preface to fifteenth edition

1.40 SECTION 1
FIGURE 1.2 Newman projections for
ethane. (a) Staggered; (b) eclipsed.
or groups and the conformation may be described as staggered. Newman projections show these
conformations clearly.
Certain physical properties show that rotation about the single bond is not quite free. For ethane
there is an energy barrier of about 3 kcal · mol1
(12 kJ · mol
1).
The potential energy of the molecule
is at a minimum for the staggered conformation, increases with rotation, and reaches a maximum at
the eclipsed conformation. The energy required to rotate the atoms or groups about the carboncarbon
bond is called torsional energy. Torsional strain is thecauseof therelativeinstability of the
eclipsed conformation or any intermediate skew conformations.
In butane, with a methyl group replacing one hydrogen on each carbon of ethane, there are several
different staggered conformations (see Fig. 1.3). There is the anti-conformation in which themethyl
groups areas far apart as they can be(dihedral angleof 180). There are two gauche conformations
in which themethyl groups areonly 60 apart; these are two nonsuperimposable mirror images of
each other. The anti-conformation is morestablethan thegauche by about 0.9 kcal · mol1
1
(4 kJ · mol ). Both are free of torsional strain. However, in a gauche conformation themethyl
groups are closer together than the sum of their van der Waals’ radii. Under these conditions van
der Waals’ forces are repulsive and raise the energy of conformation. This strain can affect not only
the relative stabilities of various staggered conformations but also the heights of the energy barriers
FIGURE 1.3 Conformations of butane. (a) Anti-staggered; (b) eclipsed; (c) gauche-staggered; (d)
eclipsed; (e) gauche-staggered; ( f ) eclipsed. (Eclipsed conformations are slightly staggered for convenience
in drawing; actually they are superimposed.)