Question

Write the reaction involved in the Etard reaction.

Hint:

Etard: toluene + chromyl chloride → benzaldehyde.

Answer 1

Concept: The Etard reaction is a neat way to change the $\mathrm{-CH_3}$ group of toluene into a $\mathrm{-CHO}$ group, stopping cleanly at the aldehyde.

Step 1: The reagent and what it does
Toluene is treated with chromyl chloride $\mathrm{(CrO_2Cl_2)}$ in carbon disulphide $\mathrm{(CS_2)}$. The chromyl chloride attacks the methyl group and forms a brown solid addition complex. This complex protects the carbon so it is not over-oxidised.

Step 2: Hydrolysis gives the aldehyde
When this brown complex is broken with water (hydrolysed), the methyl group is now turned into $\mathrm{-CHO}$, giving benzaldehyde:\[ C_6H_5CH_3 \xrightarrow{CrO_2Cl_2,\ CS_2} \text{brown complex} \xrightarrow{H_3O^+} C_6H_5CHO \]

Answer: Toluene with $\mathrm{CrO_2Cl_2}$ in $\mathrm{CS_2}$, then hydrolysis, gives benzaldehyde.