Organic Compounds Containing Oxygen: Chemistry | JEE Main
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 :
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Hint 1 of 2
How do acetals and ketals behave in aqueous basic medium?
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Step-by-step solutionView
Correct answer
Acetals and ketals are stable in basic solutions because alkoxide ions are poor leaving groups, making Assertion A true and Reason R false.
Option analysis
Why each option works or fails
A · A is true but R is false
None. This is the correct evaluation. Acetals are stable to base because the alkoxide ion (RO−) is a strong base and an extremely poor leaving group under non-acidic conditions.
B · A is false but R is true
Believing that acetals hydrolyze under both acidic and basic conditions equally, and assuming alkoxide is a good leaving group. Remember that hydrolysis of acetals requires protonation of the ether oxygen to convert the poor leaving group (RO−) into a good leaving group (ROH); this cannot happen in base.
C · Both A and R are true but R is NOT the correct explanation of A
Assuming that alkoxides have a high leaving tendency because they participate in nucleophilic substitutions. Evaluate the basicity of the leaving group: alkoxide ions (RO−) are strong conjugate bases of weak acids (alcohols) and therefore have very poor leaving ability.
D · Both A and R are true and R is the correct explanation of A
Confusing 'stability due to absence of leaving ability' with 'high leaving tendency' providing stability. A high leaving tendency promotes reaction (breakdown), not stability; acetals resist basic hydrolysis precisely because alkoxides are poor leaving groups.
Reviewed route
Solution
StepWorking
01assertion_eval
Assertion A states: 'Acetal / Ketal is stable in basic medium.' Acetals and ketals have the general structure R2C(OR')2. This structure contains two ether-like alkoxy linkages on the same carbon. Ethers, acetals, and ketals do not undergo nucleophilic displacement in basic or neutral conditions. Cleavage cannot happen because no good leaving group is present without prior protonation. Therefore, acetals and ketals are inert and stable in basic media. This makes Assertion A true.
02reason_eval
Reason R states that the high leaving tendency of the alkoxide ion gives stability to acetals or ketals in a basic medium.
An alkoxide ion (RO−) is a strong base. Hence, it is a very poor leaving group with a low leaving tendency.
For an acetal to hydrolyze in a basic medium, a nucleophile like OH− must displace RO− directly. This displacement cannot occur because RO− is a poor leaving group. It is not a good leaving group.
Thus, Reason R is false.
✓link_test
Since Reason R is false, no causal link analysis is needed. The correct verdict is that A is true but R is false.
Hints that build this answer step by step
How do acetals and ketals behave in aqueous basic medium?
They remain stable and do not undergo hydrolysis.
What is the leaving group ability of an alkoxide ion (RO⁻) in basic medium?
It is a poor leaving group because it is a strong base.
Why do acetals hydrolyze in acid if ethers are generally stable?
In an acidic medium, an oxygen atom of the acetal is protonated to form an oxonium ion. This turns the alkoxy group into a neutral alcohol molecule (R'OH). A neutral alcohol is a good leaving group. Resonance assistance from the other oxygen lone pair further stabilizes the resulting carbocation.
Why is alkoxide a poor leaving group?
Alkoxide ions (RO⁻) are the conjugate bases of weak acids (alcohols, pKa ~ 16-18). Strong bases are poor leaving groups.