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JEE Advanced levelSingle correct optionDifficulty 3 of 3g2211

Phenol has pKa 10.0 in water and 18.0 in DMSO; 4-nitrophenol has pKa 7.15 in water and 10.8 in DMSO. The 4-nitro group increases acidity by 2.85 pKa units in water but by 7.2 units in DMSO. The best explanation is that

  1. ADMSO withdraws electrons from the nitro group
  2. Bin water, hydrogen bonding stabilises the localised phenoxide strongly, so the extra delocalisation by the nitro group matters less; in DMSO this solvation is absent and the intrinsic effect shows fully
  3. C4-nitrophenol is non-aromatic in DMSO
  4. Dwater hydrogen-bonds only with the nitrophenoxide ion and not with the phenoxide ion

Answer: B

Water hydrogen-bonds to the charge-localised phenoxide oxygen and stabilises it, so it narrows the difference between phenoxide and the delocalised 4-nitrophenoxide. DMSO cannot hydrogen-bond to the anion, so the difference in intrinsic stability (delocalisation of charge into ) is exposed, and the gap widens from 2.85 to 7.2 units.
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