Conformation Of Ethane

Hydrocarbons can contain single bond as well as multiple bonds between two carbons.

But alkanes are hydrocarbons that contain only single bonds

This single bond in alkanes is a sigma bond. The electronic distribution around this bond is uniform.

This uniform distribution of electrons allows the free rotation of single bond in space.

This leads to alkane molecules having different arrangements of atoms in space.

Such molecules with different spatial arrangements of atoms are known as conformation or conformer or rotamers.

Alkanes shows conformations from ethane onwards.

Ethane is the simplest alkane that shows conformational isomerism

The conformations of ethane can be found by keeping one carbon stationary and rotating the other around the axis.

However, conformations are hard to understand on paper.

For representing such conformations projection formula are used

These are Newmann and Sawhorse projection formula.

First, let’s see the Sawhorse projections of ethane

Sawhorse projection is obtained when the molecule is viewed at an angle.

In Sawhorse projection, the ethane will be represented as.

The angle between two H atoms is

There is a possibility that hydrogen present at the front carbon is close to the hydrogen present at the back carbon.

Such type of conformation is known as eclipsed conformation.

On rotating the back , we will reach a conformation where the hydrogen atoms present at the front and back carbon are as far as possible.

Then such type of conformation is called staggered conformation.

So on the extreme ends, there can be two types of conformers. One is eclipsed and the other is staggered.

Any confirmation between eclipsed and staggered is called skew conformation

Next type of projection formula is Newmann's projection formula.

In this projection, the molecule is viewed at bond from the front as.

Newmann projection of ethane can be drawn as:

Since the atoms are close to each other, this is the eclipsed conformation

If we rotate the single bond in such a way that two hydrogen atoms are far from each other, we will get the staggered conformation

Then we will get two types of conformations named as eclipsed and staggered.

In Newmann's projection, the dihedral angle can be easily observed. The dihedral angle is the angle between bonds of similar atoms with atoms

In eclipsed it is and in staggered it is .

Any conformation with dihedral angle between and is called is skew conformation

So the dihedral angle will range from to

Using this property, we can draw the stability graph of conformers of ethane

The plot between potential energy and angle of rotation curve gives an idea about the stability of two conformers.

From the curve, we can see that eclipsed form is obtained at higher energy than staggered form.

Higher energy for eclipsed is due to repulsion between two hydrogens that are very close in eclipsed form.

Higher energy of eclipsed accounts for it’s lower stability. Therefore, staggered form is most stable.

Revision

Ethane forms two extreme conformers - one is eclipsed and other is staggered.

The conformers between them are called skew conformers

In this way the conformers can be represented by sawhorse projection formulas

In this way the conformers can be represented by Newmann projection formulas

The most stable form is a staggered one.

The End