Question

How does NBS reagent work?

Answer 1

NBS (N-bromo succinimide) is a gleaming white crystalline solid and easy as pie to work with. It will undergo many similar reactions as bromine - attached to the electron-withdrawing nitrogen of succinimide, the bromine has a partial positive charge and is therefore electrophilic.
There are two major reactions NBS is used for: allylic bromination (the most common) and also as a replacement for $B{r}_2$ in the formation of bromohydrins.
Allylic bromination is the replacement of a hydrogen on a carbon adjacent to a double bond (or aromatic ring, in which case it’s called benzylic bromination). NBS is used as a substitute for $B{r}_2$ in these cases since $B{r}_2$ tends to react with double bonds to form dibromides. The advantage of NBS is that it provides a low-level concentration of $B{r}_2$, and bromination of the double bond doesn’t compete as much.