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Cyclic compounds, which contain atoms from at least two different elements in the ring, are called heterocyclic compounds or simply heterocycles. Most of the heterocycles contain one or more carbon atoms. On the basis of electronic structure, heterocycles could be classified as aromatic, saturated and unsaturated. When the heteroatom forms a part of an aromatic ring, the compound is referred to as an aromatic heterocyclic compound or an aromatic heterocycle. Aromatic heterocycles are classified into single ring heterocycles and condensed ring heterocycles. The single ring heterocycles include, but are not limited to, pyrrole, furan, thiophene, pyridine, etc., and condensed ring heterocylces include, but not limited to, indole, quinoline, and isoquionoline.
Heterocycles are also classified on the basis of ring size, as the compounds under each ring size have common features. Three- and four-membered rings are strained systems, and are readily opened due to their small size. Hence they are useful as very reactive intermediates in organic synthesis. On the other hand, five- and six-membered rings are very stable, and are easily formed. Rings with a larger size, specifically seven-membered and larger rings, are very stable and relatively less easily formed. Heterocyclic compounds play a major and vital role in several fields of research; and a major portion of these compounds are made up of heterocyclic compounds as backbone.