Allylic bromination of alkanes using NBS
Description: When treated with N-bromosuccinimide (NBS) and light (hν) alkyl groups adjacent to alkenes will be converted into alkyl bromides.
Notes: The position next to a double bond is called the “allylic” position.
Notes: Note that in example 3, peroxides (that’s “ROOR”) and heat has the same effect as “light”.
Mechanism: NBS provides a constant, low-level concentration of Br2, which is present when HBr reacts with NBS.
When treated with light, Br2 fragments homolytically to bromine radicals (Step 1, arrow A). At any given time only a small concentration of these radicals are present.
Bromine radical then removes a hydrogen from the allylic carbon, leaving behind the resonance-stabilized allylic radical (Step 2, arrows B and C). The allylic radical then reacts with Br2, giving the allylic bromide (Step 3, arrows D and E).
Notes: The KEY to this whole process is that the allylic C-H bond is weak and allylic radicals are greatly stabilized due to resonance. This reaction does not work on ordinary alkanes unless they are tertiary (see also).