JEE Main 2022 · 25 July · evening · Single correct · +4 / −1

CH₃ - CH₂ - CN→ _(Ether)^(CH₃MgBr) A→(H₃O +) B→ _(HCl)^(Zn - Hg) C
The correct structure of C is

  1. (a) CH₃ - CH₂ - CH₂ - CH₃
  2. (b) CH₃-CH₂-C(∥(O))-CH₃
  3. (c) CH₃ - CH₂ - CH(OH) - CH₃
  4. (d) CH₃ - CH₂ - CH = CH₂
Answer and solution

Answer (a) CH₃ - CH₂ - CH₂ - CH₃

Solution

Chemistry ma’am · 4 steps

Grignard on a nitrile makes a ketone, and Clemmensen then strips that C=O right down to CH2.

  1. Grignard adds. The CH₃⁻ part of CH₃MgBr attacks the nitrile carbon. A is the imine salt CH₃CH₂C(=NMgBr)CH₃.
  2. Hydrolysis. H₃O⁺ turns the C=N into C=O and releases NH₃. So B is butan-2-one, CH₃CH₂COCH₃.

    CH₃CH₂COCH₃ →(Zn-Hg, HCl) CH₃CH₂CH₂CH₃

  3. Clemmensen reduction. Zinc amalgam with concentrated HCl reduces the carbonyl group all the way to CH₂. It does not stop at the alcohol and it does not form a double bond.
  4. So. C is butane, CH₃CH₂CH₂CH₃, which is option (a).

The idea to keep. Nitrile + RMgX, then H₃O⁺, gives a ketone. Clemmensen (Zn-Hg/HCl) and Wolff-Kishner both take C=O fully to CH₂, never stopping at the alcohol.

Answer from the NTA final answer key. Solution by PrepWiser.

Study Aldehydes & Ketones

Asked about 130 times in JEE Main since 2020. This question is from 2022.