Energy Difference Between Conformations
The various conformations of ethane do not have the same energy, this is due to electronic repulsion between hydrogens. When we rotate the molecule from a staggered conformation, the distance between the hydrogen atoms of the respective methyl groups begins to decrease, producing an increase in repulsion between the pairs of bonding electrons in the C-H bonds. The potential energy of the system increases until reaching the eclipsed conformation.
Potential Energy Diagram of Ethane
The energy differences between the various conformations can be seen by graphically representing the energy of each conformation against the rotated angle. As can be seen in the ethane diagram, the eclipsed conformations are the energy maxima and the staggered ones are the minima.
To obtain the potential energy diagram of ethane, we start from the eclipsed conformation and perform 60º turns until covering 360º.
We classify the different conformations according to energy. The staggered [1], [3], and [5] have the same energy, which we will call E1 (in the diagram we will take it as zero). The [2], [4], and [6] conformations have more energy due to hydrogen-hydrogen eclipsing. These eclipsed conformations are about 12 KJ/mol above the staggered ones.
Potential Energy Diagram of Butane
In the butane diagram, there are more conformations than just staggered and eclipsed. The staggered conformation with the methyls as far apart as possible is called anti [4] and is the most stable, having the least repulsions. Rotating the back carbon by 60º leads to an eclipsed [5] with two methyl-hydrogen interactions, another turn leads to another staggered called gauche [6]. A third turn places the methyls opposed (eclipsed) called syn conformation [1] and is the one with the highest potential energy.
