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JEE Main levelSingle correct optionDifficulty 2 of 3nc226

The compound shown has a very high basicity (the pKa of its conjugate acid is about 13). What is the best reason?

  1. AAll three nitrogens carry a lone pair that readily accepts a proton at the same time
  2. BThe C=N bond is non-polar, so the lone pair is localised
  3. CIts conjugate acid has no resonance, hence it is hard to deprotonate
  4. DIts conjugate acid is stabilised by resonance that spreads the positive charge equally over three nitrogens

Answer: D

The compound shown is guanidine. Protonation at the imine nitrogen gives the guanidinium ion, in which three equivalent resonance structures share the positive charge over three nitrogens. This large stabilisation of the cation makes guanidine a very strong organic base.
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