I was given the problem to arrange all cyclic isomers of $\ce{C6H12}$ from the lowest to the highest heat of combustion. I found twelve isomers and quickly realised that cyclohexane is the most stable and thus has the lowest heat of combustion, since it has zero strain, and that the only cyclopentane isomer - methylcyclopentane, follows after since it does not have much strain. However, what confuses me is the stability of the other isomers (four cyclobutanes as well as six cyclopropanes). I understand that the cyclobutanes are more stable than the cyclopropanes, but what about the substituents? How do they affect stability?

  • $\begingroup$ Not much. That said, multiple and/or branched substitutes would probably make a dense crowd around the ring and thus increase the strain. One long side chain would not do that. $\endgroup$ – Ivan Neretin Jul 29 '17 at 11:46
  • $\begingroup$ As a sidenote: I think your count is a bit off. I count at least 6 isomers of cyclic butane. $\endgroup$ – Martin - マーチン Jul 29 '17 at 15:39
  • $\begingroup$ ethyl-, 1,1-dimethyl, cis + trans 1,2-dimethyl-, cis + trans 1,3-dimethyl- $\endgroup$ – Martin - マーチン Jul 29 '17 at 18:40

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