Editing Chemical reaction (section)

===Addition and elimination===
The [[Addition reaction|addition]] and its counterpart, the [[elimination reaction|elimination]], are reactions that change the number of substituents on the carbon atom, and form or cleave [[covalent bond|multiple bonds]]. [[Double bond|Double]] and [[triple bond]]s can be produced by eliminating a suitable leaving group. Similar to the nucleophilic substitution, there are several possible reaction mechanisms that are named after the respective reaction order. In the E1 mechanism, the leaving group is ejected first, forming a carbocation. The next step, the formation of the double bond, takes place with the elimination of a proton ([[deprotonation]]). The leaving order is reversed in the E1cb mechanism, that is the proton is split off first. This mechanism requires the participation of a base.<ref>[[#Bruckner|Brückner]], p. 192</ref> Because of the similar conditions, both reactions in the E1 or E1cb elimination always compete with the S<sub>N</sub>1 substitution.<ref>[[#Bruckner|Brückner]], p. 183</ref>

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[[File:E2-mechanism.svg|thumb|300px|E2 elimination]]

The E2 mechanism also requires a base, but there the attack of the base and the elimination of the leaving group proceed simultaneously and produce no ionic intermediate. In contrast to the E1 eliminations, different stereochemical configurations are possible for the reaction product in the E2 mechanism, because the attack of the base preferentially occurs in the anti-position with respect to the leaving group. Because of the similar conditions and reagents, the E2 elimination is always in competition with the S<sub>N</sub>2-substitution.<ref>[[#Bruckner|Brückner]], p. 172</ref>

[[File:HBr-addition.svg|thumb|300px|Electrophilic addition of hydrogen bromide]]
The counterpart of elimination is an addition where double or triple bonds are converted into single bonds. Similar to substitution reactions, there are several types of additions distinguished by the type of the attacking particle. For example, in the [[electrophilic addition]] of [[hydrogen bromide]], an electrophile (proton) attacks the double bond forming a [[carbocation]], which then reacts with the nucleophile (bromine). The carbocation can be formed on either side of the double bond depending on the groups attached to its ends, and the preferred configuration can be predicted with the [[Markovnikov's rule]].<ref>[[#Wiberg|Wiberg]], pp. 950, 1602</ref> This rule states that "In the heterolytic addition of a polar molecule to an alkene or alkyne, the more electronegative (nucleophilic) atom (or part) of the polar molecule becomes attached to the carbon atom bearing the smaller number of hydrogen atoms."<ref>{{GoldBookRef|title=Markownikoff rule|file=M03707}}</ref>

If the addition of a functional group takes place at the less substituted carbon atom of the double bond, then the electrophilic substitution with acids is not possible. In this case, one has to use the [[hydroboration–oxidation reaction]], wherein the first step, the [[boron]] atom acts as electrophile and adds to the less substituted carbon atom. In the second step, the nucleophilic [[hydroperoxide]] or halogen [[anion]] attacks the boron atom.<ref>[[#Bruckner|Brückner]], p. 125</ref>

While the addition to the electron-rich alkenes and alkynes is mainly electrophilic, the [[nucleophilic addition]] plays an important role in the carbon-heteroatom multiple bonds, and especially its most important representative, the carbonyl group. This process is often associated with elimination so that after the reaction the carbonyl group is present again. It is, therefore, called an addition-elimination reaction and may occur in carboxylic acid derivatives such as chlorides, esters or anhydrides. This reaction is often catalyzed by acids or bases, where the acids increase the electrophilicity of the carbonyl group by binding to the oxygen atom, whereas the bases enhance the nucleophilicity of the attacking nucleophile.<ref>{{cite book | author = Latscha, Hans Peter | author2 = Kazmaier, Uli | author3 = Klein, Helmut Alfons | title = Organische Chemie: Chemie-basiswissen II | volume = 2 | edition = 6th | language = de | publisher = [[Springer Science+Business Media|Springer]] | year = 2008 | isbn = 978-3-540-77106-7 | page = 273}}</ref>

[[File:H-Add-El.Mechanismus.PNG|thumb|center|500px|Acid-catalyzed addition-elimination mechanism]]
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[[Nucleophilic addition]] of a [[carbanion]] or another [[nucleophile]] to the double bond of an [[Α,β-unsaturated carbonyl compound|alpha, beta-unsaturated carbonyl compound]] can proceed via the [[Michael reaction]], which belongs to the larger class of [[conjugate addition]]s. This is one of the most useful methods for the mild formation of C–C bonds.<ref>{{cite book|year=2004|doi=10.1002/0471264180|title=Organic Reactions|isbn=978-0-471-26418-7 |editor-last1=Denmark |editor-first1=Scott E. }}</ref><ref>{{cite web|title = Chapter 18: Enols and Enolates — The Michael Addition reaction|author = Hunt, Ian |publisher = University of Calgary|url = http://www.chem.ucalgary.ca/courses/351/Carey5th/Ch18/ch18-4-3.html}}</ref><ref>[[#Bruckner|Brückner]], p. 580</ref>

Some additions which can not be executed with nucleophiles and electrophiles can be succeeded with free radicals. As with the free-radical substitution, the [[radical addition]] proceeds as a chain reaction, and such reactions are the basis of the [[Radical polymerization|free-radical polymerization]].<ref>{{cite book | author = Lechner, Manfred | author2 = Gehrke, Klaus | author3 = Nordmeier, Eckhard | title = Macromolecular Chemistry | edition = 3rd | publisher = [[Birkhäuser]] | location =  Basel | year = 2003 | isbn = 978-3-7643-6952-1 | pages = 53–65}}</ref>