ChemActivity 4.1: Conformations of Butane
1. Build butane.
2. Unlike ethane, the staggered conformations of butane are NOT equal in energy.
a. Draw Newman projections for the three possible staggered conformations for butane.
b. Circle the most stable conformation.
3. The most stable conformation of butane is called the anti conformation. Take a moment
to think about why it would be called “anti.” What is the torsion angle between the two
methyl groups in this conformation? ____
4. When the torsion angle is approximately 60°, we say that the spatial arrangement is
gauche. Since the other two staggered conformations are both gauche, what does this tell
you about the relative energy of these two conformations (compared to each other)?
5. Both of the gauche conformations are staggered, so they are free of torsional strain;
however they are not as stable as the anti conformation. Why do you think this is?
Two of the methyl hydrogens of the gauche conformation lie within 210 pm of each
other. The distance is less than the sum of their van der Waals radii (240 pm), and there
is a repulsive force between them. This destabilization is called steric hinderance or van
der Waals strain. How does this differ from torsional strain?
6. Draw the least stable conformation of butane. What type of strain is present in this
molecule (2 types)?
7. Considering what you now know about the conformations of butane, fill in the table
below with the appropriate Newman projections. Use torsional angles on the x-axis to
help guide you.
8. What is the amount of potential energy (in kcal/mol) required to rotate from a torsion
angle from 180° (D) to 360° (G)? ____________kcal/mol
What is another name for this energy?
9. How many sets of bonds are eclipsed in the highest energy conformation (A or
G)?_____Since you know that generally, 1 kcal/mol of energy is required for each set of
eclipsed bonds (torsional strain), what would you predict the energy of A or G to be?
_____ What is the energy of A or G? ________ What type of strain contributes to the
extra ~3 kcal/mol of energy? __________