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 Hydrophobe, hydrophobic; their odor is usually fain ...

s that contain the maximum number of non-cumulated or conjugated double bonds (' mancude'). They have the general formula C_''n''H_''n'' (when ''n'' is an even number) or C_''n''H_''n''+1 (when ''n'' is an odd number). The

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 ...

accepts the use of 'annulene nomenclature' in naming carbocyclic ring systems with 7 or more carbon atoms, using the name ' 'n''nnulene' for the mancude hydrocarbon with ''n''

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 ...

atom Atoms are the basic particles of the chemical elements. An atom consists of a atomic nucleus, nucleus of protons and generally neutrons, surrounded by an electromagnetically bound swarm of electrons. The chemical elements are distinguished fr ...

s in its ring, though in certain contexts (e.g., discussions of aromaticity for different ring sizes), smaller rings (''n'' = 3 to 6) can also be informally referred to as annulenes. Using this form of nomenclature 1,3,5,7-cyclooctatetraene is nnulene and benzene is nnulene (and occasionally referred to as just 'annulene'). The discovery that 8nnulene possesses a number of key properties associated with other aromatic molecules was an important development in the understanding of

aromaticity In organic chemistry, aromaticity is a chemical property describing the way in which a conjugated ring of unsaturated bonds, lone pairs, or empty orbitals exhibits a stabilization stronger than would be expected from conjugation alone. The e ...

as a chemical concept. 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 Electron pair bond, bonding electrons instead of the usual two in a covalent bond, covalent single bond. Triple bonds are stronger than the equivalent covalent bond, sin ...

Aromaticity

Annulenes 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 ...

'' (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 In organic chemistry, ring strain is a type of instability that exists when bonds in a molecule form angles that are abnormal. Strain is most commonly discussed for small rings such as cyclopropanes and cyclobutanes, whose internal angles ar ...

due to either

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

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 (; ; ) are aromatic, provided a planar conformation can be achieved. For instance, , , and are all known aromatic species.

Gallery

File:Cyclobutadien.svg, Cyclobutadiene ( nnulene) File:Benzol.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 ...

( nnulene) File:Cyclooctatetraen.svg, Cyclooctatetraene ( nnulene) File:All-cis-cyclodecapentaene.svg, Cyclodecapentaene ( 0nnulene) File:Cyclododecahexaene.svg, Cyclododecahexaene ( 2nnulene) File:(14)Annulene.svg, Cyclotetradecaheptaene ( 4nnulene) File:(18)Annulene.svg, Cyclooctadecanonaene ( 8nnulene) File:Cyclodocosahendecaene.svg, Cyclodocosahendecaene ( 2nnulene)

References

External links

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

Aromaticity

Gallery

See also

References

External links