JEE Main 2023 · 25 January · morning · 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 : Acetal / Ketal is stable in basic medium.
Reason R : The high leaving tendency of alkoxide ion gives the stability to acetal/ketal in basic medium.
In the light of the above statements, choose the correct answer from the options given below :
- (a) A is false but R is true.
- (b) Both A and R are true but R is NOT the correct explanation of A
- (c) Both A and R are true and R is the correct explanation of A
- (d) A is true but R is false.
Answer and solution
Answer (d) A is true but R is false.
Solution
Chemistry ma’am · 4 steps
Acetals survive base because alkoxide is a bad leaving group, so Reason R has it backwards.
- Check Assertion A. An acetal or ketal has two C-OR bonds on one carbon and no acidic hydrogen or C=O for a base to attack. That is why acetals are used to protect carbonyl groups during reactions in basic medium. So A is true.
- Why acid breaks it. In acid, an -OR oxygen gets protonated to -O⁺HR. Now the group leaves as a neutral alcohol ROH, which is a good leaving group, so the acetal hydrolyses back to the carbonyl compound.
- Check Reason R. In base, nothing protonates the oxygen, so the group would have to leave as RO⁻. Alkoxide is a strong base and therefore a poor leaving group, not a good one. The stability comes from this poor leaving tendency, so R is false.
- So. A is true but R is false, which is option (d).
The idea to keep. A strong base is a poor leaving group. Alkoxide cannot leave on its own, so acetals stay intact in base and only hydrolyse once acid turns -OR into ROH.
Answer from the NTA final answer key. Solution by PrepWiser.