JEE Main 2022 · 25 June · evening · Single correct · +4 / −1
Which of the following conditions or reaction sequence will NOT give acetophenone as the major product?
- (a) (a) C₆H₅-C(∥)(O)-H + CH₃MgBr (b) Na₂Cr₂O₇, H⁺
- (b)
- (c)
- (d) C₆H₅-C(∥)(O)-Cl + CH₃MgBr + CdCl₂
Answer and solution
Answer (c)
Solution
Chemistry ma’am · 4 steps
Watch how many methyl groups end up on that carbonyl carbon: one gives the ketone, two give a 3° alcohol.
- Routes a and b. Both build the same secondary alcohol, 1-phenylethanol C₆H₅CH(OH)CH₃: one adds CH₃MgBr to benzaldehyde, the other adds C₆H₅MgBr to acetaldehyde. Oxidising a secondary alcohol with Na₂Cr₂O₇/H⁺ or PCC stops at the ketone, so both give acetophenone.
- Route d. CH₃MgBr with CdCl₂ forms dimethylcadmium, (CH₃)₂Cd. It is too weak to attack a ketone, so it replaces the Cl of benzoyl chloride once and stops at C₆H₅COCH₃.
- Route c. The first CH₃MgBr adds to ethyl benzoate and expels ethoxide, giving acetophenone. A ketone is more reactive than an ester, so the second CH₃MgBr attacks it at once, and hydrolysis gives a tertiary alcohol.
Option Key intermediate Major product (a) C₆H₅CH(OH)CH₃, 2° alcohol C₆H₅COCH₃ (b) C₆H₅CH(OH)CH₃, 2° alcohol C₆H₅COCH₃ (c) C₆H₅COCH₃, attacked again C₆H₅C(OH)(CH₃)₂, 2-phenylpropan-2-ol (d) (CH₃)₂Cd reagent C₆H₅COCH₃ - So. Only (c) does not give acetophenone as the major product. It gives the tertiary alcohol 2-phenylpropan-2-ol instead.
The idea to keep. An ester or acid chloride with a plain Grignard reagent reacts twice and ends as a tertiary alcohol. To stop at the ketone, use the milder R₂Cd made from RMgX and CdCl₂.
Answer from the NTA final answer key. Solution by PrepWiser.