Final answer:
In allylic bromination with NBS, the reaction proceeds through initiation to generate a bromine radical, followed by two propagation steps involving the formation of an allylic radical and its reaction with Br2, and it ends with termination steps. Efficient filtration of the reaction mixture post-workup is crucial for good product yield.
Step-by-step explanation:
Allylic Bromination Steps with NBS
The allylic bromination of alkenes with NBS (N-Bromosuccinimide) is a reaction that is widely used in organic chemistry to selectively brominate an allylic position. The key steps of the reaction involve the following:
- Initiation: Formation of a bromine radical by the homolytic cleavage of N-Br bond in NBS, often aided by a source of light or heat.
- Propagation Step 1: The bromine radical abstracts a hydrogen from the alkene to form an allylic radical.
- Propagation Step 2: The allylic radical then reacts with a molecule of Br2 to form an allylic bromide and another bromine radical, which can continue the chain process.
- Termination: Radical species combine in various ways to form neutral non-radical products, thus ending the chain reaction.
It is important to note that the reaction may also involve the formation of a resonance-stabilized allylic radical intermediate that can enhance the selectivity of the reaction toward the allylic position. Efficient filtration of the reaction mixture after workup is critical for good product recovery, particularly due to the pasty nature of the mixture when scales are increased, as noted in the reference studies.