Cyclohexane chair
Cyclohexane is not flat. It bends into a chair shape, which has no angle strain and no eclipsing. Each carbon has an axial position (straight up or down) and an equatorial position (pointing outward). Large groups prefer equatorial.
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Analogy
Sitting in a chair with your elbows out: staying out is roomier than poking into the next person.
Example
Axial CH₃: crowded by H atomson carbons 3 and 5 (1,3-diaxial)Equatorial CH₃: more roomEquatorial is ≈ 7 kJ/mol lower in energy
In practice
Glucose is so stable in its ring partly because all its large groups can sit equatorial.
Practice
Flashcards
- In a chair, which position do large groups prefer?Equatorial.
Interview questions
- Explain why a methyl group on cyclohexane prefers the equatorial position.
- Axial group has 1,3-diaxial interactions
- Steric crowding with axial hydrogens
- Equatorial points away from the ring
- About 7 kJ/mol more stable
Questions people ask
What is a ring flip?
A ring flip is the quick change of one chair form into the other. During it, every axial position becomes equatorial and every equatorial becomes axial. At room temperature it happens many thousands of times a second, so the two chairs are in equilibrium.
Why is a large group more stable when equatorial?
An axial group sits close to the two axial hydrogens on the same side of the ring, causing steric crowding called 1,3-diaxial strain. An equatorial group points away from the ring and avoids it. The bigger the group, the stronger the preference for the equatorial position.