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JEE Main levelSingle correct optionDifficulty 2 of 3g2016

Pentane-2,4-dione (pKa 9.0) is a much stronger acid than propanone (pKa 19.3) because

  1. Apentane-2,4-dione has two alpha carbons and hence twice as many acidic hydrogens, which accounts for the large pKa difference
  2. Bthe carbanion in propanone is stabilised by resonance with two carbonyl groups
  3. Cthe carbanion formed from pentane-2,4-dione is stabilised by resonance with two carbonyl groups
  4. Dthe extra carbonyl group makes the C-H bond non-polar

Answer: C

In pentane-2,4-dione the CH2 lies between two C=O groups, so its conjugate base delocalises the negative charge onto two oxygen atoms. Propanone has only one carbonyl to stabilise the enolate. Doubling the number of hydrogens would change Ka by a factor of only two, whereas the observed Ka ratio is about .
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