Free radical addition is a addition reaction in organic chemistry

The basic steps are:

• Initiation: H-O-O-H -> H-O* H-O* + Br-Br -> H-O-Br Br*
• Propagation: Br* + H2CCH2 -> H-Br + *CHCH2.
• Termination: H3C* + Br-Br -> Br* + BrCH3.

Free radical addition is the process by which a electron in a free radical (`1) attacks a bond, usually generating two radicals (`2 and `3), one of which is a leaving group which usually propagates to generate more free radicals (except in the case of alkenes and alkynes, which form carbocations). This process results in the radical (`1) attaching to either radical (`2) or (`3), depending on which forms the more stable product.

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 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

references

L.G. Wade's Organic Chemistry 5th Ed. (p 319) - Mechanism supplements original