Final answer:
NBS is used in allylic bromination of propene to selectively substitute a hydrogen atom at the allylic position with a bromine atom with improved selectivity due to resonance stabilization of the allylic radical.
Step-by-step explanation:
Use of NBS in Allylic Bromination of Propene
The point of using N-bromosuccinimide (NBS) to cause propene to undergo allylic bromination is to selectively substitute a hydrogen atom adjacent to a double bond (allylic position) with a bromine atom. NBS is preferred in this reaction because it is a source of low concentrations of bromine, which helps to minimize the formation of di-brominated or over-brominated products and thus improves reaction selectivity. When added to a reaction with propene, NBS facilitates the bromination at the allylic position rather than the double bond, because the allylic radical is resonance stabilized.