Cycloalkanes

What would happen if Cyclobutane followed Planar Conformation?

• Angle

• Compression and its effect

• Torsional Strain

• How is Planar Conformation different?

• 90 degree angle

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

• Max torsional strain - CH sigma bonds eclipsed

• Planar conformation gives less angle strain but equal torsional strain

Puckered/Bent Conformation in Cyclobutane

• Why doesn’t Cyclobutane adopt Planar Conformation?

• What is Puckered Conformation?

• Effect on Ring Strain

• Effect on Torsional and Angle Strain

• What is the result?

• All H eclipsed

• 1 C out of plane

• 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

• 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

Chair Conformation of Cyclohexane

• Adpoted instead of?

• Bond Angle

• Effect on Angle Strain

• Staggering

• Torsional Strain

• Ring Strain

• Adopted instead of planar

• C-C-C Bond Angle = 109.5

• Leads to free angle strain

• Perfect staggering of CH bonds on C-

• Minimal torsional strain - 60degrees

• No ring strain - No angle/torsional strain

Boat Conformation

• 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

Substituted Cyclohexanes

• Position of CH3 and its effect

• What does Axial mean?

• What does Equatorial mean?

• How do they compare

• Conformation w/ CH3 in axial position is less stable due to steric strain (vdw repulsion) experienced from H atoms at axial positions on C3 and C5

• Axial - points up

• Equatorial - Away from middle

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