Page 182 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

162 Table 2.3. Strain Energies for Cycloalkanes a

CHAPTER 2 Cycloalkane Strain energy (kcal/mol)
Stereochemistry, Cyclopropane 28.1 b
Conformation, Cyclobutane 26.3
and Stereoselectivity
Cyclopentane 7.3
Cyclohexane 1.4
Cycloheptane 7.6
Cyclooctane 11.9
Cyclononane 15.5
Cyclodecane 16.4
Cyclododecane 11.8

a. Values taken from E. M. Engler, J. D. Andose, and
P. v. R. Schleyer, J. Am. Chem. Soc., 95, 8005 (1973).
b. P. v. R. Schleyer, J. E. Williams, and K. R. Blanchard, J. Am.
Chem. Soc., 92, 2377 (1970).

considered. The small rings are dominated by angle and torsional strain. The common
rings are relatively unstrained and their conformations are most influenced by torsional
factors. Medium rings exhibit conformational equilibria and chemical properties
indicating that cross-ring van der Waals repulsions play an important role. Large rings
become increasingly flexible and possess a large number of low-energy conformations.
The combination of all types of strain for a given ring results in a total strain energy
for that ring. Table 2.3 presents data on the strain energies of cycloalkanes up to
cyclodecane.
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 Å, and the H−C−H
angle of 115 C is opened somewhat from the tetrahedral angle. 70 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. 71 1,3-Disubstituted cyclobutanes show small
energy preferences for the cis isomer, which places both substituents in equatorial-like
positions. 72 The energy differences and the barrier to inversion are both smaller than
in cyclohexane.

R'
R R R'

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. 73 In the envelope conformation, one carbon atom is

70 O. Bastiansen, F. N. Fritsch, and K. Hedberg, Acta Crystallogr., 17, 538 (1964).
71
A. Almenningen, O. Bastiansen, and P. N. Skancke, Acta Chem. Scand., 15, 711 (1961).
72 (a) K. B. Wiberg and G. M. Lampman, J. Am. Chem. Soc., 88, 4429 (1966);
(b) N. L. Allinger and L. A. Tushaus, J. Org. Chem., 30, 1945 (1965).
73
A. C. Legon, Chem. Rev., 80, 231 (1980); B. Fuchs, Top. Stereochem., 10, 1 (1978).