text: Standard conditions for using NBS in allylic and/or benzylic bromination involves refluxing a solution of NBS in anhydrous CCl with a radical initiator—usually azobisisobutyronitrile (AIBN) or benzoyl peroxide, irradiation, or both to effect radical initiation. The allylic and benzylic radical intermediates formed during this reaction are more stable than other carbon radicals and the major products are allylic and benzylic bromides. This is also called the Wohl–Ziegler reaction.
  The carbon tetrachloride must be maintained anhydrous throughout the reaction, as the presence of water may likely hydrolyze the desired product. Barium carbonate is often added to maintain anhydrous and acid-free conditions.
  In the above reaction, while a mixture of isomeric allylic bromide products are possible, only one is created due to the greater stability of the 4-position radical over the methyl-centered radical.