Conformations of Ethane
Conformations of Ethane
The different arrangements of a molecule that can be obtained by rotation around C-C single bonds are called conformations or conformational isomers.The carbon-carbon atoms are not frozen in ethane molecule. The two methyl groups can rotate infinite times about the C-C bonds. Two extreme forms of ethane are eclipsed conformation and staggered conformation. These two conformations are called rotamers because they can be converted into each other by rotation of 60° about C-C bond.
Sawhorse and Newman Projections of Eclipsed and Staggered conformation of ethane are shown below.
The staggered conformations of ethane are the low energy forms while the eclipsed conformations represent transition states. The eclipsed conformation is about 3 kcal/mol (or 12 kJ/mol) less stable (of higher energy) than the staggered conformation because of repulsions between the hydrogen atoms. To undergo rotation from a staggered to an eclipsed conformation, a mole of ethane molecules would require 3 kcal. Since this amount of energy is readily available to molecules at room temperature, the rotation can occur easily. However, even though the conformations of ethane are interconvertible at room temperature, at any given time we would expect a greater percentage of ethane molecules to be in staggered conformation because of its lower energy. So, staggered conformations are more stable than the eclipsed conformations.
Energy diagram of different conformations of ethane are shown below-
The most important thing to remember about conformational isomers is that they are just forms of a single molecule that can be interconverted by rotation about single (σ) bonds. Sufficient energy for this rotation is available at room temperature. Consequently, at room temperature it is usually possible to separate them from one another.
