Heterocyclic compounds are cyclic compounds with at least two different elements as ring members atoms.  They are the counterparts of homocyclic compounds, which have only ring atoms from the same element.
Although heterocyclic compounds may be inorganic, most contain at least one carbon atom, and one or more atoms of elements other than carbon within the ring structure, such as sulfur, oxygen or nitrogen. Since in organic chemistry non-carbons usually are considered to replace carbon atoms, they are called heteroatoms (meaning 'different from carbon and hydrogen'). Nevertheless, a ring with only heteroatoms is homocyclic. The IUPAC recommends the Hantzsch-Widman nomenclature for naming heterocyclic compounds.
Heterocyclic chemistry is the branch of chemistry dealing with synthesis, properties, and applications of heterocycles.
Classification based on electronic structure
Heterocyclic compounds can be usefully classified based on their electronic structure. The saturated heterocycles behave like the acyclic derivatives. Thus, piperidine and tetrahydrofuran are conventional amines and ethers, with modified steric profiles. The study of heterocyclic chemistry therefore focuses especially on unsaturated derivatives, and the preponderance of work and applications involves unstrained 5- and 6-membered rings. Included are pyridine, thiophene, pyrrole, and furan. Another large class of heterocycles are fused to benzene rings, which for pyridine, thiophene, pyrrole, and furan are quinoline, benzothiophene, indole, and benzofuran, respectively. Fusion of two benzene rings gives rise to a third large family of compounds, respectively the acridine, dibenzothiophene, carbazole, and dibenzofuran. The unsaturated rings can be classified according to the participation of the heteroatom in the pi-system.
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