Page 168 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

148 reactions of alkenes. Allylic strain and other conformational factors contribute to
the relative energy of competing TSs, and can lead to a preference for a particular
CHAPTER 2 stereoisomeric product.
Stereochemistry, The preferred conformations of carbonyl compounds, like 1-alkenes, are eclipsed
Conformation,
and Stereoselectivity rather than bisected, as shown below for ethanal and propanal. The barrier for methyl
group rotation in ethanal is 1.17 kcal/mol. 31 Detailed analysis has indicated that small
adjustments in molecular geometry, including -bond lengthening, must be taken
into account to quantitatively analyze the barrier. 32 The total barrier can be dissected
into nuclear-nuclear, electron-electron, nuclear-electron, and kinetic energy t ,as
described in Topic 1.3 for ethane. MP2/6-311+G(3df,2p) calculations lead to the
contributions tabulated below. The total barrier found by this computational approach
is very close to the experimental value. Contributions to the ethanal energy barrier in
kcal/mol are shown below.

V nn −10 621
V −5 492
ee
V +18 260
ne
t −0 938
total +1 209

In propanal, it is the methyl group, rather than the hydrogen, that is eclipsed
with the carbonyl group in the most stable conformation. The difference in the
two eclipsed conformations has been determined by microwave spectroscopy to be
0.9 kcal/mol. 33 A number of other aldehydes have been studied by NMR and found
to have similar rotameric compositions. 34 When the alkyl substituent becomes too
sterically demanding, the hydrogen-eclipsed conformation becomes more stable. This
is the case with 3,3-dimethylbutanal.

H O CH 3 O H O
H H H
H H H H (CH ) C H
3 3
preferred conformations for ethanal, propanal, and 3, 3-dimethylbutanal

Ketones also favor eclipsed conformations. The preference is for the rotamer in
which the alkyl group, rather than a hydrogen, is eclipsed with the carbonyl group
because this conformation allows the two alkyl groups to be anti rather than gauche
with respect to the other carbonyl substituent.

O O
R' H
R R
H H
H R'
more stable less stable

31 I. Kleiner, J. T. Hougen, R. D. Suenram, F. J. Lovas, and M. Godefroid J. Mol. Spectros., 153, 578
(1992); S. P. Belov, M. Y. Tretyakov, I. Kleiner, and J. T. Hougen, J. Mol. Spectros., 160, 61 (1993).
32
L. Goodman, T. Kundu, and J. Leszczynski, J. Am. Chem. Soc., 117, 2082 (1995).
33 S. S. Butcher and E. B. Wilson, Jr., J. Chem. Phys., 40, 1671 (1964).
34
G. J. Karabatsos and N. Hsi, J. Am. Chem. Soc., 87, 2864 (1965).