Steric Strain

Steric Strain

Steric strain becomes important in the conformations of propane, butane, and other larger molecules.  Like ethane, butane has an infinite number of conformations.  The four important to organic chemists are shown below.  When the two methyl groups are at a dihedral angle of 180^o it is known as the anti conformation which is the most stable conformation of butane.  In anti, there is no torsional or steric strain.  The next highest energy conformation of importance is the gauche conformation which lies about 3.8 kj/mol above the anti.  In the gauche conformation there is no torsional strain.  This 3.8 kj/mol is entirely a result of steric strain (repulsion) between the two methyl groups that are at a dihedral angle of 60^o.   The two eclipsed conformations are obviously much higher in energy than the staggered.  In the eclipsed conformations there is both torsional and steric strain.