Carbocyclic Rings of Other Sizes

The most important structural features that influence the conformation and reactivity of cycloalkanes differ depending on whether small (cyclopropane and cyclobutane), common (cyclopentane, cyclohexane, and cycloheptane), medium (cyclooctane through cycloundecane), or large (cyclododecane and up) rings are 

The cyclopropane ring is planar and the question of conformation does not arise. The C-C bond lengths are slightly shorter than normal, at 1.50 A, and the H—C—H angle of 115°C is opened somewhat from the tetrahedral angle. These structural features and the relatively high reactivity of cyclopropane rings are explained by the concept of bent bonds, in which the electron density is displaced from the internuclear axis (see Topic 1.3). 

Cyclobutane adopts a puckered conformation in which substituents can occupy axial-like or equatorial-like positions. 1,3-Disubstituted cyclobutanes show small energy preferences for the cis isomer, which places both substituents in equatorial-like positions. The energy differences and the barrier to inversion are both smaller than in cyclohexane. 

There is minimal angle strain in cyclopentane, but considerable torsional strain is present. Cyclopentane is nonplanar and the two minimum energy geometries are the envelope and the half-chair. In the envelope conformation, one carbon atom is 

Almenningen, O. Bastiansen, and P. N. Skancke, Acta Chem. Scand., 15, 711 (1961). 

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