Conformation of Saturated Rings

Question 1

Why do cyclohexanes adopt a chair conformation?

Answer 1

To minimize Baeyer Strain and Pitzer strain

Question 2

Substituted cyclohexanes generally favour the chair conformation that puts the largest group in….

Answer 2

…equatorial in order to minimise 1,3-diaxial interactions

Question 3

Which is the most stable conformation of the following di-substituted cyclohexane

Answer 3

• The one on the RHS
• Two large substituents are in the equatorial position

Question 3

Which is the most stable conformation of the following isomer?

Answer 3

• 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

Question 4

Which is the more stable conformation of the following di-substituted cyclohexane

Answer 4

• 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

Question 5

What is conformational locking?

Answer 5

• Cyclohexanes that cannot flip from one chair to the other

Question 6

Which of the following di-substitueted cyclohexane is more stable

Answer 6

• The one on the left
• The one on the right doesn’t actually exist

Question 7

Why is Trans Decain special?

Answer 7

It cannot rotate into another chair conformation due to the ring strain being so great

Question 8

How can we tell these two diasteroisomers appart using J-coupling values

Answer 8

• 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

Question 9

What is the Anomeric affect?

Answer 9

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

Question 10

Why is the anomeric effect only exist in 6-membered rings?

Answer 10

Strong effect of geometry in hyperconjugation due to orbital overlap