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A cyclic compound (or ring compound) is a term for a compound in the field of
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 ...
in which one or more series of atoms in the compound is connected to form a
ring (The) Ring(s) may refer to: * Ring (jewellery), a round band, usually made of metal, worn as ornamental jewelry * To make a sound with a bell, and the sound made by a bell Arts, entertainment, and media Film and TV * ''The Ring'' (franchise), a ...
. 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 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 ...
s with rings containing both carbon and non-carbon). Depending on the ring size, the
bond order In chemistry, bond order is a formal measure of the multiplicity of a covalent bond between two atoms. As introduced by Gerhard Herzberg, building off of work by R. S. Mulliken and Friedrich Hund, bond order is defined as the difference between t ...
of the individual links between ring atoms, and their arrangements within the rings, carbocyclic and heterocyclic compounds may be
aromatic In organic chemistry, aromaticity is a chemical property describing the way in which a conjugated system, conjugated ring of unsaturated bonds, lone pairs, or empty orbitals exhibits a stabilization stronger than would be expected from conjugati ...
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. natural Nature is an inherent character or constitution, particularly of the ecosphere or the universe as a whole. In this general sense nature refers to the laws, elements and phenomena of the physical world, including life. Although humans are part ...
cyclic compounds" widths="250 px" align="center"> Image:First four cycloalkanes - en.svg ,
Cycloalkane In organic chemistry, the cycloalkanes (also called naphthenes, but distinct from naphthalene) are the ring (chemistry), monocyclic Saturated and unsaturated compounds, saturated hydrocarbons. In other words, a cycloalkane consists only of hydroge ...
s, the simplest carbocycles, including
cyclopropane Cyclopropane is the cycloalkane with the molecular formula (CH2)3, consisting of three methylene groups (CH2) linked to each other to form a triangular ring. The small size of the ring creates substantial ring strain in the structure. Cyclopropane ...
,
cyclobutane Cyclobutane is a cycloalkane and organic compound with the formula (CH2)4. Cyclobutane is a colourless gas and is commercially available as a liquefied gas. Derivatives of cyclobutane are called cyclobutanes. Cyclobutane itself is of no commerc ...
,
cyclopentane Cyclopentane (also called C pentane) is a highly flammable alicyclic compound, alicyclic hydrocarbon with chemical formula C5H10, C5H10 and CAS number 287-92-3, consisting of a ring of five carbon atoms each bonded with two hydrogen atoms above and ...
, and
cyclohexane Cyclohexane is a cycloalkane with the molecular formula . Cyclohexane is non-polar. Cyclohexane is a colourless, flammable liquid with a distinctive detergent-like odor, reminiscent of cleaning products (in which it is sometimes used). Cyclohexan ...
. Note, elsewhere an
organic chemistry Organic chemistry is a subdiscipline within chemistry involving the science, scientific study of the structure, properties, and reactions of organic compounds and organic matter, organic materials, i.e., matter in its various forms that contain ...
shorthand is used where hydrogen atoms are inferred as present to fill the carbon's valence of 4 (rather than their being shown explicitly). Image:Ingenol.svg , Ingenol, a complex,
terpenoid The terpenoids, also known as isoprenoids, are a class of naturally occurring organic compound, organic chemicals derived from the 5-carbon compound isoprene and its derivatives called terpenes, diterpenes, etc. While sometimes used interchangeabl ...
natural product A natural product is a natural compound or substance produced by a living organism—that is, found in nature. In the broadest sense, natural products include any substance produced by life. Natural products can also be prepared by chemical s ...
, related to but simpler than the
paclitaxel Paclitaxel, sold under the brand name Taxol among others, is a chemotherapy medication used to treat ovarian cancer, esophageal cancer, breast cancer, lung cancer, Kaposi's sarcoma, cervical cancer, and pancreatic cancer. It is administered b ...
that follows, which displays a complex ring structure including 3-, 5-, and 7-membered non-aromatic, carbocyclic rings. Image:Taxol.svg ,
Paclitaxel Paclitaxel, sold under the brand name Taxol among others, is a chemotherapy medication used to treat ovarian cancer, esophageal cancer, breast cancer, lung cancer, Kaposi's sarcoma, cervical cancer, and pancreatic cancer. It is administered b ...
, another complex, plant-derived
terpenoid The terpenoids, also known as isoprenoids, are a class of naturally occurring organic compound, organic chemicals derived from the 5-carbon compound isoprene and its derivatives called terpenes, diterpenes, etc. While sometimes used interchangeabl ...
, also a natural product, displaying a complex multi-ring structure including 4-, 6-, and 8-membered rings (carbocyclic and heterocyclic,
aromatic In organic chemistry, aromaticity is a chemical property describing the way in which a conjugated system, conjugated ring of unsaturated bonds, lone pairs, or empty orbitals exhibits a stabilization stronger than would be expected from conjugati ...
and non-aromatic). Adding to their complexity and number, closing of atoms into rings may lock particular atoms with distinct substitution (by
functional group In organic chemistry, a functional group is any substituent or moiety (chemistry), moiety in a molecule that causes the molecule's characteristic chemical reactions. The same functional group will undergo the same or similar chemical reactions r ...
s) such that
stereochemistry Stereochemistry, a subdiscipline of chemistry, studies the spatial arrangement of atoms that form the structure of molecules and their manipulation. The study of stereochemistry focuses on the relationships between stereoisomers, which are defined ...
and
chirality Chirality () is a property of asymmetry important in several branches of science. The word ''chirality'' is derived from the Greek (''kheir''), "hand", a familiar chiral object. An object or a system is ''chiral'' if it is distinguishable fro ...
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 In chemistry, rotamers are chemical species that differ from one another primarily due to rotations about one or more single bonds. Various arrangements of atoms in a molecule that differ by rotation about single bonds can also be referred to as ...
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
organism An organism is any life, living thing that functions as an individual. Such a definition raises more problems than it solves, not least because the concept of an individual is also difficult. Many criteria, few of them widely accepted, have be ...
s, 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 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 ...
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 In chemistry, bond order is a formal measure of the multiplicity of a covalent bond between two atoms. As introduced by Gerhard Herzberg, building off of work by R. S. Mulliken and Friedrich Hund, bond order is defined as the difference between t ...
of the individual links between ring atoms, and their arrangements within the rings, cyclic compounds may be
aromatic In organic chemistry, aromaticity is a chemical property describing the way in which a conjugated system, conjugated ring of unsaturated bonds, lone pairs, or empty orbitals exhibits a stabilization stronger than would be expected from conjugati ...
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 In organic chemistry, a functional group is any substituent or moiety (chemistry), moiety in a molecule that causes the molecule's characteristic chemical reactions. The same functional group will undergo the same or similar chemical reactions r ...
– substituted atoms into place, resulting in
stereochemistry Stereochemistry, a subdiscipline of chemistry, studies the spatial arrangement of atoms that form the structure of molecules and their manipulation. The study of stereochemistry focuses on the relationships between stereoisomers, which are defined ...
and
chirality Chirality () is a property of asymmetry important in several branches of science. The word ''chirality'' is derived from the Greek (''kheir''), "hand", a familiar chiral object. An object or a system is ''chiral'' if it is distinguishable fro ...
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 In chemistry, rotamers are chemical species that differ from one another primarily due to rotations about one or more single bonds. Various arrangements of atoms in a molecule that differ by rotation about single bonds can also be referred to as ...
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 Sulfur ( American spelling and the preferred IUPAC name) or sulphur ( Commonwealth spelling) is a chemical element; it has symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms ...
(e.g., cyclooctasulfur ), sulfur and
nitrogen Nitrogen is a chemical element; it has Symbol (chemistry), symbol N and atomic number 7. Nitrogen is a Nonmetal (chemistry), nonmetal and the lightest member of pnictogen, group 15 of the periodic table, often called the Pnictogen, pnictogens. ...
(e.g.,
trithiazyl trichloride Trithiazyl trichloride is the inorganic compound with the formula . A white solid, it is a precursor to other sulfur nitrides, but has no commercial applications. Structure The molecule is a 6-membered ring of alternating nitrogen and sulfur ato ...
),
silicon Silicon is a chemical element; it has symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic lustre, and is a tetravalent metalloid (sometimes considered a non-metal) and semiconductor. It is a membe ...
(e.g., cyclopentasilane ), silicon and
oxygen Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...
(e.g., hexamethylcyclotrisiloxane ),
phosphorus Phosphorus is a chemical element; it has Chemical symbol, symbol P and atomic number 15. All elemental forms of phosphorus are highly Reactivity (chemistry), reactive and are therefore never found in nature. They can nevertheless be prepared ar ...
and nitrogen (e.g.,
hexachlorophosphazene Hexachlorophosphazene is an inorganic compound with the chemical formula . The molecule has a cyclic, unsaturated backbone consisting of alternating phosphorus and nitrogen atoms, and can be viewed as a trimer of the hypothetical compound (p ...
), phosphorus and oxygen (e.g., sodium metaphosphate ),
boron Boron is a chemical element; it has symbol B and atomic number 5. In its crystalline form it is a brittle, dark, lustrous metalloid; in its amorphous form it is a brown powder. As the lightest element of the boron group it has three ...
and oxygen (e.g.,
sodium metaborate Sodium metaborate is a chemical compound of sodium, boron, and oxygen with formula . However, the metaborate ion is trimeric in the anhydrous solid, therefore a more correct formula is or . The formula can be written also as to highlight the re ...
), boron and nitrogen (e.g.,
borazine Borazine, also known as borazole, inorganic benzene, is an inorganic compound with the chemical formula B3H6N3. In this cyclic compound, the three BH units and three NH units alternate. The compound is isoelectronic and isostructural with benz ...
), nitrogen (e.g.,
pentazole Pentazole is an aromatic molecule consisting of a five-membered ring with all nitrogen atoms, one of which is bonded to a hydrogen atom. It has the molecular formula . Although strictly speaking a homocyclic, inorganic compound, pentazole has ...
). When carbon in benzene is "replaced" by other elements, e.g., as in borabenzene, silabenzene, germanabenzene, stannabenzene, and
phosphorine Phosphorine (IUPAC name: phosphinine) is a heavier Chemical element, element analog of pyridine, containing a phosphorus atom instead of an aza- moiety. It is also called phosphabenzene and belongs to the phosphaalkene class. It is a colorless li ...
, 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).
IUPAC Gold Book The International Union of Pure and Applied Chemistry (IUPAC) publishes many books which contain its complete list of definitions. The definitions are divided initially into seven IUPAC Colour Books: Gold, Green, Blue, Purple, Orange, White, and R ...
br>''heterocyclic compounds''
/ref> Cyclic compounds that have both carbon and non-carbon atoms present are
heterocyclic 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 ...
carbon compounds, and the name refers to inorganic cyclic compounds as well (e.g.,
siloxane In organosilicon chemistry, a siloxane is an organic compound containing a functional group of two silicon atoms bound to an oxygen atom: . The parent siloxanes include the oligomeric and polymeric hydrides with the formulae and . Siloxanes ...
s, which contain only
silicon Silicon is a chemical element; it has symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic lustre, and is a tetravalent metalloid (sometimes considered a non-metal) and semiconductor. It is a membe ...
and
oxygen Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...
in the rings, and
borazine Borazine, also known as borazole, inorganic benzene, is an inorganic compound with the chemical formula B3H6N3. In this cyclic compound, the three BH units and three NH units alternate. The compound is isoelectronic and isostructural with benz ...
s, which contain only
boron Boron is a chemical element; it has symbol B and atomic number 5. In its crystalline form it is a brittle, dark, lustrous metalloid; in its amorphous form it is a brown powder. As the lightest element of the boron group it has three ...
and
nitrogen Nitrogen is a chemical element; it has Symbol (chemistry), symbol N and atomic number 7. Nitrogen is a Nonmetal (chemistry), nonmetal and the lightest member of pnictogen, group 15 of the periodic table, often called the Pnictogen, pnictogens. ...
in the rings). Hantzsch–Widman nomenclature is recommended by the IUPAC for naming heterocycles, but many common names remain in regular use.


Macrocycles

The term
macrocycle Macrocycles are often described as molecules and ions containing a ring of twelve or more atoms. Classical examples include the crown ethers, calixarenes, porphyrins, and cyclodextrins. Macrocycles describe a large, mature area of chemistry. ...
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 Cyclooctane is a cycloalkane with the molecular formula (CH2)8. It is a simple colourless hydrocarbon, but it is often a reference compound for saturated eight-membered ring compounds in general. Cyclooctane has a camphoraceous odor. Conformatio ...
), heterocyclic containing both carbon and non-carbon atoms (e.g.
lactone Lactones are cyclic carboxylic esters. They are derived from the corresponding hydroxycarboxylic acids by esterification. They can be saturated or unsaturated. Lactones are formed by lactonization, the intramolecular esterification of the corresp ...
s and
lactam A lactam is a Cyclic compound, cyclic amide, formally derived from an amino alkanoic acid through cyclization reactions. The term is a portmanteau of the words ''lactone'' + ''amide''. Nomenclature Greek_alphabet#Letters, Greek prefixes in alpha ...
s containing rings of 8 or more atoms), or non-carbon (containing only non-carbon atoms in the rings, e.g.
borazine Borazine, also known as borazole, inorganic benzene, is an inorganic compound with the chemical formula B3H6N3. In this cyclic compound, the three BH units and three NH units alternate. The compound is isoelectronic and isostructural with benz ...
). 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 Cyclooctane is a cycloalkane with the molecular formula (CH2)8. It is a simple colourless hydrocarbon, but it is often a reference compound for saturated eight-membered ring compounds in general. Cyclooctane has a camphoraceous odor. Conformatio ...
, 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.


Nomenclature

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 ...
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 Macrocycles are often described as molecules and ions containing a ring of twelve or more atoms. Classical examples include the crown ethers, calixarenes, porphyrins, and cyclodextrins. Macrocycles describe a large, mature area of chemistry. ...
is used when a ring-containing compound has a ring of 12 or more atoms. The term polycyclic is used when more than one ring appears in a single molecule.
Naphthalene Naphthalene is an organic compound with formula . It is the simplest polycyclic aromatic hydrocarbon, and is a white Crystal, crystalline solid with a characteristic odor that is detectable at concentrations as low as 0.08 Parts-per notation ...
is formally a polycyclic compound, but is more specifically named as a bicyclic compound. Several examples of macrocyclic and polycyclic structures are given in the final gallery below. The atoms that are part of the ring structure are called annular atoms.


Isomerism


Stereochemistry

The closing of atoms into rings may lock particular atoms with distinct substitution by functional groups such that the result is
stereochemistry Stereochemistry, a subdiscipline of chemistry, studies the spatial arrangement of atoms that form the structure of molecules and their manipulation. The study of stereochemistry focuses on the relationships between stereoisomers, which are defined ...
and
chirality Chirality () is a property of asymmetry important in several branches of science. The word ''chirality'' is derived from the Greek (''kheir''), "hand", a familiar chiral object. An object or a system is ''chiral'' if it is distinguishable fro ...
of the compound, including some manifestations that are unique to rings (e.g., configurational isomers).


Conformational isomerism

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 In chemistry, rotamers are chemical species that differ from one another primarily due to rotations about one or more single bonds. Various arrangements of atoms in a molecule that differ by rotation about single bonds can also be referred to as ...
is displayed. Indeed, the development of this important chemical concept arose, historically, in reference to cyclic compounds. For instance,
cyclohexane Cyclohexane is a cycloalkane with the molecular formula . Cyclohexane is non-polar. Cyclohexane is a colourless, flammable liquid with a distinctive detergent-like odor, reminiscent of cleaning products (in which it is sometimes used). Cyclohexan ...
s—six membered carbocycles with no double bonds, to which various substituents might be attached, see image—display an equilibrium between two conformations, the ''chair'' and the ''boat,'' as shown in the image. The chair conformation is the favored configuration, because in this conformation, the steric strain, eclipsing strain, and angle strain that are otherwise possible are minimized. Which of the ''possible'' chair conformations predominate in cyclohexanes bearing one or more substituents depends on the substituents, and where they are located on the ring; generally, "bulky" substituents—those groups with large ''
volumes Volume is a measure of regions in three-dimensional space. It is often quantified numerically using SI derived units (such as the cubic metre and litre) or by various imperial or US customary units (such as the gallon, quart, cubic inch). The ...
,'' or groups that are otherwise repulsive in their interactions—prefer to occupy an equatorial location. An example of interactions within a molecule that would lead to steric strain, leading to a shift in equilibrium from boat to chair, is the interaction between the two
methyl group In organic chemistry, a methyl group is an alkyl derived from methane, containing one carbon atom bonded to three hydrogen atoms, having chemical formula (whereas normal methane has the formula ). In formulas, the group is often abbreviated a ...
s in ''cis''-1,4-dimethylcyclohexane. In this molecule, the two methyl groups are in opposing positions of the ring (1,4-), and their ''cis'' stereochemistry projects both of these groups toward the same side of the ring. Hence, if forced into the higher energy boat form, these methyl groups are in steric contact, repel one another, and drive the equilibrium toward the chair conformation.


Principal uses

Because of the unique shapes, reactivities, properties, and bioactivities that they engender, cyclic compounds are the largest majority of all molecules involved in the biochemistry, structure, and function of
living organism An organism is any life, living thing that functions as an individual. Such a definition raises more problems than it solves, not least because the concept of an individual is also difficult. Many criteria, few of them widely accepted, have be ...
s, and in the man-made molecules (e.g., drugs, herbicides, etc.) through which man attempts to exert control over nature and biological systems.


Synthetic reactions


Important general reactions for forming rings

There are a variety of specialized reactions whose use is solely the formation of rings, and these will be discussed below. In addition to those, there are a wide variety of ''general'' organic reactions that historically have been crucial in the development, first, of understanding the concepts of ring chemistry, and second, of reliable procedures for preparing ring structures in high yield, and with defined orientation of ring substituents (i.e., defined
stereochemistry Stereochemistry, a subdiscipline of chemistry, studies the spatial arrangement of atoms that form the structure of molecules and their manipulation. The study of stereochemistry focuses on the relationships between stereoisomers, which are defined ...
). These general reactions include: * Acyloin condensation; * Anodic oxidations; and * the
Dieckmann condensation The Dieckmann condensation is the Intramolecular reaction, intramolecular chemical reaction of ester, diesters with base to give β-keto esters. It is named after the German chemist Walter Dieckmann (1869–1925). The equivalent intermolecular rea ...
as applied to ring formation.


Ring-closing reactions

In organic chemistry, a variety of synthetic procedures are particularly useful in closing carbocyclic and other rings; these are termed ''ring-closing reactions''. Examples include: * alkyne trimerisation; * the
Bergman cyclization The Masamune-Bergman cyclization or Masamune-Bergman reaction or Masamune-Bergman cycloaromatization is an organic reaction and more specifically a rearrangement reaction taking place when an enediyne is heated in presence of a suitable hydrogen ...
of an enediyne; * the Diels–Alder, between a conjugated
diene In organic chemistry, a diene ( ); also diolefin, ) or alkadiene) is a covalent compound that contains two double bonds, usually among carbon atoms. They thus contain two alk''ene'' units, with the standard prefix ''di'' of systematic nome ...
and a substituted
alkene In organic chemistry, an alkene, or olefin, is a hydrocarbon containing a carbon–carbon double bond. The double bond may be internal or at the terminal position. Terminal alkenes are also known as Alpha-olefin, α-olefins. The Internationa ...
, and other
cycloaddition In organic chemistry, a cycloaddition is a chemical reaction in which "two or more Unsaturated hydrocarbon, unsaturated molecules (or parts of the same molecule) combine with the formation of a cyclic adduct in which there is a net reduction of th ...
reactions; * the Nazarov cyclization reaction, originally being the cyclization of a divinyl ketone; * various radical cyclizations; *
ring-closing metathesis Ring-closing metathesis (RCM) is a widely used variation of olefin metathesis in organic chemistry for the synthesis of various Saturated and unsaturated compounds, unsaturated rings via the intramolecular olefin metathesis, metathesis of two term ...
reactions, which also can be used to accomplish a specific type of
polymerization In polymer chemistry, polymerization (American English), or polymerisation (British English), is a process of reacting monomer molecules together in a chemical reaction to form polymer chains or three-dimensional networks. There are many fo ...
; * the Ruzicka large ring synthesis, in which two
carboxyl In organic chemistry, a carboxylic acid is an organic acid that contains a carboxyl group () attached to an R-group. The general formula of a carboxylic acid is often written as or , sometimes as with R referring to an organyl group (e.g. ...
groups combine to form a
carbonyl In organic chemistry, a carbonyl group is a functional group with the formula , composed of a carbon atom double bond, double-bonded to an oxygen atom, and it is divalent at the C atom. It is common to several classes of organic compounds (such a ...
group with loss of and ; * the
Wenker synthesis The Wenker synthesis is an organic reaction converting a beta amino alcohol to an aziridine with the help of sulfuric acid. It is used industrially for the synthesis of aziridine itself. The original Wenker synthesis of aziridine itself takes ...
converting a beta amino alcohol to an
aziridine Aziridine is an organic compound consisting of the three-membered heterocycle . It is a colorless, toxic, volatile liquid that is of significant practical interest. Aziridine was discovered in 1888 by the chemist Siegmund Gabriel. Its deriva ...


Ring-opening reactions

A variety of further synthetic procedures are particularly useful in opening carbocyclic and other rings, generally which contain a double bond or other
functional group In organic chemistry, a functional group is any substituent or moiety (chemistry), moiety in a molecule that causes the molecule's characteristic chemical reactions. The same functional group will undergo the same or similar chemical reactions r ...
"handle" to facilitate chemistry; these are termed ''ring-opening reactions''. Examples include: * ring opening metathesis, which can also be used to accomplish a specific type of
polymerization In polymer chemistry, polymerization (American English), or polymerisation (British English), is a process of reacting monomer molecules together in a chemical reaction to form polymer chains or three-dimensional networks. There are many fo ...
.


Ring expansion and ring contraction reactions

Ring expansion and contraction reactions are common in
organic synthesis Organic synthesis is a branch of chemical synthesis concerned with the construction of organic compounds. Organic compounds are molecules consisting of combinations of covalently-linked hydrogen, carbon, oxygen, and nitrogen atoms. Within the gen ...
, and are frequently encountered in pericyclic reactions. Ring expansions and contractions can involve the insertion of a functional group such as the case with
Baeyer–Villiger oxidation The Baeyer–Villiger oxidation is an organic reaction that forms an ester from a ketone or a lactone from a cyclic ketone, using peroxyacids or peroxides as the oxidant. The reaction is named after Adolf von Baeyer and Victor Villiger who first ...
of cyclic ketones, rearrangements of cyclic carbocycles as seen in intramolecular Diels-Alder reactions, or collapse or rearrangement of bicyclic compounds as several examples.


Examples


Simple, mono-cyclic examples

The following are examples of simple and aromatic carbocycles, inorganic cyclic compounds, and heterocycles: Image:Benzene-6H-delocalized.svg,
Benzene Benzene is an Organic compound, organic chemical compound with the Chemical formula#Molecular formula, molecular formula C6H6. The benzene molecule is composed of six carbon atoms joined in a planar hexagonal Ring (chemistry), ring with one hyd ...
, a 6-membered carbocyclic organic compound, methine hydrogens shown, and 6 electrons shown as
delocalized In chemistry, delocalized electrons are electrons in a molecule, ion or solid metal that are not associated with a single atom or a covalent bond.IUPAC Gold Boo''delocalization''/ref> The term delocalization is general and can have slightly dif ...
through drawing of circle (aromatic). Image:Cyclooctane crown conformation.svg,
Cyclooctane Cyclooctane is a cycloalkane with the molecular formula (CH2)8. It is a simple colourless hydrocarbon, but it is often a reference compound for saturated eight-membered ring compounds in general. Cyclooctane has a camphoraceous odor. Conformatio ...
, an 8-membered carbocyclic organic compound, methylene hydrogens implied, not shown (non-aromatic). Image:Cyclooctasulfur_structural_formula_3D.svg, Cyclooctasulfur, an 8-membered inorganic cyclic compound (non-aromatic). Image:ThiazylchlorideTrimer.svg,
Trithiazyl trichloride Trithiazyl trichloride is the inorganic compound with the formula . A white solid, it is a precursor to other sulfur nitrides, but has no commercial applications. Structure The molecule is a 6-membered ring of alternating nitrogen and sulfur ato ...
, a 6-membered inorganic heterocyclic compound (non-aromatic). Image:Pentasilolane.svg, Cyclopentasilane, a 5-membered inorganic cyclic compound (non-aromatic). Image:Hexamethylcyclotrisiloxan.svg, Hexamethylcyclotrisiloxane, a 6-membered organic heterocyclic compound (non-aromatic). Image:Hexachlorotriphosphazene-2D-dimensions.png,
Hexachlorophosphazene Hexachlorophosphazene is an inorganic compound with the chemical formula . The molecule has a cyclic, unsaturated backbone consisting of alternating phosphorus and nitrogen atoms, and can be viewed as a trimer of the hypothetical compound (p ...
, a 6-membered inorganic heterocyclic compound (aromatic). Image:Borazine-dimensions-2D.svg,
Borazine Borazine, also known as borazole, inorganic benzene, is an inorganic compound with the chemical formula B3H6N3. In this cyclic compound, the three BH units and three NH units alternate. The compound is isoelectronic and isostructural with benz ...
, a 6-membered inorganic heterocyclic compound (may be aromatic). Image:Pentazole.svg,
Pentazole Pentazole is an aromatic molecule consisting of a five-membered ring with all nitrogen atoms, one of which is bonded to a hydrogen atom. It has the molecular formula . Although strictly speaking a homocyclic, inorganic compound, pentazole has ...
, a 5-membered inorganic cyclic compound (aromatic). Image:Pyrrole structure.svg,
Pyrrole Pyrrole is a heterocyclic, aromatic, organic compound, a five-membered ring with the formula . It is a colorless volatile liquid that darkens readily upon exposure to air. Substituted derivatives are also called pyrroles, e.g., ''N''-methylpyrrol ...
, a 5-membered heterocyclic organic compound, methine hydrogen atoms implied, not shown (aromatic). Image:Pyridine.svg,
Pyridine Pyridine is a basic (chemistry), basic heterocyclic compound, heterocyclic organic compound with the chemical formula . It is structurally related to benzene, with one methine group replaced by a nitrogen atom . It is a highly flammable, weak ...
, a 6 membered heterocyclic organic compound, methine hydrogen atoms implied, not shown, and
delocalized In chemistry, delocalized electrons are electrons in a molecule, ion or solid metal that are not associated with a single atom or a covalent bond.IUPAC Gold Boo''delocalization''/ref> The term delocalization is general and can have slightly dif ...
π-electrons shown as discrete bonds (aromatic). Image:Azepine-2D-skeletal.png, Azepine, a 7-membered heterocyclic organic compound (non-aromatic).


Complex and polycyclic examples

The following are examples of cyclic compounds exhibiting more complex ring systems and stereochemical features: Image:Naphtalene topo.svg ,
Naphthalene Naphthalene is an organic compound with formula . It is the simplest polycyclic aromatic hydrocarbon, and is a white Crystal, crystalline solid with a characteristic odor that is detectable at concentrations as low as 0.08 Parts-per notation ...
, technically a polycyclic, more specifically a bicyclic compound, with circles showing
delocalization In chemistry, delocalized electrons are electrons in a molecule, ion or solid metal that are not associated with a single atom or a covalent bond.IUPAC Gold Boo''delocalization''/ref> The term delocalization is general and can have slightly diff ...
of π-electrons (aromatic). Image:Cis-trans isomerism of decahydronaphthalene.svg , Decalin (decahydronaphthalene), the fully saturated derivative of
naphthalene Naphthalene is an organic compound with formula . It is the simplest polycyclic aromatic hydrocarbon, and is a white Crystal, crystalline solid with a characteristic odor that is detectable at concentrations as low as 0.08 Parts-per notation ...
, showing the two stereochemistries possible for "fusing" the two rings together, and how this impacts the shapes available to this bicyclic compound (non-aromatic). Image:Longifolene plus acsv.svg,
Longifolene Longifolene is a common sesquiterpene. It is an oily liquid hydrocarbon found primarily in the high-boiling fraction of certain pine resins. The name is derived from that of a pine species from which the compound was isolated. It is a tricyclic c ...
, a polycyclic
terpene Terpenes () are a class of natural products consisting of compounds with the formula (C5H8)n for n ≥ 2. Terpenes are major biosynthetic building blocks. Comprising more than 30,000 compounds, these unsaturated hydrocarbons are produced predomi ...
natural product A natural product is a natural compound or substance produced by a living organism—that is, found in nature. In the broadest sense, natural products include any substance produced by life. Natural products can also be prepared by chemical s ...
, and an example of a tricyclic molecule (non-aromatic). Image:Ingenol.svg , Ingenol, a polycyclic
terpene Terpenes () are a class of natural products consisting of compounds with the formula (C5H8)n for n ≥ 2. Terpenes are major biosynthetic building blocks. Comprising more than 30,000 compounds, these unsaturated hydrocarbons are produced predomi ...
natural product A natural product is a natural compound or substance produced by a living organism—that is, found in nature. In the broadest sense, natural products include any substance produced by life. Natural products can also be prepared by chemical s ...
with a tetracyclic core: with a 3- and a 5-membered carbocyclic rings, fused to two further 7-membered carbocyclic rings (non-aromatic). Image:TaxolNumberingScheme.svg ,
Paclitaxel Paclitaxel, sold under the brand name Taxol among others, is a chemotherapy medication used to treat ovarian cancer, esophageal cancer, breast cancer, lung cancer, Kaposi's sarcoma, cervical cancer, and pancreatic cancer. It is administered b ...
, a polycyclic
natural product A natural product is a natural compound or substance produced by a living organism—that is, found in nature. In the broadest sense, natural products include any substance produced by life. Natural products can also be prepared by chemical s ...
with a tetracyclic core: with a heterocyclic, 4-membered D ring, fused to further 6- and 8-membered carbocyclic (A/C and B) rings (non-aromatic), and with three further pendant
phenyl In organic chemistry, the phenyl group, or phenyl ring, is a cyclic group of atoms with the formula , and is often represented by the symbol Ph (archaically φ) or Ø. The phenyl group is closely related to benzene and can be viewed as a benzene ...
-rings on its "tail", and attached to C-2 (abbrev. Ph, C6H5; aromatics). Image:Paclitaxel_JMolBiol_2001_1045.jpg , A representative three-dimensional shape adopted by
paclitaxel Paclitaxel, sold under the brand name Taxol among others, is a chemotherapy medication used to treat ovarian cancer, esophageal cancer, breast cancer, lung cancer, Kaposi's sarcoma, cervical cancer, and pancreatic cancer. It is administered b ...
, as a result of its unique cyclic structure. Image:Cholesterol.svg,
Cholesterol Cholesterol is the principal sterol of all higher animals, distributed in body Tissue (biology), tissues, especially the brain and spinal cord, and in Animal fat, animal fats and oils. Cholesterol is biosynthesis, biosynthesized by all anima ...
, another polycyclic terpene natural product, in particular, a
steroid A steroid is an organic compound with four fused compound, fused rings (designated A, B, C, and D) arranged in a specific molecular configuration. Steroids have two principal biological functions: as important components of cell membranes t ...
, a class of tetracyclic molecules (non-aromatic). Image:Benzo-a-pyrene.svg, Benzo yrene, a pentacyclic compound both natural and man-made, and
delocalized In chemistry, delocalized electrons are electrons in a molecule, ion or solid metal that are not associated with a single atom or a covalent bond.IUPAC Gold Boo''delocalization''/ref> The term delocalization is general and can have slightly dif ...
π-electrons shown as discrete bonds (aromatic). Image:Pagodane.svg, Pagodane, a complex, highly symmetric, man-made polycyclic compound (non-aromatic).


See also

* Effective molarity *
Lactone Lactones are cyclic carboxylic esters. They are derived from the corresponding hydroxycarboxylic acids by esterification. They can be saturated or unsaturated. Lactones are formed by lactonization, the intramolecular esterification of the corresp ...
*
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 ...


References


Further reading

* Jürgen-Hinrich Fuhrhop & Gustav Penzlin, 1986, "Organic synthesis: concepts, methods, starting materials," Weinheim, BW, DEU:VCH, , se

accessed 19 June 2015. * Michael B. Smith & Jerry March, 2007, "March's Advanced Organic Chemistry: Reactions, Mechanisms, and Structure," 6th Ed., New York, NY, USA:Wiley & Sons, , se

accessed 19 June 2015. * Francis A. Carey & Richard J. Sundberg, 2006, "Title Advanced Organic Chemistry: Part A: Structure and Mechanisms," 4th Edn., New York, NY, USA:Springer Science & Business Media, , se

accessed 19 June 2015. * Michael B. Smith, 2011, "Organic Chemistry: An Acid—Base Approach," Boca Raton, FL, USA:CRC Press, , se

accessed 19 June 2015. ay not be most necessary material for this article, but significant content here is available online.* Jonathan Clayden, Nick Greeves & Stuart Warren, 2012, "Organic Chemistry," Oxford, Oxon, GBR:Oxford University Press, , se

accessed 19 June 2015. * László Kürti & Barbara Czakó, 2005, "Strategic Applications of Named Reactions in Organic Synthesis: Background and Detailed Mechanisms, Amsterdam, NH, NLD:Elsevier Academic Press, 2005ISBN 0124297854, se

accessed 19 June 2015.


External links

* * {{Authority control Cyclic compounds, Molecular geometry