Alkene Reactions: Allylic Bromination using NBS and Light

As you progress in organic chemistry, you will learn that there are multiple ways to brominate carbons. At an early stage, we have HBr, HBr with peroxide, Br2, and Br2 paired with light. However, if you want to selectively target an allylic carbon instead of an alkene bond or a/all tertiary carbons, you can use N-bromosuccinimide (NBS) in the presence of light (hv), which will target allylic carbon atoms and brominate them:

Reaction

The reaction mechanism is depicted below:

Mechanism:

In the first step, the bromine atom splits off forming a free radical bromine. In the reaction mechanism, Br2 is depicted however, NBS would behave the same way. On the NBS molecule, the Br would split away from the nitrogen atom.

In the second step, the free radical Br reacts with the allylic Hydrogen, breaking the hydrogen-carbon bond sending 1 electron to the carbon atom and the other electron goes with the hydrogen atom to the Br forming HBr.

In the third step, another NBS molecule reacts with the carbon with the single free electron to create a free radical bromine (again, Br2 is depicted here, if copying the mechanism replace this with NBS). This then binds the bromine to the allylic carbon, creating the final product

Practice this reaction using our Reaction Solver!