Basicity of Heterocyclic Aromatic Amines

The basicity of heterocyclic amines is influenced by both the resonance and the hybridization effects.

To understand the resonance effect, consider two aromatic heterocyclic amines—pyridine and pyrrole. The nonbonded electrons of the pyridine nitrogen reside in an sp² hybrid orbital. As they do not contribute to the aromatic sextet, they are available for protonation.

In pyrrole, the nonbonding electrons are present in a p orbital and are part of the aromatic π system. Here, protonation is unfavorable as aromaticity is lost in the conjugated acid.

As the pK_a of the protonated pyrrole is less than pK_a of the protonated pyridine, pyridine is more basic than pyrrole.

For the hybridization effect, consider the hybridization of the amine nitrogen. As the percent s character increases, the nonbonded electrons are held more tightly and are less available for protonation.

With less s character of the sp³-hybridized piperidine nitrogen, it is easily protonated with a higher pK_a value of the conjugate acid. Thus, piperidine is more basic than pyridine.