10.3 Allylic and Benzylic Bromination with NBS
Allylic and benzylic bromination refer to the substituion of a bromine for a hydrogen on an allylic carbon (a carbon atom 1 bond from an alkene) or on a benzylic carbon (a carbon atom 1 bond from a benzene). These two reactions are carried out using N-bromosuccinimide (abbreviated NBS).
While Br2 is the standard reagent for free radical bromination, it is not the best choice for allylic bromination of an alkene or benzylic bromination of an aromatic compound due to the production of unwanted side products which decrease the yield of the desired product. For an alkene, electrophilic addition across the alkene (commonly presented earlier in 1st semester organic chemistry) is also possible.
For an aromatic compound electrophilic aromatic substitution (commonly presented in 2nd semester organic chemistry) is also possible.
For allylic and benzylic bromination NBS (N-bromosuccinimide) is a much better choice as a very low concentration of Br2 is generated in the reaction which limits the amount of these side products produced.
NBS Allylic Bromination Mechanism
The mechanism including the initiation, propagation and possible termination steps for free radical bromination of prop-1-ene using NBS is shown below. A radical initiator such as a peroxide can be used instead of light in the initiation step to produce the bromine radical.
The major product of this reaction, 3-bromoprop-1-ene, is formed in the second propagation step. The Br2 reacting in that second propagation step is produced in low amounts by the reaction of NBS with HBr as shown at the bottom of the diagram.
'Unanticipated' Products in Allylic Bromination
The radical carbon intermediate during allylic bromination (an allyl radical) is stabilized by resonance, and if the resonance structures are not equivalent bromination will take place at more than one carbon leading to unanticipated products (at least possibly unanticipated by an undergraduate). Take, for example, the bromination of trans-prop-2-ene using NBS. The resonance structures for the radical intermediate are not equivalent leading to the production of two regioisomers.