THE CONFORMATION OF ETHANE
(C2H6)

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.

MOVIE: HOW TO SEE WEDGE-SLASH PICTURES AS NEWMAN PROJECTIONS

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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.

CONFORMATIONDIHEDRAL ANGLE (°)NEWMAN PROJECTION
ECLIPSED (SYN)0
no name30
STAGGERED (GAUCHE)60

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.


GRAPH: The Potential Energy of Ethane as a Function of the Dihedral Angle


MOVIE: The Relationship Exisiting Between Ethane's Potential Energy and its Dihedral Angle Reprised

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The Conformation of Butane Tutorial
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Bryan Modders
with Dr. Abby Parrill 
Department of Chemistry
Michigan State University
These pages may be downloaded and linked from other pages freely for academic
and educational purposes. Questions, problems, and errors should be sent to parrill@argus.cem.msu.edu