NBS is used to supply a low concentration of bromine in order to achieve allylic rather than vicinal bromination of 1-bromo-5-methyl-2-hexene.
N-Bromosuccinimide (NBS) is generally used as the reagent for the allylic bromination of alkenes. NBS produces a low concentration of bromine to allow the bromination reaction to occur.
The reaction occurs by free radical mechanism. In the allylic bromination of 1-bromo-5-methyl-2-hexene, two products are possible as shown in the image attached to this answer.
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