Planarity of rings [closed]








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Can a **7** or less than **7** membered homocyclic ring with unsaturated C atoms and fully conjugated system, loose planarity ? the smallest homocyclic non planar ring that I know is **Cyclo-Octatetra-ene** which is a 8C compound.




Setting aside rings with saturated carbon atoms, it appears that the **cyclobutadiene-dication ring** can be puckered despite being formally aromatic according to the $4n+2$ rule. Puckering may be favored by dispersion of the charge which, in the planar geometry, is highly concentrated in the four-carbon ring. From the abstract of [Firme et al.[1]](https://doi.org/10.1021/jp075869j):



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> The cyclobutadiene dication was not experimentally characterized to the date. However, some of its derivatives were. Most of them have planar geometry, but tetramethylcyclobutadiene dication has a nonplanar geometry according to ab initio calculations. ... The planar cyclobutenyl dication derivatives have bond order of chemical bonds in the ring close to unity and relatively small electronic density in the ring. The puckered cyclobutadiene dication and its puckered derivatives have relatively high electronic density in the ring.
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**Reference**


1. Firme CL, Antunes OA, Esteves PM. Electronic nature of planar cyclobutenyl dication derivatives. J Phys Chem A. 2007 Nov 22;111(46):11904-7. doi: 10.1021/jp075869j. Epub 2007 Oct 19. PMID: 17948974.


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