Question

Write short notes on Etard reaction and Gattermann-Koch reaction. (2 + 2)

Hint:

Etard: toluene + chromyl chloride \((CrO_2Cl_2)\) then hydrolysis. Gattermann-Koch: benzene + CO + HCl with anhydrous AlCl₃/CuCl. Both give benzaldehyde.

Answer 1

Etard reaction
Step 1 (Concept): The Etard reaction is used to prepare benzaldehyde from toluene. Toluene is oxidised in a controlled way by chromyl chloride (CrO₂Cl₂) so that only the methyl group is converted to the aldehyde group, and over-oxidation to benzoic acid is prevented.
Step 2 (Reagent and conditions): Toluene is treated with chromyl chloride \((CrO_2Cl_2)\) in a solvent such as CS₂ or CCl₄. A brown solid chromium complex separates out.
\[ C_6H_5CH_3 \xrightarrow{CrO_2Cl_2} \text{Chromium complex (brown solid)} \]
Step 3 (Hydrolysis): The chromium complex is then hydrolysed with water to give benzaldehyde.
\[ \text{Chromium complex} \xrightarrow{H_3O^+} C_6H_5CHO \]
So the overall product is benzaldehyde.

Gattermann-Koch reaction
Step 4 (Concept): The Gattermann-Koch reaction introduces a –CHO group directly onto the benzene ring (formylation), giving benzaldehyde from benzene.
Step 5 (Reagent and conditions): Benzene is treated with carbon monoxide (CO) and hydrogen chloride (HCl) gas in the presence of anhydrous AlCl₃ and cuprous chloride (CuCl) as catalyst.
\[ C_6H_6 + CO + HCl \xrightarrow{\text{anhyd. } AlCl_3 / CuCl} C_6H_5CHO \]
Step 6 (Result): The product is benzaldehyde. Here AlCl₃ is a Lewis acid catalyst and CuCl helps absorb CO.
\[\boxed{\text{Both reactions give benzaldehyde } (C_6H_5CHO)}\]