JEE Main 2023 · 30 January · evening · Single correct · +4 / −1
Given below are two statements: One is labelled as Assertion A and the other is labelled as Reason R.
Assertion A:
can be easily reduced using Zn-Hg / HCl to
Reason R: Zn-Hg / HCl is used to reduce carbonyl group to -CH₂- group.
In the light of the above statements, choose the correct answer from the options given below:
- (a) Both A and R are true and R is the correct explanation of A
- (b) A is false but R is true
- (c) A is true but R is false
- (d) Both A and R are true but R is not the correct explanation of A
Answer and solution
Answer (b) A is false but R is true
Solution
Chemistry ma’am · 4 steps
Clemmensen means hot concentrated HCl, and a tertiary alcohol does not survive that acid.
- Check Reason R. Zn-Hg/HCl is the Clemmensen reduction. It turns a C=O group of an aldehyde or ketone into a -CH₂⁻ group, so R is a true statement.
- Look at the substrate. The starting compound is CH₃COCH₂C(OH)(CH₃)₂. Besides the ketone, it has a tertiary -OH group. The expected product CH₃CH₂CH₂C(OH)(CH₃)₂ needs that -OH to stay untouched.
- Why A fails. Clemmensen runs in hot, concentrated HCl. The acid protonates the tertiary -OH, water leaves, and a stable tertiary carbocation forms. That carbocation loses H⁺ to give an alkene or picks up Cl⁻, so the alcohol shown in the product is not obtained.
- So. R is true but A is false, because this acid-sensitive compound cannot be reduced cleanly by Zn-Hg/HCl. The answer is (b).
The idea to keep. Before you use Clemmensen, scan the whole molecule. Any group that cannot stand hot concentrated HCl, especially a tertiary alcohol, rules it out.
Answer from the NTA final answer key. Solution by PrepWiser.