NBS, or N-bromo succinimide, is a white crystalline solid that is often used in organic chemistry reactions to add bromine atoms to a compound. It can undergo two major reactions: allylic bromination and bromohydrin formation.
The most common reaction that NBS is used for is allylic bromination. During this reaction, NBS provides the reaction with a small concentration of diatomic bromine. The bromine atoms can then attach to an organic ring system. The reaction occurs through an initiation step, a propagation step and a termination step. NBS is favorable in this reaction because it allows only one molecule of bromine to attach to the ring system. If only diatomic bromine was utilized, then two equivalents of bromine would add to the ring system.