JEE Main 2021 · 27 July · morning · Single correct · +4 / −1

Presence of which reagent will affect the reversibility of the following reaction, and change it to a irreversible reaction :
CH₄ + I₂ ⇌ _(Re versible)ʰᵛ CH₃ - I + HI

  1. (a) HOCl
  2. (b) dilute HNO₂
  3. (c) Liquid NH₃
  4. (d) Concentrated HIO₃
Answer and solution

Answer (d) Concentrated HIO₃

Solution

Chemistry ma’am · 4 steps

The HI formed is the troublemaker here, so add an oxidising agent that destroys it as soon as it forms.

  1. Why it reverses. The HI produced is a strong reducing agent. It reduces CH₃I back to CH₄, so the backward reaction keeps competing with the forward one.
  2. Remove the HI. To stop the backward reaction, HI must be removed as it forms. A strong oxidising agent such as concentrated HIO₃ (or HNO₃) oxidises it to iodine.

    5HI + HIO₃ → 3I₂ + 3H₂O

  3. Bonus. The I₂ produced here goes back into the forward reaction, so no iodine is wasted. With HI gone, the equilibrium shifts fully towards CH₃I.
  4. So. Concentrated HIO₃ makes the iodination irreversible. The answer is (d).

The idea to keep. Iodination of alkanes is reversible because HI reduces the alkyl iodide back. Carry it out with an oxidiser like HIO₃ or HNO₃ to destroy HI as it forms.

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

Study Hydrocarbons — Alkanes

Asked about 40 times in JEE Main since 2020. This question is from 2021.