In Organic chemistry, a radical substitution reaction is a substitution reaction involving free radicals as a reactive intermediate.

At least two steps exist for this reaction type. In the first step called initiation a free radical is created by homolysis. Homolysis can be brought about by heat or light but also by radical initiators such as organic peroxides or azo compounds. Light is used to create two free radicals from one diatomic species. The final step is called termination in which the radical 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 is collectively known as propagation because a new radical is created available for secondary reactions.

Radical substitution reactions

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In free radical halogenation reactions radical substitution takes place with halogen reagents and alkane substrates. Another important class of radical substitutions involve aryl radicals. One example is the hydroxylation of benzene by Fenton's reagent. Many oxidation and reduction reactions in organic chemistry have free radical intermediates, for example the oxidation of aldehydes to carboxylic acids with chromic acid. Coupling reactions can also be considered radical substitutions. Auto-oxidation is a process responsible for deterioration of paints and food and lab hazards such as diethyl ether peroxide.

More radical substitutions are listed below:

• The Barton-McCombie deoxygenation is a way to substitute a hydroxyl group for a proton.
• The Wohl-Ziegler reaction involves the allylic bromination of alkenes.
• The Hunsdiecker reaction converts silver salts of carboxylic acids to alkyl halides.
• The Dowd-Beckwith reaction involves ring expansion of cyclic β-keto esters.

Organic reactions | Radikalische Substitution