Conformations of Conformations of Cycloalkanes Cycloalkanes

CycloalkanesCycloalkanesCycloalkanesCycloalkanes

• Most common cycloalkanesMost common cycloalkanes

Adolf von Baeyer (19th century)Adolf von Baeyer (19th century)Adolf von Baeyer (19th century)Adolf von Baeyer (19th century)

• assumed cycloalkanes are planar polygonsassumed cycloalkanes are planar polygons

• distortion of bond angles from 109.5° givesdistortion of bond angles from 109.5° givesangle strain to cycloalkanes with rings eitherangle strain to cycloalkanes with rings eithersmaller or larger than cyclopentanesmaller or larger than cyclopentane

• Baeyer deserves credit for advancing the ideaBaeyer deserves credit for advancing the ideaof angle strain as a destabilizing factor.of angle strain as a destabilizing factor.

• But Baeyer was incorrect in his belief that But Baeyer was incorrect in his belief that cycloalkanes were planar.cycloalkanes were planar.

Types of StrainTypes of StrainTypes of StrainTypes of Strain

•• Torsional strainTorsional strainstrain that results from eclipsed bondsstrain that results from eclipsed bonds

•• van der Waals strain (steric strain)van der Waals strain (steric strain)strain that results from atoms being too closestrain that results from atoms being too closetogethertogether

•• angle strainangle strainstrain that results from distortion of bondstrain that results from distortion of bondangles from normal valuesangles from normal values

•• Torsional strainTorsional strainstrain that results from eclipsed bondsstrain that results from eclipsed bonds

•• van der Waals strain (steric strain)van der Waals strain (steric strain)strain that results from atoms being too closestrain that results from atoms being too closetogethertogether

•• angle strainangle strainstrain that results from distortion of bondstrain that results from distortion of bondangles from normal valuesangles from normal values

CyclopropaneCyclopropaneCyclopropaneCyclopropane

sources of strain:sources of strain:

torsional straintorsional strain

angle strainangle strain

CyclobutaneCyclobutaneCyclobutaneCyclobutane

nonplanar conformation relieves some torsional strainnonplanar conformation relieves some torsional strain

angle strain presentangle strain present

Nonplanar conformations of cyclopentaneNonplanar conformations of cyclopentaneNonplanar conformations of cyclopentaneNonplanar conformations of cyclopentane

EnvelopeEnvelope Half-chairHalf-chair

Relieve some, but not all, of the torsional strain.Relieve some, but not all, of the torsional strain.

Envelope and half-chair are of similar stabilityEnvelope and half-chair are of similar stabilityand interconvert rapidly.and interconvert rapidly.

Chair is the most stable conformation of cyclohexaneChair is the most stable conformation of cyclohexaneChair is the most stable conformation of cyclohexaneChair is the most stable conformation of cyclohexane

All of the bonds are staggered and the bond All of the bonds are staggered and the bond angles at carbon are close to tetrahedral.angles at carbon are close to tetrahedral.

Boat conformation is less stable than the chairBoat conformation is less stable than the chairBoat conformation is less stable than the chairBoat conformation is less stable than the chair

All of the bond angles are close to tetrahedralAll of the bond angles are close to tetrahedralbut close contact between flagpole hydrogensbut close contact between flagpole hydrogenscauses van der Waals strain in boat.causes van der Waals strain in boat.

180 pm180 pm

The chair conformation of cyclohexane is theThe chair conformation of cyclohexane is themost stable conformation and derivativesmost stable conformation and derivativesof cyclohexane almost always exist in the of cyclohexane almost always exist in the chair conformationchair conformation

The chair conformation of cyclohexane is theThe chair conformation of cyclohexane is themost stable conformation and derivativesmost stable conformation and derivativesof cyclohexane almost always exist in the of cyclohexane almost always exist in the chair conformationchair conformation

Conformational InversionConformational InversionConformational InversionConformational Inversion

chair-chair interconversion (ring-flipping)chair-chair interconversion (ring-flipping)

rapid process (activation energy = 45 kJ/mol)rapid process (activation energy = 45 kJ/mol)

all axial bonds become equatorial and vice versaall axial bonds become equatorial and vice versa

Reactions of CyclopropaneReactions of CyclopropaneReactions of CyclopropaneReactions of Cyclopropane