The bromination of allylic positions with N-bromosuccinimide (NBS) follows a radical pathway.
Mechanism of the Wohl-Ziegler Reaction
It is very important to keep the concentration of Br2 and HBr low to prevent side reactions derived from simple ionic addition with the alkene. These reagents are therefore generated in situ from NBS. The catalytically active species is Br2, which is almost always present in NBS samples (red colour).
A radical initiator (UV, AIBN) is needed for the homolytic bond cleavage of Br2 :
The allylic position is favoured for hydrogen abstraction, because the resulting radical intermediate is resonance stabilized:
Regeneration of Br2:
Bromination is favored to occur at the more highly substituted position, because the corresponding intermediate radicals are better stabilized.
CCl4 is the solvent of choice, because NBS is poorly soluble and resulting succinimide is insoluble and floats at the surface. This keeps the concentration of reagents low and is a signal that the reaction is finished.
However, environmental concerns have all but eliminated the use of CCl4, and its replacement, CH2Cl2, is being restricted as well. Many other solvents are reactive toward NBS, and are thus unsuitable, but acetonitrile can be used to good effect.