Page 171 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

favor the skew conformation over the planar s-cis, but the energy differences found 151
are quite small. 39 41
The case of ,ß-unsaturated carbonyl compounds is analogous to that of 1,3- SECTION 2.2
dienes, in that conjugation favors coplanarity of the C=C−C=O system. The rotamers Conformation
that are important are the s-trans and s-cis conformations. Microwave data indicate that
the s-trans form is the only conformation present in detectable amounts in 2-propenal
(acrolein). 42

H H
H H
O H
H H H O
s-trans s-cis

The equilibrium distribution of s-trans and s-cis conformations of substituted ,ß-
unsaturated ketones depends on the extent of van der Waals interaction between the
43
C(1) and the C(4) substituents. Methyl vinyl ketone has the minimal unfavorable van
der Waals repulsions and exists predominantly as the s-trans conformer.

H H
H H
O CH 3
H CH 3 H O
s-trans (73%) s-cis (27%)

When larger alkyl groups are substituted for methyl, the ratio of the s-cis form progres-
sively increases as the size of the alkyl group increases. 44

H
H CH
CH 3 3
R
O
H
H R O
R s-trans s-cis
CH 3 0.7 0.3
C H 0.55 0.45
2 5
(CH ) CH 0.3 0.7
3 2
) C 0.0 1.0
(CH 3 3

An unfavorable methyl-methyl interaction destabilizes the s-trans conformation of
4-methylpent-3-en-2-one (mesityl oxide) relative to the s-cis conformation, and the
equilibrium favors the s-cis form.

41 J. Breulet, T. J. Lee, and H. F. Schaefer, III, J. Am. Chem. Soc., 106, 6250 (1984); D. Feller and
E. R. Davidson, Theor. Chim. Acta, 68, 57 (1985).
42
E. A. Cherniak and C. C. Costain, J. Chem. Phys. 45, 104 (1966).
43 G. Montaudo, V. Librando, S. Caccamese, and P. Maravigna, J. Am. Chem. Soc., 95, 6365 (1973).
44
A. Bienvenue, J. Am. Chem. Soc., 95, 7345 (1973).