pyrrolringen

Pyrrolringen, commonly referred to as the pyrrole ring, is a five-membered heterocycle containing four carbon atoms and one nitrogen atom. The ring is aromatic and planar, with two carbon–carbon double bonds and one nitrogen atom whose lone pair participates in the π-electron system. This gives the ring six π electrons in total, contributing to its stability and reactivity. The parent molecule has the formula C4H5N, and the nitrogen bears a hydrogen in the unsubstituted ring.

The pyrrole ring is electron-rich and relatively electron-dense, which makes it less basic than many amines but highly reactive toward electrophilic substitution. Substitution in substituted pyrroles typically occurs at the 2- or 3-positions relative to the nitrogen. The nitrogen hydrogen can be replaced to form N-substituted pyrroles, and the ring can undergo oxidation, reduction, and various condensation reactions. The ring can also participate in polymerization to form polypyrrole, a conducting polymer with applications in electronics and sensing.

Pyrrole rings are widespread in nature and chemistry. They are key components of many biologically important molecules, most notably the porphyrin macrocycles that coordinate metals in heme (in hemoglobin and myoglobin) and in chlorophyll derivatives. Four pyrrole units linked by methine bridges form the core of porphyrins, illustrating the versatility of the pyrrole motif in complex natural products. In addition to biology, pyrrole derivatives are used in dyes, pharmaceuticals, and materials science due to their electronic properties and ease of functionalization.

Common methods to prepare pyrroles include the Paal-Knorr synthesis, which forms pyrroles from 1,4-dicarbonyl compounds with primary amines, among other condensation strategies.