Rotation around the C-C bond in ethane

The simplest n-alkane is ethane, C2H6. The bond between the two carbon atoms is a single ('sigma') bond, that is rotation symmetric with respect to the C-C axis, so it allows rotation around this axis. When rotating around this carbon-carbon bond, the energy of ethane varies because of the presence of the C-H bonds.
Have a look at the energy as a function of the rotation around the carbon-carbon bond in ethane. Such a plot you will also find in your textbook.



Use these animation control buttons, or click on a point in the graph.
Background


Pay attention to the following features:

Now try to answer the following question.

Indicate for which rotations (between 0 and 180 degrees) the energy of ethane reaches maxima and minima.

  1. maxima at 0, 60 and 120 degrees of rotation; minima at 30, 90 and 150 degrees of rotation
  2. maxima at 30, 90 and 150 degrees of rotation; minima at 0, 60 and 120 degrees of rotation
  3. maxima at 0 and 120 degrees of rotation; minima at 60 and 180 degrees of rotation
  4. maxima at 60 and 180 degrees of rotation; minima at 0 and 120 degrees of rotation

As you hopefully have noticed, maxima in energy occur when, looking along the carbon-carbon axis, the hydrogens in front obscure those in the back. This conformation is usually called the eclipsed conformation. Minima in the energy, on the other hand, occur when the hydrogens in the front are exactly in between (rotated 60° with respect to) the ones in the back. This conformation is called the staggered conformation.

In the next exercise, we will have a look at the same rotation in propane.


Back to index To the next exercise

Thanks to Bob Hanson for further adaptations to Internet Explorer and Netscape browsers.