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Radical Reactions

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The question this page answers: How do reactions involving free radicals work?

Deeper reading: Clayden 2e: Chapter 37 Pages 970–980, 984–998, 999–1002 — see our chapter-by-chapter practice map for Clayden.

Free radicals and their stability

What is a free radical?

Free radicals are molecules with an unpaired electron. Radicals are generally reactive species.

As with all molecules, there are two aspects of stability. Radicals are thermodynamically stabilized by conjugation and by alkyl substitution. Here are some trends:

As with all molecules, there are two aspects of stability. Radicals are thermodynamically stabilized by conjugation and by alkyl substitution. Here ar

Persistent radicals are kinetically stabilized by sterics that prevent them from being able to react despite being thermodynamically unstable. Here are examples:

Persistent radicals are kinetically stabilized by sterics that prevent them from being able to react despite being thermodynamically unstable. Here ar

Homolytic bond cleavage and BDEs

How are radicals formed?

Radicals are formed by homolytic bond cleavage.

Bond dissociation energies (BDE) are a measure of how strong bonds are in terms of homolytic cleavage, in contrast to heterolytic cleavage seen in ionic reactions:

Bond dissociation energies (BDE) are a measure of how strong bonds are in terms of homolytic cleavage, in contrast to heterolytic cleavage seen in ion

Here are some common BDEs in kcal:

C–H (99-105)C–C (88)C–Br (67)C–I (51)O–O (34)

Three mechanistic steps

Initiation, propagation, termination

Radical reactions involve three mechanistic steps:

Initiation takes place by the homolysis of weak bonds by heat and/or light energy input. In reactions, initiators with weak bonds are often used to jumpstart the radical reaction. Peroxides and AIBN are common initiators:

Initiation takes place by the homolysis of weak bonds by heat and/or light energy input. In reactions, initiators with weak bonds are often used to ju

Propagation/abstraction refers to the transfer of radicals between atoms/molecules:

Propagation/abstraction refers to the transfer of radicals between atoms/molecules:

Termination occurs when two radicals pair up to form a bond:

Termination occurs when two radicals pair up to form a bond:

Common uses of radical reactions

Halogenation, reduction, C–C bonds

Radical reactions are commonly used for:

Halogenation

Anti-Markovnikov HBr; allylic/benzylic NBS

Under radical conditions, HBr adds in an anti-Markovnikov fashion to alkenes:

Under radical conditions, HBr adds in an anti-Markovnikov fashion to alkenes:

Alkanes can also be brominated:

Alkanes can also be brominated:

More usefully, allylic and benzylic positions are easily halogenated, often using N-brominated reagents:

More usefully, allylic and benzylic positions are easily halogenated, often using N-brominated reagents:

Reduction of C–X and C–C bond formation

Swap the halide for H or C

Halides can be substituted with hydrogens:

Halides can be substituted with hydrogens:

If there are alkenes present, then C–C bond forming reactions can be conducted:

If there are alkenes present, then C–C bond forming reactions can be conducted:

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