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JEE Advanced levelOne or more correct optionsDifficulty 3 of 3g2055

Which of the following pairs have the first acid stronger than the second?

  1. A and
  2. B and
  3. C2,4-dinitrophenol and 4-nitrophenol
  4. D and

Answer: A, C, D

Pair 1: the -I effect of Cl stabilises the carboxylate (2.86 against 4.76). Pair 2: phenol (10.0) is weaker than benzoic acid (4.20), so it is not stronger. Pair 3: the second nitro group at the ortho position adds -M and -I effects (4.1 against 7.15). Pair 4: sp carbon stabilises the carbanion better than sp2 (25 against 44). Hence pairs 1, 3 and 4.
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