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Conformational Analysis of Higher Alkanes

Article ID: 42
Last updated: 26 Aug, 2011
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VISIT THE MODELS 360 SITE AT THE CHEMED DL! (search for butane to get started, is that model eclipsed, anti, or gauche around the C2-C3 bond?)
 
In order to fully understand conformations of cyclohexane and greater alkanes, one must be able to analyze and determine the relative stabilities of larger alkanes.  Below we will consider the various conformational isomers of n-butane (straight chain butane).
 
In butane it is the rotation about the C2-C3 bond that is of most interest since the relative position of the two methyl groups is important.  This can be seen most easily by rotating the molecule to view it down the C2-C3 bond, which is the basis of the Newman projections below.  Also remember, any C-C bond in a molecule could be analyzed in this way, but usually a specific bond (like C2-C3) is given to analyze for its conformational isomers.
 
There are three major conformations that result from local potential energy maxima or minima during the course of rotating around the C2-C3 axis.  In other words, below are the most and least stable conformations that help us understand the consequences of rotating around this bond.
 
ANTI
a staggered conformation with the Me groups at 180o with respect to each other. This is the most stable conformation since the Me (largest) groups are as far apart as possible.
 
GAUCHE
a staggered conformation with the Me groups at 60o with respect to each other.  Not as stable as the anti conformation because of the closer proximity of the largest groups on the C2-C3 bond.
 
SYN OR ECLIPSED
an eclipsed conformation with the Me groups at 0o with respect to each other. This is the least stable conformation due to the steric strain caused by the proximity of the Me groups and the torsional strain of the eclipsed bonds.
 
Steric and torsional strain are the dominating factors that separate these conformers in potential energy.  A general rule of thumb always applies: to achieve the most stable conformation, put the largest groups as far from each other as possible.
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Organic Chemistry -> Alkanes: Structure and Conformational Analysis -> Conformation of Alkanes and Cycloalkanes

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Conformational Analysis of Cyclohexane     Gauche Conformation

Department of Chemistry & Chemical Biology