Annulene

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Structure and AFM image of dehydrobenzo[12]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.

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References[edit]

  1. ^ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version:  (2006–) "annulene".
  2. ^ Ege, S. (1994) Organic Chemistry:Structure and Reactivity 3rd ed. D.C. Heath and Company
  3. ^ Dublin City University Annulenes Archived April 7, 2005, at the Wayback Machine.

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