Etard Reaction: Synthesis of Benzaldehyde
The Etard Reaction is a specific method for the partial oxidation of the methyl group in aromatic hydrocarbons (like Toluene) to an aldehyde group using Chromyl Chloride ($CrO_2Cl_2$). It is one of the best laboratory methods to prepare Benzaldehyde.
1. General Reaction
Toluene is treated with chromyl chloride in a non-polar solvent like Carbon Disulfide ($CS_2$) or Carbon Tetrachloride ($CCl_4$). This forms a brown chromium complex, which on hydrolysis yields Benzaldehyde.
Key Components:
- Substrate: Toluene ($C_6H_5CH_3$).
- Reagent: Chromyl Chloride ($CrO_2Cl_2$).
- Solvent: $CS_2$ or $CCl_4$ (Non-polar).
- Final Product: Benzaldehyde ($C_6H_5CHO$).
2. Detailed Mechanism
The reaction proceeds via the formation of a stable intermediate complex that prevents further oxidation to carboxylic acid.
Step 1: Formation of Chromium Complex
Chromyl chloride attacks the methyl group of toluene. Two molecules of $CrO_2Cl_2$ react with one molecule of toluene to form a precipitated Brown Chromium Complex.
$$ C_6H_5-CH \Bigg( O-Cr(OH)Cl_2 \Bigg)_2 $$
Step 2: Hydrolysis
The complex is stable in the non-polar solvent but decomposes upon the addition of water (acidic hydrolysis) to form Benzaldehyde and chromic acid species.
3. Why Benzaldehyde?
Prevention of Over-Oxidation
Strong oxidizing agents like $KMnO_4$ oxidize toluene directly to Benzoic Acid. The beauty of the Etard reaction lies in the formation of the insoluble chromium complex, which stops the oxidation at the intermediate stage. The aldehyde is released only upon hydrolysis.
4. Limitations
- It is specific to the oxidation of methyl groups attached to an aromatic ring.
- The yield can sometimes be affected by steric hindrance if substituents are present on the ring.
Etard Reaction Quiz
Test your mastery of Toluene oxidation. 10 MCQs with detailed explanations.
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