Annulenes

Annulenes are monocyclic hydrocarbons in which a continuous loop of p orbitals allows for delocalized π electrons around the ring. The general nomenclature designates [n]annulene for a ring of n carbon atoms with full conjugation, so that the ring contains n π electrons if it is planar and uninterrupted. The simplest representative is [6]annulene, benzene, which is a planar, highly stable aromatic system.

Whether an annulene is aromatic, antiaromatic, or nonaromatic depends on π-electron count and geometry. According to Hückel's rule, planar, fully conjugated rings are aromatic when they possess 4n+2 π electrons and antiaromatic when they possess 4n π electrons. Consequently, [8]annulene (cyclooctatetraene) would be antiaromatic if planar, and it indeed distorts to a nonplanar tub shape to avoid antiaromaticity. Similarly, many larger annulenes are stabilized by adopting nonplanar or distorted conformations, though some structures have been prepared in twisted or constrained forms that exhibit partial aromatic character.

Over the years, several annulenes have been synthesized in stabilized forms. [4]annulene is typically antiaromatic and unstable in a planar form, while [10]annulene and related rings have been targets for experiments aimed at achieving planar aromaticity, with partial success in constrained environments. These studies illuminate how ring strain, geometry, and through-space interactions govern aromatic stabilization in large cyclic systems.

Annulenes are of interest as benchmarks for aromaticity concepts, as potential components in materials with delocalized electronics, and as models for nonbenzenoid aromatic systems.