Students often have difficulty drawing proper chair conformations of molecules that contain 6-membered rings. Also, the projection of the 3-dimensional shape of the ring onto a 2-dimensional surface can create optical illusions. The only way to avoid the difficulties associated with drawing and perceiving such structures correctly is to practice. Here's a set of guidelines.
Draw a bond-line notation of one of the two chair conformations of a cyclohexane molecule:
The color-coding in structures 1 and 2 emphasizes the parallel disposition pairs of bonds.
Add the axial C-H bonds: a. draw a vertical line projecting upwards from the carbon at the top-right of 1 or the top-left of 2. Add an H.
b. Draw the five other axial C-H bonds. They alternate- down, up, down, up, down- from one carbon atom to the next:
Note that the 6 axial hydrogens comprise two sub-sets; 3 axial up, and 3 axial down. Bonds at the "rear" of the projection have a break in them where they intersect bonds at the "front" of the projection.
Add the equatorial C-H bonds: a. draw a horizontal line from the carbon at the top-right of 1 or the top-left of 2. Add an H. The color-coding emphasizes that this bond is parallel to each of the C-C bonds that are connected to the neighboring carbon atoms.
b. Draw the five other equatorial C-H bonds. Each one should be parallel to the C-C bonds from the neighboring carbons as indicated by the color-coding in the following diagram.
Note that the 6 equatorial hydrogens comprise two sub-sets; 3 equatorial up, and 3 equatorial down (actually horizontal in the perspectives shown here).
Now that you know how to draw a chair conformation of a 6-membered ring, let's look at the changes that occur during a chair-chair interconversion. The essential features are shown in Figure 1.
Starting with conformation 1, rotation about the C1-C2 bond moves C1 down to the position shown in conformation 2. Simultaneous rotation moves C4 up. Now look at the changes in the positions of the hydrogen atoms attached to C2: the equatorial H that projects upward becomes an axial hydrogen that is still projecting upward; the axial H pointing down becomes an equatorial H that is still oriented down. This axial-equatorial interchange occurs at every carbon atom. In other words, during a chair-chair interconversion, all the axial substituents on the ring become equatorial and all the equatorial groups become axial. However, their relative orientations do not change. Groups that point upwards in one chair conformation point upwards in the other. Groups that project down remain pointing down.
Exercise 2 Select the conformation that would be formed by chair-chair interconversion of structure 1.
d
Exercise 3 The following questions refer to structures a-d:
a. How many 1,3-diaxial interactions are there in each of these conformations? a. b. c. d.
b. How many 1,3-diaxial interactions are there in the other chair conformation of conformations a-d? a. b. c. d.
c. Is the chair conformation shown more stable than the one that is not shown?
Conformation a Yes No
Conformation b Yes No
Conformation c Yes No
Conformation d Yes No
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