Free radical addition is an addition reaction in organic chemistry involving free radicals L.G. Wade's Organic Chemistry 5th Ed. (p 319) - Mechanism supplements original.. The basic steps with examples are:

• Initiation by a radical initiator: H-O-O-H -> H-O* H-O* + Br-Br -> H-O-Br Br*.
• Chain propagation: Br* + H2CCH2 -> H-Br + *CHCH2.
• Chain termination: H3C* + Br-Br -> Br* + BrCH3.

Free radical addition depends on one of the reagents having a (relatively) weak bond. Having a weak bond means that (said reagent) will easily break apart to form radicals (often with heat or light), whereas having a strong bond will probably force any reactions to use a different mechanism.

To illustrate, consider the alkoxy radical-catalyzed, anti-Markovnikov reaction of hydrogen bromide to an alkene. In this reaction, a small amount of organic peroxide (R-O-O-R') is needed to abstract the acidic proton from HBr and generate the Br.- radical, but it is assumed that there is one molar equivalent of alkene to acid in order to balance the equation.

Initiation:

Propagation:

Note that the radical will be on the more substituted carbon.

Self-Terminating Oxidative Radical Cyclizations

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In one specific type of radical addition called self-terminating oxidative radical cyclization, alkynes are oxidized to ketones with intramolecular ring closure and the radical species are inorganic rather than carbon based. This type of reaction is self-terminating because propagating is not possible and the initiator is used in stoichiometric amounts. Self-Terminating, Oxidative Radical Cyclizations Tim Dreessen, Christian Jargstorff, Lars Lietzau, Christian Plath, Arne Stademann and Uta Wille Molecules 2004, 9, 480–497 Online article.

As an example a nitrate radical is generated by photolysis of CAN wich reacts with an alkyne to generate first a very reactive vinyl radical and then via a 1,5-hydrogen atom transfer (HAT) and 5-exo-trig ring-closure a ketyl radical. The ketyl dislodges a nitrite radical which is not reactive enough for propagation and the ketone is formed.

The radical species in effect is a single oxygen atom synthon. Other inorganic radical that show this type of reactivity are sulfate radical ions (from ammonium persulfate) and hydroxyl radicals.

See also

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The other radical reactions: radical substitution and radical polymerization.

References

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Organic reactions