Rotation around C-C bond in propane

As we have seen for ethane, maxima in energy occur when, while looking along the carbon-carbon axis, the hydrogens in front obscure those in the back. This conformation is called the eclipsed conformation. Minima in the energy, on the other hand, occur when the hydrogens in the front are rotated 60° with respect to the ones in the back. This conformation is called the staggered conformation.

Conformations of ethane
eclipsed conformation staggered conformation

We have done the same calculations for the rotation around one of the carbon-carbon bonds in propane.
The result is shown in a separate window (simply close that page afterwards).

At which values for the rotation do the maxima in energy of propane occur and is this in accordance with what you have seen in ethane?

  1. maxima at 0, 60, 120 etc. degrees, just as in ethane
  2. maxima at 0, 60, 120 etc. degrees, unlike ethane
  3. maxima at 0, 120 and 240 degrees, just as in ethane
  4. maxima at 0, 120 and 240 degrees, unlike ethane

So the periodicity of the energy curve is exactly the same as for ethane. The only difference is in the height of the maxima, the energy barrier. In propane this amounts to about 5.5 kJ/mole, whereas in ethane this is about 5.2 kJ/mole. The difference between ethane and propane is small; however, there is a difference. What do you think causes this difference?
(rotation is around the C1-C2 bond, C3 is the extra methyl group)

  1. Interactions of hydrogens on C1 with the hydrogens on C3.
  2. Interactions of hydrogens on C1 with the hydrogens on C2.
  3. Interaction of C2 with C3.

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


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