Page 172 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

152 H
CH 3 H
O CH 3
CHAPTER 2 CH 3
Stereochemistry, CH 3 CH 3 CH 3 O
Conformation,
and Stereoselectivity s-trans (28%) s-cis (72%)

2.2.2. Conformations of Cyclohexane Derivatives

The conformational analysis of six-membered ring compounds is particularly
well developed. Cyclohexane and its derivatives lend themselves to thorough analysis
because they are characterized by a small number of energy minima. The most
stable conformations are separated by barriers that are somewhat higher and more
easily measured than rotational barriers in acyclic compounds or other ring systems.
The most stable conformation of cyclohexane is the chair. Electron diffraction
studies in the gas phase reveal a slight flattening of the chair, compared with
the geometry obtained using tetrahedral molecular models. The torsion angles are

55 9 , compared with 60 for the “ideal” chair conformation, and the axial C−H

bonds are not perfectly parallel, but are oriented outward by about 7 . The C−C
bonds are 1 528Å, the C−H bonds are 1 119Å, and the C−C−C angles are
45
111 05 .

56°
H H
H H
1.528 Å
H H
111.05° H H

Two nonchair conformations of cyclohexane that have normal bond angles and
bond lengths are the twist and the boat, 46 both of which are less stable than the
chair. A direct measurement of the chair-twist energy difference has been made
47
using low-temperature IR spectroscopy. The chair was determined to be 5.5 kcal/mol
lower in energy than the twist. The twist and the boat conformations are more
flexible than the chair, but are destabilized by torsional strain, as the bonds along
the “sides” of the boat are eclipsed. In addition, the boat conformation is further
destabilized by a van der Waals repulsion between the “flagpole” hydrogens. Both this
van der Waals repulsion and the torsional strain are somewhat reduced in the twist
conformation.

45 H. J. Geise, H. R. Buys, and F. C. Mijlhoff, J. Mol. Struct., 9, 447 (1971).
46 For a review of nonchair conformations of six-membered rings, see G. M. Kellie and F. G. Riddell,
Top. Stereochem. 8, 225 (1974).
47
M. Squillacote, R. S. Sheridan, O. L. Chapman, and F. A. L. Anet, J. Am. Chem. Soc., 97, 3244 (1975).