Cyclooctadeca-1,3,5,7,9,11,13,15,17-nonayne or cyclo[18]carbon is an allotrope of carbon with molecular formula . The molecule is a ring of eighteen carbon atoms, connected by alternating triple and single bonds; thus, it is a polyyne and a cyclocarbon.
Cyclo[18]carbon is the smallest cyclo[n]carbon predicted to be relatively stable, with a computed strain energy of 72 kilocalories per mole.
It is, however, difficult to synthesize. It was first observed transiently in 1987 from a high-temperature retro-Diels-Alder reaction, but was unstable under those conditions, and could not be isolated.
A collaboration claimed to synthesize it in solid state in 2019 by electrochemical decarbonylation:
The observed geometry corresponds to alternating triple bonds and single bonds, rather than a cumulene-type structure of consecutive double bonds.
Cyclo[18] carbon is predicted to be a mild oxidant and a semiconductor.