Conformations

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.

  1. butane staggered, ethane staggered
  2. butane staggered, ethane eclipsed
  3. butane eclipsed, ethane eclipsed
  4. butane eclipsed, ethane staggered

Other factors that affect the stability of conformations are:

  1. Van der Waals forces: if atoms are closer to each other than the sum of their Van der Waals radii, they start to repel one another, resulting in Van der Waals strain. Also simply called steric hindrance.
  2. dipole-dipole interactions: atoms that are not covalently linked to one another try to position themselves in such a way as to optimise these electrostatic interactions.

Back to index Back to index To the next exercise