&nbsp;[[joule|kJ]]/[[Mole (unit)|mol]] for C=C vs. 347&nbsp;kJ/mol for C–C),<ref name="Wade"/> but not twice as strong. Double bonds are shorter than single bonds with an average [[bond length]] of 1.33 [[ångström|Å]] (133 [[picometre|pm]]) vs 1.53 Å for a typical C-C single bond.<ref>{{March6th|page=23}}</ref>

Each carbon atom of the double bond uses its three sp<sup>2</sup> [[orbital hybridization|hybrid orbitals]] to form sigma bonds to three atoms (the other carbon atom and two hydrogen atoms). The unhybridized 2p atomic orbitals, which lie perpendicular to the plane created by the axes of the three sp<sup>2</sup> hybrid orbitals, combine to form the pi bond. This bond lies outside the main C–C axis, with half of the bond on one side of the molecule and a half on the other.  With a strength of 65 kcal/mol, the pi bond is significantly weaker than the sigma bond.

Rotation about the carbon–carbon double bond is restricted because it incurs an energetic cost to break the alignment of the [[p orbital]]s on the two carbon atoms. Consequently ''cis'' or ''trans'' isomers interconvert so slowly that they can be freely handled at ambient conditions without isomerization. More complex alkenes may be named with the [[E-Z notation|''E''–''Z'' notation]] for molecules with three or four different [[substituent]]s (side groups). For example, of the [[Butene#Isomers|isomers of butene]], the two methyl groups of (''Z'')-but-2[[-ene]] (a.k.a. ''cis''-2-butene) appear on the same side of the double bond, and in (''E'')-but-2-ene (a.k.a. ''trans''-2-butene) the methyl groups appear on opposite sides. These two isomers of butene have distinct properties.
<!-- this section seems esotericTwisting to a 90° dihedral angle between two of the groups on the carbons requires less energy than the strength of a [[pi bond]], and the bond still holds. The carbons of the double bond become [[pyramidalization|pyramidal]], which allows preserving some [[p orbital]] alignment—and hence pi bonding. The other two attached groups remain at a larger dihedral angle. This contradicts a common textbook assertion that the two carbons retain their planar nature when twisting, in which case the p orbitals would rotate enough away from each other to be unable to sustain a pi bond. In a 90°-twisted alkene, the p orbitals are only misaligned by 42° and the strain energy is only around 40 kcal/mol.  In contrast, a fully broken pi bond has an energetic cost of around 65 kcal/mol.<ref name=":0">{{cite journal|last1=Barrows|first1=Susan E.|last2=Eberlein|first2=Thomas H.|year=2005|title=Understanding Rotation about a C=C Double Bond|journal=J. Chem. Educ.|volume=82|issue=9|pages=1329|bibcode=2005JChEd..82.1329B|doi=10.1021/ed082p1329}}</ref>

Some [[pyramidal alkene]]s are stable. For example, ''trans''-[[cyclooctene]] is a stable strained alkene and the orbital misalignment is only 19°, despite having a significant [[dihedral angle]] of 137° (a planar system has a dihedral angle of 180°) and a degree of pyramidalization of 18°. Even ''trans''-[[cycloheptene]] is stable at low temperatures.<ref name=":0" />-->

===Shape===
As predicted by the [[VSEPR theory|VSEPR]] model of [[electron]] pair repulsion, the [[molecular geometry]] of alkenes includes [[bond angle]]s about each carbon atom in a double bond of about 120°. The angle may vary because of [[steric strain]] introduced by [[nonbonded interactions]] between [[functional group]]s attached to the carbon atoms of the double bond. For example, the C–C–C bond angle in [[propylene]] is 123.9°.

For bridged alkenes, [[Bredt's rule]] states that a double bond cannot occur at the bridgehead of a bridged ring system unless the rings are large enough.<ref name=Bansal /> Following Fawcett and defining ''S'' as the total number of non-bridgehead atoms in the rings,<ref>{{cite journal |title=Bredt's Rule of Double Bonds in Atomic-Bridged-Ring Structures |first=Frank S. |last=Fawcett |journal=[[Chem. Rev.]] |year=1950 |volume=47 |issue=2 |pages=219–274 |doi=10.1021/cr60147a003 |pmid=24538877}}</ref> bicyclic systems require ''S''&nbsp;≥&nbsp;7 for stability<ref name=Bansal>{{cite book |title=Organic Reaction Mechanisms |chapter=Bredt's Rule |pages=14–16 |first=Raj K. |last=Bansal |edition=3rd |year=1998 |publisher=[[McGraw-Hill Education]] |isbn=978-0-07-462083-0 |chapter-url=https://books.google.com/books?id=bga3xjLVCo0C&pg=PT29}}</ref> and tricyclic systems require ''S''&nbsp;≥&nbsp;11.<ref>{{Cite book |year=2010 |chapter=Bredt's Rule |title=Comprehensive Organic Name Reactions and Reagents |volume=116 |pages=525–8 |doi=10.1002/9780470638859.conrr116 |isbn=978-0-470-63885-9}}</ref>

=== Isomerism ===
{{main|Cis–trans isomerism|E–Z notation}}

In [[organic chemistry]],the [[prefix]]es [[cis-trans isomerism|cis- and trans-]]  are used to describe the positions of functional groups attached to [[carbon]] atoms joined by a double bond. In Latin, ''cis'' and ''trans'' mean "on this side of" and "on the other side of" respectively. Therefore, if the functional groups are both on the same side of the carbon chain, the bond is said to have '''cis-''' configuration, otherwise (i.e. the functional groups are on the opposite side of the carbon chain), the bond is said to have '''trans-''' configuration.

<gallery>
Cis-2-Buten.svg|structure of cis-2-butene
Trans-2-Buten.svg|structure of trans-2-butene
Trans-2-butene.svg|(''E'')-But-2-ene
Cis-2-butene.svg|(''Z'')-But-2-ene
</gallery>

For there to be cis- and trans- configurations, there must be a carbon chain, or at least one [[functional group]] attached to each carbon is the same for both. [[E–Z notation|E- and Z- configuration]] can be used instead in a more general case where all four functional groups attached to carbon atoms in a double bond are different. E- and Z- are abbreviations of German words ''zusammen'' (together) and ''entgegen'' (opposite). In E- and Z-isomerism, each functional group is assigned a priority based on the [[Cahn–Ingold–Prelog priority rules]]. If the two groups with higher priority are on the same side of the double bond, the bond is assigned '''Z-''' configuration, otherwise (i.e. the two groups with higher priority are on the opposite side of the double bond), the bond is assigned '''E-''' configuration. Cis- and trans- configurations do not have a fixed relationship between '''E'''- and '''Z'''-configurations.

==Physical properties==
Many of the physical properties of alkenes and [[alkane]]s are similar: they are colorless, nonpolar, and combustible.  The [[physical state]] depends on [[molecular mass]]: like the corresponding saturated hydrocarbons, the simplest alkenes ([[ethylene]], [[propylene]], and [[butene]]) are gases at room temperature. Linear alkenes of approximately five to sixteen carbon atoms are liquids, and higher alkenes are waxy solids. The melting point of the solids also increases with increase in molecular mass.

Alkenes generally have stronger smells than their corresponding alkanes.  Ethylene has a sweet and musty odor.  Strained alkenes, in particular, like norbornene and [[Trans-Cyclooctene|''trans''-cyclooctene]] are known to have strong, unpleasant odors, a fact consistent with the stronger π complexes they form with metal ions including copper.<ref>{{Cite journal|last1=Duan|first1=Xufang|last2=Block|first2=Eric|last3=Li|first3=Zhen|last4=Connelly|first4=Timothy|last5=Zhang|first5=Jian|last6=Huang|first6=Zhimin|last7=Su|first7=Xubo|last8=Pan|first8=Yi|last9=Wu|first9=Lifang|date=2012-02-28|title=Crucial role of copper in detection of metal-coordinating odorants|journal=Proceedings of the National Academy of Sciences of the United States of America|volume=109|issue=9|pages=3492–7|doi=10.1073/pnas.1111297109 |pmc=3295281|pmid=22328155|bibcode=2012PNAS..109.3492D|doi-access=free}}</ref>

=== Boiling and melting points ===
Below is a list of the boiling and melting points of various alkenes with the corresponding alkane and alkyne analogues.<ref name="alkene properties">{{cite web|url=https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Supplemental_Modules_(Organic_Chemistry)/Alkenes/Properties_of_Alkenes/Physical_Properties_of_Alkenes|title=Physical Properties of Alkenes|last1=Nguyen|first1=Trung|last2=Clark|first2=Jim|date=April 23, 2019|access-date=May 27, 2019|website=Chemistry LibreTexts}}</ref><ref>{{cite web|url=http://chemistry.elmhurst.edu/vchembook/501hcboilingpts.html|title=Boiling Points and Structures of Hydrocarbons |last=Ophardt|first=Charles|date=2003|access-date=May 27, 2019|website=Virtual Chembook}}</ref> 
{| class="wikitable"
|+Melting and boiling points in [[Celsius|°C]]
!Number of<br>carbons
!
!Alkane
!Alkene
!Alkyne
|-
|rowspan="3" |2
|Name||ethane||ethylene||acetylene
|-
|Melting point||−183||−169||−80.7
|-
|Boiling point||−89||−104||−84.7
|-
| rowspan="3" |3
|Name||propane||propylene||propyne
|-
|Melting point||−190||−185||−102.7
|-
|Boiling point||−42||−47||−23.2
|-
| rowspan="3" |4
|Name||butane||1-butene||1-butyne
|-
|Melting point||−138||−185.3||−125.7
|-
|Boiling point||−0.5||−6.2||8.0
|-
| rowspan="3" |5
|Name||pentane||1-pentene||1-pentyne
|-
|Melting point||−130||−165.2||−90.0
|-
|Boiling point||36||29.9||40.1
|}

=== Infrared spectroscopy ===
In the [[infrared|IR]] spectrum, the stretching/compression of C=C bond gives a peak at 1670–1600&nbsp;[[wave number|cm<sup>−1</sup>]].  The band is weak in symmetrical alkenes. The bending of C=C bond absorbs between 1000 and 650&nbsp;cm<sup>−1</sup> wavelength

=== NMR spectroscopy ===
In <sup>1</sup>H [[NMR]] spectroscopy, the [[hydrogen]] bonded to the carbon adjacent to double bonds will give a [[chemical shift|δ<sub>H</sub>]] of 4.5–6.5&nbsp;[[parts per million|ppm]]. The double bond will also [[Electromagnetic shielding|deshield]] the hydrogen attached to the carbons adjacent to sp<sup>2</sup> carbons, and this generates δ<sub>H</sub>=1.6–2.&nbsp;ppm peaks.<ref>{{cite web|url=http://www2.ups.edu/faculty/hanson/Spectroscopy/NMR/HNMRshift.htm|title=Overview of Chemical Shifts in H-NMR|last=Hanson|first=John|website=ups.edu|access-date=May 5, 2019}}</ref> Cis/trans isomers are distinguishable due to different [[J-coupling]] effect. Cis [[Vicinal (chemistry)|vicinal]] hydrogens will have coupling constants in the range of 6–14&nbsp;[[Hz]], whereas the trans will have coupling constants of 11–18&nbsp;Hz.<ref name="NMR of Alkenes">{{cite web|url=https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Supplemental_Modules_(Organic_Chemistry)/Alkenes/Properties_of_Alkenes/Nuclear_Magnetic_Resonance_(NMR)_of_Alkenes|author=<!--Not stated-->|title=Nuclear Magnetic Resonance (NMR) of Alkenes|website=Chemistry LibreTexts|date=April 23, 2019|access-date=May 5, 2019}}</ref>

In their <sup>13</sup>C NMR spectra of alkenes, double bonds also deshield the carbons, making them have low field shift. C=C double bonds usually have chemical shift of about 100–170&nbsp;ppm.<ref name="NMR of Alkenes" />

=== Combustion ===
Like most other [[hydrocarbon]]s, alkenes [[combustion|combust]] to give carbon dioxide and water.

The combustion of alkenes release less energy than burning same [[molarity]] of saturated ones with same number of carbons. 
This trend can be clearly seen in the list of [[enthalpy of combustion|standard enthalpy of combustion]] of hydrocarbons.<ref>{{cite web|url=http://www2.ucdsb.on.ca/tiss/stretton/database/organic_thermo.htm|title=Organic Compounds: Physical and Thermochemical Data|website=ucdsb.on.ca|access-date=May 5, 2019}}</ref>
{| class="wikitable"
|+Combustion energies of various hydrocarbons
!Number of<br>carbons
!Substance
!Type
!Formula
!H<sub>c</sub><sup>ø</sup><br>(kJ/mol)
|-
| rowspan="3" |2
|[[ethane]]
|saturated
|C<sub>2</sub>H<sub>6</sub>
| −1559.7
|-
|[[ethylene]]
|unsaturated
|C<sub>2</sub>H<sub>4</sub>
| −1410.8
|-
|[[acetylene]]
|unsaturated
|C<sub>2</sub>H<sub>2</sub>
| −1300.8
|-
| rowspan="3" |3
|[[propane]]
|saturated
|CH<sub>3</sub>CH<sub>2</sub>CH<sub>3</sub>
| −2219.2
|-
|[[propene]]
|unsaturated
|CH<sub>3</sub>CH=CH<sub>2</sub>
| −2058.1
|-
|[[propyne]]
|unsaturated
|CH<sub>3</sub>C≡CH
| −1938.7
|-
| rowspan="3" |4
|[[butane]]
|saturated
|CH<sub>3</sub>CH<sub>2</sub>CH<sub>2</sub>CH<sub>3</sub>
| −2876.5
|-
|[[1-butene]]
|unsaturated
|CH<sub>2</sub>=CH−CH<sub>2</sub>CH<sub>3</sub>
| −2716.8
|-
|[[1-butyne]]
|unsaturated
|CH≡C-CH<sub>2</sub>CH<sub>3</sub>
| −2596.6
|}

==Reactions==
Alkenes are relatively stable compounds, but are more reactive than [[alkane]]s. Most reactions of alkenes involve additions to this pi bond, forming new [[sigma bond|single bonds]].  Alkenes serve as a feedstock for the [[petrochemical industry]] because they can participate in a wide variety of reactions, prominently polymerization and alkylation.  Except for ethylene, alkenes have two sites of reactivity: the carbon–carbon pi-bond and the presence of [[allylic]] CH centers.  The former dominates but the allylic sites are important too.

=== Addition to the unsaturated bonds ===
[[Image:Ear.png|400px|thumb|typical electrophilic addition reaction of [[ethylene]]]]
[[Hydrogenation]] involves the addition of [[hydrogen|H<sub>2</sub>]] ,resulting in an alkane. The equation of hydrogenation of [[ethylene]] to form [[ethane]] is:
:H<sub>2</sub>C=CH<sub>2</sub> + H<sub>2</sub>→H<sub>3</sub>C−CH<sub>3</sub>
Hydrogenation reactions usually require [[catalyst]]s to increase their [[reaction rate]]. The total number of hydrogens that can be added to an unsaturated hydrocarbon depends on its [[degree of unsaturation]].

Similarly, [[halogenation]] involves the addition of a halogen molecule, such as [[bromine|Br<sub>2</sub>]], resulting in a dihaloalkane. The equation of bromination of ethylene to form ethane is:
:H<sub>2</sub>C=CH<sub>2</sub> + Br<sub>2</sub>→H<sub>2</sub>CBr−CH<sub>2</sub>Br
Unlike hydrogenation, these halogenation reactions do not require catalysts. The reaction occurs in two steps, with a [[halonium ion]] as an intermediate.

[[File:Biadamantylidene-bromonium-ion-from-xtal-1994-2D-skeletal.png|170px|thumb|Structure of a [[bromonium ion]]]]

[[Bromine test]] is used to test the saturation of hydrocarbons.<ref>{{Cite book | title = The Systematic Identification of Organic Compounds | first1 = R.L. |last1=Shriner | first2 =C.K.F. |last2=Hermann | first3 =T.C. |last3=Morrill | first4 =D.Y. |last4=Curtin | first5 =R.C. |last5=Fuson | publisher = Wiley | date = 1997| isbn = 0-471-59748-1}}</ref> The bromine test can also be used as an indication of the [[degree of unsaturation]] for unsaturated hydrocarbons. [[Bromine number]] is defined as gram of bromine able to react with 100g of product.<ref>{{cite web|url=https://www.hach.com/asset-get.download.jsa?id=3980387617|website=Hach company|title=Bromine Number|access-date=May 5, 2019}}</ref> Similar as hydrogenation, the halogenation of bromine is also depend on the number of π bond. A higher bromine number indicates higher degree of unsaturation.

The π bonds of alkenes hydrocarbons are also susceptible to [[hydration reaction|hydration]]. The reaction usually involves [[strong acid]] as [[catalyst]].<ref>{{cite web|url=https://www.chemguide.co.uk/physical/catalysis/hydrate.html|title=The Mechanism for the Acid Catalysed Hydration of Ethene |last=Clark|first=Jim|website=Chemguide|date=November 2007|access-date=May 6, 2019}}</ref> The first step in hydration often involves formation of a [[carbocation]]. The net result of the reaction will be an [[Alcohol (chemistry)|alcohol]]. The reaction equation for hydration of ethylene is:
:H<sub>2</sub>C=CH<sub>2</sub> + H<sub>2</sub>O→{{coloredlink|black|ethanol|H<sub>3</sub>C-CH<sub>2</sub>OH}}

[[File:HBr-addition.svg|350px|thumb|Example of hydrohalogenation: addition of [[Hydrogen bromide|HBr]] to an alkene]]

[[Hydrohalogenation]] involves addition of H−X to unsaturated hydrocarbons. This reaction results in new C−H and C−X σ bonds. The formation of the intermediate carbocation is selective and follows [[Markovnikov's rule]]. The hydrohalogenation of alkene will result in [[haloalkane]]. The reaction equation of HBr addition to ethylene is:
:H<sub>2</sub>C=CH<sub>2</sub> + HBr  → {{coloredlink|black|bromoethane|H<sub>3</sub>C−CH<sub>2</sub>Br}}

=== Cycloaddition ===
{{Main|Cycloaddition}}
[[File:Diels-Alder (1,3-butadiene + ethylene) red.svg|right|thumb|a Diels-Alder reaction]]
[[File:4+2 cycloaddition cyclopentadiene O2.svg|350px|center|alt=Generation of singlet oxygen and its [4+2]-cycloaddition with cyclopentadiene]]
Alkenes add to [[diene]]s to give [[cyclohexene]]s. This conversion is an example of a [[Diels-Alder reaction]]. Such reaction proceed with retention of stereochemistry.  The rates are sensitive to electron-withdrawing or electron-donating substituents.  When irradiated by UV-light, alkenes dimerize to give [[cyclobutane]]s.<ref>{{March6th}}</ref> Another example is the [[Ene reaction#Singlet-oxygen ene reaction|Schenck ene reaction]], in which singlet oxygen reacts with an [[allyl]]ic structure to give a transposed allyl [[peroxide]]:

[[File:Schenck ene reaction.svg|200px|center|alt=Reaction of singlet oxygen with an allyl structure to give allyl peroxide]]

==== Oxidation ====
Alkenes react with [[Peroxy acid|percarboxylic acids]] and even hydrogen peroxide to yield [[epoxide]]s:
:{{chem2|RCH\dCH2  +  RCO3H  ->  RCHOCH2 + RCO2H}}

For ethylene, the [[epoxidation]] is conducted on a very large scale industrially using oxygen in the presence of silver-based catalysts:
:{{chem2|C2H4 + 1/2 O2  ->  C2H4O}}

Alkenes react with ozone, leading to the scission of the double bond.  The process is called [[ozonolysis]].  Often the reaction procedure includes a mild reductant, such as dimethylsulfide ({{chem2|SMe2}}):
:{{chem2|RCH\dCHR'  +  O3  +  SMe2  -> RCHO  +  R'CHO  +  O\dSMe2}}
:{{chem2|R2C\dCHR'  +  O3  ->  R2CHO  +  R'CHO  +  O\dSMe2}}

When treated with a hot concentrated, acidified solution of {{chem2|[[potassium permanganate|KMnO4]]}}, alkenes are cleaved to form [[ketone]]s and/or [[carboxylic acid]]s.  The stoichiometry of the reaction is sensitive to conditions. This reaction and the ozonolysis can be used to determine the position of a double bond in an unknown alkene.

The oxidation can be stopped at the [[vicinal (chemistry)|vicinal]] [[diol]] rather than full cleavage of the alkene by using [[osmium tetroxide]] or other oxidants:
:<chem>R'CH=CR2 + 1/2 O2  + H2O  -> R'CH(OH)-C(OH)R2</chem>
This reaction is called [[dihydroxylation]].

In the presence of an appropriate [[photosensitiser]], such as [[methylene blue]] and light, alkenes can undergo reaction with reactive oxygen species generated by the photosensitiser, such as [[hydroxyl radical]]s, [[singlet oxygen]] or [[superoxide]] ion. Reactions of the excited sensitizer can involve electron or hydrogen transfer, usually with a reducing substrate (Type I reaction) or interaction with oxygen (Type II reaction).<ref>{{cite journal |last1=Baptista |first1=Maurício S. |last2=Cadet |first2=Jean |last3=Mascio |first3=Paolo Di |last4=Ghogare |first4=Ashwini A. |last5=Greer |first5=Alexander |last6=Hamblin |first6=Michael R. |last7=Lorente |first7=Carolina |last8=Nunez |first8=Silvia Cristina |last9=Ribeiro |first9=Martha Simões |last10=Thomas |first10=Andrés H. |last11=Vignoni |first11=Mariana |last12=Yoshimura |first12=Tania Mateus |title=Type I and Type II Photosensitized Oxidation Reactions: Guidelines and Mechanistic Pathways |journal=Photochemistry and Photobiology |date=2017 |volume=93 |issue=4 |pages=912–9 |doi=10.1111/php.12716|pmid=28084040 |pmc=5500392 |doi-access=free }}</ref> These various alternative processes and reactions can be controlled by choice of specific reaction conditions, leading to a wide range of products. A common example is the [4+2]-[[cycloaddition]] of singlet oxygen with a [[diene]] such as [[cyclopentadiene]] to yield an [[endoperoxide]]:

===Polymerization===
{{main|polyolefin}}
Terminal alkenes are precursors to [[polymer]]s via processes termed [[polymerization]]. Some polymerizations are of great economic significance, as they generate the plastics [[polyethylene]] and [[polypropylene]]. Polymers from alkene are usually referred to as ''[[polyolefin]]s''  although they contain no olefins. Polymerization can proceed via diverse mechanisms. [[Conjugated system|Conjugated]] [[diene]]s such as [[buta-1,3-diene]] and [[isoprene]] (2-methylbuta-1,3-diene) also produce polymers, one example being natural rubber.

=== Allylic substitution ===
The presence of a C=C π bond in unsaturated hydrocarbons weakens the dissociation energy of the [[allylic]] C−H bonds. Thus, these groupings are susceptible to [[free radical substitution]] at these C-H sites as well as addition reactions at the C=C site.  In the presence of [[radical initiator]]s, allylic C-H bonds can be halogenated.<ref>{{cite journal |doi=10.15227/orgsyn.073.0240 |title=1,3,5-Cyclooctatriene |journal=Organic Syntheses |date=1996 |volume=73 |page=240|first1=Masaji|last1=Oda|first2=Takeshi|last2=Kawase|first3=Hiroyuki|last3= Kurata }}</ref> The presence of two C=C bonds flanking one methylene, i.e., doubly allylic, results in particularly weak HC-H bonds.  The high reactivity of these situations is the basis for certain free radical reactions, manifested in the chemistry of [[drying oil]]s.

===Metathesis===
Alkenes undergo [[olefin metathesis]], which cleaves and interchanges the substituents of the alkene.  A related reaction is [[ethenolysis]]:<ref name=JFH/>
:<math chem>\overset{\text{diisobutene}}{\ce{(CH3)3C-CH=C(CH3)2}} + {\color{red}\ce{CH2=CH2}} \longrightarrow \overset{\text{neohexane}}{\ce{(CH3)3C-CH=}{\color{red}\ce{CH2}}} +  \ce{(CH3)2C=}{\color{red}\ce{CH2}}</math>

=== Metal complexation===
[[File:DCDmodel.png|thumb|The [[Dewar-Chatt-Duncanson model]] for alkene-metal bonding.]]
:[[Image:Ni(cod)2.png|thumb|right|220px|Structure of [[bis(cyclooctadiene)nickel(0)]], a metal–alkene complex]]
In [[transition metal alkene complex]]es, alkenes serve as ligands for metals.<ref>{{cite web|url=http://www.ilpi.com/organomet/alkene.html|title=Alkene Complexes|last=Toreki|first=Rob|date=March 31, 2015|access-date=May 29, 2019|website=Organometallic HyperTextbook}}</ref> In this case, the π electron density is donated{{clarify|date=September 2023}} to the metal d orbitals. The stronger the donation is, the stronger the [[back bonding]] from the metal d orbital to π* anti-bonding orbital of the alkene. This effect lowers the bond order of the alkene and increases the C-C [[bond length]]. One example is the complex {{chem2|PtCl3(C2H4)]-}}. These complexes are related to the mechanisms of metal-catalyzed reactions of unsaturated hydrocarbons.<ref name=JFH>{{cite book | title=Organotransition Metal Chemistry: From Bonding to Catalysis | publisher=University Science Books | last=Hartwig |first=John | year=2010 | location=New York | pages=1160 | isbn=978-1-938787-15-7}}</ref>

===Reaction overview===
{| class="wikitable sortable" style="background-color:white;float: center; border-collapse: collapse; margin: 0em 1em;" border="1" cellpadding="2" cellspacing="0"

! width=200px|Reaction name !! Product !! class="unsortable" | Comment
|-
|valign=top | [[Hydrogenation]]
|valign=top| alkanes
| addition of hydrogen
|-
| [[Hydroalkenylation]]
| alkenes
| hydrometalation / insertion / beta-elimination by metal catalyst
|-
|valign=top | [[Halogen addition reaction]]
|valign=top| 1,2-dihalide
| electrophilic addition of halogens
|-
|valign=top | [[Hydrohalogenation]] ([[Markovnikov's rule|Markovnikov]])
|valign=top| haloalkanes
| addition of hydrohalic acids
|-
|valign=top | Anti-Markovnikov [[hydrohalogenation]]
|valign=top| haloalkanes
| free radicals mediated addition of hydrohalic acids
|-
|valign=top | [[Hydroamination]]
|valign=top| amines
| addition of {{chem2|N\sH}} bond across {{chem2|C\sC}} double bond
|-
|valign=top | [[Hydroformylation]]
|valign=top| aldehydes
| industrial process, addition of CO and {{chem2|H2}}
|-
|valign=top | [[Hydrocarboxylation]] and [[Koch reaction]]
|valign=top| carboxylic acid
| industrial process, addition of CO and {{chem2|H2O}}.
|-
|valign=top | [[Carboalkoxylation]]
|valign=top| ester
| industrial process, addition of CO and alcohol.
|-
|valign=top| [[alkylation]]
|valign=top| ester
|industrial process: alkene alkylating carboxylic acid with [[silicotungstic acid]] the catalyst. 
|-
|valign=top | [[Sharpless bishydroxylation]]
|valign=top| diols
| oxidation, reagent: osmium tetroxide, chiral ligand
|-
|valign=top| [[Woodward cis-hydroxylation|Woodward ''cis''-hydroxylation]]
|valign=top|diols
|oxidation, reagents: iodine, silver acetate
|-
|valign=top| [[Ozonolysis]]
|valign=top| aldehydes or ketones
|reagent: ozone
|-
| [[Olefin metathesis]]
| alkenes
| two alkenes rearrange to form two new alkenes
|-
| [[Diels–Alder reaction]]
| cyclohexenes
| cycloaddition with a diene
|-
| [[Pauson–Khand reaction]]
| cyclopentenones
| cycloaddition with an alkyne and CO
|-
| [[Hydroboration–oxidation]]
| alcohols
| reagents: borane, then a peroxide
|-
| [[Oxymercuration-reduction]]
| alcohols
| electrophilic addition of mercuric acetate, then reduction
|-
| [[Prins reaction]]
| 1,3-diols
| electrophilic addition with aldehyde or ketone
|-
| [[Paterno–Büchi reaction]]
| oxetanes
| photochemical reaction with aldehyde or ketone
|-
| [[Epoxidation]]
| epoxide
| electrophilic addition of a peroxide
|-
| [[Cyclopropanation]]
| cyclopropanes
| addition of carbenes or carbenoids
|-
| [[Hydroacylation]]
| ketones
| oxidative addition / reductive elimination by metal catalyst
|-
| [[Hydrophosphination]]
| phosphines
| 
|-
|}

==Synthesis==

===Industrial methods===
Alkenes are produced by hydrocarbon [[cracking (chemistry)|cracking]]. Raw materials are mostly [[natural-gas condensate]] components (principally ethane and propane) in the US and Mideast and [[naphtha]] in Europe and Asia. Alkanes are broken apart at high temperatures, often in the presence of a [[zeolite]] catalyst, to produce a mixture of primarily aliphatic alkenes and lower molecular weight alkanes. The mixture is feedstock and temperature dependent, and separated by fractional distillation. This is mainly used for the manufacture of small alkenes (up to six carbons).<ref name="Wade2">{{cite book | last = Wade | first = L.G. | title = Organic Chemistry | url = https://archive.org/details/organicchemistry00wade_388 | url-access = limited | publisher = Pearson [[Prentice Hall]] | date = 2006 |edition=6th| pages = [https://archive.org/details/organicchemistry00wade_388/page/n351 309] | isbn = 978-1-4058-5345-3 }}</ref>
[[Image:OctaneCracking.svg|500px|center|Cracking of ''n''-octane to give pentane and propene]]

Related to this is catalytic [[dehydrogenation]], where an alkane loses hydrogen at high temperatures to produce a corresponding alkene.<ref name="Wade"/> This is the reverse of the [[catalytic hydrogenation]] of alkenes.
[[Image:ButaneDehydrogenation.svg|600px|center|Dehydrogenation of butane to give butadiene and isomers of butene]]
This process is also known as [[Catalytic reforming|reforming]].  Both processes are endothermic and are driven towards the alkene at high temperatures by [[entropy]].

[[Catalytic]] synthesis of higher α-alkenes (of the type RCH=CH<sub>2</sub>) can also be achieved by a reaction of ethylene with the [[organometallic compound]] [[triethylaluminium]] in the presence of [[nickel]], [[cobalt]], or [[platinum]].

===Elimination reactions===
One of the principal methods for alkene synthesis in the laboratory is the [[elimination reaction]] of alkyl halides, alcohols, and similar compounds. Most common is the β-elimination via the E2 or E1 mechanism.<ref name="PataiBook1964">{{cite book | last = Saunders | first = W. H.  | editor = Patai, Saul | title = The Chemistry of Alkenes| chapter=Elimination Reactions in Solution|publisher = Wiley Interscience |series=PATAI'S Chemistry of Functional Groups | year = 1964 | pages = 149–201|doi=10.1002/9780470771044 | isbn = 978-0-470-77104-4 }}</ref>  A commercially significant example is the production of [[vinyl chloride]].

The E2 mechanism provides a more reliable β-elimination method than E1 for most alkene syntheses. Most E2 eliminations start with an alkyl halide or alkyl sulfonate ester (such as a [[tosylate]] or [[triflate]]). When an alkyl halide is used, the reaction is called a [[dehydrohalogenation]]. For unsymmetrical products, the more substituted alkenes (those with fewer hydrogens attached to the C=C) tend to predominate (see [[Zaitsev's rule]]). Two common methods of elimination reactions are dehydrohalogenation of alkyl halides and dehydration of alcohols. A typical example is shown below; note that if possible, the H is ''anti'' to the leaving group, even though this leads to the less stable ''Z''-isomer.<ref name=Cram1956>{{cite journal
 | last1 = Cram |first1 = D.J.
 | year = 1956
 | title = Studies in Stereochemistry. XXV. Eclipsing Effects in the E2 Reaction1
 | journal = Journal of the American Chemical Society
 | volume = 78
 | issue = 4
 | pages = 790–6
 | doi = 10.1021/ja01585a024
 | last2 = Greene
 | first2 = Frederick D.
 | last3 = Depuy
 | first3 = C. H.
}}</ref>

[[Image:E2EliminationExample.png|500px|center|An example of an E2 Elimination]]

Alkenes can be synthesized from alcohols via [[Dehydration reaction|dehydration]], in which case water is lost via the E1 mechanism. For example, the dehydration of [[ethanol]] produces ethylene:
:CH<sub>3</sub>CH<sub>2</sub>OH   →  H<sub>2</sub>C=CH<sub>2</sub> +  H<sub>2</sub>O

An alcohol may also be converted to a better leaving group (e.g., [[xanthate]]), so as to allow a milder ''syn''-elimination such as the [[Chugaev elimination]] and the [[Grieco elimination]]. Related reactions include eliminations by β-haloethers (the [[Boord olefin synthesis]]) and esters ([[ester pyrolysis]]).  A [[thioketone]] and a [[phosphite ester]] combined (the [[Corey-Winter olefination]]) or [[diphosphorus tetraiodide]] will deoxygenate [[glycol]]s to alkenes.  

Alkenes can be prepared indirectly from alkyl [[amine]]s. The amine or ammonia is not a suitable leaving group, so the amine is first either [[alkylation|alkylated]] (as in the [[Hofmann elimination]]) or oxidized to an [[amine oxide]] (the [[Cope reaction]]) to render a smooth elimination possible. The Cope reaction is a ''syn''-elimination that occurs at or below 150&nbsp;°C, for example:<ref name="CopeElimination1973">{{cite journal | last1 = Bach |first1=R.D. | title=Mechanism of the Cope elimination | journal=J. Org. Chem. | year=1973| volume=38| pages=1742–3 | doi=10.1021/jo00949a029 | last2 = Andrzejewski | first2 = Denis | last3 = Dusold | first3 = Laurence R. | issue = 9 }}</ref>

[[Image:CopeEliminationExample.svg|300px|center|Synthesis of cyclooctene via Cope elimination]]

The Hofmann elimination is unusual in that the ''less'' substituted (non-[[Zaitsev's rule|Zaitsev]]) alkene is usually the major product.

Alkenes are generated from α-halo[[sulfone]]s in the [[Ramberg–Bäcklund reaction]], via a three-membered ring sulfone intermediate.

===Synthesis from carbonyl compounds===
Another important class of methods for alkene synthesis involves construction of a new carbon–carbon double bond by coupling or condensation of a carbonyl compound (such as an [[aldehyde]] or [[ketone]]) to a [[carbanion]] or its equivalent.  Pre-eminent is the [[aldol condensation]].  Knoevenagel condensations are a related class of reactions that convert carbonyls into alkenes.Well-known methods are called ''olefinations''.  The [[Wittig reaction]] is illustrative, but other related methods are known, including the [[Horner–Wadsworth–Emmons reaction]].
 
The Wittig reaction involves reaction of an aldehyde or ketone with a [[Wittig reagent]] (or phosphorane) of the type Ph<sub>3</sub>P=CHR to produce an alkene and [[Triphenylphosphine oxide|Ph<sub>3</sub>P=O]]. The Wittig reagent is itself prepared easily from [[triphenylphosphine]] and an alkyl halide.<ref>{{cite book | last = Crowell | first = Thomas I.|chapter=Alkene-Forming Condensation Reactions  |series=PATAI'S Chemistry of Functional Groups| editor = Patai, Saul | title = The Chemistry of Alkenes| publisher = Wiley Interscience | year = 1964 | pages = 241–270|doi=10.1002/9780470771044.ch4 | isbn = 978-0-470-77104-4}}</ref>

[[Image:Wittig reaction example.svg|350px|center|A typical example of the Wittig reaction]]

Related to the Wittig reaction is the [[Peterson olefination]], which uses silicon-based reagents in place of the phosphorane.  This reaction allows for the selection of ''E''- or ''Z''-products. If an ''E''-product is desired, another alternative is the [[Julia olefination]], which uses the carbanion generated from a [[phenyl]] [[sulfone]]. The [[Takai olefination]] based on an organochromium intermediate also delivers E-products. A titanium compound, [[Tebbe's reagent]], is useful for the synthesis of methylene compounds; in this case, even esters and amides react.

A pair of ketones or aldehydes can be [[deoxygenation|deoxygenated]] to generate an alkene. Symmetrical alkenes can be prepared from a single aldehyde or ketone coupling with itself, using [[titanium]] metal reduction (the [[McMurry reaction]]). If different ketones are to be coupled, a more complicated method is required, such as the [[Barton–Kellogg reaction]].

A single ketone can also be converted to the corresponding alkene via its tosylhydrazone, using [[sodium methoxide]] (the [[Bamford–Stevens reaction]]) or an alkyllithium (the [[Shapiro reaction]]).

===Synthesis from alkenes===
The formation of longer alkenes via the step-wise polymerisation of smaller ones is appealing, as [[ethylene]] (the smallest alkene) is both inexpensive and readily available, with hundreds of millions of tonnes produced annually. The [[Ziegler–Natta process]] allows for the formation of very long chains, for instance those used for [[polyethylene]]. Where shorter chains are wanted, as they for the production of [[surfactant]]s, then processes incorporating a [[olefin metathesis]] step, such as the [[Shell higher olefin process]] are important. 
 
Olefin metathesis is also used commercially for the interconversion of ethylene and 2-butene to propylene. Rhenium- and molybdenum-containing [[heterogeneous catalysis]] are used in this process:<ref name=KO>{{cite encyclopedia|encyclopedia=Kirk-Othmer Encyclopedia of Chemical Technology |first1=Lionel |last1=Delaude |first2=Alfred F. |last2=Noels|year=2005| doi=10.1002/0471238961.metanoel.a01|place=Weinheim|publisher=Wiley-VCH|isbn = 978-0-471-23896-6|chapter = Metathesis|pages=metanoel.a01 }}</ref>
:CH<sub>2</sub>=CH<sub>2</sub>  +  CH<sub>3</sub>CH=CHCH<sub>3</sub>   →   2 CH<sub>2</sub>=CHCH<sub>3</sub>

Transition metal catalyzed [[hydrovinylation]] is another important alkene synthesis process starting from alkene itself.<ref name=Vogt2010>{{cite journal  | last = Vogt |first=D.  | year = 2010  | pages = 7166–8  | title = Cobalt-Catalyzed Asymmetric Hydrovinylation  | issue = 40  | pmid = 20672269  | journal = Angew. Chem. Int. Ed.  | volume = 49  | doi = 10.1002/anie.201003133 }}</ref> It involves the addition of a hydrogen and a vinyl group (or an alkenyl group) across a double bond.

===From alkynes===
Reduction of [[alkyne]]s is a useful method for the [[stereoselectivity|stereoselective]] synthesis of disubstituted alkenes. If the ''cis''-alkene is desired, [[hydrogenation]] in the presence of [[Lindlar's catalyst]] (a heterogeneous catalyst that consists of palladium deposited on calcium carbonate and treated with various forms of lead) is commonly used, though hydroboration followed by hydrolysis provides an alternative approach. Reduction of the alkyne by [[sodium]] metal in liquid [[ammonia]] gives the ''trans''-alkene.<ref name="ZweifelNantz">{{cite book | last1 = Zweifel | first1 = George S. |last2=Nantz |first2=Michael H.| title = Modern Organic Synthesis: An Introduction  | url = https://archive.org/details/modernorganicsyn00zwei | url-access = limited |  publisher = W. H. Freeman | year = 2007 | pages = [https://archive.org/details/modernorganicsyn00zwei/page/n373 366] | isbn = 978-0-7167-7266-8 }}</ref>

[[Image:AlkyneToAlkene.png|600px|center|Synthesis of ''cis''- and ''trans''-alkenes from alkynes]]

For the preparation multisubstituted alkenes, [[carbometalation]] of alkynes can give rise to a large variety of alkene derivatives.

===Rearrangements and related reactions===
Alkenes can be synthesized from other alkenes via [[rearrangement reaction]]s. Besides [[olefin metathesis]] (described [[#Synthesis from alkenes|above]]), many [[pericyclic reaction]]s can be used such as the [[ene reaction]] and the [[Cope rearrangement]].

[[Image:3,3copeexpansion.svg|180px|center|Cope rearrangement of divinylcyclobutane to cyclooctadiene]]

In the [[Diels–Alder reaction]], a [[cyclohexene]] derivative is prepared from a diene and a reactive or electron-deficient alkene.


== Application ==
Unsaturated hydrocarbons are widely used to produce plastics, medicines, and other useful materials. 
{| class="wikitable"
|+
!Name
!Structure
!Use
|-
|[[Ethylene]]
|[[File:Ethene structural.svg|frameless|center|upright=0.25]]
|
* Monomers for synthesizing [[polyethylene]]
|-
|[[Butadiene|1,3-butadiene]]
|[[File:1,3-butadiene.svg|frameless|center|upright=0.44]]
|
* For manufacturing [[synthetic rubber]]
|-
|[[vinyl chloride]]
|[[File:Vinyl chloride.svg|frameless|center|upright=0.25]]
|
* Precursor to [[PVC]]
|-
|[[styrene]]
|[[File:Styrene.svg|frameless|center|upright=0.4]]
|
* precursor to [[polystyrene]]
|}

==Natural occurrence==
Alkenes are prevalent in nature.
Plants are the main natural source of alkenes in the form of [[terpene]]s.<ref name="Ninkuu_2021">{{cite journal |last1=Ninkuu |first1=Vincent |last2=Zhang |first2=Lin |last3=Yan |first3=Jianpei |last4=Fu |first4=Zenchao |last5=Yang |first5=Tengfeng |last6=Zeng |first6=Hongmei |display-authors=3 |date=June 2021 |title=Biochemistry of Terpenes and Recent Advances in Plant Protection |journal=International Journal of Molecular Sciences |volume=22 |issue=11 |pages=5710 |doi=10.3390/ijms22115710 |doi-access=free |pmid=34071919 |pmc=8199371 }}</ref> Many of the most vivid natural pigments are terpenes; e.g. [[lycopene]] (red in tomatoes), [[carotene]] (orange in carrots), and [[xanthophyll]]s (yellow in egg yolk). The simplest of all alkenes, [[ethylene (plant hormone)|ethylene]] is a [[signaling molecule]] that influences the ripening of plants.

<gallery caption="Selected unsaturated compounds in nature">
File:Limonene-2D-skeletal.svg|[[Limonene]], a [[monoterpene]].
File:Alpha-Caryophyllen.svg| [[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.
</gallery>

==IUPAC Nomenclature==
Although the nomenclature is not followed widely, according to IUPAC, an alkene is an acyclic hydrocarbon with just one double bond between carbon atoms.<ref name=PAC1995.alkenes>{{GoldBookRef |title=alkenes |file=A00224 }}</ref> Olefins comprise a larger collection of cyclic and acyclic alkenes as well as dienes and polyenes.<ref name=PAC1995.olefins>{{GoldBookRef |title=olefins |file=O04281 }}</ref>

To form the root of the [[IUPAC nomenclature|IUPAC names]] for straight-chain alkenes, change the ''-an-'' infix of the parent to ''-en-''. For example, '''CH<sub>3</sub>-CH<sub>3</sub>''' is the [[alkane]] ''ethANe''. The name of '''CH<sub>2</sub>=CH<sub>2</sub>''' is therefore ''ethENe''.

For straight-chain alkenes with 4 or more carbon atoms, that name does not completely identify the compound.  For those cases, and for branched acyclic alkenes, the following rules apply:
# Find the longest carbon chain in the molecule. If that chain does not contain the double bond, name the compound according to the alkane naming rules.  Otherwise:
# Number the carbons in that chain starting from the end that is closest to the double bond.
# Define the location ''k'' of the double bond as being the number of its first carbon.
# Name the side groups (other than hydrogen) according to the appropriate rules.
# Define the position of each side group as the number of the chain carbon it is attached to.
# Write the position and name of each side group.
# Write the names of the alkane with the same chain, replacing the "-ane" suffix by "''k''-ene".

The position of the double bond is often inserted before the name of the chain (e.g. "2-pentene"), rather than before the suffix ("pent-2-ene").

The positions need not be indicated if they are unique.  Note that the double bond may imply a different chain numbering than that used for the corresponding alkane: {{chem|(H|3|C|)|3}}C–{{chem|CH|2}}–{{chem|CH|3}} is "2,2-dimethyl pentane", whereas {{chem|(H|3|C|)|3}}C–{{chem|CH}}={{chem|CH|2}} is "3,3-dimethyl 1-pentene".

More complex rules apply for polyenes and [[cycloalkene]]s.<ref name=PAC1995/>

[[Image:Alkene nomenclature.svg|550px|center|thumb|Naming substituted hex-1-enes]]

===''Cis''–''trans'' isomerism===
If the double bond of an acyclic mono-ene is not the first bond of the chain, the name as constructed above still does not completely identify the compound, because of [[cis–trans isomerism|''cis''–''trans'' isomerism]].  Then one must specify whether the two single C–C bonds adjacent to the double bond are on the same side of its plane, or on opposite sides. For monoalkenes, the configuration is often indicated by the prefixes ''cis''- (from [[Latin]] "on this side of") or ''trans''- ("across", "on the other side of") before the name, respectively; as in ''cis''-2-pentene or ''trans''-2-butene.
[[Image:Cis-trans example.svg|thumb|300px|center|The difference between ''cis-'' and ''trans-'' isomers]]

More generally, ''cis''–''trans'' isomerism will exist if each of the two carbons of in the double bond has two different atoms or groups attached to it.  Accounting for these cases, the IUPAC recommends the more general [[E–Z notation]], instead of the ''cis'' and ''trans'' prefixes.  This notation considers the group with highest [[Cahn-Ingold-Prelog priority rule|CIP priority]] in each of the two carbons. If these two groups are on opposite sides of the double bond's plane, the configuration is labeled ''E'' (from the [[German language|German]] ''entgegen'' meaning "opposite"); if they are on the same side, it is labeled ''Z'' (from German ''zusammen'', "together").  This labeling may be taught with mnemonic  "''Z'' means 'on ze zame zide'".<ref name=murr2014>{{cite book |first=John E. |last=McMurry |date=2014 |page=[https://books.google.com/books?id=KDIeCgAAQBAJ&pg=PA189 189] |title=Organic Chemistry with Biological Applications |publisher=Cengage Learning |edition=3rd |isbn=978-1-285-84291-2}}</ref>

[[Image:EZalkenes2.png|400px|center|thumb|The difference between ''E'' and ''Z'' isomers]]

===Groups containing C=C double bonds===
IUPAC recognizes two names for hydrocarbon groups containing carbon–carbon double bonds, the [[vinyl group]] and the [[allyl]] group.<ref name="PAC1995"/>
[[Image:AlkenylGroups.png|200px|center]]

==See also==
{{wiktionary}}
{{wikiquote}}
* [[Alpha-olefin]]
* [[Annulene]]
* [[Aromatic hydrocarbon]] ("Arene")
* [[Dendralene]]
* [[Nitroalkene]]
* [[Radialene]]

==Nomenclature links==
*[http://www.acdlabs.com/iupac/nomenclature/79/r79_53.htm Rule A-3. Unsaturated Compounds and Univalent Radicals] [[IUPAC Color Books#Blue Book|IUPAC Blue Book]].
*[http://www.acdlabs.com/iupac/nomenclature/79/r79_78.htm Rule A-4. Bivalent and Multivalent Radicals] IUPAC Blue Book.
*[http://www.acdlabs.com/iupac/nomenclature/79/r79_60.htm Rules A-11.3, A-11.4, A-11.5 Unsaturated monocyclic hydrocarbons and substituents] IUPAC Blue Book.
*[http://www.acdlabs.com/iupac/nomenclature/79/r79_73.htm Rule A-23. Hydrogenated Compounds of Fused Polycyclic Hydrocarbons] IUPAC Blue Book.

== References ==
{{reflist}}

{{Hydrocarbons}}
{{Alkenes}}
{{Functional Groups}}
{{BranchesofChemistry}}

{{Authority control}}

[[Category:Alkenes| ]]