Propose mechanisms to account for the observed products in the following reactions. (a)

Step 1 of 3

Wohl-Zigler bromination

Bromination of the allylic or benzylic position of an alkene using N-bromosuccinamide (NBS) is called Wohl-Zigler bromination. The reaction is initiated by peroxide or by light. NBS is the most commonly used reagent for allylic bromination of an alkene.

NBS is synthesized by adding bromine to succinamide in the presence of a strong base like sodium hydroxide. The role of NBS is to afford a low concentration of molecular bromine in solution. The bromination by NBS occurs by a radical mechanism.

The reaction is initiated by the homolytic cleavage of the N-Br bond in NBS to form a bromine free radical. These radicals abstract the allylic hydrogen from the alkene forming an allyl free radical stabilized by resonance. The allyl free radical reacts with the bromine molecule to form the most stable halide.

The formation of allyl free radical and its reaction with bromine together form the propagation step. This step is continued till all the alkenes are brominated. The termination step includes the reaction between any two free radicals.