In
chemistry
Chemistry is the scientific study of the properties and behavior of matter. It is a natural science that covers the elements that make up matter to the compounds made of atoms, molecules and ions: their composition, structure, propertie ...
, a ring is an ambiguous term referring either to a simple
cycle of atoms and bonds in a molecule or to a connected set of atoms and bonds in which every atom and bond is a member of a cycle (also called a ring system). A ring system that is a simple cycle is called a monocycle or simple ring, and one that is not a simple cycle is called a polycycle or polycyclic ring system. A simple ring contains the same number of
sigma bond
In chemistry, sigma bonds (σ bonds) are the strongest type of covalent chemical bond. They are formed by head-on overlapping between atomic orbitals. Sigma bonding is most simply defined for diatomic molecules using the language and tools of ...
s as atoms, and a polycyclic ring system contains more sigma bonds than atoms.
A molecule containing one or more rings is called a
cyclic compound
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 a ...
, and a molecule containing two or more rings (either in the same or different ring systems) is termed a
polycyclic compound
In the field of organic chemistry, a polycyclic compound is an organic compound featuring several closed rings of atoms, primarily carbon. These ring substructures include cycloalkanes, aromatics, and other ring types. They come in sizes ...
. A molecule containing no rings is called an acyclic or
open-chain compound
In chemistry, an open-chain compound (also spelled as open chain compound) or acyclic compound (Greek prefix "α", ''without'' and "κύκλος", ''cycle'') is a compound with a linear structure, rather than a cyclic one.
An open-chain compound h ...
.
Homocyclic and heterocyclic rings
A homocycle or homocyclic ring is a ring in which all atoms are of the same
chemical element
A chemical element is a species of atoms that have a given number of protons in their nuclei, including the pure substance consisting only of that species. Unlike chemical compounds, chemical elements cannot be broken down into simpler subs ...
. A
heterocycle
A heterocyclic compound or ring structure is a cyclic compound that has atoms of at least two different elements as members of its ring(s). Heterocyclic chemistry is the branch of organic chemistry dealing with the synthesis, properties, a ...
or heterocyclic ring is a ring containing atoms of at least two different elements, i.e. a non-homocyclic ring. A
carbocycle or carbocyclic ring is a homocyclic ring in which all of the atoms are
carbon
Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon makes u ...
. An important class of carbocycles are
alicyclic rings, and an important subclass of these are
cycloalkanes.
Rings and ring systems
In common usage the terms "ring" and "ring system" are frequently interchanged, with the appropriate definition depending upon context. Typically a "ring" denotes a simple ring, unless otherwise qualified, as in terms like "
polycyclic ring", "
fused ring", "
spiro ring" and "
indole ring
Indole is an aromatic heterocyclic organic compound with the formula C8 H7 N. It has a bicyclic structure, consisting of a six-membered benzene ring fused to a five-membered pyrrole ring. Indole is widely distributed in the natural environmen ...
", where clearly a polycyclic ring system is intended. Likewise, a "ring system" typically denotes a polycyclic ring system, except in terms like "monocyclic ring system" or "
pyridine ring system". To reduce ambiguity,
IUPAC
The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering Organizations working for the advancement of the chemical sciences, especially by developing nomenclature and terminology. It is ...
's recommendations on organic nomenclature avoid the use of the term "ring" by using phrases such as "monocyclic parent" and "polycyclic ring system".
[{{BlueBook2013]
See also
*
Cyclic compound
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 a ...
*
Polycyclic compound
In the field of organic chemistry, a polycyclic compound is an organic compound featuring several closed rings of atoms, primarily carbon. These ring substructures include cycloalkanes, aromatics, and other ring types. They come in sizes ...
*
Heterocyclic compound
A heterocyclic compound or ring structure is a cyclic compound that has atoms of at least two different elements as members of its ring(s). Heterocyclic chemistry is the branch of organic chemistry dealing with the synthesis, properties, a ...
*
Bicyclic molecule
In chemistry, a bicyclic molecule () is a molecule that features two joined rings. Bicyclic structures occur widely, for example in many biologically important molecules like α-thujene and camphor. A bicyclic compound can be carbocyclic (all o ...
*
Spiro compound
In organic chemistry, spiro compounds are compounds that have at least two molecular rings with only one common atom. The simplest spiro compounds are bicyclic (having just two rings), or have a bicyclic portion as part of the larger ring sy ...
References
Molecular geometry