Lecture Summary 11 September 20, 2004 - Cook Group

Lecture Summary 11
September 20, 2004
Chapter 4 - Stereochemistry of Alkanes and Cycloalkanes
Conformations of Cyclopropane
Cyclopropane has a high degree of angle strain due to the highly distorted bond angles. The angle
between the carbons is 60°. This cannot be accommodated by sp 3 -hybridized atoms. Thus, the
bonds actually are bent sigma bonds.
60°
Cyclopropane only has three atoms in the ring, so it must be a flat structure. Indeed, it is the only
cycloalkane that is completely flat. This causes a high degree of torsional strain because all the
bonds are eclipsed.
Conformations of Cyclobutane
Cyclobutane is almost as high in ring strain energy as cyclopropane, but being a bigger ring, it has
just enough flexibility to pucker slightly. This helps to alleviate a little bit of the torsional strain. The
bonds are not quite eclipsed.
©2004 Gregory R. Cook page 1 Chem 341
North Dakota State University

Conformations of Cyclopentane
Cyclopentane has even more flexibility and can bend more. This removes a large amount of the
angle strain and the torsional strain. Note that at any given time cyclopentane has some bonds
more eclipsed than others.
Conformations of Cyclohexane
Cyclohexane has no ring strain. In the chair conformation, all the bonds are staggered and all the
angles are ideal. Note that the bonds that point straight up or down from the plane of the ring are
in axial positions and the bonds that point out away from the plane of the ring are called equatorial
positions.
Cyclohexanes are flexible enough to undergo a ring flip from one chair conformation to another at
room temperature (and below). When the chair flips, all the equatorial positions become axial and
all the axial positions become equatorial.
©2004 Gregory R. Cook page 2 Chem 341
North Dakota State University

If the cyclohexane ring is substituted with subsituents like a methyl group, the two chair
conformations are no longer equal in energy (like the all H cyclohexane). The conformation with
the methyl group in the axial position is higher in energy because the methyl group has steric
interactions with the other axial groups (1,3-diaxial interactions).
1,3-diaxial interaction
axial methyl
H
H
H
H
CH 3
H
ring flip
H
H
H
H
H
equatorial methyl
CH 3
1.8 kcal/mol more
stable conformation
axial methylcyclohexane side and top views
equatorial methylcyclohexane side and top views
©2004 Gregory R. Cook page 3 Chem 341
North Dakota State University