In an idealized molecule, there are six hydrogens on the periphery of the carbon ring (referred to as equatorial) and three axially located (referred to as axiomatically). As you rotate the molecule in JSMOL, you can see that the equatorial hydrogens flip around to become axial and vice versa. This interconversion between the two chair conformations of cyclohexane is called “ring flipping.”
Which Chair Conformation Is Higher in Energy?
In methylcyclohexane, each of the twelve hydrogens is structurally different. Six of the hydrogens are on the periphery of the ring, and three of them are axially located above and below the symmetry plane.
The axial hydrogens are closer to the center of the molecule than the equatorial hydrogens. This results in increased steric strain. The axial hydrogens have an angle of about 60 degrees to the center of the ring.
However, if we look closely at the molecule, we can notice that there is a significant 1,3-diaxial interaction between the methyl and ethyl groups. This is due to Van der Waals repulsion between the electron clouds on each of these groups. This introduces significant strain into the molecule and makes it unstable.
This is one of the reasons why the staggered conformation is more stable than the eclipsed conformation. It minimizes the repulsion between the methyl and ethyl group.
Now that we have seen how these relationships are established, let’s take a look at a few more molecules. First, we’ll look at ethane and propane.
What we can learn from these structures is that there are three main types of conformational isomerism. The staggered conformation is the most stable, because it maximizes angles between the C-H bonds on the front and back carbons.
It also has the lowest total energy. The total energy of the molecule is the sum of the individual energies for the staggered and the eclipsed forms.
In addition, the boat form is slightly less stable because it suffers from a flagpole interaction between the axial hydrogens on the front and rear carbons. The torsional strain of the flagpole interaction is not so great, but the steric energy between the methyl and ethyl atoms is still quite high.
Therefore, the boat is less stable than both of the other conformations and has a rotational barrier of approximately 30.0 kcal/mol. This is about 1.0 kcal/mol per eclipsed H-H interaction.
The next alkane, butane, is a bit more complicated. It has four possible conformations, and the energy of these four conformations are correlated to the angle of rotation.