Acyclic compound (orring compound) is a term for acompound in the field ofchemistry in which one or more series of atoms in the compound is connected to form aring. Rings may vary in size from three to many atoms, and include examples where all the atoms are carbon (i.e., arecarbocycles), 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, thebond order of the individual links between ring atoms, and their arrangements within the rings, carbocyclic and heterocyclic compounds may bearomatic or non-aromatic; in the latter case, they may vary from being fullysaturated to having varying numbers of multiple bonds between the ring atoms. Because of the tremendous diversity allowed, in combination, by thevalences 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 distinctsubstitution (byfunctional groups) such thatstereochemistry andchirality 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 thatconformational 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, andbioactivities that they engender, are the majority of all molecules involved in the biochemistry, structure, and function of livingorganisms, and in man-made molecules such as drugs, pesticides, etc.
A cyclic compound or ring compound is acompound in which at least some its atoms are connected to form a ring.[1] Rings vary in size from three to many tens or even hundreds of atoms. Examples of ring compounds readily include cases where:
Common atoms can (as a result of theirvalences) form varying numbers of bonds, and many common atoms readily form rings. In addition, depending on the ring size, thebond order of the individual links between ring atoms, and their arrangements within the rings, cyclic compounds may bearomatic or non-aromatic; in the case of non-aromatic cyclic compounds, they may vary from being fullysaturated to having varying numbers of multiple bonds. As a consequence of the constitutional variability that isthermodynamically possible in cyclic structures, the number of possible cyclic structures, even of small size (e.g., <17 atoms) numbers in the many billions.[3]
Moreover, the closing of atoms into rings may lock particularfunctional group–substituted atoms into place, resulting instereochemistry andchirality being associated with the compound, including some manifestations that are unique to rings (e.g.,configurational isomers);[4] 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 thatconformational isomerism is displayed.[4]
The vast majority of cyclic compounds areorganic, and of these, a significant and conceptually important portion are composed of rings made only of carbon atoms (i.e., they are carbocycles).[citation needed]
Inorganic atoms form cyclic compounds as well. Examples includesulfur andnitrogen (e.g.heptasulfur imideS7NH,trithiazyl trichloride(NSCl)3,tetrasulfur tetranitrideS4N4),silicon (e.g.,cyclopentasilane(SiH2)5),phosphorus and nitrogen (e.g.,hexachlorophosphazene(NPCl2)3), phosphorus andoxygen (e.g.,metaphosphates(PO−3)3 and other cyclicphosphoric acid derivatives),boron and oxygen (e.g.,sodium metaborateNa3(BO2)3,borax), boron and nitrogen (e.g.borazine(BN)3H6).[citation needed] When carbon in benzene is "replaced" by other elements, e.g., as inborabenzene,silabenzene,germanabenzene,stannabenzene, andphosphorine, aromaticity is retained, and soaromatic inorganic cyclic compounds are also known and well-characterized.[citation needed]
A heterocyclic compound is a cyclic compound that has atoms of at least two differentelements as members of its ring(s).[5] Cyclic compounds that have both carbon and non-carbon atoms present areheterocyclic carbon compounds, and the name refers to inorganic cyclic compounds as well (e.g.,siloxanes, which contain onlysilicon andoxygen in the rings, andborazines, which contain onlyboron andnitrogen in the rings).[5] Hantzsch–Widman nomenclature is recommended by the IUPAC for naming heterocycles, but many common names remain in regular use.[citation needed]
The termmacrocycle is used for compounds having a rings of 8 or more atoms.[6][7] Macrocycles may be fully carbocyclic (rings containing only carbon atoms, e.g.cyclooctane), heterocyclic containing both carbon and non-carbon atoms (e.g.lactones andlactams 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 stableconformations, with preference to reside in conformations that minimizetransannular nonbonded interactions within the ring (e.g., with the chair and chair-boat being more stable than the boat-boat conformation forcyclooctane, because of the interactions depicted by the arcs shown).[citation needed] 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.[8] Conformational analysis of odd-membered rings suggests they tend to reside in less symmetrical forms with smaller energy differences between stable conformations.[9]
IUPAC nomenclature has extensive rules to cover the naming of cyclic structures, both as core structures, and as substituents appended toalicyclic structures.[citation needed] The termmacrocycle is used when a ring-containing compound has a ring of 12 or more atoms.[6][7] The termpolycyclic is used when more than one ring appears in a single molecule.Naphthalene 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.[10]
The closing of atoms into rings may lock particular atoms with distinctsubstitution by functional groups such that the result isstereochemistry andchirality of the compound, including some manifestations that are unique to rings (e.g.,configurational isomers).[4]
Depending on ring size, the three-dimensional shapes of particular cyclic structures—typically rings of 5-atoms and larger—can vary and interconvert such thatconformational isomerism is displayed.[4] Indeed, the development of this important chemical concept arose, historically, in reference to cyclic compounds. For instance,cyclohexanes—six memberedcarbocycles with no double bonds, to which various substituents might be attached, see image—display an equilibrium between two conformations, thechair and theboat, as shown in the image.
The chair conformation is the favored configuration, because in this conformation, thesteric strain,eclipsing strain, andangle strain that are otherwise possible are minimized.[4] Which of thepossible 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 largevolumes, or groups that are otherwise repulsive in theirinteractions[citation needed]—prefer to occupy an equatorial location.[4] An example of interactions within a molecule that would lead tosteric strain, leading to a shift in equilibrium from boat to chair, is the interaction between the twomethyl groups incis-1,4-dimethylcyclohexane. In this molecule, the two methyl groups are in opposing positions of the ring (1,4-), and theircis 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.[4]
Because of the unique shapes, reactivities, properties, andbioactivities that they engender, cyclic compounds are the largest majority of all molecules involved in the biochemistry, structure, and function ofliving organisms, and in the man-made molecules (e.g., drugs, herbicides, etc.) through which man attempts to exert control over nature and biological systems.
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 ofgeneral 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 highyield, and with defined orientation of ring substituents (i.e., definedstereochemistry). These general reactions include:
In organic chemistry, a variety of synthetic procedures are particularly useful in closing carbocyclic and other rings; these are termedring-closing reactions. Examples include:
A variety of further synthetic procedures are particularly useful in opening carbocyclic and other rings, generally which contain a double bond or otherfunctional group "handle" to facilitate chemistry; these are termedring-opening reactions. Examples include:
Ring expansion and contraction reactions are common inorganic synthesis, and are frequently encountered inpericyclic reactions. Ring expansions and contractions can involve the insertion of a functional group such as the case withBaeyer–Villiger oxidation of cyclic ketones, rearrangements of cyclic carbocycles as seen inintramolecularDiels-Alder reactions, or collapse or rearrangement ofbicyclic compounds as several examples.
The following are examples of simple and aromatic carbocycles, inorganic cyclic compounds, and heterocycles:
The following are examples of cyclic compounds exhibiting more complex ring systems and stereochemical features:
![A representative three-dimensional shape adopted by paclitaxel, as a result of its unique cyclic structure.[11]](/mt/?noimg=&dark=on&url=https%3a%2f%2fwikipedia.org%2fwiki%2fFile%3aPaclitaxel_JMolBiol_2001_1045.jpg)
![Benzo[a]pyrene, a pentacyclic compound both natural and man-made, and delocalized π-electrons shown as discrete bonds (aromatic).](/mt/?noimg=&dark=on&url=https%3a%2f%2fwikipedia.org%2fwiki%2fFile%3aBenzo-a-pyrene.svg)
