Radical — decarboxylative halogenation
Hunsdiecker reaction
How the Hunsdiecker 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: RCOOAg (dry), Br₂. Conditions: CCl₄, reflux, anhydrous. Bonds formed: C–Br; AgBr; CO₂; bonds broken: C–COO (decarboxylation); Br–Br. Stereochemistry: Radical at carbon: stereochemistry lost. Rate and selectivity: Radical chain; Ag salt must be dry.
At a glance
Step 1: Acyl hypobromite forms
A silver carboxylate reacts with Br₂; the carboxylate oxygen displaces bromide, forming an acyl hypobromite.
Step 2: O–Br homolysis, then CO₂ loss
Homolysis of the weak O–Br bond generates a carboxyl radical, which rapidly loses CO₂ — decarboxylation is fast because the resulting alkyl radical is far more stable than the original.
Step 3: Bromine transfer propagates the chain
The alkyl radical abstracts a bromine atom from another acyl hypobromite, giving the alkyl bromide and continuing the chain — net result: one carbon shorter than the starting acid, with a new C–Br bond.
Abbreviations and terms
- R
- R group. Any carbon substituent (alkyl, aryl…) whose identity does not matter for the mechanism.
- Δ
- Heat. Reaction is run at elevated temperature.
- hν
- Light. Photochemical activation.
How it compares with related reactions
| Hunsdiecker reaction | Barton nitrite ester reaction | Sandmeyer reaction | |
|---|---|---|---|
| Reagents | RCOOAg (dry), Br₂ | Alkyl nitrite R–O–N=O (from ROH + NOCl, pyridine), hν (> 300 nm) | ArN₂⁺ Cl⁻, CuCl (cat.) |
| Conditions | CCl₄, reflux, anhydrous | Benzene or toluene, 0–25 °C, photolysis | Aqueous HCl, 0–60 °C |
| Stereochemistry | Radical at carbon: stereochemistry lost | Chair-like six-membered TS: only the δ-carbon on the same face is functionalised | Not applicable (sp² aryl) |
| Rate / selectivity | Radical chain; Ag salt must be dry | 1,5-HAT is far faster than intermolecular H abstraction | Radical-chain / redox cycle; N₂ loss drives it |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Hunsdiecker reaction?
RCOOAg (dry), Br₂. Conditions: CCl₄, reflux, anhydrous.
Which bonds are formed and which are broken?
Formed: C–Br; AgBr; CO₂. Broken: C–COO (decarboxylation); Br–Br.
What is the stereochemical outcome?
Radical at carbon: stereochemistry lost
What controls the rate or the selectivity?
Radical chain; Ag salt must be dry
Study guides that help with this reaction
- Radical chain reactions: initiation, propagation and termination
- Curved-arrow notation: rules and the mistakes students make