Energy 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 3.7 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... 

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. 

Plot of energy versus dihedral angle for conformations of butane. Test yourself on the concepts in this figure at OrganicChemistryNow. 

Conformations of Butane (page 190) 6.7 Plot of Energy versus Dihedral Angle for Conformations of Butane (page 191)... 

This is the common definition of conformers. The IUPAC definition also requires that a conformer correspond to a distinct potential energy minimum, such as the anti and gauche conformations of butane. 

TABLE 3.11 Relative Energies (kcal mol of the Conformers of Butane... 

TABLE 3.13 Energy Difference (kcal mol Between the G and T Conformers of Butane and the GGG and TTT Conformers of Hexane"... 

Use SpartanView to compare the energies of gauche and anti conformers of butane. 1,2-ethanediol, and 1,2-dimethoxyethane. Which molecules prefer the anti conformation. about the C-C bond, and which prefer the gauche Examine the electrostatic potential map of any molecule that prefers the gauche conformation, and explain why this conformation is preferred. 

In the vast majority of appropriately constructed molecules, for example derivatives of butane, the anti conformation, e.g. 12, in which the groups X are each flanked by two hydrogens, is the most stable however, there are a few exceptions. The anti conformation of butane is more stable than the cis by ca. 18.8 kJ mol1. An energy profile of rotation around the central C-C bond of butane is given in Figure 1.7. 

The boat conformation of cyclohexane (18) can be constructed from a molecular model of the chair form by holding the right-hand three carbons C(2), C(3) and C(4) of 15, clamped from the top with the hand and moving the left-hand three carbons upward. A Newman projection of the boat form looking along the C(l)-C(2) bond, and shown in 19, is reminiscent of the highest energy cis conformation of butane. 

Draw structural representations of two conformations of butane. Indicate the one having the lower potential energy. 

Figure 1.2.3 Energy diagram for the conformers of butane. The most prominent stretched and cyclic (Fischer) conformers are given below.

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