Eclipsed conformation of butane

At Its most basic level separating the total strain of a structure into its components is a qualita tive exercise For example a computer drawn model of the eclipsed conformation of butane using ideal bond angles and bond distances (Figure 3 8) reveals that two pairs of hydrogens are separated by a distance of only 175 pm a value considerably smaller than the sum of their van der Waals radii (2 X 120 pm = 240 pm) Thus this conformation is destabilized not only by the torsional strain associ ated with its eclipsed bonds but also by van der Waals strain... 

FIGURE 3 8 Ball and spoke and space filling models of methyl-methyl eclipsed conformation of butane... 

Now let s look at the three eclipsed conformations of butane. Again, let s keep the back where it is, and let s just rotate the front carbon atom with its three groups ... 

Torsional strain is the second factor in cyclopropane s large ring strain. The three-membered ring is planar, and all the bonds are eclipsed. A Newman projection of one of the carbon-carbon bonds (Fig. 3-16) shows that the conformation resembles the totally eclipsed conformation of butane. The torsional strain in cyclopropane is not as great as its angle strain, but it helps to account for the large total ring strain. 

Thus, the energy-minimized eclipsed conformation represents a balance between a decrease in steric strain and an increase in angle strain. The calculated strain in the energy-minimized eclipsed conformation of butane is 21 kJ (5.0 kcal)/mol, relative to the staggered conformation. 

The total energy cost associated with the degenerate eclipsed conformations of butane amounts to 16 kJ/mol. 

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. 

The methyl-methyl eclipsed conformation of butane (a) ball-and-spoke model, (b) space-filling model, and (c) electrostatic potential map. The molecule itself is the same size in each. 

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