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.

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?
Write a structural formula for the most stable conformation of each of the following compounds:  (c)  cis-1-Isopropyl-3-methylcyclohexane 
Write a structural formula for the most stable conformation of each of the following compounds:  (d)  trans-1-Isopropyl-3-methylcyclohexane 
Write a structural formula for the most stable conformation of each of the following compounds:  (e)  cis-1-tert-Butyl-4-ethylcyclohexane 
Write a structural formula for the most stable conformation of each of the following compounds:  (f)  cis-1,1,3,4-Tetramethylcyclohexane 
Write a structural formula for the most stable conformation of each of the following compounds: 
Identify the more stable stereoisomer in each of the following pairs, and give the reason for your choice:  (a) cis- or trans-1-Isopropyl-2-methylcyclohexane  
Identify the more stable stereoisomer in each of the following pairs, and give the reason for your choice:  (b) cis- or trans-1-Isopropyl-3-methylcyclohexane 
Draw the flipped conformation of the chair and circle the lower energy conformer.
Identify the more stable stereoisomer in each of the following pairs, and give the reason for your choice:  (c) cis- or trans-1-Isopropyl-4-methylcyclohexane 
Identify the more stable stereoisomer in each of the following pairs, and give the reason for your choice:   
Identify the more stable stereoisomer in each of the following pairs, and give the reason for your choice: 
Identify the more stable stereoisomer in each of the following pairs, and give the reason for your choice: 
One stereoisomer of 1,1,3,5-tetramethylcyclohexane is 15 kJ/mol (3.7 kcal/mol) less stable than the other. Indicate which isomer is the less stable, and identify the reason for its decreased stability.   
Draw the lowest energy chair conformer for each structure (A and B). Rank these two low energy conformers as more or less stable.  
The following are representations of two forms of glucose. The six-membered ring is known to exist in a chair conformation in each form. Draw clear representations of the most stable conformation of each. Are they two different conformations of the same molecule, or are they stereoisomers that cannot be interconverted by rotation about single bonds? Which substituents (if any) occupy axial sites? 
For each of the following pairs of compounds, identify the compound that would have the higher heat of combustion:
For each of the following pairs of compounds, identify the compound that would have the higher heat of combustion:
For each of the following pairs of compounds, identify the compound that would have the higher heat of combustion:
Write the structures of two chair conformations of 1- tert-butyl-1-methylcyclohexane. Which conformation is more stable? Explain your answer.  
Menthol, isolated from various mint oils, is used in the treatment of minor throat irritation. Draw both chair conformations of menthol and indicate which conformation is lower in energy.
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”.
Draw both chair conformations for each of the following compounds. In each case, identify the more stable chair conformation: (a) Methylcyclohexane
Draw both chair conformations for each of the following compounds. In each case, identify the more stable chair conformation: (b) trans-1,2-Diisopropylcyclohexane
Draw both chair conformations for each of the following compounds. In each case, identify the more stable chair conformation: (c) cis-1,3-Diisopropylcyclohexane
Draw both chair conformations for each of the following compounds. In each case, identify the more stable chair conformation: (d) trans-1,4-Diisopropylcyclohexane
For each of the following pairs of compounds, determine which compound is more stable (you may find it helpful to draw out the chair conformations):
For each of the following pairs of compounds, determine which compound is more stable (you may find it helpful to draw out the chair conformations):
Consider that cyclobutane exhibits a puckered geometry. Judge the relative stabilities of the 1,2-disubstituted cyclobutanes and of the 1,3-disubstituted cyclobutanes. (You may find it helpful to build handheld molecular models of representative compounds.)  
For each of the following pairs of compounds, determine which compound is more stable (you may find it helpful to draw out the chair conformations):
For each of the following pairs of compounds, determine which compound is more stable (you may find it helpful to draw out the chair conformations):
Glucose (a sugar) is produced by photosynthesis and is used by cells to store energy. Draw the most stable conformation of glucose:
Consider the cis and trans isomers of 1,3-di-tert-butylcyclohexane (build molecular models). What unusual feature accounts for the fact that one of these isomers apparently exists in a twist boat conformation rather than a chair conformation?  
Which is the most stable conformation for  cis-1-bromo-3-methylcyclohexane?
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
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.
Write the two chair conformations of each of the following and in each part designate which conformation would be the more stable: (a) cis-1-tert-butyl-3-methylcyclohexane.  
Write the two chair conformations of each of the following and in each part designate which conformation would be the more stable: (b) trans-1-tert-butyl-3-methylcyclohexane.  
Write the two chair conformations of each of the following and in each part designate which conformation would be the more stable:  (c) trans-1-tert-butyl-4-methylcyclohexane,  
Write the two chair conformations of each of the following and in each part designate which conformation would be the more stable: (d) cis-1-tert-butyl-4-methylcyclohexane.  
Indicate the most and the least stable conformations. Give your reasoning.  
Which diastereomer is most stable?  
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
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.
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?
Draw the most stable chair conformer of the most stable isomer of 1,3,5-trimethylcyclohexane.
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):
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.
A hydroxyl group is a somewhat “smaller” substituent on a six-membered ring than is a methyl group. That is, the preference of a hydroxyl group for the equatorial orientation is less pronounced than that of a methyl group. Given this information, write structural formulas for all the isomeric methylcyclohexanols, showing each one in its most stable conformation. Give the substitutive IUPAC name for each isomer. 
(a) Menthol, used to flavor various foods and tobacco, is the most stable stereoisomer of 2-isopropyl-5-methylcyclohexanol. Draw its most stable conformation. Is the hydroxyl group cis or trans to the isopropyl group? To the methyl group? 
(b) Neomenthol is a stereoisomer of menthol. That is, it has the same constitution but differs in the arrangement of its atoms in space. Neomenthol is the second most stable stereoisomer of 2-isopropyl-5-methylcyclohexanol; it is less stable than menthol but more stable than any other stereoisomer. Write the structure of neomenthol in its most stable conformation. 
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.
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.
Put each threesome below in order of stability from high to low using > or = signs. If two species are equal use = signs.
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)
Draw the most stable configuration. trans-1-bromo-3-ethylcyclohexane
Even though the methyl group occupies an equatorial site, the conformation shown is not the most stable one for methylcyclohexane. Explain. 
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.
Draw the lowest energy-chair conformation for the syn- and anti- products shown.
Draw the most stable chair conformation of cyclohexane
Draw the most stable chair conformation of cyclohexane
By assuming that the heat of combustion of the cis isomer was larger than the trans, structural assignments were made many years ago for the stereoisomeric 2-, 3-, and 4-methylcyclohexanols. This assumption is valid for two of the stereoisomeric pairs but is incorrect for the other. For which pair of stereoisomers is the assumption incorrect? Why? 
What chair conformation represents the most stable chair conformation of Molecule A? (Top Hat)
For the following six-member ring, draw both chair conformations and indicate which is the most stable.