JEE Main 2022 · 27 June · morning · Single correct · +4 / −1
Which of the following reactions will yield benzaldehyde as a product?
- (a) (B) and (C)
- (b) (C) and (D)
- (c) (A) and (D)
- (d) (A) and (C)
Answer and solution
Answer (c) (A) and (D)
Solution
Chemistry ma’am · 3 steps
Stop the carbon at the aldehyde level: either reduce gently from above or oxidise gently from below.
- What to look for. An aldehyde sits between an alcohol and an acid. So check each route for one thing: does it stop exactly at C₆H₅CHO, or does it overshoot or fail to start?
Reaction What happens Product (A) C₆H₅COOH: SOCl₂, quinoline, then H₂/Pd/BaSO₄ SOCl₂ gives C₆H₅COCl. Rosenmund reduction with poisoned Pd stops at the aldehyde. benzaldehyde (B) C₆H₅CH₂OH: CrO₃/H₂SO₄ Jones reagent is a strong oxidant in aqueous acid. It takes the primary alcohol all the way to the acid. benzoic acid (C) C₆H₅COOCH₃: NaBH₄, then PCC NaBH₄ is too mild to reduce an ester, so no alcohol forms and PCC has nothing to oxidise. no benzaldehyde (D) C₆H₅CH₃: CrO₃ in (CH₃CO)₂O, then H₃O⁺, heat The methyl group becomes benzylidene diacetate, C₆H₅CH(OCOCH₃)₂, which is protected from further oxidation. Acid hydrolysis frees the aldehyde. benzaldehyde - Why D stops. Acetic anhydride traps the aldehyde as a diacetate the moment it forms, so CrO₃ cannot push it on to benzoic acid. Only the hydrolysis step releases C₆H₅CHO.
- So. Only (A) and (D) give benzaldehyde, which is option (c).
The idea to keep. To stop at an aldehyde, use a reagent that cannot overshoot: Rosenmund from acid chlorides, PCC from alcohols, CrO₃ in acetic anhydride from toluene. And NaBH₄ leaves esters alone.
Answer from the NTA final answer key. Solution by PrepWiser.