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JEE Advanced levelSingle correct optionDifficulty 3 of 3o1051

The pKa of acetic acid is 4.76 in water but 12.3 in DMSO. This large difference arises because

  1. ADMSO has a higher dielectric constant than water, so less ionisation occurs
  2. BDMSO is a much stronger base than water and removes the acid proton
  3. Cacetic acid forms dimers only in DMSO
  4. Dacetate is well stabilised by hydrogen bonding in water but is poorly solvated in aprotic DMSO, making it much more basic and nucleophilic there

Answer: D

Dissociation yields a carboxylate anion. Water hydrogen bonds to and stabilises it, favouring ionisation. DMSO cannot donate hydrogen bonds, so the anion is destabilised relative to the neutral acid and ionisation falls by about 7.5 pKa units, which also means acetate in DMSO is a stronger base and nucleophile. The dielectric constant of DMSO (about 47) is lower than that of water (about 78), so (C) is not valid.
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