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 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, annulene, cyclooctadecanonaene or  annulene), non-aromatic (annulene), or anti-aromatic (cyclobutadiene, annulene). Only cyclobutadiene and benzene are fully planar, though  and annulene with all trans double bonds (placing 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 does exhibit some ring strain due to steric hindrance.
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.
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