Eclipsed conformation mechanism

Eclipsed conformation, 94 molecular model of, 94 Edman, Pehr Victor, 1031 Ed man degradation, 1031-1032 mechanism of. 1032 Eicosanoid, 1067-1070... 

A very different explanation was put forward by Pophristic and Goodman. They proposed that the rotational barrier in ethane results not from steric destabilization of the eclipsed conformation but, instead, from stabilization of the staggered conformation arising from delocalization of the cr bonding electrons. We will discuss the mechanism of this proposed stabilization in Chapter 4, but there is one point to be made here. When observable physical properties such as torsional energy are attributed to nonobservables—that is, to concepts such as steric effects that are inherently associated with other nonobservables such as molecular orbitals—it is difficult to establish the origin of the physical property... 

The difference in energy between staggered and eclipsed conformations of ethane is referred to as torsional strain, and its cause has been somewhat debated over the years. Based on recent quantum mechanical calculations, it is now believed that the staggered conformation possesses a favorable interaction between an occupied, bonding MO and an unoccupied, antibonding MO (Figure 4.7). 

As calculated by molecular mechanics, E, n for the eclipsed conformation of butane is -1-9.3 kJ/mol. On the basis of this and the Estrain values just cited, calculate the activation energy for rotation about the C(2)—C(3) bond. 

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