Pyridines

This chapter evaluates pyridine, a polar liquid which is miscible with both organic solvents and water. It can formally be derived from benzene by replacement of a CH group by a nitrogen atom. Pyridine is a highly aromatic heterocycle, but the effect of the heteroatom makes its chemistry quite distinct from that of benzene. The aromatic sextet of six π-electrons is complete without invoking participation of the lone pair on the nitrogen. Hence, the lone pair of pyridine is available for bonding without disturbing the aromaticity of the ring. The effect of the heteroatom is to make the pyridine ring very unreactive to normal electrophilic aromatic substitution. Conversely, pyridines are susceptible to nucleophilic attack. The chapter then considers the synthesis, electrophilic substitution, nucleophilic substitution, and anion chemistry of pyridines.