All About the Chair Conformation

Transcription

1 All About the Chair Conformation Background Before we begin, here are some terms to know: 1. Conformation: the shape that a molecule can adopt due to rotation around one or more single bonds 2. Angle strain: strain due to deviation from one or more ideal bond angles 3. Torsional strain: strain caused by van der Waals repulsion which can be reduced or eliminated by rotation around a single bond Now, what s so special about the chair conformation? Molecules will try to adopt the most stable conformation that minimizes strain. For cyclohexane, C 6 H 12, this would be the chair conformation. Let s see why. versus If cyclohexane was planar, its structure would look like the picture to the left. All the carbons in cyclohexane are sp 3 hybridized, so the ideal bond angle for a tetrahedral atom would be degrees. However, the planarity of the ring would force the carbons to have bond angles of 120 degrees, which makes it unstable. There would also be torsional strain, as all 12 carbon-hydrogen bonds would be fully eclipsed. To gain more stability, cyclohexane adopts the chair conformation instead. The chair conformation is a six-membered ring in which atoms 2, 3, 5, and 6 lie in the same plane, atom 1 lies above the plane, and atom 4 lies below the plane. We will examine how to draw and number the structure later. With this conformation, the bond angles are degrees, much closer to the ideal degrees. All the carbon-hydrogen bonds are also fully staggered, eliminating the torsional strain. Together, these features make the chair conformation very stable.

2 How to Draw the Chair Conformation The chair conformation consists of three sets of parallel lines, drawn in one set at a time. It is important to draw the structure clearly so that the substituents can be placed correctly on the ring. Now let s look at axial and equatorial bonds. Axial bonds are parallel to the axis of the ring, while equatorial bonds are perpendicular to the axis of the ring and lie along the equator of the chair. Notice that each carbon has one axial and one equatorial bond. When drawing the bonds, it is important to clearly distinguish between axial and equatorial. The equatorial bonds should be drawn parallel to the lines representing the carbon-carbon bonds in the ring. After the equatorial bonds are drawn, fill in the axial bonds by drawing lines up or down. Without any other substituents, the final chair conformation would have hydrogen atoms at the ends of all the axial and equatorial bonds. The structure would look like this:

3 Now let s number the structure and place substituents on the ring. The carbons can be numbered any way, as long as you are consistent in counting either clockwise or counterclockwise. One of the more common ways to number the carbons is like this: Now for the substituents. Solid wedges indicate that the bond projects outward towards the viewer, and broken (dashed) wedges indicate that the bond recedes away from the viewer. For simplicity in drawing, we will label atoms on solid wedges as being up on the ring and atoms on dashed wedges as being down on the ring. This makes sense, because in the chair conformation, each attachment to a carbon will have either an up or down orientation. It is important to note, however, that there is no correlation between up/down and axial/equatorial. Let s take a look at an example. First we number the carbons on the structure to the left. The OH group is on carbon 1. We see that it is on a solid wedge, so we place it up on carbon 1 on the chair. For the methyl group on carbon 3, we see that it is on dashed wedge, so we place it on down on carbon 3 on the chair. In this case, the OH group happens to be axial up and the methyl group happens to be equatorial down. But there is no relation between our naming of up/down and axial/equatorial. For example, if the OH was on carbon 2, then it would be up on the equatorial position instead. It just depends which carbon the substituent is on. Flipping the Chair When the chair is flipped, all the axial positions become equatorial, and all the equatorial positions become axial. It is also important to note that even after the chair flips, all the up substituents remain up and all the down substituents remain down.

4 In this example, the OH on carbon 1 switches from axial to equatorial, but it is still in the up position. The methyl group on carbon 3 switches from equatorial to axial, but it is still in the down position. To draw the flipped conformation, follow the same steps as before, but take note of the difference in drawing the parallel lines. In the picture before, the difference is indicated in the color red. Also use the same method to number the carbons as before. This time, however, the more common way to number the carbons is starting with carbon 1 on the bottom right corner. Also take note of the arrow between the conformations that indicate equilibrium. The cyclohexane structure continuously flips from one chair conformation to the other. Without any substituents, both forms are equal in energy and are equivalent to each other. However, once substituents are placed on the ring, the two forms may not be equal in energy, and the molecule may spend more time in one conformation compared to the other. The next section will address this difference in stability. Comparing the Stability of Chairs Substituents on the chair conformation prefer to be in the equatorial position. The reason is that when substituents are in the axial position, there tends to be more unfavorable interactions with other axial atoms on the same side. These unfavorable interactions are called 1,3-diaxial interactions. When substituents are in the equatorial position, they are farther away from each other. This increases the stability of the conformation. In the pictures below, the methyl in the equatorial position is more stable because it avoids interaction with the hydrogen atoms. The larger the group is, the more it will tend to remain in the equatorial position. Therefore, when trying to determine which chair is more stable, place the larger group in the equatorial position.

5 versus Which structure is more stable, the left or the right? The C(CH 3 ) 3 group is larger than the chlorine atom so it will have a tendency to remain in the equatorial position. Therefore, the structure on the left is more stable. Also note that when you have two groups and they can be either both axial or both equatorial, it is more stable when they are both equatorial. versus The conformation on the right is more stable because both substituents are equatorial. Determining Cis and Trans Cis means on the same side and trans means on opposite sides. When both substituents are both up or both down, they are cis to each other. When one group is up and one group is down, we call them trans to each other. Let s look at three examples. 1) 2) 3) 1) Both substituent groups are on dashed wedges. They are both down. They are cis. 2) The OH group is on a dashed wedge, whereas the methyl group is on a solid wedge. One is down, while the other is up. They are trans. 3) Both groups are on solid wedges, so they are both up. They are cis. Works Cited - Illustrated Glossary on course website - Professor Lavelle s Chem 14B Organic Chemistry course reader - Google images