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Radical substitution
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{{short description|Substitution reaction in organic chemistry involving free radicals}} {{Use dmy dates|date=March 2024}} In [[organic chemistry]], a '''radical-substitution reaction''' is a [[substitution reaction]] involving [[radical (chemistry)|free radical]]s as a [[reactive intermediate]].<ref>March Jerry; (1985). Advanced organic chemistry reactions, mechanisms and structure (3rd ed.). New York: John Wiley & Sons, inc. {{ISBN|0-471-85472-7}}</ref> The reaction always involves at least two steps, and possibly a third. :[[File:FreeRadicalSubstitutionGeneralDescr (1).svg|Free-radical substitution general description]] In the first step called '''initiation''' ('''2''','''3'''), a [[free radical]] is created by [[homolysis (chemistry)|homolysis]]. Homolysis can be brought about by heat or [[ultraviolet light]], but also by [[radical initiator]]s such as [[organic peroxide]]s or [[azo compound]]s. UV Light is used to create two free radicals from one diatomic species. The final step is called '''termination''' ('''6''','''7'''), in which the radical [[Recombination (chemistry)|recombines]] with another radical species. If the reaction is not terminated, but instead the radical group(s) go on to react further, the steps where new radicals are formed and then react are collectively known as '''propagation''' ('''4''','''5'''). This is because a new radical is created, able to participate in secondary reactions. == Radical substitution reactions == In [[free radical halogenation]] reactions, radical substitution takes place with [[halogen]] reagents and [[alkane]] substrates. Another important class of radical substitutions involve [[aryl radical]]s. One example is the hydroxylation of [[benzene]] by [[Fenton's reagent]]. Many oxidation and reduction reactions in organic chemistry have free radical [[reactive intermediate|intermediate]]s, for example the oxidation of [[aldehyde]]s to [[carboxylic acid]]s with [[chromic acid]]. [[Coupling reaction]]s can also be considered radical substitutions. Certain aromatic substitutions takes place by [[radical-nucleophilic aromatic substitution]]. [[Auto-oxidation]] is a process responsible for deterioration of paints and food, as well as production of certain lab hazards such as [[diethyl ether peroxide]]. More radical substitutions are listed below: * The [[Barton–McCombie deoxygenation]] involves substitution of a [[hydroxyl]] group for a proton. * The [[Wohl–Ziegler reaction]] involves allylic bromination of alkenes. * The [[Hunsdiecker reaction]] converts silver salts of carboxylic acids to alkyl halides. * The [[Dowd–Beckwith ring-expansion reaction|Dowd–Beckwith reaction]] involves ring expansion of cyclic β-keto esters. * The [[Barton reaction]] involves synthesis of nitrosoalcohols from nitrites. * The [[Minisci reaction]] involves generation of an alkyl radical from a carboxylic acid and a silver salt, and subsequent substitution at an aromatic compound ==References== {{Reflist}} {{DEFAULTSORT:Radical Substitution}} [[Category:Free radical reactions]] [[Category:Reaction mechanisms]]
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