Page 163 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

staggered conformation at any given instant, but each molecule can rapidly traverse 143
9 −1

through the eclipsed conformation. The rate of rotation is about 6×10 s at 25 C.
SECTION 2.2
Conformation
H a H H a
a H c H a
H c H H H b
H c H b b c H b

Shortly,wewilllearnthatforsomehydrocarbonmolecules,vanderWaalsrepulsions
are a major factor in conformational preferences and energy barriers, but that is not the
case for ethane. Careful analysis of the van der Waals radii show that the hydrogens do not
15
come close enough to account for the barrier to rotation. Furthermore, the barrier of just
under 3 kcal is applicable to more highly substituted single bonds. The barrier becomes
significantly larger only when additional steric components are added, so the barrier must
be an intrinsic property of the bond and not directly dependent on substituent size. The
barrier to rotation is called the torsional barrier. There are analogous (although smaller)
barriers to rotation about C−N and C−O bonds. Topic 1.3 probes further into the origin
of the torsional barrier in small molecules. The conclusion reached is that the main factor
∗
responsible for the torsional barrier is - delocalization (hyperconjugation), which
favors the staggered conformation.

H

H
hyperconjugation
in anti conformation

The interplay between the torsional barrier and nonbonded (van der Waals) inter-
actions can be illustrated by examining the conformations of n-butane. The relationship
between energy and the torsion angle for rotation about the C(2)−C(3) bond is
presented in Figure 2.11. The potential energy diagram of n-butane resembles that
of ethane in having three maxima and three minima, but differs in that one of the
minima is lower than the other two and one of the maxima is of higher energy than
the other two. The minima correspond to staggered conformations. Of these, the anti
is lower in energy than the two gauche conformations. The energy difference between
the anti and gauche conformations in n-butane is about 0.6 kcal/mol. 16 The maxima
correspond to eclipsed conformations, with the highest-energy conformation being the
one with the two methyl groups eclipsed with each other.
The rotational profile of n-butane can be understood as a superimposition of
van der Waals repulsion on the ethane rotational energy profile. The two gauche
conformations are raised in energy relative to the anti by an energy increment resulting
from the van der Waals repulsion between the two methyl groups of 0.6 kcal/mol. The

15 E. Eliel and S. H. Wilen, Stereochemistry of Organic Compounds, Wiley, New York, 1994, p. 599.
16
G. J. Szasz, N. Sheppard, and D. H. Rank, J. Chem. Phys., 16, 704 (1948); P. B. Woller and
E. W. Garbisch, Jr., J. Am. Chem. Soc., 94, 5310 (1972).