JEE Main · 2020 · 05 Sep 2020 · Shift 1 · Compounds Containing Nitrogen

Among these nitrogen bases the pyrrole-type is weakest, because its nitrogen lone pair is locked into the aromatic sextet and is not free to bind a proton. The pyridine-type nitrogen, holding its lone pair in an $s{p}^2$ orbital, is somewhat more basic but still less so than a saturated amine. The saturated cyclic amine, whose $s{p}^3$ lone pair is fully available, is more basic again, and the imidazole-type base is strongest because protonating it gives a cation stabilised by two equivalent resonance forms. Basicity therefore increases from the pyrrole to the pyridine to the saturated amine to the imidazole.