Cyclic compound

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 five 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 majority of all molecules involved in the biochemistry, structure, and function of living organisms, and in man-made molecules such as drugs, pesticides, etc.

Structure and classification

A cyclic compound or ring compound is a compound in which at least some its atoms are connected to form a ring. Rings vary in size from three to many tens or even hundreds of atoms. Examples of ring compounds readily include cases 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 with rings containing both carbon and non-carbon).

Common atoms can (as a result of their valences) form varying numbers of bonds, and many common atoms readily form rings. In addition, depending on the ring size, the bond order of the individual links between ring atoms, and their arrangements within the rings, cyclic compounds may be aromatic or non-aromatic; in the case of non-aromatic cyclic compounds, they may vary from being fully saturated to having varying numbers of multiple bonds. As a consequence of the constitutional variability that is thermodynamically possible in cyclic structures, the number of possible cyclic structures, even of small size (e.g., <17 atoms) numbers in the many billions.

Moreover, the closing of atoms into rings may lock particular functional group–substituted atoms into place, resulting in stereochemistry and chirality being associated with the compound, 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 five atoms and larger — can vary and interconvert such that conformational isomerism is displayed.

Carbocycles

The vast majority of cyclic compounds are organic, and of these, a significant and conceptually important portion are composed of rings made only of carbon atoms (i.e., they are carbocycles).

Inorganic cyclic compounds

Inorganic atoms form cyclic compounds as well. Examples include sulfur (e.g., cyclooctasulfur S₈), sulfur and nitrogen (e.g., trithiazyl trichloride (NSCl)₃), silicon (e.g., cyclopentasilane (SiH₂)₅), silicon and oxygen (e.g., hexamethylcyclotrisiloxane [(CH₃)₂SiO]₃), phosphorus and nitrogen (e.g., hexachlorophosphazene (NPCl₂)₃), phosphorus and oxygen (e.g., sodium metaphosphate Na₃(PO₂)₃), boron and oxygen (e.g., sodium metaborate Na₃(BO₂)₃), boron and nitrogen (e.g., borazine (BN)₃H₆), nitrogen (e.g., pentazole N₅H). When carbon in benzene is "replaced" by other elements, e.g., as in borabenzene, silabenzene, germanabenzene, stannabenzene, and phosphorine, aromaticity is retained, and so aromatic inorganic cyclic compounds are also known and well-characterized.

Heterocyclic compounds

A heterocyclic compound is a cyclic compound that has atoms of at least two different elements as members of its ring(s). Cyclic compounds that have both carbon and non-carbon atoms present are heterocyclic carbon compounds, and the name refers to inorganic cyclic compounds as well (e.g., siloxanes, which contain only silicon and oxygen in the rings, and borazines, which contain only boron and nitrogen in the rings). Hantzsch–Widman nomenclature is recommended by the IUPAC for naming heterocycles, but many common names remain in regular use.

Macrocycles

From the perspectives of conformational freedom and reactivity, rings in carbon compounds can viewed as containing:

• 3-4 atoms, with significant ring strain, so-called small rings;
• 5-7 atoms, so-called common rings;
• 8-11 atoms, so-called medium rings; and
• 12 or more atoms, so-called large rings, or macrocycles.

(In older literature, a breakpoint for the large rings is sometimes stated as 8 or more atoms, although this is the rarer perspective in the primary literature.)

Macrocycles may be fully carbocyclic (rings containing only carbon atoms, e.g. cyclododecane), heterocyclic or hybrid (rings containing both carbon and non-carbon atoms, e.g. lactones and lactams), or purely inorganic (containing only non-carbon atoms in the rings, e.g. {Pd₈₄}^Ac). Heterocycles with carbon in the rings may have limited non-carbon atoms in their rings (e.g., in lactones and lactams whose rings are rich in carbon but have limited number of non-carbon atoms), or be rich in non-carbon atoms and displaying significant symmetry (e.g., in the case of chelating macrocycles).

Medium rings (8-11 atoms) are more strained than macrocycles, with between 9-13 (kcal/mol) strain energy. For instance, the chair and chair-boat of cyclooctane are more stable than its boat-boat conformation, because of the interactions depicted.

Macrocycles can access a number of stable conformations, with preference to reside in conformations that minimize transannular nonbonded interactions within the ring. Analysis of factors important in the conformations of larger macrocycles can be modeled using medium ring conformations. Conformational analysis of odd-membered rings suggests they tend to reside in less symmetrical forms with smaller energy differences between stable conformations.