Question

Write chemical equations of the Etard reaction and the Gattermann-Koch reaction.

Hint:

Both make benzaldehyde: Etard oxidises toluene with \(CrO_2Cl_2\); Gattermann-Koch formylates benzene with \(CO+HCl/AlCl_3\).

Answer 1

Step 1: Etard reaction. Toluene is oxidised at its methyl group to benzaldehyde using chromyl chloride \((CrO_2Cl_2)\) in carbon disulphide. A brown chromium complex forms first and is then hydrolysed to the aldehyde.
\[C_6H_5CH_3 \xrightarrow{CrO_2Cl_2,\ CS_2} \text{chromium complex} \xrightarrow{H_3O^+} C_6H_5CHO\]
Controlled oxidation stops at the aldehyde stage (it does not go to the acid).
Step 2: Gattermann-Koch reaction. Benzene reacts with carbon monoxide and hydrogen chloride in the presence of anhydrous \(AlCl_3\) (with \(CuCl\) as a helper) to give benzaldehyde. It behaves as a Friedel-Crafts type formylation.
\[C_6H_6 + CO + HCl \xrightarrow{anhyd.\,AlCl_3/CuCl} C_6H_5CHO + HCl\]
Step 3: Summary. Both reactions are important routes to benzaldehyde: Etard from toluene, Gattermann-Koch from benzene.
\[\boxed{\text{Both give } C_6H_5CHO}\]