Сақиналы молекулалар және олардың түрлері

Molecule with a ring of bonded atoms

A cyclic compound (or 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 with rings containing both carbon and non carbon). 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 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.

The term macrocycle is used for compounds having a rings of 8 or more atoms. Macrocycles may be fully carbocyclic (rings containing only carbon atoms, e. g. cyclooctane), heterocyclic containing both carbon and non carbon atoms (e. g. lactones and lactams containing rings of 8 or more atoms), or non carbon (containing only non carbon atoms in the rings, e. g. diselenium hexasulfide). 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). Macrocycles can access a number of stable conformations, with preference to reside in conformations that minimize transannular nonbonded interactions within the ring (e. g., with the chair and chair boat being more stable than the boat boat conformation for cyclooctane, because of the interactions depicted by the arcs shown). Medium rings (8 11 atoms) are the most strained, with between 9 13 (kcal/mol) strain energy, and 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.

IUPAC nomenclature has extensive rules to cover the naming of cyclic structures, both as core structures, and as substituents appended to alicyclic structures. The term macrocycle is used when a ring containing compound has a ring of 12 or more atoms.