Exercise 3: butane: the gauche conformation

So far, you have seen the rotation around the carbon-carbon axis of ethane (exercise 1) and propane (exercise 2). In this exercise, we will be looking at the rotation around the C2-C3 axis of n-butane. Going from ethane to propane, nothing really unexpected happened, the energy profile looked quite the same, except for the slightly larger rotational barrier in propane. For butane, you will see a really different energy profile.

So, let's see what happens when rotating n-butane around the C2-C3 axis.

Whereas the energy profiles for rotation in ethane and propane were sinusoidal (i.e. only one maximum and one minimum energy, both repeating every 120°), the profile for butane contains local minima and maxima. Let's step through them, starting with the easy ones:

• At 0° there is an energy maximum: the hydrogens on C2 and C3 are in an eclipsed conformation, and the methyl groups attached to C2 and C3 are crowding together.
• At 180°, the absolute energy minimum is found. This is a staggered conformation in which the methyl groups connected to C2 and C3 are as far apart as possible (dihedral angle of 180°).
• At 60° and 300°, local minima in energy are found. You would indeed expect minima, because the conformation is staggered. However, the methyl groups attached to C2 and C3 are closer to each other than at 180° of rotation, which causes the higher energy. This conformation, where the functional groups attached to two adjacent carbon atoms are 60° apart, is called a gauche conformation
• At 120° and 2°, local maxima in energy exist. These maxima are not as high as the maximum at 0°, because although the conformation is eclipsed, the two methyl groups are not as close to each other as at 0° rotation.

To see if you understand the above explanation, answer the question regarding the models below:

The next part is the quiz to test your knowlegde. After the quiz, you can calculate the rotational energy-barriers of substituted alkanes of your choice.