Conformations of Acyclic Alkanes

Acyclic and cyclic alkanes are continuously in motion, flipping, twisting, bending and rotating externally and internally.  If you could take a lot pictures of a molecule with a very very fast camera you would notice most of the pictures would correspond to conformations of low energy.  Indeed molecules will spend most of their time in low energy stable conformations and much shorter amounts of time in high energy unstable conformations.

Chemists depict the conformations of molecules by two different drawing methods; Sawhorse and Newman projections.

What controls the 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.  Ultimately must be minimized for a molecule to rest in a stable state or conformation.    

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

 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 60^o while in the eclipsed structure the dihedral angle is 0^o.

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 bond lines up parallel with an adjacent "empty" σ* antibonding orbital. 

Steric Strain

Steric strain becomes important in the conformations of propane, butane, and other larger molecules.  Like ethane, butane has an infinite number of conformations.  The four important to organic chemists are shown below.  When the two methyl groups are at a dihedral angle of 180^o it is known as the anti conformation which is the most stable conformation of butane.  In anti, there is no torsional or steric strain.  The next highest energy conformation of importance is the gauche conformation which lies about 3.8 kj/mol above the anti.  In the gauche conformation there is no torsional strain.  This 3.8 kj/mol is entirely a result of steric strain (repulsion) between the two methyl groups that are at a dihedral angle of 60^o.   The two eclipsed conformations are obviously much higher in energy than the staggered.  In the eclipsed conformations there is both torsional and steric strain.