Going Round in Circles

So far we've seen straight chains, and branched chains, of atoms. Another common shape for molecules is a ring. The most stable rings are made up of 5 or 6 atoms, usually mostly or entirely carbon atoms, bonded together to make a pentagon or hexagon. Larger or smaller rings are also known, but these are the two most common sizes.

Check out these two representations of cyclohexane, the cycloalkane with six carbon atoms. Cyclo- is a prefix meaning bonded around into a ring. Instead of a ball and stick model, I've opted to show just the sticks, so that it will be easier to see the molecule's structure.

Go ahead and use the mouse to rotate the figures around. Notice that the ring is not a flat hexagon, but has a puckered shape. This happens because the carbon atoms are tetrahedral, with an angle of about 109° between any two bonds, while the angle inside a hexagon is 120°. The form shown above is called the chair form of cyclohexane, because of its resemblance to the shape of a chair.

Notice that some of the hydrogen atoms in the chair form are sticking out to the sides, while others are pointing straight up or down from the plane of the ring. The sideways hydrogens are called equatorial, while the up and down ones are called polar.

Let's see cyclohexane again, but with the hydrogens hidden. (You may have to rotate the models to fully see their shapes.) One of these models shows the chair form again, while the other shows the boat form.

In fact, cyclohexane freely interconverts between the chair and boat forms, as well as any number of intermediate shapes. But the most stable shape is the chair form.

In cyclopentane, the tetrahedral angle of approximately 109° is very close to the internal angle of a pentagon which is 108°. Consequently, cyclopentane rings are more flat than cyclohexane rings. A perfectly flat ring is said to be planar. But cyclopentane is not perfectly planar because it puckers somewhat in order to keep its hydrogen atoms farther apart. Here is a model of cyclopentane:

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