Are (cyclic) $\ce{C}$-chain molecules as in the picture likely to occur or do they exist at all?

C-chain and so on...


I'm considering only double bonds between $\ce{C}$ atoms. This rules out most common allotropes.

  • $\begingroup$ Diamond, graphite... are carbon only $\endgroup$ – user15489 Jun 12 '15 at 7:41
  • $\begingroup$ And buckeyballs and carbon nanotubes $\endgroup$ – Ben Norris Jun 12 '15 at 11:04
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    $\begingroup$ I don think those in particular will exist due to high angle strain. But larger ones nay. However I reckon they would be more stable as alternating triple and single bonds rather than all double bonds. $\endgroup$ – Ali Caglayan Jun 12 '15 at 11:07
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    $\begingroup$ Regarding $\ce{C2}$ with quadruple bonds, see this. $\endgroup$ – M.A.R. Jun 12 '15 at 11:37
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    $\begingroup$ Can't seem to find anything that exactly matches, but the closest might be "cyclic allenes" - this paper| may be relevant. $\endgroup$ – gilleain Jun 12 '15 at 14:20

The term you are looking for is "allotropes", and carbon has a whole bunch of them, but the three most readily spotted in nature are graphite, diamond and fullerene, however, none of them are simple chains such as those postulated in the answer.

Graphite exists in large honeycomb flakes held together on two axes by covalent bonds, and on the third by weaker Van der Waals forces, which is why they come off readily under mechanical shear, leading to their popularity in pencil lead.

Diamond are huge, multiply interlocked complexes of crystalline carbon, although often with impurities locked into their structure, perverting the crystal structure and yielding interesting colors as a byproduct.

Fullerene is a molecular allotrope, consisting (usually) of 80 carbon atoms arranged in a buckyball. Initially thought to be a lab-grown species, it has since been identified as a natural result of certain combustion processes.

Now, none of these are chains like what you describe; while some of the compounds you sketch out are likely possible and present, they are also likely to be highly reactive, perhaps even to the point of decomposing spontaneously.

While amorphous carbon chains may very well exist, without a crystalline structure to shield their electrons, their reactivity would likely be such that they would be unable to exist for very long at all.

Simplistically speaking, the long stable chains of carbon found in hydrocarbons aren't entirely characteristic of carbon, as such, but rather, characteristic of hydrogenated carbon.

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    $\begingroup$ Presumably a very large chain of linear acetylenic carbon or carbyne could close on itself and form a cyclic molecule made purely with carbon. I think this allotrope would be stable with respect to spontaneous decomposition in carefully controlled conditions, but would be extremely reactive $\endgroup$ – Nicolau Saker Neto Jun 12 '15 at 15:51
  • $\begingroup$ If you Google "carbyne ring" you get several relevant hits including this rather speculative paper: evgeny-belenkov.narod.ru/CR07_52_0343_en.pdf $\endgroup$ – Jan Jensen Jun 17 '15 at 8:42

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