annulene

Annulenes are a class of hypothetical or real organic compounds that consist of a conjugated planar ring of alternating single and double bonds, having the formula CnHn. The term was coined by Georg Wittig. The simplest annulene is benzene, C6H6, which is aromatic. Other annulenes, such as cyclobutadiene (C4H4) and cyclopentadienyl anion (C5H5-), exhibit varying degrees of aromaticity or antiaromaticity, which can be predicted by Hückel's rule. Hückel's rule states that a planar, cyclic, conjugated system is aromatic if it has 4n+2 pi electrons, and antiaromatic if it has 4n pi electrons, where n is an integer.

Larger annulenes, such as [10]annulene (C10H10) and [18]annulene (C18H18), have been synthesized and studied. Many larger annulenes are non-planar to relieve strain, which affects their aromatic character. The study of annulenes is important in understanding aromaticity and the electronic properties of conjugated systems. They serve as models for exploring the interplay of ring size, planarity, and electron count in determining chemical stability and reactivity. Some substituted annulenes have found applications in materials science due to their unique electronic and optical properties.