annulene

Annulenes are a class of completely conjugated cyclic hydrocarbons, meaning they have a ring structure with alternating single and double bonds throughout. The term "annulene" comes from the Latin word "annulus," meaning ring.1

These compounds are often discussed in relation to the Hückel rule, which helps predict whether such cyclic systems will exhibit aromaticity, a special stability. According to this rule, annulenes with 4n + 2 pi electrons (where n is a whole number) are considered aromatic. For example, benzene (C₆H₆), with 6 pi electrons, is aromatic. However, compounds like cyclobutadiene (4 pi electrons) and cyclooctatetraene (8 pi electrons) do not fit this rule and are nonaromatic.2

Annulenes are differentiated by a numerical prefix indicating the number of pi electrons. While some larger annulenes like-annulene are predicted to be aromatic and show some aromatic characteristics, others can be highly reactive due to ring strain or internal hydrogen repulsions.32