Cyclohexane in 3-D

Introduction

Cyclohexane is a saturated six-membered ring that is found in many different three-dimensional conformations.

The conformation of cyclohexane in solution is really a matter of temperature, the energy of the conformation, and percentages of a population. In a population of molecules (a solution) different amounts of each conformation exist. At higher temperatures, there is more energy available to increase the proportion of cyclohexane molecules in the higher energy states, especially energy needed to overcome the transition states between stable forms. The stable forms of cyclohexane are shown in the energy diagram as the "valley" bottoms, and the transition states as the tops of the "hills" of the curve (see diagram at left).

At room temperature, cyclohexane can exist in a number of conformations, two of the simplest being the chair and boat conformation. Other conformations include the half-chair and twist-chair.

Chair conformation

The conformation with the lowest energy is the chair form. Note that is looks like a lawn chair. What angle do you see between the carbon atoms?

As cyclohexane interconverts between two possible chair conformations, it must first change into the boat form. To get into a boat conformation, an energy barrier must be overcome. The structure that represents this high-energy transition state structure is a twist-chair or half-chair conformation. What part of the molecule appears strained? What angle do you see between these carbon atoms?

Boat conformation

The twist-boat has the lowest energy of the boat conformations. Can you identify a part of the molecule where there is steric strain?

The boat conformation represents a transition state between two differently twisted boat forms. Why do you think this more symmetrical arrangement is more unstable?

Once in one of the boat conformations, cyclohexane can again go through the twist-chair transition state to again form the stable chair structure.

Review Question 1:

Load the structure below by selecting "View."

Why do you think the half-chair transition state conformation has such a high energy? Hint: what are the angles favored by each sp₃-hybridized carbon?