Editing Alkene (section)

=== 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}}