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JEE Main levelSingle correct optionDifficulty 2 of 3g1042

4-Nitrophenol is a stronger acid than 3-nitrophenol because

  1. Athe nitro group is farther from the OH in the 4-isomer so its -I effect is larger
  2. Bin the 4-isomer the nitro group exerts both -I and -M effects on the phenoxide, but in the 3-isomer only the -I effect operates
  3. Cin the 3-isomer the nitro group exerts a +M effect
  4. Dthe 3-isomer has no -I effect from the nitro group

Answer: B

A -M group delocalises the negative charge of the phenoxide only from the ortho and para positions. A nitro group at C-4 has -M plus -I, and at C-3 it cannot conjugate and operates only through -I, so 4-nitrophenol (pKa 7.2) is more acidic than 3-nitrophenol (8.4).
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