In
chemistry
Chemistry is the scientific study of the properties and behavior of matter. It is a physical science within the natural sciences that studies the chemical elements that make up matter and chemical compound, compounds made of atoms, molecules a ...
, 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) or sigma overlap are the strongest type of covalent chemical bond. They are formed by head-on overlapping between atomic orbitals along the internuclear axis. Sigma bonding is most simply defined for diat ...
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 ...
, 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 ring (chemistry), rings of atoms, primarily carbon. These ring substructures include cycloalkanes, aromaticity, aromatics, and other ring ...
. A molecule containing no rings is called an acyclic or
open-chain compound
In chemistry, an open-chain compound (or open chain compound) or acyclic compound (Greek prefix ''α'' 'without' and ''κύκλος'' 'cycle') is a compound with a linear structure, rather than a Cyclic compound, cyclic one.
An open-chain compound ...
.
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 chemical substance whose atoms all have the same number of protons. The number of protons is called the atomic number of that element. For example, oxygen has an atomic number of 8: each oxygen atom has 8 protons in its ...
. 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 organic chemistry is the branch of organic chemistry dealing with the synthesis, proper ...
or heterocyclic ring is a ring containing atoms of at least two different elements, i.e. a non-homocyclic ring. A
carbocycle
In organic chemistry, an alicyclic compound contains one or more all-carbon rings which may be either saturated or unsaturated, but do not have aromatic character. Alicyclic compounds may have one or more aliphatic side chains attached.
Cycloa ...
or carbocyclic ring is a homocyclic ring in which all of the atoms are
carbon
Carbon () is a chemical element; it has chemical symbol, symbol C and atomic number 6. It is nonmetallic and tetravalence, tetravalent—meaning that its atoms are able to form up to four covalent bonds due to its valence shell exhibiting 4 ...
. An important class of carbocycles are
alicyclic rings, and an important subclass of these are
cycloalkanes
In organic chemistry, the cycloalkanes (also called naphthenes, but distinct from naphthalene) are the monocyclic saturated hydrocarbons. In other words, a cycloalkane consists only of hydrogen and carbon atoms arranged in a structure containing ...
.
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", 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
In chemical nomenclature, the IUPAC nomenclature of organic chemistry is a method of naming organic compound, organic chemical compounds as recommended by the International Union of Pure and Applied Chemistry (IUPAC). It is published in the ''No ...
avoid the use of the term "ring" by using phrases such as "monocyclic parent" and "polycyclic ring system".
[{{BlueBook2013]
See also
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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 ...
*
Polycyclic compound
In the field of organic chemistry, a polycyclic compound is an organic compound featuring several closed ring (chemistry), rings of atoms, primarily carbon. These ring substructures include cycloalkanes, aromaticity, aromatics, and other ring ...
*
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 organic chemistry is the branch of organic chemistry dealing with the synthesis, proper ...
*
Bicyclic molecule
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 of the ring ...
*
Spiro compound
In organic chemistry, spiro compounds are Organic compound, compounds that have at least two Cyclic compound, molecular rings sharing one common atom. Simple spiro compounds are bicyclic (having just two rings). The presence of only one common at ...
References
Molecular geometry