Conformational energy of ethane

Figure 1.2. Conformational energies of ethane, butane and 1,2-dichloroethane. [in part, reprinted with permission from Hine, J. Physical Organic Chemistry, 2nd Ed., McGraw-Hill, New York, 1962, p 36. Copyright 1962, McGraw-Hill.]...

Figure 2.2 Conformational energy of ethane as a function of torsion angle.

Matsubara T, Sieber S, Morokuma K (1996) A test of the new integrated MO + MM (IMOMM) method for the conformational energy of ethane and n-butane. Int J Quantum Chem 60 1101-1109... 

Problem 4.42 How would the energy-conformation diagrams of ethane and propane differ ... 

The simplest example is ethane the torsional strain energy of ethane is at a minimum when the torsion angles are 60°, 180° and 300° and at a maximum when they are 0°, 120° and 240°, and these are the staggered and eclipsed conformations, respectively. 

The torsional energy of ethane is lowest in the staggered conformation. The eclipsed conformation is about 12.6 kJ/mol (3.0 kcal/mol) higher in energy. At room temperature, this barrier is easily overcome and the molecules rotate constantly. 

A Pairwise Additivity Scheme for Conformational Energies of Substituted Ethanes. 

Figure 1.11 Views of two conformational isomers of ethane, staggered and eclipsed. The end-on view is called a Newman projection. Below them is a plot of the energy as the C-C bond rotates.

Conformational isomers of ethane (H3C-CH3). Eclipsed and staggered conformations for ethane are possible by virtue of the unrestricted rotation about the carbon-carbon single bond. There is a potential energy difference between the two forms, the staggered form being at the minimum and the eclipsed form at the maximum. 

The energy of ethane as a function of dihedral angle. The eclipsed conformations are approximately 12.6 kj (3.0 kcal)/mol higher in energy than the staggered conformations. 

An energy diagram showing the conformational analysis of ethane. 

In principle, ethane has an infinite number of conformations that differ by only tiny increments in their torsion angles. Not only is the staggered conformation more stable than the eclipsed, it is the most stable of all of the conformations the eclipsed is the least stable. Figure 3.4 shows how the potential energy of ethane changes for a 360° rotation about the... 

The graph in Figure 4.8 shows how the potential energy of ethane changes with dihedral angle as one CH3 group rotates relative to the other. The staggered conformation is the most stable... 

Now let us consider a conformational analysis of ethane. Clearly, infinitesimally small changes in the dihedral angle between C—H bonds at each end of ethane could lead to an infinite number of conformations, including, of course, the staggered and eclipsed conformations. These different conformations are not aU of equal stability, however, and it is known that the staggered conformation of ethane is the most stable conformation (i.e., it is the conformation of lowest potential energy). The fundamental reason for this has recently come to light. 

All staggered conformations are degenerate, and the same is true for aU eclipsed conformations. As such, the energy diagram has a shape that is similar to the energy diagram for the conformational analysis of ethane ... 

The origin of a torsional barrier can be studied best in simple cases like ethane. Here, rotation about the central carbon-carbon bond results in three staggered and three eclipsed stationary points on the potential energy surface, at least when symmetry considerations are not taken into account. Quantum mechanically, the barrier of rotation is explained by anti-bonding interactions between the hydrogens attached to different carbon atoms. These interactions are small when the conformation of ethane is staggered, and reach a maximum value when the molecule approaches an eclipsed geometry. 

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