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Conformational Analysis of Simple Alkanes

Article ID: 43
Last updated: 01 Aug, 2011
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Conformational analysis is presented early in your organic chemistry studies to help illustrate the fact that all organic molecules are in constant motion. Most single bonds can and do rotate freely at room temperature. In order to perform conformational analysis on more complicated structures, it is important to understand conformational isomerization within simple alkanes.  Below we will consider the various conformational isomers of ethane.
 
Rotation about the C-C bond in ethane produces different conformations. Although an infinite number of conformations are possible, the staggered and eclipsed conformations, which represent the most and least stable respectively, are the two most important. Looking at this a different way, the least stable conformation (eclipsed) is also the highest in potential energy, while the most stable conformation (staggered) is the lowest in potential energy.  For more complex alkanes, more conformations exist.
                                                          
STAGGERED
ECLIPSED
 
Above are two common ways to show conformations: 1) using Newman projections which place the two atoms in the rotating bond in front of and behind a circle (see the examples above for the C-C bond in ethane), and 2) using wedge and dash drawings, which resemble typical Lewis structures with wedges showing a bond coming out of the screen and dashes showing a bond going into the screen (all normal lines exist in the plane of the screen).
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Conformational Analysis of Higher Alkanes     Cycloalkane Conformations

Department of Chemistry & Chemical Biology