How do you calculate the energy difference between two conformers?

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]

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Problem 1 Problem 2 Problem 3 Problem 4 Problem 5 Problem 6 Problem 7 Problem 8 Problem 9 Problem 10 Problem 11 Problem 12 Problem 13 Problem 14 Problem 15 Problem 16 Problem 17 Problem 18 Problem 19 Problem 20 Problem 21 Problem 22 Problem 23 Problem 24 Problem 25 Problem 26 Problem 27 Problem 28 Problem 29 Problem 30 Problem 31 Problem 32 Problem 33 Problem 34 Problem 35 Problem 36 Problem 37 Problem 38 Problem 39 Problem 40 Problem 41 Problem 42 Problem 43 Problem 44 Problem 45 Problem 46 Problem 47 Problem 48 Problem 49 Problem 50 Problem 51 Problem 52 Problem 53 Problem 54 Problem 55 Problem 56 Problem 57 Problem 58 Problem 59 Problem 60 Problem 61 Problem 62 Problem 63 Problem 64 Problem 65 Problem 66 Problem 67 Problem 68 Problem 69 Problem 70 Problem 71 Problem 72 Problem 73 Problem 74 Problem 75 Problem 76 Problem 77 Problem 78 Problem 79 Problem 80 Problem 81

Problem 73 Easy Difficulty

Answer

The energy difference between the two conformers $=3.48-0.87=2.61 \mathrm{kral} / \mathrm{mol}$.

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Chemistry 102

Organic Chemistry

Chapter 2

An Introduction to Organic Compounds

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We have video lessons for 1.19% of the questions in this textbook

Paula Yurkanis Bruice

Organic Chemistry

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How do you calculate energy conformation?

In general, to calculate the total energy of a given conformation, add all the torsional and steric strain: For example: Calculate the energy difference between these two conformations of butane: The first conformation has one CH3/CH3 gauche interaction which brings 3.8 kJ/mol energy of destabilization.

How do you know which chair conformation has more energy?

To Determine Chair Conformation Stability, Add Up The A-Values For Each Axial Substituent. The Lower The Number, The More Stable It is.

What is equilibrium conformer energy?

4. Equilibrium Conformer- Replaces the submitted molecule with its lowest-energy conformation. Often performed prior to calculating structure using a quantum chemical model. 5. Conformer Distribution - Creates a new file consisting of a selection of low- energy conformers.

What is conformation energy?

CONFORMATIONAL ENERGY The relationship between molecular structure and potential energy is a major area of study in organic chemistry. It has applications in conformational analysis because we are interested in relating the structure of conformers to their energy, and therefore to their relative stabilities.