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I am admittedly not a chemist. I was playing with a molecule set recently and I created a ring molecule where I double-bonded four carbon molecules to each other in a ring.

cyclobuta-1,2,3,4-tetraene

I was in my university's chemistry department, and a passing biochemist said that such a molecule was impossible, but they couldn't explain why.

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    $\begingroup$ There is a thing called "ring strain". $\endgroup$ Commented Mar 20, 2023 at 6:17
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    $\begingroup$ Rings containing 4 carbons do exist but not with all double bonds. Cyclobutane is commercially available and many other compounds contain a cyclobutane unit. But it has strained bonds compared to butane. And adding double bonds makes the strain worse. Cyclobutadiene (two double bonds) is unstable above temperatures of 37 K! though some derivatives are known. Trying to add another two double bonds will increase strain further pushing it into "impossible" territory. $\endgroup$
    – matt_black
    Commented Mar 20, 2023 at 9:46
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    $\begingroup$ Related: Hydrocarbons with only 4 carbon atoms $\endgroup$
    – andselisk
    Commented Mar 20, 2023 at 11:21
  • $\begingroup$ Recently asked for oxygen: chemistry.stackexchange.com/questions/172272/… $\endgroup$ Commented Mar 20, 2023 at 12:38
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    $\begingroup$ @Mithoron Because 0K < 37K? :) $\endgroup$ Commented Mar 20, 2023 at 21:32

4 Answers 4

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A four C ring does exist in many compounds, but it is not possible for double bonds.

Cumulated double bonds $\ce{C=C=C}$ must be linear, in 1 line. They can be bent a little, but it would cause the strain in the bonds, causing instability.

They definitely cannot be bent in right angles. The right angle geometry excludes possibility of overlap of orbitals to form such double bonds.

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  • $\begingroup$ Knowing ~nothing of chemistry, would a Carbon with four single bonds have the least strain with them ~120 degrees apart? So you could make a low-strain loop of single bonds with.... 6 Carbons? $\endgroup$ Commented Mar 21, 2023 at 23:58
  • $\begingroup$ Knowing ~nothing of chemistry, your answer makes me think that it can occur, but would simply be "unstable" and ~immediately disconnect? And therefore that maybe a 5-Carbon ring of single-bonds would be less unstable, and therefore maybe "possible"? $\endgroup$ Commented Mar 22, 2023 at 0:01
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    $\begingroup$ With single bonds, it would not be carbon only, but other atoms would be there, typically, but not limited to, hydrogen. Like (CH2)4. $\endgroup$
    – Poutnik
    Commented Mar 22, 2023 at 5:05
  • $\begingroup$ @MooingDuck 6 carbon loops with single bonds or with up to 3 double bonds are very common - for example in graphene and benzene. $\endgroup$
    – jpa
    Commented Mar 22, 2023 at 9:05
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    $\begingroup$ @MooingDuck, it's possible with four single bonds (plus hydrogens or other substituents). It's also possible with two single and two double (plus hydrogens, etc) -- the name of the base molecule of this form is cyclobutadiene. $\endgroup$ Commented Mar 22, 2023 at 14:29
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Ring C4 does exist, but it is usually called "rhombic C4". It is not square, having the shape of a rhombus. The length of each side is about 1.52 Angstroms and the shorter diagonal is 1.45 Angstroms.

There are two lone pairs, 5 sigma bonds, and one pair of delocalized electrons.

enter image description here

(drawing from Isomers and excitation energies of C4 J. Chem. Phys. 84, 3284 (1986) )

See the following references for more information:

Electronic structure calculations on the C4 cluster J. Chem. Phys. 124, 234304 (2006)

Conversion of Linear to Rhombic C4 in the Gas Phase:  A Joint Experimental and Theoretical Study J. Am. Chem. Soc. 2000, 122, 29, 7105–7113

Observation of laser excitation of rhombic C4 using the coulomb explosion method Zeitschrift für Physik D Atoms, Molecules and Clusters volume 26, pages 340–342 (1993)

Cn(n=2−4): current status Phil. Trans. R. Soc. A. volume 376 (2018)

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Ring strain and principles of aromaticity render such a molecule unstable.

Each double bond contributes 2 π electrons, and with four double bonds in one cycle in common, there is a total of 8. Compounds with (4n + 2) π electrons (where n is an integer number) are considered as Hueckel aromatic with an additional energetic stabilization e.g., in benzene compared to a hypothetical cyclo-1,3,6-cyclohexatriene. Compounds with 4n π electrons (e.g., 8) however are considered as anti aromatic – not only that they do not benefit the stabilization, they are extra unstable.

It does not say that there is not such a molecule (chemistry in stars / in mass spectroscopy allows for molecules and ions you can't store in bottles on the shelf), but – because of lack of overlap of atomic orbitals to establish bonds and thermodynamics (enthalpy of formation) – very unlikely.

Consider yourself lucky that the molecule set's bonds did not break constructing a cyclobuta-1,2,3,4-tetraene.

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    $\begingroup$ I'm not sure I can follow the aromaticity argument. Why would the electrons be confined to the same pi system, when there are plenty of other orbitals to use? I'm quite confident that apparent instability has more profound reasons. $\endgroup$ Commented Mar 20, 2023 at 8:07
  • $\begingroup$ Yeah, it if you go from cyclobutadiene, then two additional "bonds" are perpendicular to the original ones. At such angles they would be very tenuous (lack of overlap, indeed). Thing is while single cyclobutadiene molecule is perfectly stable by itself, just reactive, this most likely would ring open faster than it was made. $\endgroup$
    – Mithoron
    Commented Mar 20, 2023 at 16:13
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Cyclo-carbon with 4 atoms is potentially more likely to exist as cyclobutadiyne, which has two triple bonds instead of four double bonds.

It has been mentioned at least once before in the literature in an article with theoretical computations of hypothetical forms of carbon.

Lopanov, A. N., and E. A. Fanina. "Enthalpies of the transitional forms of carbon." Solid Fuel Chemistry 49.2 (2015): 99-108.

A cyclo-carbon with 18 atoms is known to exist and has been synthesized.

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  • $\begingroup$ The cyclo(18)carbon is an example where discrete single and triple bonds are conjugated, a pattern like $\ce{-C#C-C#C}$. (Cyclooctratetraene is an example that this pattern of saturated/unsaturated C-C bonds equally is known for alkenes, $\ce{-C=C-C=C}$. In the presented structure of cyclobutatetraene however, double bonds would be as consecutive / adjacent as in allenes or cumulenes. $\endgroup$
    – Buttonwood
    Commented Apr 2, 2023 at 11:16

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