BS132 - Lecture 2 - Cycloalkanes

What would happen if Cyclobutane followed Planar Conformation?

• Angle

• Compression and its effect

• How is Planar Conformation different?

• 90 degree angle

• Less compression of angle - better overlap so stronger bonds than cyclopropane

• Gives less angle strain but equal torsional strain

Describe what happens at Maximum Torsional Strain

CH sigma bonds eclipsed

Why doesn’t Cyclobutane adopt Planar Conformation?

All H eclipsed

What is Puckered Conformation?

1 C out of plane

Puckered/Bent Conformation in Cyclobutane

• Effect on Ring Strain

• Effect on Torsional and Angle Strain

• What is the result?

• Relieves overall ring strain

• Decreases torsional strain, increases angle strain

• Relief of torsional strain > increase in angle strain compared to planar structure to give lower ring strain

Puckered Conformation in Cyclopentane

• Envelope

• Effects on angle and torsional strain

• 1 C out of plane, gives envelope conformation

• Increases angle strain, relieves torsional strain

Ring Strain in Cyclohexane

• How does it compare to others?

• Angle Strain

• Torsional Strain and what it means

• Lowest ring strain - 0

• No angle strain so internal angles have to be 109.5

• Minimal torsional strain - CH bonds on neighbouring Cs are staggered, not eclipsed

What is Chair Conformation of Cyclohexane adopted instead of?

Planar

What is the C-C-C bond angle in Cyclohexane?

109.5

Chair Conformation of Cyclohexane

• Effect on Angle Strain

• Staggering

• Torsional Strain

• Ring Strain

• Leads to free angle strain

• Perfect staggering of CH bonds on C-

• Minimal torsional strain - 60 degrees

• No ring strain - No angle/torsional strain

Boat Conformation

• Angle strain?

• Torsional strain

• Transannular Strain

• No angle strain, other types contribute to ring strain

• Torsional strain due to eclipsing of 8 H atoms at base

• Transannular (across ring steric) strain due to vdw repulsion between flagpole H atoms

Separation of Cyclohexane

• Room Temperature

• How significant is chair conformation

• Separation of cyclohexane confromers impossible at room temp

• 99% of molecules in chair confirmation at any given moment

Equatorial vs Axial

Equatorial more stable chair - steric strain introduced between 2 Hs on C3 and C5 (known as 1,3-diaxial interactions)

What does Equatorial mean?

Points away from middle

Conformation of CH3 and its effect in Substituted Cyclohexanes

Axial position, less stable due to steric strain (vdw repulsion) experienced from H atoms at axial positions on C3 and C5