Editing Radical initiator

{{Short description|Molecules that can produce radicals and catalyze radical reactions}}
In [[chemistry]], '''radical initiators''' are substances that can produce [[radical (chemistry)|radical species]] under mild conditions and promote [[radical reaction]]s.<ref>{{JerryMarch}}</ref> These substances generally possess weak bonds&mdash;bonds that have small [[bond dissociation energy|bond dissociation energies]]. Radical initiators are utilized in industrial processes such as [[polymer]] synthesis. Typical examples are molecules with a nitrogen-halogen bond, [[azo compound]]s, and [[organic peroxide|organic and inorganic peroxides]].<ref>{{March6th}}</ref>

==Main types of initiation reaction==
*Halogens undergo [[homolysis (chemistry)|homolytic fission]] relatively easily. [[Chlorine]], for example, gives two chlorine radicals (Cl•) by irradiation with [[ultraviolet light]]. This process is used for [[Free-radical halogenation|chlorination]] of [[alkane]]s.
*Azo compounds (R-[[nitrogen|N]]=N-R') can be the precursor of two [[carbon]]-centered radicals (R• and R'•) and nitrogen gas upon heating and/or by irradiation. For example, [[Azobisisobutyronitrile|AIBN]] and [[ABCN]] yield isobutyronitrile and cyclohexanecarbonitrile radicals, respectively.
:[[File:AIBN-radical-2D.png|400px|AIBN initiator]]

*[[Organic peroxide]]s each have a peroxide bond (-[[oxygen|O]]-O-), which is readily cleaved to give two oxygen-centered radicals. The oxyl radicals are unstable and believed to be transformed into relatively stable carbon-centered radicals. For example, [[di-tert-butyl peroxide|di-''tert''-butyl peroxide]] (''t''-[[butyl|Bu]][[peroxide|OO]]''t''-Bu) gives two ''t''-butoxy radicals (''t''-BuO•) and the radicals become [[methyl]] radicals (C[[hydrogen|H]]<sub>3</sub>•) with the loss of [[acetone]]. [[Benzoyl peroxide]] (([[phenyl|Ph]]C)OO)<sub>2</sub>) generates benzoyloxyl radicals (PhCOO•), each of which loses [[carbon dioxide]] to be converted into a phenyl radical (Ph•). [[Methyl ethyl ketone peroxide]] is also common, and [[acetone peroxide]] is on rare occasions used as a radical initiator, too.
*Inorganic peroxides function analogously to organic peroxides. Many polymers are often produced from the alkenes upon initiation with [[peroxydisulfate]] salts. In solution, peroxydisulfate dissociates to give sulfate radicals:<ref name = Ullmann>{{Ullmann | title = Peroxo Compounds, Inorganic | author =  Harald Jakob, Stefan Leininger, Thomas Lehmann, Sylvia Jacobi, Sven Gutewort | doi = 10.1002/14356007.a19_177.pub2}}</ref>
:[O<sub>3</sub>SO-OSO<sub>3</sub>]<sup>2−</sup> {{eqm}} 2 [SO<sub>4</sub>]<sup>−</sup>
The sulfate radical adds to an alkene forming radical sulfate esters, e.g. <sup>.</sup>CHPhCH<sub>2</sub>OSO<sub>3</sub><sup>−</sup>, that add further alkenes via formation of C-C bonds.  Many styrene and fluoroalkene polymers are produced in this way.

* In [[atom transfer radical polymerization]] (ATRP), carbon-halides reversibly generate organic radicals in the presence of [[transition metal]] [[catalyst]]. 
[[File:ATRP general.png|thumb|600px|center|General ATRP Reaction. <span style="color: blue">A.</span> Initiation. <span style="color: blue">B.</span> Equilibrium with dormant species. <span style="color: blue">C.</span>Propagation]]

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==Safety==
Some radical initiators such as [[azo compound]]s and [[peroxide]]s can detonate at elevated  temperatures so they must be stored cold.

==References==
{{Reflist}}

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[[Category:Radical initiators| ]]