Radical Substitution: Allylic Bromination

NBS maintains a low bromine concentration throughout the reaction, preventing unwanted addition reactions. Molecular bromine at room temperature causes dibromo addition to alkenes. NBS acts as a bromine radical source, enabling selective allylic substitution in non-polar solvents while keeping both HBr and Br2 concentrations minimal, allowing radical bromination to outcompete ionic addition.

Allylic bromination involves initiation, propagation, and termination. During initiation, light or peroxide causes NBS to undergo homolytic cleavage, forming a bromine radical. Propagation includes two steps: the bromine radical abstracts allylic hydrogen to form an allylic radical and HBr, then Br2 reacts with the allylic radical to yield allyl bromide. Termination occurs when radicals couple to form nonradical products.

HBr generated in the first propagation step reacts with NBS in an ionic reaction to produce Br2, which is essential for the second propagation step. This regeneration of bromine maintains the radical chain reaction. By consuming HBr, NBS ensures that bromine concentration stays low while continuously supplying Br2 needed for the reaction to proceed efficiently.

The allylic radical intermediate formed during bromination is resonance stabilized, allowing the unpaired electron to delocalize across multiple carbon atoms. This resonance stabilization enables the radical to abstract bromine from either resonance site, generating two different allyl bromide products. The distribution of products depends on the stability and reactivity of each resonance form.

Allylic substitution occurs in non-polar solvents at high temperatures with very low bromine concentration, producing 3-bromopropene. In contrast, room temperature treatment with bromine in polar solvents favors ionic addition, forming a dibromo product. Using NBS as the reagent achieves selective allylic substitution at room temperature by maintaining minimal bromine concentration throughout.

Light or peroxide initiates the radical chain reaction by providing energy to cleave the weak N–Br bond in NBS homolytically, generating the initial bromine radical. This radical then abstracts allylic hydrogen from the alkene, starting the propagation cycle. Without photochemical activation or peroxide, NBS cannot efficiently generate the bromine radicals needed to begin the reaction.

At low bromine concentration in non-polar solvents, the ionic addition pathway cannot effectively compete with radical bromination. Radical reactions proceed through chain propagation with minimal bromine present, while ionic addition requires higher Br2 concentrations to form stable bromonium intermediates. By using NBS to maintain low Br2 levels, radical substitution becomes the dominant pathway, selectively forming allyl bromide.