Ch. 4 - Alkanes and CycloalkanesWorksheetSee all chapters
All Chapters
Ch. 1 - A Review of General Chemistry
Ch. 2 - Molecular Representations
Ch. 3 - Acids and Bases
Ch. 4 - Alkanes and Cycloalkanes
Ch. 5 - Chirality
Ch. 6 - Thermodynamics and Kinetics
Ch. 7 - Substitution Reactions
Ch. 8 - Elimination Reactions
Ch. 9 - Alkenes and Alkynes
Ch. 10 - Addition Reactions
Ch. 11 - Radical Reactions
Ch. 12 - Alcohols, Ethers, Epoxides and Thiols
Ch. 13 - Alcohols and Carbonyl Compounds
Ch. 14 - Synthetic Techniques
Ch. 15 - Analytical Techniques: IR, NMR, Mass Spect
Ch. 16 - Conjugated Systems
Ch. 17 - Aromaticity
Ch. 18 - Reactions of Aromatics: EAS and Beyond
Ch. 19 - Aldehydes and Ketones: Nucleophilic Addition
Ch. 20 - Carboxylic Acid Derivatives: NAS
Ch. 21 - Enolate Chemistry: Reactions at the Alpha-Carbon
Ch. 22 - Condensation Chemistry
Ch. 23 - Amines
Ch. 24 - Carbohydrates
Ch. 25 - Phenols
Ch. 26 - Amino Acids, Peptides, and Proteins

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

Determining the Best Position

Concept #1: Axial or Equatorial: Which position is better?

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 #1: Draw the following chair in the most stable conformation.

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. 

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

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!

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