Conformations of Butane

The trans conformation corresponds to a torsion angle of 180°, the gauche(+) conformation to oni + 60° and the gauche -) conformation to -60°. These approximately correspond to the torsion anj of the three minimum energy conformations of butane. 

FIGURE 3 6 The gauche and anti conformations of butane shown as ball and spoke mod els left) and as Newman projections right) The gauche conformation is less stable than the anti because of the van der Waals strain between the methyl groups... 

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... 

We can relate the conformational preference for an equatorial methyl group m methylcyclohexane to the conformation of a noncyclic hydrocarbon we discussed ear her butane The red bonds m the following structural formulas trace paths through four carbons beginning at an equatorial methyl group The zigzag arrangement described by each path mimics the anti conformation of butane... 

Section 3 2 The two staggered conformations of butane are not equivalent The anti conformation is more stable than the gauche... 

Anti conformation of butane Gauche conformation of butane... 

Figure 3.7 illustrates the potential energy relationships fflnong the various conformations of butane. The staggered conformations are more stable than the eclipsed. At any instant, almost all the molecules exist in staggered conformations, and more are present in the anti conformation than in the gauche. The point of maximum potential... 

Gauche conformation (Section 3.7) The conformation of butane in which the two methyl groups lie 60° apart as viewed in a Newman projection. This conformation has 3.8 kj/mol steric strain. 

It may be observed that the gauche conformation of butane (L) or any other similar molecule is chiral. The lack of optical activity in such compounds arises from the fact that L and its mirror image are always present in equal amounts and interconvert too rapidly for separation. 

We have seen that Newman projections are a powerfnl way to show the different conformations of a molecule. We mentioned earlier that there are staggered conformations and eclipsed conformations. In fact, there are three staggered and three eclipsed conformations. Let s draw all three staggered conformations of butane. The best way to do this is to keep the back carbon atom motionless (so the fan in the back is not spinning), and let s slowly turn the groups in the front (only the front fan is spinning) ... 

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 ... 

When the methyl group is axial, each path mimics the gauche conformation of butane. 

Figure 5-10 Sawhorse and Newman conventions for showing the staggered conformations of butane. Only one gauche form is shown. Cyclohexane is shown to emphasize the resemblance of its stable conformation to the gauche conformation of butane.

Exercise 12-3 Figure 5-8 indicates that the difference in energy between the conformation of butane with eclipsed methyls and the gauche form is about 5 kcal mole-1. Use this number to estimate the contribution of eclipsing to the instability of planar cyclohexane. Then calculate the instability of planar cyclohexane by including the angle strain from Exercise 12-2 in your estimate. 

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