Figure 2.5. Potential Energy vs. Degree of Rotation about the C-2 to C-3 Bond in ButaneKEY CONCEPT
Notice that the anti staggered isomer (A and G) has the lowest energy, whereas the totally
eclipsed isomer (D) has the highest energy.These conformational interconversion barriers are small ( between anti staggered butane
and totally eclipsed butane) and are easily overcome at room temperature. Nevertheless, at very
low temperatures, conformational interconversions are dramatically slow. If the molecules do not
possess sufficient energy to cross the energy barrier, they may not rotate at all (as happens to all
molecules at absolute zero).
Cyclic Conformations
Cycloalkanes can be either fairly stable compounds or fairly unstable—depending on ring strain.
Ring strain arises from three factors: angle strain, torsional strain, and nonbonded strain
(sometimes referred to as steric strain). Angle strain results when bond angles deviate from their
ideal values by being stretched or compressed. Torsional strain results when cyclic molecules must
assume conformations that have eclipsed or gauche interactions. Nonbonded strain (van der