The benzylic C-H bonds weaker than most sp3 hybridized C-H. This is because the radical formed from homolysis is resonance stabilized.
Resonance stabilization of the benzylic radical
Because of the weak C-H bonds, benzylic hydrogens can form benzylic halides under radical conditions.
NBS as a Bromine Source
NBS (N-bromosuccinimide) is the most commonly used reagent to produce low concentrations of bromine. When suspended in tetrachloride (CCl4), NBS reacts with trace amounts of HBr to produce a low enough concentration of bromine to facilitate the allylic bromination reaction.
Allylic Bromination Mechanism
Step 1: Initiation
Once the pre-initiation step involving NBS produces small quantities of Br2, the bromine molecules are homolytically cleaved by light to produce bromine radicals.
Step 2 and 3: Propagation
Step 4: Termination
Prof. Steven Farmer (Sonoma State University)
- Layne A. Morsch - University of Illinois Springfield