What is the structural reagent used in the Etard reaction to oxidize toluene selectively to benzaldehyde?

Answer and explanation

Correct answer: C. Chromyl chloride (CrO2Cl2) in carbon disulfide (CS2), followed by aqueous hydrolysis

  1. Step 1: Direct oxidation of toluene with strong oxidants like alkaline or acidified oxidizes the benzylic methyl group completely to benzoic acid.
  2. Step 2: To halt oxidation at the benzaldehyde stage, the Etard reaction employs chromyl chloride () in a non-polar solvent like or .
  3. Step 3: Chromyl chloride oxidizes the methyl group to a brown insoluble chromium complex: .
  4. Step 4: Subsequent mild aqueous acidic hydrolysis decomposes this complex into benzaldehyde ().
Answer
Chromyl chloride (CrO2Cl2) in carbon disulfide (CS2), followed by aqueous hydrolysis
Why not the others: 'Alkaline potassium permanganate...' oxidizes toluene all the way to benzoic acid; 'Chromium trioxide in conc. H2SO4 (Jones reagent)' also over-oxidizes to benzoic acid; 'Lead tetraacetate...' cleaves vicinal diols, not benzylic methyl groups.

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