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JEE Advanced levelOne or more correct optionsDifficulty 3 of 3g2107

Which of the following statements about imidazole (pKaH 7.0) and pyridine (pKaH 5.2) are correct?

  1. AImidazole is a stronger base than pyridine
  2. BImidazole is protonated at N-3, the nitrogen without H
  3. CIn the imidazolium ion the two nitrogen atoms are equivalent by resonance
  4. DProtonation at N-1 (the N-H nitrogen) is the major pathway

Answer: A, B, C

N-3 has the free in-plane lone pair and is protonated, giving imidazolium in which the charge is delocalised over both N atoms in two equivalent resonance forms, a stabilisation absent in pyridinium. This gives pKaH 7.0 > 5.2. N-1's lone pair is in the aromatic sextet, so D is false.
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