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JEE Main levelSingle correct optionDifficulty 2 of 3g2190

Carboxylic acids and phenols are far weaker acids in dimethyl sulfoxide (DMSO) than in water. The best explanation is that

  1. ADMSO solvates the carboxylate or phenoxide anion poorly because it cannot donate hydrogen bonds
  2. BDMSO is a stronger acid than water
  3. CDMSO has a lower dielectric constant than hexane
  4. DDMSO solvates anions more strongly than water does

Answer: A

DMSO is a polar aprotic solvent: it solvates cations well through its oxygen but has no O-H or N-H to hydrogen bond to anions. The conjugate base is therefore not stabilised as it is in water, and ionisation is less favourable. For example, acetic acid has pKa 4.76 in water but about 12.3 in DMSO.
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