THE CONFORMATION OF ETHANE
PART I: Understanding how to use NEWMAN PROJECTIONS
With Newman Projections can we better understand conformations
In the illustrations above, you see carbon 1 rotate toward you and the second carbon rotate away from you. In the Newman Projection carbon 1 is in front while the second carbon is behind and cannot be seen.
The Newman Projection you have just observed is called an ECLIPSED CONFORMATION, also known as SYN-CONFORMATION. The eclipsed conformation is the highest energy conformation for ethane. The lowest energy conformation is the STAGGERED CONFORMATION, also known as GAUCHE CONFORMATION. The STAGGERED CONFORMATION is accomplished when you rotate the second carbon (the one you cannot see in the Newman Projection) 60°, either clockwise or counterclockwise, moving the three hydrogens that are bonded to it. The angle we refer to when discussing degree rotations is called the DIHEDRAL ANGLE, which is defined as the angle created by two intersecting planes. For an in depth explanation of what the DIHEDRAL ANGLE is, click here.
Along with the ECLIPSED and STAGGERED CONFORMATIONS, there also exists a broad range of other conformations.
|CONFORMATION||DIHEDRAL ANGLE (°)||NEWMAN PROJECTION|
PART II: Comparing Potential Energies Based on the DIHEDRAL ANGLE The dihedral angle has a pronounced influence on the energy within the ethane molecule
When the dihedral angle=0°(ECLIPSED CONFORMATION) the energy is higher by 3 kcal/mol than if the dihedral angle=60°(STAGGERED CONFORMATION). This is due to the dynamic intramolecular repulsions of the hydrogens existing in close proximity to other hydrogens on the same intersecting plane.
Since syn conformation has a greater potential energy, it is less stable than the gauche conformation.
The Conformation of Butane Tutorial
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with Dr. Abby Parrill
Department of Chemistry
Michigan State University
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