Annulenes are monocyclic

hydrocarbon In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic, and their odors are usually weak or ...

s that contain the maximum number of non-cumulated or conjugated double bonds (' mancude'). They have the general formula C_nH_n (when ''n'' is an even number) or C_''n''H_''n''+1 (when ''n'' is an odd number). The IUPAC naming conventions are that annulenes with 7 or more carbon atoms are named as 'n''nnulene, where ''n'' is the number of

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 mak ...

atom Every atom is composed of a nucleus and one or more electrons bound to the nucleus. The nucleus is made of one or more protons and a number of neutrons. Only the most common variety of hydrogen has no neutrons. Every solid, liquid, gas, ...

s in their ring, though sometimes the smaller annulenes are referred to using the same notation, and benzene is sometimes referred to simply as annulene. The first three even annulenes are

cyclobutadiene Cyclobutadiene is an organic compound with the formula . It is very reactive owing to its tendency to dimerize. Although the parent compound has not been isolated, some substituted derivatives are robust and a single molecule of cyclobutadiene is ...

benzene Benzene is an organic chemical compound with the molecular formula C6H6. The benzene molecule is composed of six carbon atoms joined in a planar ring with one hydrogen atom attached to each. Because it contains only carbon and hydrogen atoms ...

, and

cyclooctatetraene 1,3,5,7-Cyclooctatetraene (COT) is an unsaturated derivative of cyclooctane, with the formula C8H8. It is also known as nnulene. This polyunsaturated hydrocarbon is a colorless to light yellow flammable liquid at room temperature. Because of ...

( nnulene). Some annulenes, namely cyclobutadiene, cyclodecapentaene ( 0nnulene), cyclododecahexaene ( 2nnulene) and cyclotetradecaheptaene ( 4nnulene), are unstable, with cyclobutadiene extremely so. In the related annulynes, one double bond is replaced by a

triple bond A triple bond in chemistry is a chemical bond between two atoms involving six bonding electrons instead of the usual two in a covalent single bond. Triple bonds are stronger than the equivalent single bonds or double bonds, with a bond order o ...

Aromaticity

Annulenes may be ''

aromatic In chemistry, aromaticity is a chemical property of cyclic ( ring-shaped), ''typically'' planar (flat) molecular structures with pi bonds in resonance (those containing delocalized electrons) that gives increased stability compared to satur ...

'' (benzene, nnulene and 8nnulene), ''non-aromatic'' ( and 0nnulene), or '' anti-aromatic'' (cyclobutadiene, nnulene). Cyclobutadiene is the only annulene with considerable antiaromaticity, since planarity is unavoidable. With nnulene, the molecule takes on a tub shape that allows it to avoid conjugation of double bonds. 0nnulene is of the wrong size to achieve a planar structure: in a planar conformation, ring strain due to either

steric hindrance Steric effects arise from the spatial arrangement of atoms. When atoms come close together there is a rise in the energy of the molecule. Steric effects are nonbonding interactions that influence the shape ( conformation) and reactivity of ions ...

of internal hydrogens (when some double bonds are ''trans'') or bond angle distortion (when the double bonds are all ''cis'') is unavoidable. Thus, it does not exhibit appreciable aromaticity. When the annulene is large enough, 8nnulene for example, there is enough room internally to accommodate hydrogen atoms without significant distortion of bond angles. 8nnulene possesses several properties that qualify it as aromatic. However, none of the larger annulenes are as stable as benzene, as their reactivity more closely resembles a conjugated polyene than an aromatic hydrocarbon. In general, charged annulene species of the form _{4''n''+2+''q''}H_{4''n''+2+''q''}sup>''q'' (''n'' = 0, 1, 2, ..., ''q'' = 0, ±1, ±2, 4''n'' + 2 + ''q ≥'' 3) are aromatic, provided a planar conformation can be achieved. For instance, C₅H₅^–, C₃H₃⁺, and C₈H₈^2– are all known aromatic species.

Gallery

File:Cyclobutadien.svg,

Cyclobutadiene Cyclobutadiene is an organic compound with the formula . It is very reactive owing to its tendency to dimerize. Although the parent compound has not been isolated, some substituted derivatives are robust and a single molecule of cyclobutadiene is ...

( nnulene) File:Benzol.svg,

Benzene Benzene is an organic chemical compound with the molecular formula C6H6. The benzene molecule is composed of six carbon atoms joined in a planar ring with one hydrogen atom attached to each. Because it contains only carbon and hydrogen atoms ...

( nnulene) File:Cyclooctatetraen.svg,

Cyclooctatetraene 1,3,5,7-Cyclooctatetraene (COT) is an unsaturated derivative of cyclooctane, with the formula C8H8. It is also known as nnulene. This polyunsaturated hydrocarbon is a colorless to light yellow flammable liquid at room temperature. Because of ...

( nnulene) File:Cyclododecahexaene.svg, Cyclododecahexaene ( 2nnulene) File:(14)Annulene.svg, Cyclotetradecaheptaene ( 4nnulene) File:(18)Annulene.svg,

Cyclooctadecanonaene Cyclooctadecanonaene or 8nnulene is an organic compound with chemical formula . It belongs to the class of highly conjugated compounds known as annulenes and is aromatic. The usual isomer that 8nnulene refers to is the most stable one, containi ...

( 8nnulene) File:Cyclodocosahendecaene.svg, Cyclodocosahendecaene ( 2annulene)

References

External links

NIST Chemistry WebBook
- 8nnulene
Structure
of 4and 8nnulene {{Authority control Physical organic chemistry

Aromaticity

Gallery

See also

References

External links