Cyclic alkanes

An infinite alkane has CCC angle α≈113° coresponding to unstrained sp3 hybridization. The geometric structure of cyclic alkanes is influenced in order of importance by: this angle strain, steric crowding of hydrogens, eclipsing strain of CC 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 CcenterC 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 n8 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

References

Reusch W, Ring conformers