Cyclohexane - Equatorial Preference

This is the #1 thing you need to know about cyclohexane. So let’s get right into it.

Determining the Best Position

Concept: Axial or Equatorial: Which position is better?

5m
Video Transcript

The equatorial preference has to do with the fact that one of the two positions, remember that there's the axial position and there's 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.
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?
Now let's imagine that I put different shapes here. Let's say that I just put a bunch of maybe green circles on the equatorial positions and let's say that I put some blue balls, oh man, this just got really weird. Blue circles on the axial positions. That sounds like it hurts.
We've got 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. That's how clear I want it to be.
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 it's going to be the blue balls are like really close together. It's like awkward and stuff. They do not want to be there. On top of that, they're like sitting on sticks. It's terrible. Whereas, the equatorial positions they've got all this room to spread out. It's awesome. Look how far apart they are.
In fact, if you want to think about the equatorial position, it kind of looks like its the equator of the earth. If this was a big globe, the equatorial positions would be like on the equator, the axial positions would be like on the North Pole and the South Pole. So you don't want to be stuck on the South Pole or the North Pole. You want to be in paradise, like on an island drinking a Corona. 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.
What that means is that the ring is always going to flip in order to accommodate the preference of the largest substituent.
In this case, I have a tertbutyl group and that tertbutyl group can be on two different chairs. It could be on one chair that has it in the axial position. But any time that you flip a chair, you wind up flipping positions. If you flip your chair, you also wind up flipping positions. Now this would become equatorial over here.
It goes from axial to equatorial. Which of these do you think is going to be the most stable? 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 that 1% is going to exist in the axial position. Why? Because the axial is so much more torsionally strained with these H's here. See they're just bumping into each other, whereas the equatorial position is way better.
As I just said, when chairs flip remember that axials are always going to become equatorial and equatorials become axial. 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. 

  • Blue = Axial. This position sucks, it’s really cramped up. Large groups can’t stand it.
  • Green = Equatorial. This position is awesome. Large groups want to flip to this position

Determining Equatorial Preference

We always want to draw our chairs with the largest groups equatorial. If they are axial, we need to flip the chair. 

Example: Draw the following chair in the most stable conformation.

4m

Don’t worry about drawing this problem out correctly on the first try, as long as you know how to flip it to the correct chair, that’s all that matters. 

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

3m

Hint: If you don’t know what neopentyl is, it’s ok. Obviously it has 5 carbons, so keep that in mind when deciding equatorial preference!

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

5m

Cyclohexane - Equatorial Preference Additional Practice Problems

In the following equilibrium transformation, towards which direction does the reaction proceed the furthest as drawn (ie. left or right)?

a) A

b) B

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The change is free energy on flipping from a cyclohexane conformer with the indicated substituent equatorial to the conformer with the substituent axial is indicated in the table below: 

For each of the following cyclohexane derivatives draw the molecule in the most stable conformation. Be sure to show hydrogen atoms on the substituted ring carbons.

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Using the cyclohexane scaffold shown below, draw the structural formula of all-cis-1-ethyl-2,4 difluorocyclohexane (all-cis means that all three substituents point in the same direction). Now, draw both of the chair conformations of this molecule and circle the more stable conformation (equatiorial preference energies: –F: 0.2 kcal mol−1; –CH3CH2: −1.9 kcal mol−1).

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Draw the most stable chair conformation for the following molecule. Use the templates provided. The chloride and the numbering of carbons are already drawn in for you in the template. (*Note: D=deuterium, an isotope of hydrogen):

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Circle which isomer is more stable, cis-1,3-dimethylcyclohexane or trans-1,3-dimethylcyclohexane is more stable. To receive credit, you must include work to justify your answer.

                       cis-1,3-dimethylcyclohexane          or         trans-1,3-dimethylcyclohexane

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Draw the most stable chair conformation of the molecule shown below and explain.

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Draw the most stable chair conformer of the most stable isomer of 1,3,5-trimethylcyclohexane.

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Consider (1R, 3S)-1,3-diiodocyclohexane. Draw the two chair conformations and determine which conformation is more stable.

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For the pair drawn below, choose the MORE STABLE molecule.

Does the  UNSTABLE molecule chosen below have  ANGLE STRAIN?

Does the  UNSTABLE molecule chosen below have  TORSIONAL STRAIN?

Does the  UNSTABLE molecule chosen below have  STERIC STRAIN?

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Draw the LEAST stable chair conformation of 2,3-dipropyl-1,4-dimethylcyclohexane with the following info: the propyl groups are trans to one another; the C1 methyl is cis to the C2 propyl; the C4 methyl is cis to the C3 propyl.

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Perform a ring flip on the following compound and determine which structure is more stable.

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Draw the most stable configuration.

trans-1-bromo-3-ethylcyclohexane

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Draw the LEAST stable chair conformation of 2,3-dipropyl-1,4-dimethylcyclohexane with the following info: the propyl groups are trans to one another; the C1 methyl is cis to the C2 propyl; the C4 methyl is cis to the C3 propyl.

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A. Draw the two chair conformations of cis-1,3-di-tert-butyl-cyclohexane.

B. Circle the most stable chair conformation.

C. Using complete sentences, explain why your circled chair conformation is the most stable chair conformation.

D. For the circled chair conformation, state the axial or equatorial orientation for each of the non-hydrogen substituents on cyclohexane.

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One chair conformation of the sugar galactose is given below. Complete the tetrahedral representation by filling in the missing substituent groups. It is only necessary to show bonds to groups that are not hydrogen in your tetrahedral representation. Use wedges, dashed wedges, and normal lines as appropriate to show the stereochemistry clearly and unambiguously. Consider the oxygen atom that is part of the 6-membered ring to be your “reference” atom! Then, draw the conformation that will result upon “chair flip” of the given chair. Write your answer in the empty box on the right side of the page. Remember, the 6-membered ring includes an oxygen atom! Indicate which chair is more stable by marking the appropriate box with the symbol “X”.

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A “planar representation” of a substituted cyclohexane ring is given below. Complete the chair conformation by filling in the missing substituent groups.  It is only necessary to show bonds to groups that  are not hydrogen in your chair conformation. To help you get started, a reference carbon is marked with an asterisk in the “planar” representation. This reference carbon is also marked with an asterisk in the chair “skeleton” that is provided. You don’t need to draw the other chair conformation, but please indicate which statement is true about the relative stability of the chair conformations by marking the appropriate box with the symbol “X”.

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For the pair of molecules drawn below, choose the letter that corresponds to the MORE STABLE molecule.

Does the UNSTABLE molecule chosen have ANGLE STRAIN? (YES or NO)

Does the UNSTABLE molecule chosen have TORSIONAL STRAIN? (YES or NO)

Does the UNSTABLE molecule chosen have STERIC STRAIN? (YES or NO)

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For the following cyclohexane derivative, draw the most stable chair conformation on the template provided. Be sure to attach the substituents to the corresponding numbered carbon on the template.

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Put each threesome below in order of stability from high to low using > or = signs. If two species are equal use = signs.

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Focusing on just one of the six membered rings from the structure in  avermectin, draw this portion as a cyclohexane chair below. Perform a  chair flip, and circle the more stable conformation. Provide a one sentence rationale for which conformation you circled.

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Consider a single enantiomer of trans-1-methyl-3-tert-butylcyclohexane. (For this problem, it does not matter which one.) There are two chair forms. Draw both of them. Which one is more stable? Why?

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Draw the lowest energy chair conformer for each structure (A and B). Rank these two low energy conformers as more or less stable.

 

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Draw the flipped conformation of the chair and circle the lower energy conformer.

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Complete the chair forms of the molecules shown in the box, on the templates provided as it goes through a ring flip. Indicate which conformer is the most stable.

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The preferred conformation of cis-1-isopropyl-2-methylcyclohexane is one in which:

a. the isopropyl group is axial and the methyl group is equatorial

b. the methyl group is axial and the isopropyl group is equatorial

c. both groups are axial

d. both groups are equatorial

e. the molecule exists in a twist-boat conformation

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