Regardless of the format of organic chemistry classes (e.g. online, hybrid, F2F), many students struggle with chair conformations. Because it is our job, as educators, to help convey challenging material as clearly as possible, I like to provide rules of thumb (ROTs) to my students, which are a major component of my teaching style.
Below are 3 ROTs that I use in class to help my students better conceptualize chair conformations and substituent stability:
ROT #1: Chair conformations with high stability (low energy) need as many of the largest/highest priority groups to be placed in the equatorial position.
For instance, tert-butylcyclohexane can be arranged with the substituent in the equatorial bond or the axial bond. Yet one conformation is preferred over the other due to the energies associated with the substituent when positioned on the chair. The tert-butyl group needs to be placed in the equatorial bond, as it is the lowest energy, or highest stability, conformation. If the tert-butyl group is placed in the axial bond, then the chair has the highest energy, or the least stable conformation.
ROT #2: Always place the largest/highest priority group in the equatorial position. However, keep in mind that the geometry of the cyclohexane needs to be conserved. Groups in the trans orientation stay trans (in the opposing plane) and vice versa.
This ROT is most appropriate for a multi-substituted cyclohexane. For instance, (1R, 2S, 4R) 2,4-dimethyl-1-tert-butylcyclohexane can be arranged with a number of the substituents in the equatorial bond or the axial bond. Yet one conformation is preferred over the other due to the energies associated with the substituent. The tert-butyl group needs to be placed in the equatorial bond as it is the lowest energy, or highest stability, chair. However, the orientation of the methyl groups needs to be conserved in order to follow the stereochemistry of the structure. This means that the methyl at the 2 position is forced to be in the axial position, and the methyl at the 4 position is forced to be in the equatorial position. Having a group in the axial position tends to scare students and give them the impression that they might have arranged the structure wrong. However, this is the best orientation for the chair conformation of (1R, 2S, 4R) 2,4-dimethyl-1-tertbutylcyclohexane.
What happens if students review a cyclohexane that has no stereochemistry provided? In that case, students should follow ROT #1: put all the groups in the equatorial position to generate the most stable chair conformation.
For example, let’s look at the molecule of 1,2,4-trimethylcyclohexane. There is no specification of stereochemistry. This means that there are a number of combinations: 8, to be exact. However, only one conformation is the lowest energy (highest stability) chair. The methyl groups all go in equatorial positions. (1R, 2R, 4R) 1,2,4-trimethylcyclohexane does not have the methyl groups in the same plane. Rather, the conformation is driven by stability.
ROT #3: When a group is positioned up, it stays up when subjected to a ring flip. However, groups in the axial position will convert to an equatorial position when subjected to a ring flip, and vice versa.
This ROT is meant to help students (and professors) who struggle with ring flips. It is not an easy topic to teach. And to attempt to convey this theme over an online format can be even more complicated.
For instance, let’s review the molecule of (1R, 3S, 4S) 4-ethyl-1-iodo-3-methylcyclohexane. The iodine substituent (position 1) and methyl group (position 3) are cis to each other and should go in the equatorial positions to generate a stable chair conformation. However, they are pointed up when arranged. Keep in mind that when the chair is subjected to a ring flip, the groups still remain up. However, they are switched to the axial positions. The big takeaway here is that regardless of the positions—equatorial or axial—they stayed up!
In the same molecule, (1R, 3S, 4S) 4-ethyl-1-iodo-3-methylcyclohexane, the ethyl group at the 4 position is down and equatorial for the most stable chair. It is down because it is trans to the other two substituents. Keep in mind that when the chair is subjected to a ring flip, the ethyl group still remains down. However, it’s switched to the axial position. The big takeaway here is that regardless of the position—equatorial or axial—the ethyl group stays down! This idea that “up stays up” and “down remains down” is something that I stress in class, only because I know how challenging it can be for students to grasp the concept of a chair flip.
Even though ROT #3 doesn’t directly help students with stability, it does help them understand the process needed to decipher between two conformations and, out of the two, to determine which is the most stable.
As an instructor, I try to see how I can best help my students maneuver through difficult material. Because flipping chairs can be one of the more challenging topics in organic chemistry, I use these 3 ROTs about chair conformations to help my students master the mechanics of organic chemistry.
-Kerri Taylor, Columbus State University
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