Annulene
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), 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,[citation needed] 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.[citation needed] 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.
Gallery
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Cyclobutadiene ([4]annulene)
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Benzene ([6]annulene)
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Cyclooctatetraene ([8]annulene)
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Cyclotetradecaheptaene ([14]annulene)
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Cyclooctadecanonaene ([18]annulene)
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Cyclodocosahendecaene ([22]-annulene)
See also
References
- ^ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "annulene". doi:10.1351/goldbook.A00368
- ^ Ege, S. (1994) Organic Chemistry:Structure and Reactivity 3rd ed. D.C. Heath and Company
- ^ Dublin City University Annulenes Archived April 7, 2005, at the Wayback Machine
External links
- NIST Chemistry WebBook - [18]annulene
- Structure of [14] and [18]annulene