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

For most classes all you will need to know how to do is use equatorial preference to predict the most stable chair conformation.

 

However, sometimes you will be required to use energetics to calculate the exact percentages of each chair in solution. This is a multistep process, so here I’m going to walk you through it from scratch. 

Calculating Flip Energy

First we have to introduce the concept of an A-value, which is simply the energy difference between the equatorial (most stable) and axial (least stable) positions.

Concept #1: Explaining how A-Values are related to cyclohexane flip energy

We can use these values to calculate how much energy it is going to take to flip a chair into its least stable form.

Note: The above chair flip in the video is slightly off. Remember that the direction of the groups (up vs. down) should not change when going from axial to equatorial or vice versa.

 

All the math is still correct here, but I should have drawn the groups down instead of up on the second chair. :)

 

[Refer to the videos below for examples of this]

Practice: Calculate the difference in Gibbs free energy between the alternative chair conformations of trans-4-iodo-1-cyclohexanol. 

Practice: Calculate the difference in Gibbs free energy between the alternative chair conformations of cis-2-ethyl-1-phenylcyclohexane. 

Additional Problems
Table I: Interaction Energy Groups                                kcal/mol (each interaction) H : H eclipsed                           1.0 CH3 : H eclipsed                       1.4 CH3 : CH3CH2  eclipsed           2.7   H : CH(CH3)2 eclipsed              2.0 H : C(CH3)3 eclipsed                 2.8 CH3 : H  1,3-diaxial                   0.9 CH3CH2 : H 1,3-diaxial            0.95 (CH3)2CH : H  1,3-diaxial          1.1 (CH3)3C : H  1,3-diaxial             2.7 CH3 : CH3   gauche                    0.9 CH3 : CH3CH2  gauche                 1.0 CH3 : (CH3)2CH  gauche            1.2 (CH3)3C : (CH3)3C  1,3-diaxial    4.1  
Consider the cyclohexane derivative below and answer all associate questions. a) Complete the Newman projection looking down the direction indicated. Add all H's and alkyl groups in the correct positions directly on the scaffolds below. b) What is the R/S configuration of all three stereocenters, use the numbering above to indicate which center you are referring to. c) What is the conformation of each group in II (axial, equtorial)?  C1 t-butyl ______  C3 t-butyl _______  methyl  _________ d) Calculate all the interactions for each conformer. Use the table on the next page to assist you. CIRCLE which of the two forms is more stable. I _______________________________________ kcal/mol II _______________________________________ kcal/mol  
A conformation for a cyclohexane derivative is shown below. Calculate the interaction energy for this form. Show calculations.   _______________________________________________ kcal/mol What is the name of this form? _______________   Interaction Energy Table: Interaction                                                              Energy (kcal/mol) H:H eclipsed                                                                     1.0 CH3:H eclipsed                                                                 1.4 -CH- : -CH- eclipsed "flagpole" interactions                      2.5 CH3: iPr gauche                                                                1.3 CH3: tBu gauche                                                              2.1 CH3: H 1,3-diaxial                                                           1.0 tBu: H 1,3-diaxial                                                              2.0 iPr: H 1,3-diaxial                                                               1.3 Staggered interactions                                                       0
Calculate ΔH for the process of going from the most stable chair conformation to the least stable conformation. Use the table provided. Show complete work. 
Prove that about 97.5% of isopropylcyclohexane is in the equatorial chair conformation at 300K, given that the equatorial chair is more stable than the axial conformer by 9.2 kJ/mol.