Ethane Conformations
In the molecular model of ethane shown on the right, we can see that the methyl groups can easily rotate relative to each other. The energy required for this rotation is only 3 kcal/mol. It can be said that there is free rotation around the single bonds.
Staggered and Eclipsed Conformations of Ethane
There are two ways to draw the ethane molecule: staggered and eclipsed conformations. In the staggered conformation, each hydrogen atom of the first carbon is located between two hydrogen atoms of the second carbon, which avoids repulsions and makes this conformation low in energy. In the eclipsed conformation, all hydrogen atoms of the first carbon are directly opposite those of the second. The rotation of the methyl group around the C-C bond allows the transition from the staggered to the eclipsed conformation and vice versa.
The multiple shapes of ethane created by rotation around the C-C bond are called conformations, and the study of these is known as conformational analysis.
Conformational Isomers of Butane
In butane, rotation around the C2-C3 bond also generates staggered and eclipsed conformations, among which the following stand out: syn conformation, anti conformation, and gauche conformation. These molecular models represent the anti, syn, and gauche conformations of butane.
The anti conformation is the most stable conformation of butane, with the methyl groups far apart and not interacting. The gauche conformation is less stable, with an interaction between methyl groups that are positioned at 60 degrees, called the gauche interaction.