Changes in energy as one methyl of ethane is rotated relative to the other can be represented by a sinple graph:

The zero of energy is the staggered form (dihedral 180^o); the maximum is the eclipsed isomer (dihedral 0^o), with intermediate angles having intermediate energies.

Here are the two extreme conformers in JMol windows:

The staggered conformation

The eclipsed conformation

• Left click and drag in the window to change the orientation of the molecule, so that, for example, you can see the view along a bond

• Click the radio buttons below the structure to change the atoms display: off, 20% of van der Waals, or full vader Waals radius.

• Right click in the window to bring up a menu with numerous options for viewing and manipulating the structures

Rotation around the central C2-C3 bond in butane likewise involves conformers of different energies:

In this case there are two different staggered conformers (anti and gauche), and two different eclipsed conformers (methyl-methyl and methyl-hydrogen).

Here are the two eclipsed and two staggered conformations of butane, for rotation around the C2-C3 bond..

The anti- conformation	Methyl eclipsed with hydrogen
Skew butane - methyls at 60 degrees to each other	Methyl eclipsed with methyl

Using the ideas and energies applied above, we can analyze the conformations of any acyclic alkane:

• Draw a Newman projection of any one conformer

• Generate the other five distinct conformers by successive 60^o rotations
  • You should find three eclipsed and three staggered conformers

• All eclipsed conformers will be higher in energy than all staggered conformers

• Estimate the energies of the conformers by counting:
  • 1.0 kcal/mol for each H-H eclipsed

  • 1.2 kcal/mol for each H-CH₃ eclipsed

  • 5.5 kcal/mol for each CH₃-CH₃ eclipsed

  • 0.8 kcal/mol for each CH₃-CH₃ gauche