Overall reaction
R–CONH₂ + Br₂ + 4 NaOH → R–NH₂ + Na₂CO₃ + 2 NaBr + 2 H₂O
A primary amide loses its carbonyl carbon to give an amine with one carbon fewer.
Name reaction · mechanism
Hofmann Bromamide Reaction (Hofmann Rearrangement)
The Hofmann bromamide reaction converts a primary amide into a primary amine with one carbon fewer, using bromine and sodium hydroxide. The amide is N-brominated, the N-bromoamide anion rearranges to an isocyanate as the alkyl group moves from carbon to nitrogen, and the isocyanate is hydrolysed and decarboxylated.
Mechanism step by step
- Deprotonation. Hydroxide removes an N–H
- N-Bromination. The amide anion attacks Br₂
- Second deprotonation. The N–H of the N-bromoamide is acidic
- Rearrangement. CH₃ moves to N as Br⁻ leaves: isocyanate
- Hydroxide adds. Attack on the isocyanate carbon
- Proton transfer. Carbamate anion
- Decarboxylation. CO₂ is lost; water then protonates N
Each step is checked electron by electron: the curly arrows give exactly the structures drawn.
Key points
- The product amine has one carbon fewer than the amide.
- The migrating group keeps its configuration.
- Uses: aromatic amines (benzamide → aniline) and degrading carbon chains.
- Related: Curtius (acyl azide) and Lossen rearrangements via the same isocyanate.
For JEE and NEET
- The product amine has one carbon fewer than the amide (e.g. acetamide → methylamine, benzamide → aniline).
- Intermediate: isocyanate, R–N=C=O; the migrating group keeps its configuration.
- Only primary amides (–CONH₂) react.
- Related rearrangements via the same isocyanate: Curtius (acyl azide) and Lossen (hydroxamic acid).
FAQ
Why does the amine have one carbon fewer?
The carbonyl carbon becomes the carbon of the isocyanate, which is lost as CO₂ (carbonate) after hydrolysis.
What is the key intermediate?
The isocyanate R–N=C=O, formed when R migrates from carbon to nitrogen as bromide leaves the N-bromoamide anion.
What does benzamide give?
Aniline (C₆H₅NH₂).
Draw it in MolDraw
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