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

A compound ‘A’ on reaction with ‘X’ and ‘Y’ produces the same major product but different by product 'a' and 'b′. Oxidation of 'a' gives a substance produced by ants.
'X' and 'Y' respectively are

JEE Main PYQ figure
  1. (a) KMnO₄ / H⁺ and dil. KMnO_(4′) 273 K
  2. (b) KMnO₄ (dilute), 273 K and KMnO₄ / H⁺
  3. (c) KMnO₄ / H⁺ and O₃, H₂ O / Zn
  4. (d) O₃, H₂ O / Zn and KMnO₄ / H⁺
Answer and solution

Answer (d) O₃, H₂ O / Zn and KMnO₄ / H⁺

Solution

Chemistry ma’am · 4 steps

Look at the CH₂= end: the reagent that leaves it as HCHO is X, because HCHO oxidises to formic acid.

  1. Read compound A. A is CH₂=C(CH₃)-CH₂-C(CH₃)₃. Both reactions break the C=C bond and give the same ketone, CH₃COCH₂C(CH₃)₃, from the substituted carbon. So both X and Y must cleave the double bond. They differ only in what the terminal CH₂ becomes.
    ReagentEffect on the double bondKetone formed?Fate of the =CH₂ carbon
    O₃, then Zn/H₂Oreductive cleavageyesstops at HCHO
    KMnO₄/H⁺oxidative cleavageyesHCHO → HCOOH → CO₂ + H₂O
    dil. KMnO₄, 273 Kadds two OH groups, no cleavageno, gives a diolstays as CH₂OH in the diol
  2. Identify a. Ants produce formic acid, HCOOH. The only by-product that oxidises to it is formaldehyde, HCHO. So a is HCHO, and X is ozonolysis with Zn/H₂O.
  3. Identify b. Hot acidic KMnO₄ gives the same ketone, but it oxidises the CH₂ end all the way to CO₂ and water. So b is CO₂, and Y is KMnO₄/H⁺.
  4. So. X is O₃, H₂O/Zn and Y is KMnO₄/H⁺, which is option (d).

The idea to keep. A terminal =CH₂ gives HCHO on reductive ozonolysis but CO₂ with acidic KMnO₄. Cold dilute KMnO₄ never cleaves the double bond. It only makes a diol.

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

Study Hydrocarbons — Alkenes & Alkynes

Asked about 90 times in JEE Main since 2020. This question is from 2022.