Chapter 5: Conformations of Acyclic Alkanes and Cyclohexanes

Contents

5.0 Introduction
5.1 Acyclic Alkanes
5.2 Conformational Analysis of Ethane
5.3 Conformational Analysis of Butane
5.4 Conformation Stability of Acyclic Alkanes
5.5 Conformations of Cyclohexanes
5.6 Visualizing the 3D Structure of Cyclohexane
5.7 Drawing Cyclohexane in a Chair Conformation
5.8 Conformations of Substituted Cyclohexanes and Potential Energy
5.9 More than one Substituent on Cyclohexane 
5.10 Chapter Summary 

Learning Objectives

• Visualizing and drawing three-dimensional structures of acyclic and cycloalkanes.
• Drawing wedge and dash, sawhorse and Newman projections. 
• Recognizing staggered, eclipsed, boat and chair conformations.
• Recognizing anti, gauche and 1,3-diaxial interactions.
• Understanding the effect torsional and steric strain has on the stability of different conformations of alkanes.
• Recognizing axial and equatorial positions and understanding the effect substituents have on the stability of the conformation.

5.0 Introduction

Acyclic alkanes and cyclohexanes can form different conformations through sigma bond rotations. Different conformations have different energies because of steric interactions and internal strain. In this chapter you will learn how to draw different conformations of acyclic alkanes and cyclohexanes and determine their relative potential energies.

5.1 Acyclic Alkanes

In acyclic alkanes, sigma bonds are free to rotate which allows molecules to adopt different orientations in space. Conformations or rotamers are the temporary molecular shapes that result from rotation around single bonds.

Some of these spatial arrangements of a molecule are more energetically favorable than others. Conformational analysis is the study of the energy changes associated with a molecule undergoing rotation around sigma bonds.

To show the three dimensional form of the molecule on paper, dash and wedge are used. In Figure 5.2, dash and wedge has been used to show the spatial arrangement of the hydrogens in ethane. 

Video: Drawing ethane in wedge-and-dash, sawhorse, and Newman projections

Other tools for three dimensional visualization of molecules on paper include sawhorse and Newman projections (Figure 5.3). In ethane, rotation around the C-C bond, changes the position of the hydrogens on one carbon compared to the second carbon. The angle that exists at any moment between one hydrogen on one carbon and a hydrogen on the second carbon (red and blue hydrogens ) is called dihedral angle. For staggered conformation, the dihedral angle is 60º while for eclipsed conformation, it is 0º (Figure 5.4).

3D Molecule*: ethane

To change the conformation, rotate the carbon-carbon sigma bond. The eclipsed conformation occurs when the dihedral angle is 0º.

Video: Drawing the conformations of ethane

5.2 Conformational Analysis of Ethane

The staggered conformation is 12 kJ/mol more stable than the eclipsed conformation. The eclipsed conformation is less stable because of the electron repulsion between the C-H bonds between adjacent carbons. Torsional strain is the difference in energy between the two conformations.

As the C-C sigma bond rotates, the potential energy of the molecule changes. The eclipsed conformation is 12 kJ/mol less stable and thus has a higher potential energy. The potential energy diagram illustrates the change in energy as the ethane molecules rotate around the C-C sigma bond. At room temperature, approximately 99% of the molecules will be in the staggered conformation.

5.3 Conformational Analysis of Butane

There are two staggered conformations of butane—anti and gauche—which will produce conformations with different energies. There are also two different eclipsing conformations: one with eclipsing hydrogens and eclipsing hydrogen-methyls and the other eclipsing methyls and eclipsing hydrogens. These conformations will also have different potential energies.

3D Molecule*: butane

When considering the potential energy of the different conformations, two factors must be considered: torsional strain, as described above, and steric strain or hindrance. Steric hindrance is caused when clouds of electrons come close together and repulsion occurs. In butane, this occurs when the two methyl groups get close to each other.

Video: Conformational analysis of butane

5.4 Conformation Stability of Acyclic Alkanes

The most stable staggered conformations will have the least amount of gauche interactions with the larger groups. These groups will be as far away from each other as possible to minimize steric interaction. The least stable conformation will have the greatest amount of torsional and steric strain. Therefore this conformation will be eclipsed, with the larger groups eclipsing each other.

Example: Order the Newman conformations of 1,2-dibromo-1-fluoroethane (Figure 5.9) from most stable to least stable.

Work through to figure out the answer: Bromine is larger than fluorine. The most stable conformation will have the least amount of torsional and steric strain and will have the bromine and fluorine gauche to each other. The least stable will have the greatest amount of torsional and steric strain and will have the two bromines eclipsing each other.

5.5 Conformations of Cyclohexanes

Heats of combustion, per CH₂ group, have shown that cyclohexane is the most stable monocyclic alkane. This is because it has the least amount of torsional strain and angle strain. Torsional strain is caused by eclipsing C-H bonds on adjacent carbons. Angle strain is caused by bond angles that are different from the desired 109.5º bond angle of a sp³ carbon. For example, cyclopropane is the least stable due to bond angles of 60º and eclipsing hydrogens. 

The half-chair is 45 kJ/mol less stable than the chair conformation. Cyclohexane exists primarily as the chair conformation.

5.6 Visualizing the 3D Structure of Cyclohexane

One of the major difficulties students have in organic chemistry is the visualization of three-dimensional structures on a two-dimensional surface. Shown below is a flat structure of cyclohexane and an optimized ACD/Labs 3D view of cyclohexane.

3D Molecule*: cyclohexane

You must learn how to go from one structure to the other.

Watch this video on how to visualize axial and equatorial hydrogens in Figures 5.13 and following diagrams.

Video: Visualizing cyclohexane’s axial and equatorial positions

Ring flip occurs when all of the sigma bonds undergo rotation to form another chair conformation. The second conformation has exactly the same energy, but what was equatorial in the first conformation has become axial in the second conformation and vice versa. Watch the video for an explanation.

Video: Visualizing cyclohexane ring flip

5.7 Drawing Cyclohexane in a Chair Conformation

Only one method is shown. You may come up with your own preferred way of drawing cyclohexane, but the end result should look the same. Watch the video for an explanation of steps.

Video: Drawing cyclohexane in a chair conformation

5.8 Conformations of Substituted Cyclohexanes and Potential Energy

First consider methyl cyclohexane. There are two conformations, one with the methyl group in the axial position and the other in the equatorial position. At room temperature, 95% of the molecules are in the conformation that has the methyl group in the equatorial position. This conformation has a lower potential energy and is more stable than the conformation with the methyl group axial. Experiments have shown that as the substituent gets larger, the equilibrium is shifted to more molecules that have the substituent in the equatorial position.

3D Molecule*: methylcyclohexane

Why is the equatorial conformation more stable? Let us examine the two conformations of methyl cyclohexane.

When the methyl group is axial, there is steric interaction—electron repulsion—with the axial hydrogens that are two carbons away on the same side. This is called 1,3-diaxial steric interaction or 1,3 diaxial strain. When the methyl group is equatorial, there is far less electron repulsion between adjacent atoms. The conformation with the methyl group axial is 7.3 kJ/mol less stable.

Another reason, which is clearer when you look at the Newman projection, is that when the methyl group is axial, there is a gauche interaction with the adjacent C-C bond. This is less stable than the anti-periplanar interaction of the methyl group and the C-C bond when the methyl group is equatorial. Watch this video with t-butylcyclohexane to help visualize this interaction.

So far, we have discussed the conformations of methyl cyclohexane. Energetics of other monosustituted cyclohexanes follow the same trend and in all cases, the equatorial conformation is preferred. Below, t-butylcyclohexane is shown.

Video: Tert-butylcyclohexane chair conformations and potential energy. This video does not feature audio.

5.9 More than one Substituent on Cyclohexane

If all of the substituents are the same size, the most stable conformation will have the most number of substituents in the equatorial position. This is because of the reduced 1,3-diaxial strain and gauche interactions. If the substituents are of different sizes, the larger group will dominantly be in the equatorial position. You must be able to take a flat two-dimensional cyclohexane structure, draw two chair conformations, and then determine which conformation is the most stable.

Example 1: Draw two chair conformations of trans-1,2-dibromocyclohexane and determine which conformation is most stable.

To help transform the flat drawing into the chair conformations, the atoms on the top of the drawing are colored blue and the bottom red. Top and bottom are arbitrary. Labelling top and bottom helps to describe the process.

Step 1: Draw two chair conformations labeling carbons 1 and 2.

Step 2: Replace the hydrogens with bromine atoms keeping the trans stereochemistry. In both conformations, on carbon 1, replace the top blue hydrogen with a bromine atom and the bottom red hydrogen with a bromine atom.

Step 3: Which conformation is most stable? Conformation A has both bromines axial and conformation B has both bromines equatorial. Conformation B will be most stable, because in conformation A, both bromines are experiencing 1,3-diaxial and gauche interactions.

Video: What is the most stable conformation of trans-1,2-dimethylcyclohexane?

Example 2: Draw two chair conformations of cis-1,2-dibromocyclohexane and determine which conformation is most stable.

Step 1: Draw two chair conformations. This time, only the bonds on carbons 1 and 2 are drawn to simplify the drawing.

Step 2: Add the bromines. You need to think about what relationship the bromines have with each other. They are on the same side of the cyclohexane molecule and have hydrogens below them.

In both conformations one bromine is axial and the other is equatorial. Therefore, both conformations are of equal energy.

3D Molecule*: cis-1,2-dibromocyclohexane 

Examples:

Which conformation is preferred when two substituents are different in size and one substituent occupies the equatorial position and the other the axial position? In general, the conformation that has the larger group in the equatorial position will be favored. As shown in Figure 5.27, the preferred conformation has the larger bromine atom in the equatorial position. However, this is only a guideline and in some cases hard to determine. The energy differences between different conformation for questions in this chapter will always be apparent. 

3D Molecule*: cis-1,2,4-trimethylcyclohexane

5.10 Chapter Summary

Acyclic Alkanes

Acyclic alkanes have sigma bonds that are free to rotate, allowing them to adopt different orientations in space. These different orientations are referred to as conformations or rotamers. Conformational analysis is the study of the energy changes associated with a molecule undergoing rotation about sigma bonds. There are three ways to represent 3D alkanes on a 2D plane.

When discussing the various conformations of an alkane one must understand how adjacent atoms or groups can interact. When adjacent atoms/bonds have a dihedral angle of 60° then they are referred to as “staggered”; when adjacent atoms/bonds have a dihedral angle of 0° then they are referred to as “eclipsed”. When two substituents have a dihedral angle of 180° in the staggered conformation they are referred to as “anti”; when two substituents have a dihedral angle of 60° in the staggered conformation they are referred to as “gauche”. An example of butane is shown below in both the wedge-dash and Newman projections:

When considering the potential energy of the different conformations two factors must be considered: torsional strain as described above (i.e. staggered vs eclipsed) and steric strain or steric hindrance. Steric hindrance is caused when clouds of electrons come close together and repulsion occurs. In butane this occurs when the two methyl groups get close to each other. An energy diagram is shown below.

Cyclohexane

Cyclohexane is the most stable monocyclic alkane since it has the least amount of torsional strain and angle strain. Torsional strain is caused by eclipsing C-H bonds on adjacent carbons; angle strain is by bond angles that are different from the desired 109.5° bond angle of a bond angle of a sp³ hybridized carbon atom. Cyclohexane has many different conformations shown below on a potential energy diagram:

The most stable conformation of cyclohexane is the chair conformation. Each carbon atom in the chair conformation has an axial (a) and an equatorial (e) substituent. An example of a chair conformation and the corresponding Newman projection is shown below.

Ring flipping occurs when all of the sigma bonds undergo rotation to form the other chair conformation.

In the chair conformation larger groups prefer to be in the equatorial position as this is lower in energy. In the axial position larger groups undergo 1,3-diaxial strain and a gauche interaction with the adjacent C-C bond, shown below.

When a cyclohexane molecule has more than one substituent, the most stable conformation will have the least number of substituents in the axial position or will have the largest substituent(s) in the equatorial position.

End of Chapter 5

Image Credits

[1] Image courtesy of Petr Kratochvil in the Public Domain.
[*] 3D Molecule: courtesy of QR Chem. QR Chem is a resource created by students and Professor Neil Garg at UCLA.