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 [10]-annulene, C₁₀H₁₀, [12]-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, [10]-annulene has (4n + 2) pi electrons with n = 2, but is not aromatic because it is not planar. [14]-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 [18]-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, [30]-annulene, can also exist in a planar form but is highly unstable. See also pseudoaromatic.

Annulenes