Bromination of aromatic alkanes to give alkyl bromides

by James

Description: When treated with N-bromosuccinimide (NBS) and light (hv) alkyl groups adjacent to aromatic groups will be converted into alkyl bromides.
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Notes: A carbon adjacent to an aromatic group is said to be in the “benzylic” position. This reaction does not affect the aromatic ring. 

This reaction is essentially the same as allylic bromination.

Examples: 

Notes: Note that only the carbons adjacent to the aromatic group are affected here.

Mechanism: NBS provides a constant, low level concentration of Br2 which is present when HBr reacts with NBS.

When treated with light, Br2 fragments to bromine radicals (Step 1, arrow A). At any given time only a small concentration of these radicals is present.

Bromine radical then removes a hydrogen from the benzylic carbon, leaving behind the resonance stabilized benzylic radical (Step 2, arrows B and C). the benzylic radical then reacts with Br2, giving the benzylic bromide (Step 3, arrows D and E).

 

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