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JEE Advanced levelSingle correct optionDifficulty 3 of 3g2048

Assertion: Pentane-2,4-dione (pKa 9.0) is a stronger acid than ethyl 3-oxobutanoate (pKa 10.7). Reason: The ester carbonyl is a weaker electron acceptor than the ketone carbonyl because of the +M effect of the OR oxygen.

  1. AAssertion is false but Reason is true.
  2. BBoth Assertion and Reason are true and the Reason is the correct explanation of the Assertion.
  3. CAssertion is true but Reason is false.
  4. DBoth Assertion and Reason are true but the Reason is not the correct explanation of the Assertion.

Answer: B

Both statements are true. In ethyl 3-oxobutanoate the carbonyl carbon of the ester is partly stabilised by donation from the OC2H5 oxygen lone pair, so it withdraws electrons less effectively than a ketone carbonyl. The enolate of the ketoester is therefore less stabilised, which explains the higher pKa.
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