Page 99 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

78 Topic 1.1. The Origin of the Rotational (Torsional) Barrier in Ethane
and Other Small Molecules
CHAPTER 1
Chemical Bonding
and Molecular Structure One of the most general structural features of saturated hydrocarbons is the
preference for staggered versus eclipsed conformations. This preference is seen with the
simplest hydrocarbon with a carbon-carbon bond—ethane. The staggered conformation
is more stable than the eclipsed by 2.9 kcal/mol, as shown in Figure 1.33. 102
The preference for the staggered conformation continues in larger acyclic and
also cyclic hydrocarbons, and is a fundamental factor in the conformation of saturated
hydrocarbons (see Section 2.2.1). The origin of this important structural feature has
been the subject of ongoing analysis. 103 We consider here the structural origin of
the energy barrier. A first step in doing so is to decide if the barrier is the result
of a destabilizing factor(s) in the eclipsed conformation or a stabilizing factor(s) in
the staggered one. One destabilizing factor that can be ruled out is van der Waals
repulsions. The van der Waals radii of the hydrogens are too small to make contact,
even in the eclipsed conformation. However, there is a repulsion between the bonding
electrons. This includes both electrostatic and quantum mechanical effects (exchange
repulsion) resulting from the Pauli exclusion principle, which requires that occupied
orbitals maintain maximum separation (see Section 1.1.2). There is also a contribution
from nuclear-nuclear repulsion, since the hydrogen nuclei are closer together in the
eclipsed conformation. The main candidate for a stabilizing interaction is delocal-
ization (hyperconjugation). The staggered conformation optimizes the alignment of the
∗
and orbitals on adjacent carbon atoms.

Fig. 1.33. Potential energy as a function of rotation angle for ethane.

102 K. S. Pitzer, Disc. Faraday Soc., 10, 66 (1951); S. Weiss and G. E. Leroi, J. Chem. Phys., 48, 962
(1968); E. Hirota, S. Saito, and Y. Endo, J. Chem. Phys., 71, 1183 (1979).
103
R. M. Pitzer, Acc. Chem. Res., 16, 207 (1983).