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Wolff Rearrangement: Carbene to Ketene

By Chemca Editorial Team • Last Updated: January 2026 • 7 min read

The Wolff Rearrangement is a reaction involving the conversion of an $\alpha$-diazoketone into a Ketene via the loss of nitrogen. This rearrangement is the key step in the Arndt-Eistert synthesis for homologating carboxylic acids.

1. General Reaction

An $\alpha$-diazoketone, upon heating, irradiation with UV light ($h\nu$), or treatment with a silver catalyst (like $Ag_2O$), rearranges to form a highly reactive Ketene.

$$ R-CO-CHN_2 \xrightarrow{\Delta \text{ or } h\nu \text{ or } Ag_2O} \underbrace{R-CH=C=O}_{\text{Ketene}} + N_2 \uparrow $$

Conditions:

• Thermal: Heating in a high-boiling solvent.
• Photochemical: Irradiation with UV light.
• Catalytic: Using Silver Oxide ($Ag_2O$) or Silver Benzoate.

2. Detailed Mechanism

The reaction proceeds through a carbene intermediate or a concerted mechanism.

Step 1: Loss of Nitrogen

The diazoketone loses a molecule of nitrogen gas ($N_2$) to generate an $\alpha$-Oxo Carbene (Acyl Carbene). This species has a sextet of electrons on the carbon.

$$ R-C(=O)-CH=N^+ =N^- \xrightarrow{-N_2} R-C(=O)-\ddot{C}H \text{ (Acyl Carbene)} $$

Step 2: 1,2-Rearrangement (Wolff Rearrangement)

The alkyl or aryl group ($R$) migrates with its bonding electrons from the carbonyl carbon to the electron-deficient carbene carbon. This migration restores the octet and forms the Ketene.

$$ R-C(=O)-\ddot{C}H \rightarrow O=C=CH-R \text{ (Ketene)} $$

Note: In the concerted pathway, $N_2$ loss and migration happen simultaneously.

3. Reactions of Ketenes

Ketenes are very reactive electrophiles and react rapidly with nucleophiles.

Reaction with Water

Forms Carboxylic Acids.

$$ R-CH=C=O + H_2O \rightarrow R-CH_2-COOH $$

Reaction with Alcohol

Forms Esters.

$$ R-CH=C=O + R'OH \rightarrow R-CH_2-COOR' $$

Reaction with Amine

Forms Amides.

$$ R-CH=C=O + R'NH_2 \rightarrow R-CH_2-CONHR' $$

Cycloaddition

Reacts with alkenes to form Cyclobutanones ([2+2]).

4. Application: Ring Contraction

Wolff rearrangement is excellent for Ring Contraction of cyclic diazoketones.

Example:

2-Diazocyclohexanone rearranges to form a ketene intermediate, which upon hydration yields Cyclopentanecarboxylic acid.

(6-membered Ring) $\xrightarrow{h\nu, -N_2}$ [Carbene] $\xrightarrow{\text{Ring Contraction}}$ (5-membered Ring Ketene) $\xrightarrow{H_2O}$ Acid

5. Stereochemistry

Retention of Configuration

The migration of the group $R$ is intramolecular and occurs with complete Retention of Configuration. If the migratory carbon is chiral, its stereochemistry is preserved in the product.

Wolff Rearrangement Quiz

Test your concepts on Carbenes and Ketenes. 10 MCQs with explanations.

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