Free Radical Reactions at Allylic Centers

Allyl free radicals form more easily than the corresponding alkyl radical. The enhanced reactivity of an allylic C—H bond is shown in its reaction with low concentrations of bromine. For example, cyclohexene reacts with very low concentrations of bromine when energy is added by a source of light such as a sun lamp. The light causes homolytic cleavage of the Br—Br bond, and the net result of the reaction is replacement of an allylic hydrogen with a bromine atom. 

Any of the four equivalent aUyhc hydrogen atoms of cyclohexene can react under these conditions. The vinyl C—H bonds do not react because the C—H bond energy of sp -hybridized carbon atoms is larger than the bond energy of aUyhc C—bonds. The four C—H bonds located at non-allylic sites are also less reactive than the aUyhc C—H bonds. We expect this result based on the difference in the bond dissociation energies cited above for propane versus propene. 

The reaction of cyclohexene with bromine is potentially rather complicated. We know that alkenes react with bromine by an electrophilic addition mechanism. Might not this reaction occur in competition with the aUylic bromination reaction at low concentrations of bromine The answer is no because the free-radical chain reaction is much faster than the addition reaction if the concentration of bromine is low. The free-radical chain reaction for reaction of cyclohexene with bromine has the foUowing steps. 

Free Radical Reactions at Allylic Centers Catalyzed by A/-Bromosuccinimide 

Hydrogen bromide continually forms in the reaction. As its concentration increases, might it not add to the double bond in competition with the free-radical allylic bromination The answer is, it could if it was to accumulate as a reaction product. However, this possibility can be bypassed completely. We can use a reagent called N-bromosuccinimide (NBS) to carry out allylic bromination. NBS generates a low concentration of bromine and also prevents the continued production of HBr that would add to the double bond in an electrophihc addition reaction. 

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