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JEE Main levelSingle correct optionDifficulty 2 of 3g1268

Compared with 4-nitrophenol (pKa about 7.2), the compound shown is a weaker acid (pKa about 8.3). The reason is

  1. Asteric inhibition of resonance: the flanking methyl groups twist the nitro group out of the ring plane
  2. Bthe methyl groups donate electrons to the oxygen directly by hyperconjugation only
  3. Cthe nitro group becomes a donor
  4. Dthe methyl groups increase the O-H bond polarity

Answer: A

In the compound shown the two methyl groups flank the nitro group and force it to twist out of the plane of the ring. This steric inhibition of resonance reduces the -M effect of the nitro group, so the phenoxide is less stabilised and the acid is weaker.
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