Return to Contents Chapter 3: Conformations of Alkanes and Cycloalkanes  Ch 3 contents

Alkanes

Let's start with simple alkanes so we can prepare ourselves for the more complex world that lies beyond.....

Methane: CH4
 

Although σ bonds are able to rotate about the internuclear axis, the spherical symmetry of H atoms means that methane has a single unique conformation. 

methane

 

Ethane: CH3-CH3

Rotation about the C-C bond in ethane (see the JMOL animation to the right) 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. 
Try rotating the 3D model of the animation so that you are looking directly along the C-C bond to see the interconversion of the two conformations.
The differences between these two conformations are most apparent when viewed directly down the C-C bond, as in a Newman projection, see below: 
STAGGERED
Look at how the each H-C-H bond angle is bisected by a C-H bond on the adjacent C atom. This is the most stable conformation for ethane since the torsional strain is minimised. The staggered conformation is 12kJ/mol (2.9 kcal/mol) more stable than the eclipsed conformation  (below).

ECLIPSED
Think like an astronomer....each of  the C-H bonds is aligned with a C-H bond on the adjacent C atom so that the H attached to the front C obscures (eclipses)  those on the rear C

The animation below shows how the potential energy of the ethane molecule varies for a full rotation about the central C-C bond.
Use the controls to show the energies of the important conformations.



Propane: CH3-CH2-CH3
 

Although there are 2 C-C bonds in propane, they are equivalent and rotation produces conformations that are similar to those of ethane except that the "extra" methyl group is interacting with the H atoms. 

These can be seen by in the 3D model of propane to the right. 
 

   

 


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Dr. Ian Hunt, Department of Chemistry, University of Calgary