Reactions Involving Free Radicals

Reactions Involving Free Radicals

Free radical reactions involve one electron species, frequently generated by homolysis (shown below)

Radical reactions can be divided into three steps:1) Initiation2) Propagation3) Termination

Weaker bonds are more readily cleaved by homolysis

Some bonds can be cleaved by heating

Radical Initiators

AIBN

Benzoyl PeroxideLink

Radical Bromination

Link

Allylic (and benzylic) bromination with NBS (N-Bromosuccinimide)

NBS is regarded as a source of trace amounts of Br2 via the mechanism shown below.

Reductions utilizing tributyltin hydride as a hydrogen atom donor

The Sn-H bond is relatively weak (82 kcal/mole), relative to the C-H bond (99 kcal/mole)By contrast, tin forms stronger bonds to bromine, iodine, and sulfur than does carbon.

Sn-S 111 kcalSn-Br 132 KcalSn-I 56 kcal

C-S 65 kcalC-Br 68 kcalC-I 51 kcal

The Barton-McCombie Deoxygenation

Mechanism

Radical Reaction with Allyltributylstannane

The Barton Decarboxylation

Free Radical Polymerization

Link