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JEE Advanced levelSingle correct optionDifficulty 3 of 3o3203

1-Chloro-2,4-dinitrobenzene reacts with sodium methoxide in methanol (SNAr). Which of the following changes would be expected to DECREASE the rate?

  1. Ausing dimethyl sulfoxide instead of methanol as the solvent
  2. Breplacing chlorine by fluorine
  3. Creplacing the 4-nitro group by a 4-methyl group
  4. Dadding a third nitro group at C-6

Answer: C

Rate depends on stabilisation of the Meisenheimer complex. A para methyl group donates electrons and removes one acceptor, so the complex is less stable and the rate falls. A polar aprotic solvent leaves methoxide less solvated (faster), fluorine accelerates the rate-determining addition, and a second ortho nitro group adds more stabilisation.
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