Overall reaction
R–CON₃ → (heat) R–N=C=O + N₂ → (R′OH) R–NH–COOR′
An acyl azide loses N₂ and rearranges to an isocyanate, trapped as a carbamate.
Name reaction · mechanism
Curtius Rearrangement
In the Curtius rearrangement an acyl azide loses nitrogen on heating while its R group moves from carbon to nitrogen, giving an isocyanate. Alcohols trap the isocyanate as carbamates (Boc and Cbz amines), and water gives the amine with one carbon fewer after loss of CO₂.
Mechanism step by step
- Resonance form. The acyl azide as N⁻–N⁺≡N
- Rearrangement. Phenyl moves to N as N₂ leaves (concerted)
- Methanol adds. Attack on the isocyanate carbon
- Proton transfer. The carbamate
Each step is checked electron by electron: the curly arrows give exactly the structures drawn.
Key points
- The acyl azide comes from an acyl chloride and NaN₃ (or DPPA with the acid).
- Migration keeps the configuration of the migrating carbon.
- Water instead of an alcohol gives the amine (via the carbamic acid and loss of CO₂).
- Related: Hofmann (N-bromoamide) and Lossen (hydroxamic acid) rearrangements.
For JEE and NEET
- Concerted loss of N₂ with migration; the migrating group keeps its configuration.
- Same isocyanate intermediate as the Hofmann and Lossen rearrangements.
- Water → amine (via carbamic acid, loss of CO₂).
- Acyl azides are potentially explosive.
FAQ
Curtius or Hofmann?
Both give an isocyanate. Hofmann starts from a primary amide with Br₂/NaOH; Curtius from an acyl azide on heating, under neutral conditions.
What traps the isocyanate?
Alcohols give carbamates, amines give ureas, water gives the amine after CO₂ loss.
Does the migrating carbon keep its stereochemistry?
Yes, migration occurs with retention of configuration.
Draw it in MolDraw
The button above opens the complete figure — overall reaction, numbered mechanism and key points — as ordinary editable structures. Change substituents, recolour atoms, or export a PNG, SVG or chemfig code for notes and slides. The same figure is in the Templates panel under Name reactions.