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Recently, we have been learning Chemistry in my physics class at school. We learned that Carbon is one of the elements that has the most variety in the compounds it can formed, being able to form four covalent bonds. This got me thinking- I know you can form Carbon into hexagonal rings- graphite does this- but shouldn't you be able to form almost any kind of polygon from carbon, with each one double bonded to the two next to it?

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Carbon bonds have certain preferred angles. For example, the tetrahedrally symmetrical methane, which has sp3 hybridization,, $\ce{CH4}$, has an angle of 109.5 0. In benzene, where there is sp2 hybridization, the angle is 120 0.

These are the preferred angles for bonds. In some molecules, the geometry forces them to odd angles, such as cyclopropene, with a 60 0 bend. This imposes stress, much like making Tinker toy models where you have to bend the sticks a bit. Overdo this and the stick (bond) snaps.

So, here are some cyclic (ring) compounds with the following number of carbons, and at least one alkene bond:

3: cyclopropene

4: cyclobutene

5: cyclopentene

6: cyclohexene

7: cycloheptene

  1. cyclooctene

Notice in the drawings of the molecular structure that larger rings are increasingly asymmetrical, degenerating to a twisted mess. So yes, very large carbon "rings" are possible, but eventually cannot hold their shapes. Some compounds contain more than on C=C bond, such as 1,3-cyclohexadiene, but I know of no cyclic compounds where all carbons contain a double (alkene) bond.

You can extend this, though, to build up three-dimensional polyhedra, such as cubene or the elegant (though not double-bonded) buckminsterfullerene.

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  • $\begingroup$ The OP asked about cycloalkene rings (name?) not cycloalkane rings. $\endgroup$ – MaxW Mar 29 '16 at 5:10

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