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Tutorial topics:


Viewing Basics

Molecular Shapes



Substituted Cyclohexane


SN2 Reaction

SN1 Reaction



Cycloalkane Conformations

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.

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)

Rotate Me!

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.

Rotate Me!

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?

cyclopentane half-chair

cyclopentane "envelope"


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This page was last updated on September 4, 2001 by Dr. Linfeng Xie.
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