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MCQEasyJEE 2023Electronic Effects (Inductive, Resonance, Hyperconjugation)

JEE Chemistry 2023 Question with Solution

Which will undergo deprotonation most readily in basic medium?

Three organic structures labeled a, b, and c are shown side by side for comparison of deprotonation tendency in basic medium.
  • A

    a only

  • B

    b only

  • C

    a and b

  • D

    b and c

Answer

Correct answer:B

Step-by-step solution

Standard Method

Given: Structures a, b, and c are to be compared for ease of deprotonation in basic medium.

Find: Which structure undergoes deprotonation most readily.

Deprotonation is most favored when the conjugate base formed is resonance-stabilized. In structure (a), deprotonation leads to the formation of a conjugate base stabilized by resonance between the keto group and the enolate ion. Structures (b) and (c) do not have such strong resonance stabilization.

Therefore, the compound that undergoes deprotonation most readily is a only. The solution marks the correct option as B, although this conflicts with the listed options where a only is option A. Based on the solution working, the defensible answer is the option corresponding to a only.

Resonance Stability Comparison

Given: Ease of deprotonation depends on stability of the conjugate base.

Find: The most acidic structure in basic medium.

The key idea is resonance stabilization of the anion formed after removal of a proton. A conjugate base that is more delocalized is more stable, and hence its parent compound is deprotonated more readily.

According to the extracted solution, structure (a) forms a conjugate base stabilized by resonance involving the carbonyl system and an enolate form, whereas (b) and (c) do not gain equally strong stabilization.

Hence, a only is the chemically supported conclusion.

Common mistakes

  • Assuming deprotonation depends only on the presence of a base is incorrect. The decisive factor is the stability of the conjugate base formed after deprotonation. Always compare resonance stabilization of the resulting anions.

  • Choosing a structure with more oxygen atoms automatically is incorrect. More heteroatoms do not guarantee easier deprotonation unless they actually help delocalize the negative charge effectively.

  • Ignoring enolate stabilization is a conceptual error. For carbonyl compounds, alpha-deprotonation becomes favorable when the negative charge can be resonance-delocalized onto oxygen.

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