Chair Conformation of Cyclohexane Axial and Equatorial

This section will cover the chair conformation of cyclohexane axial and equatorial. First, we must lay the groundwork but introduce what is unique about the chair conformation of cyclohexane.

Cyclohexane is a very unique ring because it is strain-free (no ring strain), so it is very stable. Because of this, it is often used in organic chemistry. We see cyclohexane drawn in 2 ways:

Picture14

Both can be used to draw the exact same molecule, but they are simply different ways of representing it. For this majority of this section, we will focus on the chair conformation.

 

Chair flipping

Chairs can change conformations through a process called chair flipping, creating 2 conformations for the same chair. The two conformations exist in equilibrium but often don’t have the same energy as one another; therefore, it is common for the equilibrium to favor one side or the other. The equilibrium will tend to lie toward the more stable chair conformation.

Picture15

A common exam question tests the student’s ability to “flip” the ring of a cyclohexane chair conformation. The arrows in the figure below are meant to show how the structure physically moves to get from one conformation to the other. The arrows do not represent electrons moving. Note how the carbons move from one flipped structure to the other (following the red and blue circles). The carbons are also numbered to more easily depict the locations of the carbons before and after the flip.

 

Picture16

 

Keep it Simple

When flipping a ring, a substituent that was axial becomes equatorial and a substituent that was equatorial becomes axial; therefore, the position of the substituent changes. Also, notice that a substituent that is pointing upwards before the ring flip, always ends up pointing upwards after the flip, even if it changes from axial to equatorial, or vice versa.

Picture17

This should be used as a check following a chair flip problem as it is a simple way to make sure no silly mistakes were made.

 

Substituents in a Chair

There are two general positions that a substituent can be in for a chair conformation of cyclohexane: axial and equatorial. Axial substituents are labeled in red below. Equatorial substituents are in blue.

This image shows a chair conformation of cyclohexane axial and equatorial.

 

Keep it Simple

Here’s a good trick for the chair conformation of cyclohexane (axial and equatorial tricks):

For remembering the location of equatorial substituents: equatorial substituents are always parallel to a portion of the chair. In the figure below, equatorial substituents and the portion of the ring they are parallel to are color coded to depict this.

Picture19

One the other hand, all axial substituents point either straight up or straight down.

These two tips vastly simplify the task of remember the axial and equatorial positions of the chair conformation of cyclohexane.

 

Substituents in axial positions come very close to the axial substituent 3 carbons away, which causes an unfavored interaction between the substituents called 1,3-diaxial interactions. These 1,3-diaxial interactions cause axial substituents much higher energy than equatorial substituents.

 

Picture20

 

Because equatorial positions have less energy (due to the lack of 1,3-diaxial interactions), they are much more stable, so substituents prefer to be in the equatorial positions. Your textbook will have a table containing the different energy values for substituents in the axial position. By adding up the energy values of all axial substituents for each chair, one can calculate the difference in stability between 2 chairs.

 

Going from 2-D structures to Chair Conformations

Often, the structure for a problem is given in a 2-D conformation, but the problem asks for the answer in a chair. It’s very simple to go from 2-D to chair using the correct methodology as the following example explains.

 

Example

Draw the following structure in its most stable chair conformation:

Picture21

Answer
  1. First, arbitrarily number the carbons. This numbering has nothing to do with naming the molecule, but it is only used to help keep track of where the substituents are in relation to one another.

Picture22

  1. We then draw a regular chair conformation and a chair conformation in its flipped formed. The first chair can be arbitrarily numbered, but it’s important the numbering stays with the same carbon during the course of the flip.

Picture23

  1. We now add substituents to each. At each carbon on the cyclohexane, there is a one substituent that points up and one that points down, which is something we will utilize in this step. If the substituent is a wedge () on the 2-D cyclohexane, then place the substituent so it is going upward on the chair at the corresponding carbon (e.g. the chlorine off carbon 1 should be added to carbon 1 of both chairs). If it is a dash (), then place the substituent so it is facing downward on the corresponding carbon. Do this for each chair shown above:

Picture24

  1. Both of these answers would be correct if we just had to convert the 2-D to the 3-D structure; however, questions often ask for the most stable structure. By looking at the table in one’s textbook, we can see that a chlorine in the axial position has 2.0 kJ/mol of extra energy associated with it due to 1,3-diaxial interactions. Since the left structure has both chlorine atoms in the axial position, the total additional energy due to 1,3-diaxial interactions is 4.0 kJ/mol. The right molecule, on the other hand, has no axial substituents and therefore no extra energy. This is our most stable molecule then:

Picture25

By drawing both possible chair structures, we were able to see all possible conformations the chair could take, so we could assess which was the most stable.