Page 170 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

150 H H O
H
CHAPTER 2 H
CH H
Stereochemistry, 3 H H
Conformation,
and Stereoselectivity
Moreover, electron diffraction studies of 3-pentanone indicate the methyl-eclipsed
conformation shown below to be the most stable rotamer. 37
O
CH 3 CH 3
H H
H H
preferred conformation
for 3-pentanone

The pattern, then, is that methyl and unbranched alkyl groups prefer to be eclipsed
with the carbonyl group.
1,3-Dienes adopt conformations in which the double bonds are coplanar, so as
to permit optimum -orbital overlap and electron delocalization. The two alternative
planar conformations for 1,3-butadiene are referred to as s-trans and s-cis. In addition
to the two planar conformations, there is a third conformation, referred to as the skew
conformation, which is cisoid but not planar. Various types of structural studies have
shown that the s-trans conformation is the most stable one for 1,3-butadiene. 38 A
small amount of the skew conformation is also present in equilibrium with the major
conformer. 39 The planar s-cis conformation incorporates a van der Waals repulsion
between the hydrogens on C(1) and C(4), which is relieved in the skew conformation.

H
H H
H H H H CH 2
H H CH
H 2
H H skew
H
H
s-trans s-cis

The barrier for conversion of the skew conformation to the s-trans is 3.9 kcal/mol.
The energy maximum presumably refers to the conformation in which the two bonds
are mutually perpendicular. The height of this barrier gives an approximation of the
stabilization provided by conjugation in the planar s-trans conformation. Various MO
calculations find the s-trans conformation to be 2–5 kcal/mol lower in energy than
either the planar or skew cisoid conformations. 40 Most high-level MO calculations

37 C. Romers and J. E. G. Creutzberg, Rec. Trav. Chim., 75, 331 (1956).
38
A. Almenningen, O. Bastiansen, and M. Traetteburg, Acta Chem. Scand., 12, 1221 (1958);
K. K. Kuchitsu, T. Fukuyama, and Y. Morino, J. Mol. Struct., 1, 643 (1967); R. L. Lipnick and
E. W. Garbisch, Jr., J. Am. Chem. Soc., 95, 6370 (1973).
39 K. B. Wiberg and R. E. Rosenburg, J. Am. Chem. Soc., 112, 1509 (1990).
40
A. J. P. Devaquet, R. E. Townshend, and W. J. Hehre, J. Am. Chem. Soc., 98, 4068 (1976); K. B. Wiberg,
P. R. Rablen, and M. Marquez, J. Am. Chem. Soc., 114, 8654 (1992); M. Head-Gordon and J. A. Pople,
J. Phys. Chem., 97, 1147 (1993).