heterocycle

A heterocycle is a cyclic compound in which one or more carbon atoms in the ring are replaced by heteroatoms, typically nitrogen, oxygen, or sulfur. The presence of heteroatoms alters the electronic structure, reactivity, and physical properties of the ring, and heterocycles are among the most common motifs in chemistry, biology, and materials science.

Most heterocycles are aromatic, including five-membered rings such as pyrrole, furan, and thiophene; and the related heterocycles imidazole, oxazole, and thiazole. Six-membered rings such as pyridine, pyridazine, pyrimidine, and pyrazine are also common. Many heterocycles occur in fused or condensed forms, for example indole, quinoline, isoquinoline, and purine. Nonaromatic heterocycles exist as well, such as certain azacycles and oxazepanes. In aromatic heterocycles, the heteroatoms contribute lone pairs that participate in the aromatic π-system and can influence basicity and hydrogen-bonding properties.

Heterocycles are pervasive in nature and technology. They are core components of nucleic acids (purine and pyrimidine bases), many vitamins, alkaloids, and enzymes. They underlie countless pharmaceuticals, agrochemicals, dyes, and ligands. The chemistry of heterocycles includes diverse reactions such as electrophilic and nucleophilic substitutions, annulation to form fused systems, and metal complexation.

Understanding heterocycles involves considerations of ring size, aromaticity, and the position of heteroatoms, which together determine reactivity and biological activity. Because of their versatility, heterocycles remain a central focus in synthetic chemistry and drug discovery.