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Torsional Strain

What factors are important to the relative stability of different conformations?  There are a number of different strains that a molecule can impose on itself,  which you must be able to recognize.  

There are two types of strain important in acyclic systems;

  1. Torsional strain - destabilization of a molecule upon rotation about a bond.  Torsional strain is a maximum when atoms (typically protons) on adjacent atoms are eclipsed.
  2. Steric strain - destabilization of a portion of a molecule because of van der Waals repulsion.  This strain increases as two groups are brought closer together.  

In cyclic systems, you must also consider an additional strain.

  1. Angle strain - the deviation of bond angles of cyclic molecules from typical values expected from VSEPR.

Torsional Strain

Torsional strain is best explained by examining the two extreme conformations of ethane.  The C-C σ bond of ethane results from the overalp of two sp3 hybrid orbitals.  Because of the symmetry of this bond, the ethane bond can rotate about its single bond.  There are an infinite number of possible conformations, but only two extremes.  These are called eclipsed and staggered conformations.

Conformations of Ethane

staggered eclipsed

0 kj/mol

 

12 kj/mol

 

 The eclipsed conformation is 12 kj/mol higher in energy than the staggered.  Hence we would say that the eclipsed conformer is destabilized relative to the staggered conformer.  This is torsional strain and is recognized by the three sets of eclipsed hydrogens.  Thus each H/H eclipse cost the structure about 4 kj/mol.

In the staggered conformation, the dihedral angle between adjacent H atoms on adjacent atoms is 60o while in the eclipsed structure the dihedral angle is 0o.

The applet below illustrates, what a Newman project is, how to determine the dihedral angle, and the differences between the staggered and eclipsed conformations of ethane.

What causes torsional strain?

Torsional strain is a result of the lack of hyperconjugation in eclipsed ethane.  Hyperconjugation is a stabilizing effect.  In the staggered conformation a C-H σ bonds align with adjacent "empty" σ* antibonding orbitals.