Conformations: Sawhorse and Newman Projections (of ethane)

Introduction

The spatial arrangements of atoms in a molecule that can be obtained through rotation around one or more single bonds like Carbon-Carbon(C-C) bonds are known as conformation or conformational isomers. Rotational energy functions as a barrier in single bond rotation. The 12 KJ/mol barrier must be overcome to interchange from one conformer to another. There are various types of conformational isomers like Ethane and Butane.

Conformations

Conformational isomers can be presented in two ways.

1. Sawhorse representations
2. Newman’s projection

Sawhorse representations – Sawhorse shows how the spatial orientation of all C-H bonds is arranged at an angle. 

Newman’s projection – In Newman’s projection, the two carbon atoms are represented by a circle and a carbon-carbon bond is viewed along its axis. Line representation towards the center of the circle represents the bonds attached to the front carbon. Lines going to the edge of the circle are used to view the bonds attached to the rear carbon. Newman’s projections are easy to draw and clearly show the relationships among substituents on the different carbon atoms.

Conformation is the arrangement of the atoms or groups in a molecule that can be obtained through rotation around one or more single bonds. In methane, the new Carbon-Carbon bond brings several complications making organic chemistry enjoyable. Methane is a single hydrocarbon compound. 

Methane has a 3-dimensional shape all the time. Even if we draw it from different perspectives, it retains the same shape. The same cannot be said for ethane. Here, the two CH3 particles are connected by a C-C sigma bond. Rotation can occur through the bond without disturbing the connection. The rotation around the C-C bond gives rise to different 3-dimensional shapes. These different shapes are known as conformational isomers.

In ethane conformation, since the hydrogen atoms are spherical, the rotation around the six carbon-hydrogen bonds does not result in any shape change. However, many different molecular conformations are possible due to rotation about the carbon-carbon bond. 

These conformations can be drawn either in the Newman’s projection form or the sawhorse projection form for deeper understanding. Newman’s projection provides a view of the carbon-carbon bond in ethane. The ‘front’ atom can be shown as a dot and the ‘back’ atom as a larger circle. Solid lines from the two carbons at 120°angles are used to indicate the six carbon-hydrogen bonds. Actual tetrahedral geometry looks like when viewed from this perspective and flattened into two dimensions.

If we now rotate the front CH3 group 60° clockwise, the molecule is in the highest energy ‘eclipsed’ conformation, and the hydrogens on the front carbon are as close as possible to the hydrogens on the back carbon.

Sawhorse projection for conformation of ethane

• A sawhorse projection is the sight of a molecule with a particular Carbon-Carbon bond.
• Both front and back carbon groups are drawn with sticks at an angle of 120 degrees.
• In the staggered form of ethane,

1. The electrons of carbon-hydrogen bonds are as far apart as possible.
2. Minimum repulsive forces
3. Minimum energy
4. Maximum stability of molecules

• In the eclipsed form of ethane,

1. The electrons of carbon-hydrogen bonds come closer to each other.
2. Increased repulsive force
3. More energy
4. Minimum stability of molecules

• Sawhorse views the C-C bond at an angle to show the spatial location of all C-H bonds.

Newman’s Projection formulae for different conformations of ethane:

• The rotation of ethane molecules around its carbon-carbon single bond.
• The two extreme conformation results in staggered and eclipsed conformation.
• In staggered conformation projection,

1. The hydrogens are drawn at an angle of 60 degrees.
2. Energy minima.

• In the eclipsed conformation of the Newman’s projection,

1. The 3 front hydrogens are directly in front of the 3 hydrogens in the back.
2. Energy maxima.

• In Newman’s projection, the C-C bond is seen along its axis, and the circle represents the two carbon atoms.

The Newman’s Projection’s advantage is that it is easy to draw and shows clear relationships between different carbon atoms. We can further discuss the stability of the conformations of ethane. Newman’s projection of ethane occurs in two forms: staggered conformation and eclipsed conformation.

How the Staggered Vs. Eclipsed Conformations of ethane differ:

Conformations are the different forms of projections or arrangements of substituents of a molecule that can be used to depict a molecule. Newman projections demonstrate the conformation of the molecule when seen through the C-C bond in the front-back direction.

Conclusion

With respect to the stability of the two conformations, there are minimum repulsive forces, minimum energy and maximum stability in the staggered conformation of ethane. This is because the electron clouds of the carbon-hydrogen bonds are as far as possible and when the staggered conformation is converted to the eclipsed conformation the electron clouds come closer. This results in increased repulsions, and therefore the molecules will possess more energy and stability in the staggered conformation than the eclipsed form of conformation.