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.
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°
To see if you understand the above explanation, answer the question
regarding the models below:
Above are four models of 1,2-dichloroethane. Which of the lines below lists
the conformations in order of decreasing energy?
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.