Chapter 3: Conformations of Alkanes and Cycloalkanes

Alkanes

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

• Methane
• Ethane
• Propane

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

Ethane: CH₃-CH₃
 
 

Rotation about the C-C bond in ethane (see CHIME animation to the right) produces different conformations. Although an infinite number of conformations are possible, the staggered conformation and the eclipsed conformation which represent the most and least stable respectively are the two most important.
The differences between these two conformations can be seen by rotating the ethane molecule to view it directly down the C-C bond, as in a Newman projection, see below: Try rotating the CHIME animation above to see the interconversion of the two conformations.
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 shown 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 high energy conformations are the eclipsed conformers where the C-H bonds are aligned
	The plot to the left shows how the potential energy varies for a full rotation about the central C-C bond.
	The most stable (low energy) conformations are the staggered conformers where the C-H bonds are at maximum separation

Propane: CH₃-CH₂-CH₃