A monocyclic 6 carbon ring with 6 double bonds

Question

I would like to know if this molecule has ever been synthesized, if not why not, and what would its IUPAC name be?

Answer 1

There are several monocyclic $\ce{C6}$ isomers. Three of them are pictured below

• A - the compound you've drawn above, cyclohexahexaene
• B - cyclohexatriyne
• C - cyclohexa-2,5-diyne-bis-1,4-ylidene

While all of the carbons in these molecules are (more or less) $\ce{sp}$ hybridized, the molecules are different from one another due to differences in bond lengths and bond angles. For example, all of the bonds in A are exactly the same length due to symmetry, whereas the triple bonds in B will be shorter than the adjacent single bonds. Therefore, A-C are isomers, not resonance structures.

Ideally the atoms attached to an $\ce{sp}$ hybridized carbon form a linear arrangement (180° bond angle) with the central $\ce{sp}$ carbon, acetylene and the central carbon in allene serve as examples.

(image source)

(image source)

However in the cyclic $\ce{C6}$ compounds A-C, the $\ce{sp}$ hybridized carbons are forced by the ring to deviate from the desired 180° arrangement to something down around 120°. Therefore, quite a bit of angle strain is incorporated into A-C making them extremely strained and unavailable through laboratory preparation to date.

As mentioned above, A-C have different bond lengths and bond angles therefore they are isomers, not resonance structures. A fascinating question is,

• Do A-C each sit in a potential well, separated from one another by an energy barrier and if so, which one is the most stable (energy minimum)?

or

• Is there another monocyclic species lower in energy than A-C? Perhaps a molecule that could be described by resonance structures A'-C', where the prime denotes that all of the bond lengths and angles have been adjusted and made equal in A, B and C.

Answer 2

This is a type of cyclocarbon, specifically Benzotriyne. Processes using it or synthesizing it have been described in Towards the total synthesis of cyclo[n]carbons and the generation of cyclo[6]carbon, among others.

It has not yet been successfully synthesized. Wikipedia states (without citations) that

Recent investigations have concluded that benzotriyne is unlikely to exist due to the large angle strain.

Answer 3

Not really with six carbon atoms, but with eighteen. Cyclo[18]carbon, preferred IUPAC name Cyclooctadeca-1,3,5,7,9,11,13,15,17-nonayne, is predicted to be the minimal stable carbocycle. According to the WP article, it has been synthesized at low temperature by decomposition of an organic precursor[1] and "claimed" in the solid state[2].

References

1. 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. https://doi.org/10.1126/science.245.4922.1088. PMID 17838807.

2. Kaiser, Katharina; Scriven, Lorel M.; Schulz, Fabian; Gawel, Przemyslaw; Gross, Leo; Anderson, Harry L. (15 Aug 2019). "An sp-hybridized molecular carbon allotrope, cyclo[18]carbon". Science 365 (6459): 1299–1301. arXiv:1908.05904. https://doi.org/10.1126/science.aay1914.