Rearrangement — nitrogen extrusion
Curtius rearrangement: acyl azide → isocyanate
How the Curtius rearrangement 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: Acyl azide (from RCO₂H + DPPA, or RCOCl + NaN₃). Conditions: Heat, 60–100 °C in toluene or benzene. Bonds formed: C=O of isocyanate; N≡N; bonds broken: C–R (migrates), N–N. Stereochemistry: Retention at the migrating group. Rate and selectivity: First-order in acyl azide; concerted, no free nitrene.
At a glance
Step 1: Heat: R migrates as N₂ leaves
Mild heating causes the acyl azide to expel N₂ gas. As the C–N bond to the departing nitrogen breaks, the R group migrates from carbon to the adjacent nitrogen — concerted with N₂ loss.
Step 2: Isocyanate and dinitrogen
This 1,2-shift proceeds with retention of configuration at the migrating group and forms an isocyanate (R–N=C=O) directly, without ever passing through a free nitrene intermediate.
Step 3: Hydrolysis and decarboxylation
Isocyanates react with water to give unstable carbamic acids that spontaneously lose CO₂, yielding a primary amine — a classic way to shorten a carboxylic acid by one carbon while installing nitrogen.
Abbreviations and terms
- R
- R group. Any carbon substituent (alkyl, aryl…) whose identity does not matter for the mechanism.
- H2
- Hydrogen gas. Reductant for catalytic hydrogenation.
- Δ
- Heat. Reaction is run at elevated temperature.
How it compares with related reactions
| Curtius rearrangement | Bamberger rearrangement | Beckmann rearrangement | |
|---|---|---|---|
| Reagents | Acyl azide (from RCO₂H + DPPA, or RCOCl + NaN₃) | N-Phenylhydroxylamine, dilute H₂SO₄ (aq) | H₂SO₄, PCl₅, TsCl or PPA |
| Conditions | Heat, 60–100 °C in toluene or benzene | H₂O, 25–60 °C | Acid, 100–130 °C (industrial: oleum) |
| Stereochemistry | Retention at the migrating group | None; para-selective | Group anti to OH migrates, with retention |
| Rate / selectivity | First-order in acyl azide; concerted, no free nitrene | Loss of water to give the nitrenium ion is rate-limiting | Migration is rate-limiting; leaving-group ability of N–OX matters |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Curtius rearrangement?
Acyl azide (from RCO₂H + DPPA, or RCOCl + NaN₃). Conditions: Heat, 60–100 °C in toluene or benzene.
Which bonds are formed and which are broken?
Formed: C=O of isocyanate; N≡N. Broken: C–R (migrates), N–N.
What is the stereochemical outcome?
Retention at the migrating group
What controls the rate or the selectivity?
First-order in acyl azide; concerted, no free nitrene
Study guides that help with this reaction
- Carbonyl chemistry: addition, acyl substitution and enolates
- Pericyclic reactions explained: cycloadditions, electrocyclisations and sigmatropic shifts
- Stereochemistry in mechanisms: inversion, retention, syn and anti