Page 183 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

displaced from the plane of the other four. In the half-chair conformation, three carbons 163
are coplanar, with one of the remaining two being above the plane and the other below.
The planar portions of both conformations have torsional strain owing to C−H and SECTION 2.2
C−C bond eclipsing. The energy differences between the conformers are small and Conformation
there is rapid interconversion of conformers. 74 All of the carbon atoms move rapidly
through planar and nonplanar positions, owing to a process called pseudorotation.

envelope half-chair

As ring size increases, there are progressively more conformations that have
to be considered. For cycloheptane, four conformations have been calculated to be
particularly stable. 75 NMR investigations indicate that the twist-chair is the most
76
77
stable. Various derivatives adopt mainly twist-chair conformations. The individual
twist-chair conformations interconvert rapidly by pseudorotation. 78 The most recent
MM4 and CCSD/6-311++G ∗∗ computations (see Section 2.3) indicate the following
relative energies. 79 Figure 2.17 shows the conformations.

chair boat twist-chair twist-boat

MM4 CCSD/6-311++G ∗∗
Twist-chair 0 0
Chair 1.4 0.9
Boat 3.8 3.3
Twist-boat 3.5 3.3

Relative energy in kcal/mol

74
W. J. Adams, H. J. Geise, and L. S. Bartell, J. Am. Chem. Soc., 92, 5013 (1970); J. B. Lambert,
J. J. Papay, S. A. Khan, K. A. Kappauf, and E. S. Magyar, J. Am. Chem. Soc., 96, 6112 (1974).
75 J. B. Hendrickson, J. Am. Chem. Soc., 89, 7036 (1967); D. F. Bocian and H. L. Strauss, J. Am. Chem.
Soc., 99, 2866 (1977); P. M Iavanov and E. Osawa, J. Comput. Chem., 5, 307 (1984).
76 J. B. Hendrickson, R. K. Boeckman, Jr., J. D. Glickson, and E. Grunwald, J. Am. Chem. Soc., 95, 494
(1973).
77
F. H. Allen, J. A. K. Howard, and N. A. Pitchard, Acta Crystallog., B49, 910 (1993).
78 D. F. Bocian, H. M. Pickett, T. C. Rounds, and H. L. Strauss, J. Am. Chem. Soc., 97, 687 (1975).
79
K. B. Wiberg, J. Org. Chem., 68, 9322 (2003).