Annulene

Annulenes are completely conjugated monocyclic hydrocarbons. They have the general formula CnHn (n is an even number) or CnHn+1 (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 ([8]annulene). Some annulenes, namely cyclobutadiene, cyclodecapentaene or [10]annulene, cyclododecahexaene or [12]annulene and [14]annulene, are unstable, with cyclobutadiene extremely so.

Annulenes may be aromatic (benzene, [14]annulene, cyclooctadecanonaene or [18] annulene), non-aromatic ([8]annulene), or anti-aromatic (cyclobutadiene). 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, with [14] and [18]annulene following Hückel's rule with 4n+2 pi electrons. [14]annulene does exhibit some ring strain due to steric hindrance. None of the larger annulenes are as stable as benzene.

In annulynes, one double bond is replaced by an alkyne bond.