Annulene
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Annulenes are completely conjugated monocyclic hydrocarbons. They are named as [n]annulene, where n is the number of carbon atoms in their ring. The first three annulenes are cyclobutadiene, benzene, and cyclooctatetraene (respectively [4], [6], and [8]annulene). Some of these, namely [4], [10], and [14]annulene, are unstable, with [4]annulene extremely so. Only [4] and [6]annulene are planar. Annulenes may be aromatic ([6]annulene), non-aromatic ([8]annulene), or anti-aromatic ([4]annulene).