Preparation of Alkyl Halides from Alkenes; Allylic Bromination

Before delving into allylic bromination, let's briefly recap two important reactions of alkenes that yield alkyl halides. We've previously explored the addition of halogens (X₂, where X = Cl or Br) to alkenes, a reaction that proceeds via a cyclic halonium ion intermediate and results in the anti-addition of the two halogen atoms across the double bond. We also examined the addition of (HX, where X = Cl, Br, or I) to alkenes.  This reaction follows Markovnikov's rule, with the proton adding to the carbon of the double bond that already has more hydrogens, and the halide ion adding to the more substituted carbon, forming a more stable carbocation intermediate (except when HBr is used with peroxides, in which case anti-Markovnikov addition is observed). These additions provide versatile routes to dihaloalkanes and monohaloalkanes, respectively.

Allylic Bromination

However, these methods are limited to creating halides on carbons that were originally part of the double bond. A different approach is needed to halogenate allylic positions, which are the carbons adjacent to a double bond. Allylic bromination, typically achieved using N-bromosuccinimide (NBS), offers a selective way to introduce a bromine atom at these allylic sites.

While alkenes can undergo addition reactions with Br₂, leading to vicinal dibromides, NBS promotes a different reaction pathway, favoring substitution at the allylic site.

The reaction proceeds via a radical mechanism, initiated by the homolytic cleavage of the weak N-Br bond in NBS, generating a bromine radical.  This is a radical initiation step as we've discussed in the past.

This bromine radical abstracts an allylic hydrogen atom, creating an allylic radical in a propagation step.

This allylic radical is resonance-stabilized, meaning the unpaired electron is delocalized over the allylic system. This resonance stabilization is key to understanding the selectivity of the reaction. The H-Br generated from this step reacts with N-bromosuccinimide to form Br₂.

Because the allylic radical is resonantly delocalized, it can react with Br₂ (present in small amounts due to the reaction of HBr with NBS) at either of the two allylic positions.  In this case, the allylic radical is symmetrical and a single product is formed.

If the original allylic radical is not symmetrical, this will result in a mixture of allylic bromides if there are other allylic positions available for radical formation. The major product will be the more stable allylic radical. If the allylic radical is symmetrical only one product will be formed. It's important to note that while NBS is the reagent of choice, the actual brominating agent is Br₂, which is generated in situ from the reaction of HBr with NBS. This method offers a distinct advantage over direct bromination with Br₂, which would primarily lead to addition across the double bond. The allylic bromination reaction with NBS is highly selective for substitution at the allylic position due to the stability of the intermediate allylic radical.