Equatorial Preference - Online Tutor, Practice Problems & Exam Prep

In cyclohexane conformations, equatorial positions are preferred over axial due to reduced steric hindrance and torsional strain. Bulky substituents, like tert-butyl groups, favor equatorial placement, leading to greater stability—over 99% of the compound exists in this form. When the chair conformation flips, axial substituents become equatorial and vice versa, impacting the overall stability of the molecule. Understanding these conformational dynamics is crucial for predicting reactivity and stability in organic synthesis.

Equatorial Preference

Equatorial preference has to do with the fact that one of the two positions, remember that there is the axial position and there is the equatorial position, one of them is going to be much more crowded or what we call torsionally strained than the other. Now, usually if you just have hydrogens in there, it's not a big deal. But if you start adding bulkier groups in there, it's actually going to affect it. Okay? So let's just look at the different positions. Remember, we have our axial positions, they're going straight up and down with the corners and remember that we have our equatorial positions going slightly opposite. Are you guys cool with that so far?

And now let's imagine that I put different shapes here. So let's say that I just put a bunch of maybe like green circles on the equatorial positions, and let's say that I put some blue circles on the axial positions. That sounds like it hurts. So, we have these ones on the positions and I just want to analyze the ones at the top. Let's just say that we look at this blue circle, this blue circle, and this blue circle versus this green circle, this green circle, and this green circle. Are you guys following so far?

In fact, let's go ahead. You don't have to do this, but I'm just going to erase the other ones so you guys don't get distracted. So you guys can really see what's going on here. See, so that's how clear I want it to be. So basically, we've got our axial positions and our equatorial positions. Which of these do you think is going to be the most spread out? And then which of them do you think is going to be the most tight together? And it turns out that the blue circles are really close together. Okay. So it's like awkward and stuff. They do not want to be there. Alright. On top of that, they're like sitting on sticks. It's terrible. So whereas the equatorial positions, they've got all this room to spread out. It's awesome. Look how far apart they are. Okay?

In fact, if you want to think about the equatorial position, it kind of looks like it's the equator of the earth. If this is like a big globe, the equatorial positions would be like on the equator, the axial positions would be like on the north pole. Alright? And the south pole. So you don't want to be stuck on the south pole or north pole. You want to be like in paradise, like on an island drinking a Corona. Okay? So the axial positions suck. That's what I'm trying to say. Especially when you put large groups there, you do not want to be in the axial position. So what that means is that the ring is always going to flip in order to accommodate the preference of the largest substituent. So, in this case, I have a tert-butyl group and that tert-butyl group can be on 2 different chairs. It could be on one chair that has it in the axial position, but anytime that you flip a chair, you wind up flipping positions. Okay? So if you flip your chair, you also wind up flipping positions. So now this would become equatorial over here. Alright? So it goes from axial to equatorial.

Which of these do you think is going to be the most stable? And it turns out that it's going to be way more stable in the equatorial position. In fact, over 99% of this compound is going to exist in the equatorial position, and less than 1% is going to exist in the axial position. Why? Because the axial is so much more torsionally strained with these hydrogens here. See, they're just bumping into each other. Whereas, the equatorial position is way better. Okay. So as I just said, when chairs flip, remember that axials are always going to become equatorial and equatorials become axial. Okay. So any time you flip, you're going to be giving something in the axial position an opportunity to become equatorial. But you also have to change the shape of the chair as well.

Draw the following chair in the most stable conformation.

So what I want you guys to do is look at the following molecule and analyze if it's in the most stable position. In fact, don't even analyze it. I'm already telling you. It's not in its most stable conformation. Can you tell me why it's not in the most stable? Because what I have is this. I have an axial position and I have an equatorial position. Right? I have 2 different groups. I have a big group and a small group. Okay? No matter what, if I flip this molecule, I'm always going to have one equatorial and one axial because the equatorial becomes axial and the axial becomes equatorial. So no matter what, I have to have one of them in the axial position. Do you agree with me? At least something has to be axial. But right now, I have the wrong group in the axial position. I have the big group. Instead, I should have the small group in the axial position. So what we want to do is we want to draw a chair flip to accommodate the equatorial preference. What that means is that I want to show that now I'm going to draw the opposite chair and then I'm going to draw the CH₂CH₃ now in the equatorial position.

So what that means is that I want to draw a chair that has this corner facing down, so I'm going to draw it like this. Okay? So now what that means is that this corner, basically I had these were 3 carbons away or like 1 in if this was my number 1, then this would be 2 and this would be 3. Well, now I'm going to make that this is my 1, which means that that 2, 3, my 3 would be over here. Okay? So what that means is that now this equatorial position that was a methyl on the 3 is now going to be axial facing down. Why is that? Because remember that the axial position always follows the corner. In this case, the corner is facing down, so I would put it there. Okay. And now since my number 1 is here, I would now place this one equatorially. And this would actually be a much more stable conformation. Why? Because of the fact that you're going to be able to have the larger group, the bigger group in the equatorial position and the smaller group in the axial position.

Alright? So, some of you guys might be wondering, Johnny, but how do you know to pick those carbons? So for example, let me see if I can add myself back. Alright, there we go. So some of you guys might be wondering, Johnny, why didn't you go around the other way? Why didn't you say that this was your 2 and this was your 3 and then draw the CH₃ here? And honestly, it doesn't matter. In fact, I'm going to have an entire section where I'm just going to talk about how those two chairs are the same thing and it doesn't matter exactly how you draw it as long as you're being consistent. Okay? But just so you know, it would still give me the same thing where the CH₃ would be axial, the ethyl would be equatorial, and this one's way more stable. Alright? And you should be used to drawing; you should get used to drawing chair flips because some professors really want you to be able to do that. All right? So let's go ahead and move on to the next topic.

Draw the MOST STABLE conformation of cis-1-tert-butyl-4-methylcyclohexane.

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Draw the LEAST STABLE conformation of trans-1-tert-butyl-3-neopentylcyclohexane.

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Here’s what students ask on this topic:

What is equatorial preference in cyclohexane conformations?

Equatorial preference in cyclohexane conformations refers to the tendency of substituents to occupy the equatorial positions rather than the axial positions. This preference arises because equatorial positions experience less steric hindrance and torsional strain compared to axial positions. In the axial position, substituents are closer to other hydrogen atoms, leading to increased repulsion and instability. Therefore, bulky groups, like tert-butyl, prefer the equatorial position, making the molecule more stable. Understanding this concept is crucial for predicting the stability and reactivity of cyclohexane derivatives in organic synthesis.

Why are equatorial positions more stable than axial positions in cyclohexane?

Equatorial positions in cyclohexane are more stable than axial positions due to reduced steric hindrance and torsional strain. In the axial position, substituents are aligned with hydrogen atoms on the same side of the ring, leading to 1,3-diaxial interactions, which cause repulsion and instability. In contrast, equatorial positions allow substituents to be more spread out, minimizing these interactions. This spatial arrangement reduces the overall energy of the molecule, making the equatorial position more favorable, especially for bulky groups.

How does the chair flip affect the positions of substituents in cyclohexane?

When cyclohexane undergoes a chair flip, the positions of substituents switch between axial and equatorial. In this process, all axial substituents become equatorial, and all equatorial substituents become axial. This flip impacts the molecule's stability, as bulky substituents prefer the equatorial position due to reduced steric hindrance and torsional strain. Therefore, the chair conformation with bulky groups in the equatorial position is more stable. Understanding this dynamic is essential for predicting the most stable conformation of cyclohexane derivatives.

What is the impact of bulky substituents on the stability of cyclohexane conformations?

Bulky substituents significantly impact the stability of cyclohexane conformations. These groups prefer the equatorial position because it offers more space and reduces steric hindrance and torsional strain. When a bulky group, such as a tert-butyl group, is in the axial position, it experiences 1,3-diaxial interactions with hydrogen atoms, leading to increased repulsion and instability. As a result, the chair conformation with the bulky substituent in the equatorial position is much more stable, often existing in over 99% of the population. This preference is crucial for predicting the behavior of cyclohexane derivatives in organic reactions.

How do 1,3-diaxial interactions influence the stability of cyclohexane conformations?

1,3-diaxial interactions significantly influence the stability of cyclohexane conformations. These interactions occur when substituents in the axial position are close to hydrogen atoms on the same side of the ring, leading to repulsion and increased torsional strain. This repulsion makes the axial position less stable, especially for bulky groups. In contrast, the equatorial position minimizes these interactions, providing more space and reducing strain. Therefore, substituents prefer the equatorial position, enhancing the overall stability of the cyclohexane conformation.