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JEE Advanced levelSingle correct optionDifficulty 3 of 3g1046

The pKa of 4-nitrophenol is about 7.2, whereas that of 3,5-dimethyl-4-nitrophenol is about 8.3. The best explanation is that in the dimethyl compound

  1. Athe methyl groups make the OH bond stronger by hyperconjugation only
  2. Bthe nitro group converts to a +M group
  3. Cthe methyl groups change the -I effect of the OH group
  4. Dthe two methyl groups ortho to the nitro group twist it out of the ring plane, inhibiting its -M effect

Answer: D

The nitro group in 4-nitrophenol is coplanar with the ring and withdraws the negative charge of the phenoxide by -M. In the 3,5-dimethyl compound the two methyls flank the nitro group, which twists out of plane (steric inhibition of resonance), so the -M effect is lost and the acidity falls.
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