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, though sometimes the smaller annulenes are referred to using the same notation, and benzene is sometimes referred to simply as annulene.
The first three even annulenes are cyclobutadiene, benzene, and cyclooctatetraene (annulene). Some annulenes, namely cyclobutadiene, cyclodecapentaene or annulene, cyclododecahexaene or annulene and cyclotetradecaheptaene (annulene), are unstable, with cyclobutadiene extremely so.
Annulenes may be aromatic (benzene), non-aromatic (annulene), or anti-aromatic (cyclobutadiene). Only cyclobutadiene and benzene are fully planar, though annulene (and 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  and annulene following Hückel's rule with 4n+2 π electrons. 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, 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 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.
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- 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
- Oth, Jean F. M.; Bünzli, Jean-Claude; De Julien De Zélicourt, Yves (1974-11-06). "The Stabilization Energy of  Annulene. A thermochemical determination". Helvetica Chimica Acta. 57 (7): 2276–2288. doi:10.1002/hlca.19740570745. ISSN 0018-019X.