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'''Annulenes''' are completely [[conjugated system|conjugated]] [[cyclic compound|monocyclic]] [[hydrocarbon]]s. They have the general formula C<sub>n</sub>H<sub>n</sub> (when '''''n''''' is an even number) or C<sub>n</sub>H<sub>n+1</sub> (when '''''n''''' is an odd number). The [[IUPAC nomenclature|IUPAC naming conventions]] are that annulenes with 7 or more carbon atoms are named as [''n'']annulene, where ''n'' is the number of [[carbon]] [[atom]]s in their ring,<ref>{{GoldBookRef |title = annulene |file = A00368}}</ref> though sometimes the smaller annulenes are referred to using the same notation, and benzene is sometimes referred to simply as annulene.<ref>Ege, S. (1994) ''Organic Chemistry:Structure and Reactivity'' 3rd ed. [[D.C. Heath and Company]]</ref><ref>Dublin City University ''[http://www.dcu.ie/~chemist/pratt/annulene/annulene.htm Annulenes]''</ref>
'''Annulenes''' are completely [[conjugated system|conjugated]] [[cyclic compound|monocyclic]] [[hydrocarbon]]s. They have the general formula C<sub>n</sub>H<sub>n</sub> (when '''''n''''' is an even number) or C<sub>n</sub>H<sub>n+1</sub> (when '''''n''''' is an odd number). The [[IUPAC nomenclature|IUPAC naming conventions]] are that annulenes with 7 or more carbon atoms are named as [''n'']annulene, where ''n'' is the number of [[carbon]] [[atom]]s in their ring,<ref>{{GoldBookRef |title = annulene |file = A00368}}</ref> though sometimes the smaller annulenes are referred to using the same notation, and benzene is sometimes referred to simply as annulene.<ref>Ege, S. (1994) ''Organic Chemistry:Structure and Reactivity'' 3rd ed. [[D.C. Heath and Company]]</ref><ref>Dublin City University ''[http://www.dcu.ie/~chemist/pratt/annulene/annulene.htm Annulenes] {{wayback|url=http://www.dcu.ie/~chemist/pratt/annulene/annulene.htm |date=20050407131525 }}''</ref>


The first three annulenes are [[cyclobutadiene]], [[benzene]], and [[cyclooctatetraene]] ([8]annulene). Some annulenes, namely cyclobutadiene, [[cyclodecapentaene]] or [10]annulene, [[cyclododecahexaene]] or [12]annulene and [[cyclotetradecaheptaene]] ([14]annulene), are unstable, with cyclobutadiene extremely so.
The first three annulenes are [[cyclobutadiene]], [[benzene]], and [[cyclooctatetraene]] ([8]annulene). Some annulenes, namely cyclobutadiene, [[cyclodecapentaene]] or [10]annulene, [[cyclododecahexaene]] or [12]annulene and [[cyclotetradecaheptaene]] ([14]annulene), are unstable, with cyclobutadiene extremely so.
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==External links==
==External links==
*[http://webbook.nist.gov/cgi/cbook.cgi?ID=C2040735&Units=SI NIST Chemistry WebBook] - [18]annulene
*[http://webbook.nist.gov/cgi/cbook.cgi?ID=C2040735&Units=SI NIST Chemistry WebBook] - [18]annulene
*[http://www.ccc.uga.edu/~cwannere/big.pdf Structure] of [14] and [18]annulene
*[https://web.archive.org/web/20050827223458/http://www.ccc.uga.edu:80/~cwannere/big.pdf Structure] of [14] and [18]annulene
{{Annulenes}}
{{Annulenes}}


Revision as of 01:15, 15 October 2016

Annulenes are completely conjugated monocyclic hydrocarbons. They have the general formula CnHn (when n is an even number) or CnHn+1 (when n is an odd number). The IUPAC naming conventions are that annulenes with 7 or more carbon atoms are named as [n]annulene, where n is the number of carbon atoms in their ring,[1] though sometimes the smaller annulenes are referred to using the same notation, and benzene is sometimes referred to simply as annulene.[2][3]

The first three annulenes are cyclobutadiene, benzene, and cyclooctatetraene ([8]annulene). Some annulenes, namely cyclobutadiene, cyclodecapentaene or [10]annulene, cyclododecahexaene or [12]annulene and cyclotetradecaheptaene ([14]annulene), are unstable, with cyclobutadiene extremely so.

Annulenes may be aromatic (benzene, cyclooctadecanonaene or [18] annulene), non-aromatic ([10]annulene), or anti-aromatic (cyclobutadiene, [12]annulene). Only cyclobutadiene and benzene are fully planar, though [14] and [18]annulene with all trans double bonds (placing the hydrogens inside the ring) can achieve the planar conformation needed for aromaticity, with [14] and [18]annulene following Hückel's rule with 4n+2 π electrons. [14]annulene does exhibit some ring strain due to steric hindrance.

Many of the larger annulenes, [18]annulene for example, are large enough to minimize the van der Waals strain of internal hydrogens and thermodynamically qualify 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 annulynes, one double bond is replaced by a triple bond.

See also

Wikimedia Commons has media related to Annulenes.

References

  1. ^ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "annulene". doi:10.1351/goldbook.A00368
  2. ^ Ege, S. (1994) Organic Chemistry:Structure and Reactivity 3rd ed. D.C. Heath and Company
  3. ^ Dublin City University Annulenes Template:Wayback
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