Annulene

The discovery that [18]annulene possesses a number of key properties associated with other aromatic molecules was an important development in the understanding of aromaticity as a chemical concept.

In the related annulynes, one double bond is replaced by a triple bond.

Aromaticity

Annulenes may be aromatic ([6]annulene (benzene) and [18]annulene), non-aromatic ([8] and [10]annulene), or anti-aromatic (cyclobutadiene, [4]annulene). Cyclobutadiene is the only annulene with considerable antiaromaticity, since planarity is unavoidable. With [8]annulene, the molecule takes on a tub shape that allows it to avoid conjugation of double bonds. [10]Annulene is of the wrong size to achieve a planar structure: in a planar conformation, ring strain due to either steric hindrance of internal hydrogens (when some double bonds are trans) or bond angle distortion (when the double bonds are all cis) is unavoidable. Thus, it does not exhibit appreciable aromaticity.

When the annulene is large enough, [18]annulene for example, there is enough room internally to accommodate hydrogen atoms without significant distortion of bond angles. [18]Annulene possesses several properties that qualify it 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 [C_4n+2+qH_4n+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, C₅H−5, C₃H+3, and C₈H2−8 are all known aromatic species.