pyridine

Pyridine is benzene with one of its CH units swapped for a nitrogen atom. The result is a flat six-membered aromatic ring, C5H5N, that looks and behaves much like benzene but with a built-in nitrogen that gives it a personality of its own. It is the simplest and most important nitrogen-containing aromatic ring, and a constant presence in biology and medicine.

Here is what the nitrogen does and does not do. Like each carbon in benzene, the nitrogen is sp2 hybridized and contributes one electron to a ring double bond, so the pi system still holds exactly six electrons (three double bonds) and pyridine is fully aromatic. The crucial point is the nitrogen's lone pair: it does not enter the pi cloud. Instead it sits in an sp2 orbital that lies in the plane of the ring, pointing outward like a finger, completely separate from the aromatic system above and below. Because this lone pair is free and available, pyridine acts as a base and a nucleophile through its nitrogen, accepting a proton or attacking an electrophile without disturbing the aromatic ring at all. Its conjugate acid, the pyridinium ion, has a pKa around 5, so pyridine is a modest base, weaker than an ordinary amine but a useful one.

Because the electronegative nitrogen pulls electron density toward itself, the pyridine ring is somewhat electron-poor compared with benzene, which makes it less reactive toward electrophiles and more reactive toward nucleophiles. Pyridine and its relatives appear in vitamin B3 (niacin), in the energy-shuttling coenzyme NAD, and as the core of countless drugs and agrochemicals. As a liquid it is also a common laboratory base and solvent. Pyridine is the textbook case of an aromatic ring with an outward-pointing, basic lone pair.