heterotsyklisen

Heterocyclic compounds, or heterocycles, are cyclic structures in which at least one atom in the ring is not carbon. Commonly, the heteroatoms are nitrogen, oxygen, or sulfur, but other elements can occur. Heterocycles are most often five- or six-membered rings, and many structures are fused to form larger systems. In Finnish, the adjective for these compounds is heterotsyklinen.

Many heterocycles are aromatic, with a delocalized pi electron system that follows Hückel’s rule. The identity and position of the heteroatoms influence properties such as basicity, acidity, and reactivity. For example, pyridine is a weak base, whereas pyrrole’s nitrogen lone pair participates in the aromatic sextet and is not basic. Non-aromatic heterocycles also exist and display diverse reactivity patterns.

Biology and medicine are rich with heterocycles. They are core motifs in nucleic acids (purine and pyrimidine bases), vitamins, and many enzymes. In medicinal chemistry, heterocyclic scaffolds are among the most common and versatile in drugs, affecting binding to targets and pharmacokinetic properties.

Examples and classes include five-membered rings such as pyrrole, furan, and thiophene; six-membered rings such as pyridine, pyridazine, pyrimidine, and pyrazine; and fused heterocycles like quinoline, isoquinoline, and indole. Other notable rings include imidazole and thiazole.

Synthesis and nomenclature of heterocycles rely on diverse methods, including cyclization, condensation, and annulation. Nomenclature uses systematic rules for heteroatom-containing rings; aromatic heterocycles are often called heteroarenes. The study of heterocycles remains central to organic chemistry and materials science.