JEE Advanced 2025 Paper 1 Q04 Chemistry Organic Chemistry Acidity of Hydrocarbons Hard

JEE Advanced 2025 Paper 1 · Q04 · Acidity of Hydrocarbons

Consider the depicted hydrogen ($\mathbf{H}$) in the hydrocarbons given in the figure above. The most acidic hydrogen ($\mathbf{H}$) is

  1. A. See option A in figure
  2. B. See option B in figure
  3. C. See option C in figure
  4. D. See option D in figure
Reveal answer + step-by-step solution

Correct answer:B

Solution

The acidity of a C–H bond is judged by the stability of the conjugate-base carbanion that results from its deprotonation. Aromatic stabilisation of a 6π cyclopentadienide (Cp$^-$) ring is the dominant effect here.

(A) The marked H is on an isopropyl group attached to a fulvene-type ring. Removing it gives a localised, non-aromatic carbanion outside the ring — not stabilised by Cp$^-$ formation. (B) The marked H is allylic/doubly-allylic and crucially its loss generates an aromatic cyclopentadienide ($6\pi$, Cp$^-$) anion via tautomerisation/conjugation through the exocyclic double bond. This is the strongest stabilisation among the four — the most acidic. (C) The marked H is a vinylic / sp$^2$ ring H — carbanion is not aromatic and is high in energy. (D) Saturated cyclopentane ring; deprotonation gives no aromatic stabilisation.

Hence the most acidic H is in (B) because its conjugate base is an aromatic cyclopentadienide anion. Answer: (B).

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