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JEE Advanced levelSingle correct optionDifficulty 3 of 3nc225

The compound shown has two types of ring atoms that could be protonated, one of which is an N-H. Which statement correctly identifies the site of protonation and the reason it is a stronger base than pyridine?

  1. AThe nitrogen bearing hydrogen; its lone pair is more electron rich than the other nitrogen
  2. BThe nitrogen without hydrogen; the conjugate acid is stabilised by delocalisation of charge over both nitrogen atoms
  3. CThe nitrogen without hydrogen; the conjugate acid is stabilised because it has no resonance
  4. DNeither nitrogen, since protonation always destroys the aromatic sextet

Answer: B

The compound shown is imidazole. The N-H nitrogen lone pair is part of the aromatic sextet, whereas the other nitrogen has its lone pair in the ring plane and is protonated. The imidazolium ion has the charge shared equally by two nitrogens (amidinium type), so imidazole (conjugate acid pKa about 7) is more basic than pyridine (about 5).
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