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 even 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). Only cyclobutadiene and benzene are fully planar, though [18]annulene (and [14]annulene, to some extent) can achieve planarity with a combination of cis and trans double bonds (placing some of 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. [10]Annulene is too small to achieve a planar structure. In a planar conformation, ring strain due to either steric hindrance of internal hydrogens or bond angle distortion is unavoidable. Thus, it does not exhibit appreciable aromaticity.

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.[4] 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 [C4n+2+qH4n+2+q]q (n = 0, 1, 2, ..., q = 0, ±1, ±2, 4n + 2 + q ≥ 3) are aromatic, provided a planar conformation can be achieved. For instance, C5H5, C3H3+, and C8H82– are all known aromatic species.

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". 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 Archived April 7, 2005, at the Wayback Machine
  4. ^ Oth, Jean F. M.; Bünzli, Jean-Claude; De Julien De Zélicourt, Yves (1974-11-06). "The Stabilization Energy of [18] Annulene. A thermochemical determination". Helvetica Chimica Acta. 57 (7): 2276–2288. doi:10.1002/hlca.19740570745. ISSN 0018-019X.

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