In this module, we will learn which factors influence the stability of a
conformation. Knowledge of these factors enables one to make predictions
about the most probable conformation of a molecule.
An important factor that decreases the conformational freedom of a molecule is the tendency of atoms to have bond angles that match the angles of its bonding orbitals (e.g. 109.5° in sp3-hybridised carbon). Deviation from the ideal bond angle is accompanied by angle strain.
Another degree of conformational freedom in a molecule like propane is the
rotation around the C-C bonds. An energy minimum exists when the groups on one
carbon are rotated 60° with respect to the groups on the adjacent carbon
atoms. This conformation is called the staggered conformation. Deviation
from this conformation causes torsional strain, which reaches a maximum
in the eclipsed conformation.
Normally, the flexibility of a molecule is due to torsional degrees of freedom. For each sp3-sp3 single bond we have three staggered conformations, whose relative energy depends on the substituents present. Remember that in butane the anti conformation is slightly more stable than the gauche conformation.
At the right, butane and ethane are shown. Rotate the molecules using the left mouse button to determine the conformation of both molecules. Check your answer here.
Other factors that affect the stability of conformations are: