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Arndt-Eistert Synthesis: Acid Homologation

By Chemca Editorial Team • Last Updated: January 2026 • 9 min read

The Arndt-Eistert Synthesis is a specific series of chemical reactions used to convert a carboxylic acid into its next higher homologue (an acid containing one additional carbon atom). The key transformation involves the insertion of a methylene group ($-CH_2-$) into the carbon chain.

1. General Reaction

A carboxylic acid is converted to an acid chloride, then to an $\alpha$-diazoketone, which rearranges to a ketene, and finally hydrolyzes to the homologous acid.

$$ R-COOH \xrightarrow{1. \ SOCl_2} R-COCl \xrightarrow{2. \ CH_2N_2} R-CO-CHN_2 \xrightarrow{3. \ Ag_2O, \ \Delta, \ H_2O} R-CH_2-COOH $$

Key Steps:

1. Formation of Acid Chloride ($RCOCl$).
2. Formation of Diazoketone ($RCOCHN_2$) using Diazomethane.
3. Wolff Rearrangement (Crucial Step).
4. Hydrolysis.

2. Detailed Mechanism

The beauty of this reaction lies in the carbene-mediated rearrangement.

Step 1: Activation of Acid

The carboxylic acid is converted to the more reactive acid chloride using Thionyl Chloride ($SOCl_2$).

$$ R-COOH + SOCl_2 \rightarrow R-COCl + SO_2 + HCl $$

Step 2: Formation of Diazoketone

The acid chloride reacts with Diazomethane ($CH_2N_2$). Two equivalents are used: one to form the diazoketone and one to neutralize the HCl produced (forming methyl chloride).

$$ R-COCl + 2CH_2N_2 \rightarrow R-CO-CHN_2 + CH_3Cl + N_2 $$

Step 3: Wolff Rearrangement (The Heart of the Reaction)

Upon heating with Silver Oxide ($Ag_2O$) catalyst or light ($h\nu$), the diazoketone loses Nitrogen gas ($N_2$) to form a transient Acyl Carbene.

This unstable carbene undergoes a 1,2-rearrangement (alkyl migration) to form a stable Ketene ($R-CH=C=O$).

$$ R-CO-CHN_2 \xrightarrow{-N_2} [R-CO-\ddot{CH}] \text{ (Carbene)} $$ $$ [R-CO-\ddot{CH}] \xrightarrow{\text{Rearrangement}} R-CH=C=O \text{ (Ketene)} $$

Step 4: Hydrolysis

The ketene is highly reactive towards nucleophiles. Water attacks the ketene to form the higher carboxylic acid.

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

3. Variations (Different Nucleophiles)

Instead of water, other nucleophiles can be used in the final step to obtain different derivatives of the homologous acid.

• Alcohol ($R'OH$): Yields Esters ($R-CH_2-COOR'$).
• Ammonia ($NH_3$): Yields Amides ($R-CH_2-CONH_2$).
• Amine ($R'NH_2$): Yields Substituted Amides.

4. Example: Synthesis of Phenylacetic Acid

Starting from Benzoic Acid ($C_6H_5COOH$).

$$ C_6H_5COOH \rightarrow C_6H_5COCl \rightarrow C_6H_5COCHN_2 \rightarrow C_6H_5-CH=C=O \rightarrow C_6H_5CH_2COOH $$

5. Comparison

Reaction	Conversion	Carbon Count
Arndt-Eistert	Acid $\to$ Homologous Acid	Increases by 1 (+1)
Hoffmann Bromamide	Amide $\to$ Amine	Decreases by 1 (-1)
Hunsdiecker	Silver Salt $\to$ Alkyl Bromide	Decreases by 1 (-1)

Arndt-Eistert Quiz

Test your concepts on Homologation. 10 MCQs with explanations.

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