Radical — allylic/benzylic bromination
Wohl-Ziegler bromination with NBS
How the Wohl-Ziegler bromination with NBS works, in 3 steps. Each step shows the structures and the curved arrows that move the electrons.
In short. A 3-step mechanism. Typical reagents: NBS, cat. AIBN or benzoyl peroxide (or hν). Conditions: CCl₄ (or PhCF₃), reflux. Bonds formed: C–Br (allylic/benzylic); H–Br; bonds broken: allylic C–H; Br–Br. Stereochemistry: Planar radical, racemic product; allylic shift possible. Rate and selectivity: Radical chain; selectivity from weak allylic C–H (BDE ≈ 88).
At a glance
Step 1: Br• abstracts allylic H
A radical initiator generates a bromine radical, which abstracts the weakest C–H bond in the molecule — the allylic or benzylic hydrogen, since the resulting radical is stabilized by resonance with the adjacent π system.
Step 2: NBS regenerates a trickle of Br₂
NBS serves mainly to maintain a very low, steady concentration of Br₂ (by reacting with the HBr byproduct), preventing the alkene itself from being consumed by simple electrophilic addition.
Step 3: Radical takes Br from Br₂
The resonance-stabilized allylic (or benzylic) radical abstracts a bromine atom from Br₂, giving the substitution product and continuing the radical chain — selective for that one C–H bond over all others in the molecule.
Abbreviations and terms
- NBS
- N-Bromosuccinimide. Source of a low, steady concentration of Br₂/Br• for allylic/benzylic bromination.
- AIBN
- Azobisisobutyronitrile. Radical initiator: on heating releases N₂ and two 2-cyano-2-propyl radicals.
- HBr
- Hydrobromic acid. Strong acid; bromide is a good nucleophile.
- hν
- Light. Photochemical activation.
How it compares with related reactions
| Wohl-Ziegler bromination with NBS | Barton nitrite ester reaction | Hunsdiecker reaction | |
|---|---|---|---|
| Reagents | NBS, cat. AIBN or benzoyl peroxide (or hν) | Alkyl nitrite R–O–N=O (from ROH + NOCl, pyridine), hν (> 300 nm) | RCOOAg (dry), Br₂ |
| Conditions | CCl₄ (or PhCF₃), reflux | Benzene or toluene, 0–25 °C, photolysis | CCl₄, reflux, anhydrous |
| Stereochemistry | Planar radical, racemic product; allylic shift possible | Chair-like six-membered TS: only the δ-carbon on the same face is functionalised | Radical at carbon: stereochemistry lost |
| Rate / selectivity | Radical chain; selectivity from weak allylic C–H (BDE ≈ 88) | 1,5-HAT is far faster than intermolecular H abstraction | Radical chain; Ag salt must be dry |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Wohl-Ziegler bromination with NBS?
NBS, cat. AIBN or benzoyl peroxide (or hν). Conditions: CCl₄ (or PhCF₃), reflux.
Which bonds are formed and which are broken?
Formed: C–Br (allylic/benzylic); H–Br. Broken: allylic C–H; Br–Br.
What is the stereochemical outcome?
Planar radical, racemic product; allylic shift possible
What controls the rate or the selectivity?
Radical chain; selectivity from weak allylic C–H (BDE ≈ 88)
Study guides that help with this reaction
- Radical chain reactions: initiation, propagation and termination
- Curved-arrow notation: rules and the mistakes students make