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Cycloalkane
Conformations
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In the previous tutorials on conformations, we learned
the factors that affect the energy (stability) of conformations as
a result of rotation around a C-C bond of acyclic (non-cyclic) alkanes.
In this tutorial, we will examine the different conformations of cyclic
alkanes, in particular, cyclohexane.
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Compounds with only single bonds attempt to maintain
a bond angle of near 109°. This "optimum" bond
angle is sometimes impossible due to the strains in a molecular
structure. For example, the C-C-C bond angle in cyclopropane
("live" structure is shown below) is only 60°,
with a 49° deviation from the normal tetrahedral angle
of 109°. This causes the energy of cyclopropane to increase,
and the extra energy is attributed to the angle
strain. (Note: cyclopropane also has very high
torsional strain due to eclipsed C-H bonds)
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Cyclopropane
Rotate Me!
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It is important to understand the conformations of cyclohexane
since it six-membered ring is abundant in drugs and many natural
products. The most stable conformation of cyclohexane is the
chair conformation, in which all bond angles are close to
the tetrahedral 109°. Due to the chair conformation,
there are two different positions: axial and equatorial. The
axial H's are shown here with white color and the equatorial
H's with red color. Note that all axial H's are pointing straight
up and down, while the equatorial ones pointing towards the
outside of the ring.
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Cyclohexane
Rotate Me!
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There are actually two chair conformations for cyclohexane,
which interconvert through a process called "ring flip".
This can be viewed in the following "live" models by right-clicking
on the left image and select "animation/start". The image
displayed on the right can be used as a reference to which the animated
one is compared. Note that as the ring flips, the red hydrogens change
from the equatorial positions (as shown on the right) to the axial
positions and then back. To freeze the animation,
right-click and then select "animation/stop".
From the above "live" ring-flipping, we
conclude that substituents on axial positions change to occupy equatorial
positions when a ring flips, and vice versa. This process is normally
extremely fast at room temperature!
As cyclohexane goes from one chair conformation to
the other (note that the two chairs are equivalent), it has to go
through several other forms (conformations): half-chair, twist boat,
and boat. Shown below are the "live" molecules for these
three conformations. After visualizing them, test yourself on the
following questions:
a) Why is the half-chair conformation very unstable
(with high energy)?
b) Why is twist boat less stable than the chair conformation?
c) What is responsible for the fact that twist boat is slightly
more stable than the boat conformation?
d) Identify the flag-pole hydrogens in the boat conformation.
Next we will explore the shape of other cycloalkanes:
cyclopropane, cyclobutane, and cyclopentane.
Note that there are two shapes for cyclobutane and
two for cyclopentane. Which cyclobutane model is of lower energy?
