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 and nitrogen (e.g. heptasulfur imide S₇NH, trithiazyl trichloride (NSCl)₃, tetrasulfur tetranitride S₄N₄), silicon (e.g., cyclopentasilane (SiH₂)₅), phosphorus and nitrogen (e.g., hexachlorophosphazene (NPCl₂)₃), phosphorus and oxygen (e.g., metaphosphates (PO−3)₃ and other cyclic phosphoric acid derivatives), boron and oxygen (e.g., sodium metaborate Na₃(BO₂)₃, borax), boron and nitrogen (e.g. borazine (BN)₃H₆). When carbon in benzene is "replaced" by other elements, e.g., as in borabenzene, silabenzene, germanabenzene, stannabenzene, and phosphorine, 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).^[5] Cyclic compounds that have both carbon and non-carbon atoms present are heterocyclic carbon compounds, and the name refers to inorganic cyclic compounds as well (e.g., siloxanes, which contain only silicon and oxygen in the rings, and borazines, which contain only boron and nitrogen in the rings).^[5] Hantzsch–Widman nomenclature is recommended by the IUPAC for naming heterocycles, but many common names remain in regular use.

Macrocycles

The term macrocycle 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 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.^[8] Conformational analysis of odd-membered rings suggests they tend to reside in less symmetrical forms with smaller energy differences between stable conformations.^[9]

Chelating macrocyclic structures of interest in inorganic and supramolecular chemistry, an example array. A, the crown ether, 18-crown-6; B, the simple tetra-aza chelator, cyclam; C, an example porphyrin, the unsubstituted porphine; D, a mixed amine/imine, the Curtis macrocycle; E, the related enamine/imine Jäger macrocycle, and F, the tetracarboxylate-derivative DOTA macrocycle.

Nomenclature

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.^[6][7] The term polycyclic 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]

Stereochemistry

The closing of atoms into rings may lock particular atoms with distinct substitution by functional groups such that the result is stereochemistry and chirality of the compound, including some manifestations that are unique to rings (e.g., configurational isomers).^[4]

Conformational isomerism

Chair and boat conformers in cyclohexanes. Two conformers of cyclohexane, the chair at left, and the boat at right (in German, respectively, Sessel and Wanne, the latter meaning "bath").

cis-1,4-Dimethylcyclohexane, in chair form, minimising steric interactions between the methyl groups in the directly opposing 1,4-positions of the cyclohexane ring.

General description. The structures are shown in line angle representation, though in the image at left, the lines projecting from the cyclohexane are not terminal methyl groups; rather, they indicate possible positions that might be occupied by substituents (functional groups) attached to the ring. In the image at left, those groups projecting upward and downward are termed axial substituents (a), and those groups projecting around the conceptual equator are termed equatorial substituents (e). Note, in general, the axial substituents are closer in space to one another (allowing for repulsive interactions); moreover, in the boat form, axial substituents in directly opposing positions (12 o'clock and 6 o'clock, termed "1,4-") are very close in space, and therefore give rise to even greater repulsion. These and other types of strain are used to explain the observation that the chair conformation of cyclohexanes is the favored conformation.^[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 that conformational isomerism is displayed.^[4] Indeed, the development of this important chemical concept arose, historically, in reference to cyclic compounds. For instance, cyclohexanes—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.^[4] 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, or groups that are otherwise repulsive in their interactions—prefer to occupy an equatorial location.^[4] 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 groups 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.^[4]

Important general reactions for forming rings

Dieckmann ring-closing reaction

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). These general reactions include:

• Acyloin condensation;
• Anodic oxidations; and
• the Dieckmann condensation as applied to ring formation.

Ring-closing reactions

In organic chemistry, a variety of synthetic procures are particularly useful in closing carbocyclic and other rings; these are termed ring-closing reactions. Examples include:

• alkyne trimerisation;
• the Bergman cyclization of an enediyne;
• the Diels–Alder, between a conjugated diene and a substituted alkene, and other cycloaddition reactions;
• the Nazarov cyclization reaction, originally being the cyclization of a divinyl ketone;
• various radical cyclizations;
• ring-closing metathesis reactions, which also can be used to accomplish a specific type of polymerization;
• the Ruzicka large ring synthesis, in which two carboxyl groups combine to form a carbonyl group with loss of CO₂ and H₂O;
• the Wenker synthesis converting a beta amino alcohol to an aziridine
• other reactions, such as an amino group reacting with a hydroxy group, as in the biosynthesis of solanine

Ring-opening reactions

A variety of further synthetic procedures are particularly useful in opening carbocyclic and other rings, generally which contain a double bound or other functional group "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.

Ring expansion and ring contraction reactions

Ring expansion and contraction reactions are common in organic synthesis, 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 of cyclic ketones, rearrangements of cyclic carbocycles as seen in intramolecular Diels-Alder reactions, or collapse or rearrangement of bicyclic compounds as several examples.

Simple, mono-cyclic examples

The following are examples of simple and aromatic carbocycles, inorganic cyclic compounds, and heterocycles:

• Simple mono-cyclic compounds: Carbocyclic, inorganic, and heterocyclic (aromatic and non-aromatic) examples.
• 
  Cycloheptane, a simple 7-membered carbocyclic compound, methylene hydrogens shown (non-aromatic).
• 
  Benzene, a 6-membered carbocyclic organic compound. methine hydrogens shown, and 6 electrons shown as delocalized through drawing of circle (aromatic).
• 
  Cyclo-octasulfur, an 8-membered inorganic cyclic compound (non-aromatic).
• 
  Diselenium hexasulfide, an 8-membered inorganic heterocyclic compound (non-aromatic).
• 
  Cyclopentasilane, a 5-membered inorganic cyclic compound (non-aromatic).
• 
  Hexamethylcyclotrisiloxane, a 6-membered organic heterocyclic compound (non-aromatic).
• 
  Hexachlorophosphazene, a 6-membered inorganic heterocyclic compound (aromatic).
• 
  Borazine, a 6-membered inorganic heterocyclic compound (may be aromatic).
• 
  Pentazole, a 5-membered inorganic cyclic compound (aromatic).
• 
  Azetidine, a 4-membered nitrogen (aza) heterocyclic compound, methylene hydrogen atoms implied, not shown (non-aromatic).
• 
  Caprolactam, a 7-membered heterocyclic organic compound (non-aromatic).
• 
  Pyridine, a 6 membered heterocyclic compound, methine hydrogen atoms implied, not shown, and delocalized π-electrons shown as discrete bonds (aromatic).

Complex and polycyclic examples

The following are examples of cyclic compounds exhibiting more complex ring systems and stereochemical features:

• Complex cyclic compounds: Macrocyclic and polycyclic examples
• 
  Naphthalene, technically a polycyclic, more specifically a bicyclic compound, with circles showing delocalization of π-electrons (aromatic).
• 
  Decalin (decahydronaphthalene), the fully saturated derivative of naphthalene, showing the two stereochemistries possible for "fusing" the two rings together, and how this impacts the shapes available to this bicyclic compound (non-aromatic).
• 
  Longifolene, a terpene natural product, and an example of a tricyclic molecule (non-aromatic).
• 
  Paclitaxel, a polycyclic natural product with a tricyclic 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-rings on its "tail", and attached to C-2 (abbrev. Ph, C₆H₅; aromatics).
• 
  A representative three-dimensional shape adopted by paclitaxel, as a result of its unique cyclic structure.^[11]
• 
  Cholesterol, another terpene natural product, in particular, a steroid, a class of tetracyclic molecules (non-aromatic).
• 
  Benzo[a]pyrene, a pentacyclic compound both natural and man-made, and delocalized π-electrons shown as discrete bonds (aromatic).
• 
  Pagodane, a complex, highly symmetric, man-made polycyclic compound (non-aromatic).
• 
  Brevetoxin A, a natural product with ten rings, all fused, and all heterocyclic, and a toxic component associated with the organisms responsible for red tides. The R group at right refers to one of several possible four-carbon side chains (see main Brevetoxin article; non-aromatic).