Cyclic alkanes

An infinite alkane has C-C-C 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 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 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


Reusch W, Ring conformers