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