Remember: "Etard reaction uses Chromyl chloride ($\text{CrO}_2\text{Cl}_2$)".
This single reagent association is highly reliable for identifying the reaction in competitive examinations.
Step 1: Understanding the Question:
The question is a direct name reaction identification problem.
We need to identify the chemical equation that correctly represents the Etard reaction from the given options.
Step 2: Key Formula or Approach:
The Etard reaction is a well-known organic reaction used for the selective partial oxidation of a methyl group on an aromatic ring to an aldehyde group.
The specific reagent for the Etard reaction is chromyl chloride ($\text{CrO}_2\text{Cl}_2$).
Step 3: Detailed Explanation:
• Let us analyze each of the options:
• Option (A):
Toluene ($\text{C}_6\text{H}_5\text{CH}_3$) reacts with chromyl chloride ($\text{CrO}_2\text{Cl}_2$) in carbon disulfide ($\text{CS}_2$) as a solvent to form a brown chromium complex.
This complex, upon subsequent hydrolysis with dilute acid ($\text{H}_3\text{O}^+$), yields benzaldehyde ($\text{C}_6\text{H}_5\text{CHO}$).
This is the correct definition and representation of the Etard reaction.
• Option (B):
Toluene reacts with chromic oxide ($\text{CrO}_3$) in acetic anhydride to form a gem-diacetate, which is then hydrolyzed to benzaldehyde. This is a related oxidation reaction but is not called the Etard reaction.
• Option (C):
Benzene reacts with CO and HCl in the presence of anhydrous aluminium chloride. This is the Gattermann-Koch reaction.
• Option (D):
Benzoyl chloride ($\text{C}_6\text{H}_5\text{COCl}$) is hydrogenated in the presence of palladium catalyst supported on barium sulfate. This is the Rosenmund reduction.
Step 4: Final Answer:
The reaction in Option (A) represents the Etard reaction.
