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NEET levelSingle correct optionDifficulty 1 of 3g2006

2,4,6-Trinitrophenol (picric acid, pKa 0.4) is far more acidic than phenol (pKa 10.0) mainly because

  1. Athe three nitro groups at the ortho and para positions stabilise the phenoxide ion by -M and -I effects
  2. Bthe nitro groups increase the electron density on the oxygen atom of the phenoxide ion
  3. Cthe molecule has three acidic hydroxyl groups
  4. Dthe nitro groups donate electrons to the ring by +M effect

Answer: A

Picric acid has one OH group only. The nitro groups at positions 2, 4 and 6 withdraw electron density through -M and -I effects, delocalising the negative charge of the picrate ion. This lowers the energy of the conjugate base and makes the acid much stronger.
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