Cyclocarbon

A cyclo[n]carbon is a chemical compound consisting solely of a number n of carbon atoms covalently linked in a ring. Since the compounds are composed only of carbon atoms, they are allotropes of carbon. Possible bonding patterns include all double bonds (a cyclic cumulene) or alternating single bonds and triple bonds (a cyclic polyyne).[1][2][3]

As of 2020, the only cyclocarbon that has been synthesized is cyclo[18]carbon.

Cyclo[6]carbon
Main article: Benzotriyne

The (hypothetical) six-carbon member of this family (C6) is also called benzotriyne.

Cyclo[18]carbon
Main article: Cyclo[18]carbon

The smallest cyclo[n]carbon predicted to be thermodynamically stable is C18, with a computed strain energy of 72 kilocalories per mole.[1][3] An IBM/Oxford team claimed to synthesize its molecules in solid state in 2019[4]:

Synthesis of cyclocarbon

According to these IBM researchers, the synthesized cyclocarbon has alternating triple and single bonds, rather than being made of entirely of double bonds. This supposedly makes this molecule a semiconductor.[5]

References
  1. George A. Adamson; Charles W. Rees (1996). "Towards the total synthesis of cyclo[n]carbons and the generation of cyclo[6]carbon". J. Chem. Soc., Perkin Trans. 1 (13): 1535–1543. doi:10.1039/P19960001535.
  2. Yves Rubin; Carolyn B. Knobler; Francois Diederich (1990). "Precursors to the cyclo[n]carbons: from 3,4-dialkynyl-3-cyclobutene-1,2-diones and 3,4-dialkynyl-3-cyclobutene-1,2-diols to cyclobutenodehydroannulenes and higher oxides of carbon". J. Am. Chem. Soc. 112 (4): 1607–1617. doi:10.1021/ja00160a047.
  3. François Diederich; Yves Rubin; Carolyn B. Knobler; Robert L. Whetten; Kenneth E. Schriver; Kendall N. Houk; Yi Li (8 September 1989). "All-Carbon Molecules: Evidence for the Generation of Cyclo[18]carbon from a Stable Organic Precursor". Science. 245 (4922): 1088–1090. Bibcode:1989Sci...245.1088D. doi:10.1126/science.245.4922.1088. PMID 17838807.
  4. Kaiser, Katharina (15 Aug 2019). "An sp-hybridized molecular carbon allotrope, cyclo[18]carbon". Science. 365 (6459): 1299–1301. arXiv:1908.05904. doi:10.1126/science.aay1914. PMID 31416933.
  5. Castelvecchi, Davide (15 August 2019). "Chemists make first-ever ring of pure carbon". Nature. 572 (7770): 426. doi:10.1038/d41586-019-02473-z.
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