Fundamentals of Organic ChemistryChemistryGeneral Organic Chemistry
Imidazole has a p K_ a of approximately 7 with respect to its conjugate acid. Which nitrogen atom is protonated to form this conjugate acid, and what is the primary reason for its basicity?
Options
- AN1 is protonated because its lone pair is part of the aromatic system.
- BN1 is protonated because imidazole is an aromatic heterocycle where n=1 as per Hückel's rule.
- CN3 is protonated because its lone pair resides in an sp ² hybridized orbital and is not involved in aromaticit
- DN1 is protonated because the nitrogen atom is sp ³ hybridized.
Correct answer
C. N3 is protonated because its lone pair resides in an sp ² hybridized orbital and is not involved in aromaticit
Step-by-step solution
In imidazole, there are two nitrogen atoms: N1 and N3. The nitrogen atom N1 (pyrrole-like) is bonded to a hydrogen atom. Its lone pair of electrons resides in an unhybridized p-orbital and is delocalized into the ring to complete the aromatic sextet ( 6 electrons), satisfying Hückel's rule ( 4n+2 ). Because this lone pair is involved in aromaticity, it is not readily available for protonation. Protonating N1 would disrupt the aromatic stability of the molecule. The nitrogen atom N3 (pyridine-like) is part of a doub