Conformation of Saturated Rings
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Why do cyclohexanes adopt a chair conformation?
To minimize Baeyer Strain and Pitzer strain
Substituted cyclohexanes generally favour the chair conformation that puts the largest group in….
…equatorial in order to minimise 1,3-diaxial interactions
Which is the most stable conformation of the following di-substituted cyclohexane
• The one on the RHS • Two large substituents are in the equatorial position
Which is the most stable conformation of the following isomer?
• The one on the right • Due to the A value of the methyl group being bigger, meaning its larger • Hence is it more stable for the methyl to be in the equatorial position
Which is the more stable conformation of the following di-substituted cyclohexane
• The one on the RHS, because the methyl has the bigger A value • Hence will be less sterically hindered and less 1,3-diaxial interactions
What is conformational locking?
• Cyclohexanes that cannot flip from one chair to the other
Which of the following di-substitueted cyclohexane is more stable
• The one on the left • The one on the right doesn’t actually exist
Why is Trans Decain special?
It cannot rotate into another chair conformation due to the ring strain being so great
How can we tell these two diasteroisomers appart using J-coupling values
• For the isomer on the left: • JHₐₓ,Hₐₓ are 180° from another, hence >10Hz • JHₐₓ,Hₑₚ are 60° from another, hence 2-5Hz • For the isomer on the right: • JHₐₓ,Hₑₚ are 60° from another, hence 2-5Hz • JHₑₚ,Hₑₚ are 60° from another, hence 2-5Hz
What is the Anomeric affect?
Electronegative groups at the anomeric position, adopt an axial position, despite destabilising 1,3-dixial interaction, due to strong hyperconjugation The anomeric position is the position next to the oxygen in the ring The filled orbital on the oxygen will interact with the σ’ orbital of the C-MeO bond
Why is the anomeric effect only exist in 6-membered rings?
Strong effect of geometry in hyperconjugation due to orbital overlap