Cyclic Compound

any one of a class of compounds, mainly organic compounds, containing one or more rings (cycles) consisting of three or more atoms.

Five- and six-member rings occur most often because of the ease of their formation and the absence of ring strain. Depending on the nature of the atoms composing the ring, a distinction is made between carbocyclic, or isocyclic, compounds and heterocyclic compounds (seeCARBOCYCLIC COMPOUNDS and HETERO-CYCLIC COMPOUNDS). In carbocyclic compounds, the rings consist only of carbon atoms, as in for example, alicyclic compounds, such as cyclopropane (I) and cyclopentane (II), and aromatic compounds, such as benzene (III) (seeALICYCLIC COMPOUNDS and AROMATIC COMPOUNDS). In heterocyclic compounds, the rings contain, in addition to carbon atoms, one or more atoms of other elements, called heteroatoms, mainly, oxygen, nitrogen, and sulfur; examples of such compounds are ethylene oxide (IV), pyridine (V), and tetrazole (VI). In polycyclic compounds, the rings may be separated from each other, as in stilbene (VII), may be attached by a single bond, as in diphenyl (VIII), may have a single common atom [(IX), seeSPIRAN], or may have two common atoms (fused-ring compounds), as in naphthalene (X) and deca-hydronaphthalene (XI). Catenanes (XII) are a special case of cyclic compounds; their molecules are constructed in the manner of a simple interlinked chain.

Examples of inorganic cyclic compounds are borazole (a boron compound), phosphonitrile chlorides (phosphorus compounds), and cyclosiloxanes (silicon compounds). (See alsoORGANICCHEMISTRY.)