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JEE Main levelSingle correct optionDifficulty 2 of 3g2071

In 1H-imidazole (the ring has N-1 bearing an H atom and N-3 without H), protonation by an acid occurs at

  1. AN-1, because it carries an H atom
  2. BN-3, whose lone pair lies in an sp orbital in the ring plane
  3. CN-1 and N-3 equally, giving two different cations
  4. Dneither nitrogen; protonation occurs only at carbon

Answer: B

The lone pair of N-1 is part of the aromatic sextet, whereas the lone pair of N-3 is in the plane and free. Protonation at N-3 gives the imidazolium ion (pKaH 7.0), in which both N atoms carry H and the charge is shared equally by resonance.
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