A
cyclic compound (
ring compound) is a term for a
compound in the field of
chemistry in which one or more series of atoms in the compound is connected to form a
ring. Rings may vary in size from three to many atoms, and include examples where all the atoms are carbon (i.e., are
carbocycles), none of the atoms are carbon (inorganic cyclic compounds), or where both carbon and non-carbon atoms are present (
heterocyclic compounds). Depending on the ring size, the
bond order of the individual links between ring atoms, and their arrangements within the rings, carbocyclic and heterocyclic compounds may be
aromatic or non-aromatic, in the latter case, they may vary from being fully
saturated to having varying numbers of multiple bonds between the ring atoms. Because of the tremendous diversity allowed, in combination, by the
valences of common atoms and their ability to form rings, the number of possible cyclic structures, even of small size (e.g., <17 total atoms) numbers in the many billions. Adding to their complexity and number, closing of atoms into rings may lock particular atoms with distinct
substitution (by
functional groups) such that
stereochemistry and
chirality of the compound results, including some manifestations that are unique to rings (e.g.,
configurational isomers). As well, depending on ring size, the three-dimensional shapes of particular cyclic structures—typically rings of 5-atoms and larger—can vary and interconvert such that
conformational isomerism is displayed. Indeed, the development of this important chemical concept arose, historically, in reference to cyclic compounds. Finally, cyclic compounds, because of the unique shapes, reactivities, properties, and
bioactivities that they engender, are the largest majority of all molecules involved in the biochemistry, structure, and function of
living organisms, and in the man-made molecules (e.g., drugs, herbicides, etc.).