An infinite alkane has C-C-C angle α≈113° coresponding to unstrained sp^{3} hybridization.
The geometric structure of cyclic alkanes is influenced in order of importance by: this angle strain, steric
crowding of hydrogens, eclipsing strain of C-C bonds. The last condition requires chair conformation, which
can not be strain free in cycles with odd number of atoms. To remove angle strain a molecule puckers. If η
is the out of plane angle of this puckering and 2π/*n* is the planar projection of C-center-C angle then sin α/2
= cos η cos π/*n*. The angle η must be as small as possible due to steric
crowding of hydrogens. Obviously for *n*=3, 4, 5 the angle strain is unavoidable and the first two
conditions require plain configuration (for *n*=3 the geometry is rigid). But for *n*=5 the molecule
is slightly puckered to remove the eclipsing strain. For *n*≥6 the first and the third conditions are
always satisfied. For odd *n* chair conformation is possible only in disordered structure. For *n*≥8 η>50°
leading to steric
crowding of hydrogens. Thus molecules with *n*≥7 are disordered (see e.g. *n*=15). All the conditions can be satisfied
only for *n*=6.

Derivatives are illustrated by the case *n*=3: arizidine, oxirane,
tris(methylene)-cyclopropane, spiropentane.

*See also* Annulenes

Reusch W, Ring conformers