Free radical addition
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Free radical addition is an 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:
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Propagation:
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TheDaoOf42_rxn_1_step_3.jpg
Image:TheDaoOf42 rxn 1 step 3.jpg
Note that the radical will be on the more substituted carbon
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TheDaoOf42_rxn_1_step_4.jpg
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References
L.G. Wade's Organic Chemistry 5th Ed. (p 319) - Mechanism supplements original