The basicity of heterocyclic amines varies over a wide range and reflects both the hybridization of the nitrogen orbital containing the lone pair electrons and the effects of delocalization. Pyridine is a substantially weaker base than alkylamines. The electron pair of pyridine occupies an sp2-hybridized orbital, and lies closer to the nitrogen nucleus than the electron pair in the sp3-hybridized orbital of alkylamines. As a result, pyridine is a weaker base (larger pKb) than an alkylamine.
Pyrrole is an exceedingly weak base. The pair of electrons of the nitrogen atom interacts with the four electrons of the two carbon-carbon double bonds to give an aromatic six-π-electron system similar to that of benzene. Thus, the electron pair is not readily available for protonation because it is required to maintain the sextet of electrons required for aromaticity in the ring. Pyrrolidine has a pKb similar to acyclic amines.
Imidazole is an aromatic ring found in many biological molecules. One of its nitrogen atoms resembles that of pyrrole, and is not basic. The second nitrogen atom, which is structurally similar to the nitrogen atom of pyridine, acts as a base. However, imidazole is about 100 times more basic than pyridine. The increased basicity results from resonance stabilization of the positive charge of the conjugate acid.
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