Conformations
15.2 Conformations and energy.
Different conformations have different potential energies. The staggered conformation is
at a lower potential energy than the eclipsed conformation, and is favored. In ethane,
the barrier to rotation is approximately 25 kJ/mol, indicating that each pair of eclipsed
hydrogens raises the energy by about 8 kJ/mol. This number also applies to other organic
compounds which have hydrogen atoms at similar distances from each other. At very low
temperatures all conformations revert to the stabler, lower energy staggered conformation.
15.3 Steric effects
Steric effects have to do with size. Two bulky objects run into each other and invade each
others space. If we replace one or more hydrogen atoms on the above Newman projections
with a methyl or other group, the potential energy goes up especially for the eclipsed
conformations.
Lets look at a Newman projection of butane as it rotates counterclockwise around its axes.