Radical — diazonium substitution
Sandmeyer reaction: diazonium → aryl chloride
How the Sandmeyer 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⁺ Cl⁻, CuCl (cat.). Conditions: Aqueous HCl, 0–60 °C. Bonds formed: Ar–Cl; N≡N (gas); bonds broken: Ar–N; Cu(I) → Cu(II) → Cu(I). Stereochemistry: Not applicable (sp² aryl). Rate and selectivity: Radical-chain / redox cycle; N₂ loss drives it.
At a glance
Step 1: Single-electron transfer from Cu(I)
Cu(I) transfers a single electron to the diazonium salt, reducing it and triggering loss of N₂ gas to form an aryl radical.
Step 2: N₂ leaves; aryl radical takes Cl
The aryl radical abstracts a chlorine atom from CuCl₂ (or reacts with a copper-chloride species), forming the C–Cl bond and regenerating Cu(I) to continue the cycle.
Step 3: Aryl chloride, catalyst regenerated
The overall transformation swaps a diazonium group for a halide — a reliable route to aryl chlorides and bromides that's hard to make directly by EAS.
Abbreviations and terms
- Ar
- Aryl group. Any aromatic ring substituent such as phenyl.
- HCl
- Hydrochloric acid / hydrogen chloride. Strong acid; chloride is the nucleophile in additions/substitutions.
- EAS
- Electrophilic aromatic substitution. An electrophile replaces H on an arene via a Wheland intermediate.
How it compares with related reactions
| Sandmeyer reaction | Barton nitrite ester reaction | Hunsdiecker reaction | |
|---|---|---|---|
| Reagents | ArN₂⁺ Cl⁻, CuCl (cat.) | Alkyl nitrite R–O–N=O (from ROH + NOCl, pyridine), hν (> 300 nm) | RCOOAg (dry), Br₂ |
| Conditions | Aqueous HCl, 0–60 °C | Benzene or toluene, 0–25 °C, photolysis | CCl₄, reflux, anhydrous |
| Stereochemistry | Not applicable (sp² aryl) | Chair-like six-membered TS: only the δ-carbon on the same face is functionalised | Radical at carbon: stereochemistry lost |
| Rate / selectivity | Radical-chain / redox cycle; N₂ loss drives it | 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 Sandmeyer reaction?
ArN₂⁺ Cl⁻, CuCl (cat.). Conditions: Aqueous HCl, 0–60 °C.
Which bonds are formed and which are broken?
Formed: Ar–Cl; N≡N (gas). Broken: Ar–N; Cu(I) → Cu(II) → Cu(I).
What is the stereochemical outcome?
Not applicable (sp² aryl)
What controls the rate or the selectivity?
Radical-chain / redox cycle; N₂ loss drives it
Study guides that help with this reaction
- Radical chain reactions: initiation, propagation and termination
- Curved-arrow notation: rules and the mistakes students make