Ethane Conformational Analysis

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Definition of Conformations

Definition: CONFORMATIONS--Different spatial arrangements (structures) of a molecule which are generated by the relatively easy rotation around a single bond (usually a C-C single bond).

Things to recall:

Single bonds are sigma bonds, which by definition are symmetrical with respect to the bond axis. Thus rotation does not change the extent of overlap and thus does not change the strength of the bond. Consequently rotation around a sigma bond is relatively easy , in contrast to a pi bond, where rotation around the bond completely destroys the pi bond.
The rotation we're speaking of is "internal rotation", that is the rotation of one part of the molecule relative to the other. For example in ethane, the rotation of one methyl group relative to the other.The angle of rotation is called a dihedral angle , and it is represented by a Greek theta symbol.
Since the different structures which are generated by this rotation are all very similar (they have the same number and kind of bonds) and are very easily interconverted, they are not referred to as isomers but as different conformations of the same molecule. These conformations have similar energies, but not precisely the same energy.
Since rotation around the C-C bond of ethane is continuous, there are an infinite number of conformations, but only two are of major interest. These are the lowest energy structure, called the staggered conformation, and the highest energy structure, called the eclipsed conformation.
The staggered structure is important because, being the structure of lowest energy, it is the structure actually adopted by ethane (the ground state structure). It defines the shape of ethane molecules. The eclipsed structure is important not because it is highly populated , but because its energy relative to the staggered structure defines how much energy is required to complete a rotation in ethane. This is called the energy barrier to rotation, and it is about 3 kcal/mol. That is, the eclipsed structure is 3 kcal higher in energy than the staggered structure.

Torsional Strain

Torsional Strain: An energy increase caused by the eclipsing of bonds.
Strain: Any increase in energy of a molecule, particularly an increase relative to that which would normally be expected.
Eclipsing: The relationship between two bonds when the dihedral angle is zero.
Conformational Energy Diagram: A plot of energy,E (vertically) vs. dihedral angle,theta (horizontally). It should include the structures,energies, and names of the energy minima and maxima. You should be able to draw such a diagram for ethane.

Conformational Energy Diagram for Ethane

Basis for Torsional Strain

TORSIONAL STRAIN IS CONSIDERED TO BE CAUSED BY THE REPULSIONS BETWEEN ELECTRONS IN THE TWO BONDS WHICH ARE ECLIPSED. THESE ELECTRONS ARE CLOSER TOGETHER IN THE ECLIPSED FORM THAN IN THE STAGGERED FORM, AND SO THE REPULSIONS ARE GREATER. RECALL THAT REPULSIONS BETWEEN OPPOSITELY CHARGED PARTICLES REPRESENT AN INCREASE IN POTENTIAL ENERGY.

Conformational Analysis of Ethane

Conformational Analysis

It is important to note that in ethane there are three sets of bond eclipses taken pairwise. Since the total torsional strain is 3 kcal, it is usually considered that each pair of bond eclipses engenders 1 kcal of torsional strain. This kind of analysis is useful for estimating the torsional strain present in other, more complex, molecules, as we shall see and in understanding the ground state structures and shapes of molecules, which we call conformational analysis.

Newman Projection Structures

Newman Projection Representations

You should be able to represent the staggered and eclipsed structures of ethane using Newman Projection Structures as shown above.