Question

Propose a mechanism for the bromination of ethyl benzene shown above.

Short answer

Initiation:

Propagation:

Step-by-step solution

Bromination

Bromine radicals are not as reactive as chlorine radicals, so bromination is more selective than chlorination. Bromine reacts exclusively at the benzylic position.

Either elemental bromine (much cheaper) or N-bromosuccinimide may be used as the reagent for benzylic bromination. N-Bromosuccinimide is preferred for allylic bromination because Br₂ can get added to the double bond easily.

Mechanism for the bromination of ethyl benzene

Initiation:

The formation of free radicals in presence of light.

Propagation:

Here, the free radical formed, stabilizes itself by taking an electron from other species (compound) and creating a free radical in that compound.

The radical created in the new compound tries to get stabilize with the help of resonance.

Later, one bromine atom gets attached to the radical carbon to stabilize the compound and decrease its energy.

The bromine radical, left radical in the end undergoes the free-radical reaction again.