Editing Alkene (section)

==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
| 
|-
|}