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.

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.

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.