Radical biaryl coupling
Gomberg-Bachmann reaction
How the Gomberg-Bachmann reaction 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: ArN₂⁺ X⁻, arene (solvent, large excess), aq. NaOH. Conditions: Biphasic, 0–25 °C. Bonds formed: Ar–Ar′ C–C bond; bonds broken: C–N (as N₂), arene C–H. Stereochemistry: None; mixture of o/m/p isomers. Rate and selectivity: Low yields (<40 %) from side reactions; radical chain.
At a glance
Step 1: Diazonium loses N₂
An aryldiazonium salt, under basic aqueous conditions, loses N₂ homolytically to generate an aryl radical directly — the same kind of radical intermediate formed in the Sandmeyer reaction, just trapped differently here.
Step 2: Radical adds to the arene
This aryl radical adds to a second arene's π system — the same type of radical addition seen in radical polymerization propagation, just adding to an aromatic ring instead of a vinyl monomer.
Step 3: H• loss restores aromaticity
Loss of a hydrogen atom (often to another diazonium-derived radical, continuing a chain) restores aromaticity, giving an unsymmetrical biaryl — a radical-based alternative to the transition-metal-catalyzed biaryl couplings covered elsewhere in this library.
Abbreviations and terms
- Ar
- Aryl group. Any aromatic ring substituent such as phenyl.
- X
- Halogen / leaving group. Cl, Br or I (sometimes any leaving group).
- NaOH
- Sodium hydroxide. Strong aqueous base and nucleophile.
How it compares with related reactions
| Gomberg-Bachmann reaction | Atom transfer radical polymerization (ATRP) | Barton nitrite ester reaction | |
|---|---|---|---|
| Reagents | ArN₂⁺ X⁻, arene (solvent, large excess), aq. NaOH | Alkyl halide initiator (R–Br), Cu(I)Br/ligand (bipyridine, PMDETA), monomer | Alkyl nitrite R–O–N=O (from ROH + NOCl, pyridine), hν (> 300 nm) |
| Conditions | Biphasic, 0–25 °C | 60–110 °C, degassed | Benzene or toluene, 0–25 °C, photolysis |
| Stereochemistry | None; mixture of o/m/p isomers | Low [R•] (~10⁻⁸ M); controlled growth | Chair-like six-membered TS: only the δ-carbon on the same face is functionalised |
| Rate / selectivity | Low yields (<40 %) from side reactions; radical chain | Keq = kact/kdeact ≈ 10⁻⁸–10⁻⁶; Đ ≈ 1.1–1.3 | 1,5-HAT is far faster than intermolecular H abstraction |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Gomberg-Bachmann reaction?
ArN₂⁺ X⁻, arene (solvent, large excess), aq. NaOH. Conditions: Biphasic, 0–25 °C.
Which bonds are formed and which are broken?
Formed: Ar–Ar′ C–C bond. Broken: C–N (as N₂), arene C–H.
What is the stereochemical outcome?
None; mixture of o/m/p isomers
What controls the rate or the selectivity?
Low yields (<40 %) from side reactions; radical chain
Study guides that help with this reaction
- Radical chain reactions: initiation, propagation and termination
- Curved-arrow notation: rules and the mistakes students make