pyrrole

Pyrrole is a five-membered aromatic ring containing one nitrogen, with the formula C4H5N. It looks like a small cousin of pyridine, yet it behaves almost oppositely, and the reason is a single, instructive choice the nitrogen makes about where to keep its lone pair. Pyrrole is the building block of some of biology's most important molecules.

Counting pyrrole's pi electrons reveals the twist. The ring has only two carbon-carbon double bonds, which give four pi electrons, two short of the aromatic six. To become aromatic, the nitrogen donates its entire lone pair into the pi system: it places that lone pair in a p orbital aligned with the ring, where it joins the loop and supplies the missing two electrons. Now the ring has six pi electrons (4n+2, n=1), is flat and fully conjugated, and is aromatic. But there is a price. With its lone pair committed to the aromatic cloud, the nitrogen has nothing left over to donate to a proton, so pyrrole is an extremely weak base, far weaker than pyridine or an ordinary amine. In fact pyrrole is faintly acidic at its N-H, because losing that proton leaves an aromatic anion.

Pyrrole rings are the bricks of life's pigments and carriers. Four pyrrole-derived rings linked in a larger ring form the porphyrin that grips iron in heme (carrying oxygen in your blood) and magnesium in chlorophyll (capturing sunlight in plants); the same motif appears in vitamin B12. Pyrrole is the cleanest example of a heteroatom donating its lone pair into the pi system to achieve aromaticity, and of the surprising consequence that this makes the nitrogen non-basic.