CONFORMATIONAL ISOMERS
59
Dihedral angle (degrees)
Relative energy
0
12 kJ mol −1
60
120
180
240
300
eclipsed
staggered
eclipsed
staggered
staggered
ethane
Figure 3.1 Energy diagram: ethane conformations
Similarly, of the low-energy staggered conformers, there will be two equivalent ones where the
carbon–methyl bonds are inclined at 60
◦ to each
other, and one in which the carbon–methyl bonds
are inclined at 180
◦ . We can also predict that
the latter conformer, which has the methyl groups
as far away from each other as possible, will
be of lower energy than the alternative staggered
conformers, where there must be at least some
spatial interaction between the methyl groups. The
staggered conformer with maximum separation of
methyl groups is termed the anti conformer (Greek:
anti = against), whilst the two other ones are termed
gauche conformers (French: gauche = left). The
energy diagram observed (Figure 3.2) reflects these
predictions, and the energy difference between the
Relative energy
0
120
60
0
60
120
3.8
15.9
18.8 kJ mol −1
180
180
butane
anti
anti
gauche
gauche
Me Me dihedral angle (degrees)
Figure 3.2 Energy diagram: butane conformations