It proceeds apparently via a flat structure. There is not much energy difference between this planar conformation and the envelope conformation, although eclipsing C-H bonds destabilize the former. Again, a pretty small barrier via a flat structure. These restricted angles cause a high strain energy. The ring strain in turn creates increased reactivity and sometimes, unusual reactions.
Medium rings have a large variety of structures.
For example, the cyclodecyl cation, by iso- tope perturbation, has actually been shown to have a symmetrical bridged structure, rather than a rapidly equilibrating one. Highly stained rings - the syntheses of most of these has been accomplished by organic chemists. Fullerenes - the third form of elemental carbon.