Q1(b). NBS: preparation and allylic bromination of propene

Introduction. N-Bromosuccinimide (NBS) is the standard reagent for allylic bromination (Wohl–Ziegler reaction). It supplies bromine radicals while holding the concentration of Br₂ very low, so substitution at the allylic C–H wins over addition to the C=C.

Preparation of NBS

Succinimide (the N–H is acidic, flanked by two C=O) is dissolved in cold aqueous NaOH (ice bath, 0–5°C) and bromine is added. The imide anion takes Br⁺ from Br₂; NBS precipitates and is filtered off.

Scheme: Succinimide → N-bromosuccinimide.

Reaction with propene

Scheme: Allylic substitution: the C=C is kept.

Mechanism: radical chain

1. Initiation: light breaks the weak N–Br bond (or a trace of Br₂): Br₂ →2 Br•
2. Propagation 1: Br• abstracts an allylic H from the CH₃ group. The allylic C–H is the weakest C–H in propene, because the allyl radical is resonance-stabilised: CH₂=CH-CH₂• <→ •CH₂-CH=CH₂.
3. Propagation 2: the allyl radical takes Br from Br₂: CH₂=CH-CH₂• + Br₂ → CH₂=CH-CH₂Br + Br•
4. Role of NBS: the HBr formed in step 2 reacts with NBS (ionic) to give succinimide and Br₂: NBS + HBr → succinimide + Br₂. So Br₂ is regenerated continuously but only at a low concentration.
5. Why no addition: addition of Br• to the C=C is reversible, and capture of the adduct radical needs Br₂; at very low [Br₂] the radical loses Br again, while allylic H abstraction is irreversible. Termination: two radicals combine.

Conclusion. NBS is made from succinimide with Br₂ in cold aqueous NaOH. With propene in sunlight it gives 3-bromopropene (allyl bromide) by a radical chain through the resonance-stabilised allyl radical; NBS keeps Br₂ low and so suppresses addition.

Application. NBS allylic bromination is used to make allylic bromides for synthesis (e.g. 3-bromocyclohexene), and the same reagent in aqueous DMSO gives bromohydrins.