Organic Chemistry I For Dummies
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Although the hydrocarbon cyclohexane is typically drawn as if it were flat, in reality the structure is not flat at all. Most of the time, the structure exists in what is called the chair conformation. This conformation is called the chair because it looks (sort of) like a reclining lounge chair, as shown here.

The chair conformation of cyclohexane.

The chair conformation of cyclohexane
Other conformations for cyclohexane do exist — and they include the boat, half-chair, and twist-boat conformations — but the most important conformation is the chair because this conformation is typically the most stable conformation.

The importance of drawing chairs

Many students find that drawing chairs can be somewhat difficult at first, but mastering the correct drawing of this conformation is essential for your success in organic chemistry. To begin, start by drawing two lines that are parallel to each other but not perfectly horizontal, as shown here. Next, add a downward-pointing V tip to one end (this is the tail of the chair). Finally, add an upward-pointing V tip to the other end (this is the nose of the chair).

The steps involved in drawing the chair conformation of cyclohexane.

The steps involved in drawing the chair conformation of cyclohexane

Axial hydrogens, equatorial hydrogens

A cyclohexane chair contains two kinds of hydrogens — axial hydrogens and equatorial hydrogens. Axial hydrogens are those hydrogens that stick straight up or straight down parallel to an imaginary axis through the chair; equatorial hydrogens are hydrogens that stick out along the equator of the chair. Both types are shown here.

The axial and equatorial hydrogens on cyclohexane.

The axial and equatorial hydrogens on cyclohexane

In drawing the hydrogens on a chair cyclohexane, it's often easiest to draw the axial hydrogens first. At any point on the chair that sticks up, put the axial hydrogen sticking straight up; at any point on the chair that sticks down, draw the axial hydrogen straight down. After the axial hydrogens are drawn, adding in the equatorial hydrogens around the equator of the chair is a fairly straightforward task.

The ring flip

At room temperature, cyclohexane doesn't stay in one-chair conformation, but rapidly interconverts into an alternative chair conformation with a ring flip, shown here.

The ring flip of cyclohexane.

The ring flip of cyclohexane
With the chair on the left of the figure, the nose of the cyclohexane chair goes down, and the tail of the cyclohexane goes up to make the new chair cyclohexane conformer. With a ring flip, all hydrogens that were originally axial become equatorial, and all hydrogens that were equatorial become axial. It's a good idea to get out your molecular models and try this ring flip yourself, so you can see it in three dimensions.

When a ring flip doesn't change the molecule

With unsubstituted cyclohexane (a cyclohexane that has only hydrogens attached to it), undergoing a ring flip doesn't change the molecule. With substituted cyclohexanes, however, the two chair conformers may not be identical. For example, isopropylcyclohexane is shown here. One chair conformer puts the isopropyl group in the axial position. After undergoing a ring flip, the isopropyl becomes equatorial.

The chair conformers of isopropylcyclohexane.

The chair conformers of isopropylcyclohexane

Ring flips change all axial positions to equatorial and all equatorial positions to axial.

These two conformers are not identical, and they don't have the same energy. When a large group is axial, the large group invades the space of the hydrogens on carbons two positions away, introducing 1,3-diaxial strain. This interaction increases the energy of the axial conformer. Therefore, as a rule, large groups prefer to be equatorial because this conformation has no 1,3-diaxial strain — as shown in the figure, the right-pointing arrow is longer, indicating the direction of the equilibrium favors the more stable chair conformation.

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Arthur Winter, PhD, is the author of the popular Organic Chemistry Help! website and Organic Chemistry I For Dummies. His professional focus is on the chemistry of magneto-organic materials.

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