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JEE Main levelSingle correct optionDifficulty 2 of 3g2014

The pKa values of 4-nitrophenol and 3-nitrophenol in water are 7.15 and 8.35 respectively. The higher acidity of 4-nitrophenol is best explained by

  1. Athe -M effect of the nitro group, which acts from the para position but not from the meta position
  2. Bintramolecular hydrogen bonding between OH and NO2 in the para isomer
  3. Cthe -I effect acting only at the para position and not at the meta position
  4. Dthe +I effect of the nitro group being stronger at the para position

Answer: A

In the 4-nitrophenoxide ion the negative charge can be delocalised onto the nitro oxygens by resonance (-M). From the meta position no such resonance structure exists, so only the -I effect operates. Hence the para isomer is the stronger acid. The nitro group has no +I effect, and in the para isomer OH and NO2 are too far apart for hydrogen bonding.
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