Cyclo(6)carbon
Cyclocarbon is an allotrope of carbon with molecular formula. This theoretical molecule is a ring of six carbon atoms, connected by alternating double bonds. It is, therefore, a member of the cyclocarbon|cyclocarbon family.
There have been a few attempts to synthesize cyclocarbon, e.g. by pyrolysis of mellitic anhydride, but without success until 2023, when it was successfully synthesised by atom manipulation of hexachlorobenzene, although more thorough research has yet to be reported in peer-reviewed scientific work. Another synthesis was reported in 2025 by the dehalogenation of hexaiodobenzene on a sodium chloride layer on an Au surface. The molecule's structure presents double bonds in sequence in a bent fashion, which is not explained by valence bond theory for organic molecules, that describes double bonds by combinations of hybridised sp2 orbitals, limiting the molecule's geometry in terms of increase of energy due to bond angles. Cyclocarbon would present then low probability of formation due to its high energy state from a thermodynamical perspective, which is reflected on the lack of evidence of its successful synthesis.
Calculations suggest that the cyclic cumulene structure, called cyclohexahexaene, is the potential energy minimum of the cyclocarbon framework, rather than a benzotriyne with alternating single and triple bonds.