The carbon chain in alkanes may connect with themselves to form rings. These hydrocarbons are the cycloalkanes. The simplest cycloalkane with three carbon atoms joined in a ring is cyclopropane. Cyclopropane has a high energy relative to propane because the three sp3 carbon atoms are distorted from their normal 109.5o bond angles to 60o.
Chime representation of cyclopropane. |
Cyclopropane can be represented as a line-bond structure showing all the carbon and hydrogen atoms or as a triangle. In this geometric representation, each corner of the figure represents a carbon atom and the hydrogens needed to form the four bonds to each carbon are omitted for clarity.
Very few naturally occurring molecules contain three membered rings due to the severe ring strain. An important exception is the natural insecticide pyrethrin I.
Cyclobutane, the four carbon ring, is also highly strained because the sp3 carbon atoms are forced from their normal 109.5o bond angles. Planar cyclobutane would have 90o bond angles, however, by twisting out of the plane it can assume a lower energy conformation. Two and three dimensional representations of cyclobutane are shown below.
Compounds containing the highly strained four-membered ring are rarely found in nature. One example is the insect pheromone grandisol, a sex attractant for boll weevils. The bicyclic compound a-pinene, a major constituent of turpentine, reacts readily with iodine in a highly exothermic reaction driven by relief of ring strain. During the reaction, the four-membered ring rearranges to a more stable, less strained, five-membered ring.
In the five membered cyclopentane ring, the bond angles are much less strained than in three and four membered rings. The cyclopentane molecule is puckered and looks somewhat like an envelope.
Five membered rings are found in many natural products. The chemical that makes catnip attractive to cats, nepetalactone,is an example of a naturally occurring cyclopentane derivative.
Cyclohexane (C6H12) is the smallest hydrocarbon that has tetrahedral 109.5o bond angles. This molecule is not planar but exists as a three dimensional molecule. The lowest energy arrangement of the atoms in cyclohexane is the chair conformation which is shaped like a patio chair. In this conformation, if you look straight along any bond, the carbon atoms that make up the rest of the ring are in staggered gauche-butane arrangements. A higher energy conformation occurs when adjacent bonds are eclipsed. This conformation is called a boat. These conformations are shown in the following diagrams which show the carbon skeleton (hydrogens are omitted for clarity). These two chair conformations are equal in energy and the barrier to rotation is small enough that this interconversion occurs readily at room temperature.
 |
| This model shows the gauche-butane interactions of chair cyclohexane. The shadow projection shows why it is called a chair. |
 |
|
Chair and boat conformations of cyclohexane. Hydrogen atoms omitted for clarity. |
|
 |
|
Newman projection of chair cyclohexane. |
Chime animations of chair and boat cyclohexane. |
|
The cyclohexane ring has two different kinds of hydrogen atoms. In the chair conformation the six hydrogens pointing perpendicular to the ring are designated axial and the six hydrogens near the plane of the ring are called equatorial.
Cyclohexane can flip from one chair conformation to another by rotation about C-C single bonds. In this interconversion, the axial hydrogens become equatorial and equatorial hydrogens assume axial positions.
|
| Click on the image for a video of the interconversions of the two chair conformations of cyclohexane. |
If we replace any of the six hydrogen atoms of cyclopropane with a methyl group, we obtain methylcyclopropane (click here for the rules for naming cycloalkanes).
Methylcyclopropane
With dimethylcyclopropane, we can place the second methyl group on the same carbon atom as the first and obtain 1,1-dimethylcyclopropane, or we can place it on one of the adjacent carbon atoms and obtain 1,2-dimethylcyclopropane. These are constitutional isomers.
The situation with 1,2-dimethylcyclopropane becomes even more complex because of the three-dimensional shape of the cyclopropane ring and the restriction the ring makes to free rotation about C-C bonds. The two methyl groups can be placed on the same side of the ring (cis- isomer) or on opposite sides (trans- isomer). These two isomers are sometimes called geometric isomers.
Cis- and trans-1,2-dimethylcyclopropane. |
|
.
Substituted cyclohexanes pose more challenges because of the existence of axial and equatorial bonds in the more stable chair conformation. Methylcyclohexane can exist in either the equatorial or axial conformation.
Methylcyclohexane in equatorial and axial conformations. The other hydrogen atoms are omitted for clarity.
These two conformations are not equivalent in energy. The equatorial conformation is more stable than the axial by 7.5 kJ mol-1. This energy difference corresponds to an equilibrium mixture composed of 95% equatorial methylcyclohexane at room temperature. The stability of the equatorial isomer relative to the axial is attributed to steric strain due to interaction of the axial methyl with the axial hydrogens on C-3 an C-5. Examine the Chime representations of the two isomers to visualize this repulsion. It is helpful to display the models with the van der Waals radii to illustrate the importance of electronic crowding and repulsion in the axial isomer.
|
|
Methyl cyclohexane in axial and equatorial conformations. |
|
With the dimethylcyclohexanes the situation becomes even more complex. Consider 1,4-dimethylcyclohexane. With two substituents on different carbon atoms of a ring, we again have the possibility for cis- and trans- isomers. If we place one methyl group in the equatorial position at C1 the cis- arrangement requires the methyl group at C4 to be in an axial position. When this isomer converts to the other chair form, the cis- arrangement of the groups does not change. Furthermore, because each conformation has one axial and one equatorial methyl group, the energy of the two conformations is the same.
In the case of the trans- isomer, if we place the methyl groups at C1 in the more stable equatorial position, the methyl group at C4 must occupy an equatorial position also. When the ring flips into the other chair conformation, the two methyl groups remain trans- but now occupy axial positions. These two conformations are no longer equivalent in energy. The diequatorial isomer is more stabile than the diaxial form by twice the steric strain energy of methylcyclohexane (2 x 7.5 kJ mol-1 = 15.0 KJ mol-1). This corresponds to an equilibrium mixture composed of about 99.5% of the diequatorial isomer at room temperature.
Example |
Draw both chair conformations of the following disubstituted cyclohexanes. Predict which conformation, if any, is the more stable. |
1 |
|
2 |
|
3 |
|
4 |
Cyclic organic compounds found in natural sources usually have 5 or 6 member rings. However many compounds may be found with larger rings. We have seen that cyclohexane does not have bond strain like the smaller three and four membered rings. Rings made up of seven and eight carbon atoms, called medium rings, possess steric strain due to van der Waals interaction with nonbonded atoms. The floppy nature of these medium rings can be shown by examining models of their conformations.
Large ring systems are also possible. Muscone, a natural product found in the scent gland of the musk deer, is used in musk perfumes and fragrances. This molecule possesses a 15 membered ring. Today commercial muscone is usually manufactured by chemical synthesis, not extracted from its natural source.
Many organic compounds are made up of several simple rings joined together. For example, decalin (bicyclo[4.4.0]decane) has two cyclohexane rings fused together. Two geometric isomers, cis-decalin and trans-decalin are possible.
|
|
Steroids are important natural products and include cholesterol and the hormones testosterone, estrogen, and progesterone. These steroids are tetracyclic compounds made up of three six-membered rings and one five-membered ring. The structure of cholesterol is shown below. Notice the chair conformation of the six-membered rings in the Chime representation.
Chemists respond to the beauty and challenge of synthesizing hydrocarbon compounds with several rings fused together to make more complex structures. One goal of synthetic chemistry is the preparation of the Platonic rectangular solids, prismane and cubane are simple examples of these structures. If two cyclopentane rings are joined together, the resulting structure looks somewhat like a child's drawing of a house and was named housane. Similarly, the polycyclic compound basketane looks like a basket. Adamentane is a 10 carbon polycyclic structure and has an atomic arrangement like the carbon atoms in diamond. Adamentane's very high melting (314oC) reveals the close packing of the molecules in the solid. Dodecahedrane is a twelve sided regular figure. Its symmetry gives the compound a very high melting point (> 450oC ).
| Name | Structure | |
| prismane |  |
|
| cubane |  |
|
| housane |  |
|
| basketane |  |
|
| adamentane |  |
|
| dodecahedrane |
|
