Organic hydrocarbons that have molecules containing simple single rings of carbon atoms linked by alternating single and double bonds. Such compounds have even numbers of carbon atoms. Cyclo-octatetraene, C8H8, is the next in the series following benzene. Higher annulenes are usually referred to by the number of carbon atoms in the ring, as in -annulene, C10H10, -annulene, C12H12, etc. The lower members are not stable as a result of the interactions between hydrogen atoms inside the ring. This is true even for molecules that have the necessary number of pi electrons to be aromatic compounds. Thus, -annulene has 4n+2 pi electrons with n=2, but is not aromatic because it is not planar. -annulene also has a suitable number of pi electrons to be aromatic (n=3) but is not planar because of interaction between the inner hydrogens.
The compound -annulene is large enough to be planar and obeys the Hückel rule (4n+2=18, with n=4). It is a brownish red fairly stable reactive solid. NMR evidence shows that it has aromatic character. The annulene with n=7, -annulene, can also exist in a planar form but is highly unstable. See also pseudoaromatic.