Chapter 3Chapter 3Conformations of Alkanes and Conformations of Alkanes and

CycloalkanesCycloalkanes

Conformations or Conformers or Rotamers;

Different spatial arrangements of a molecule that are generated by rotation about single bonds.

eclipsed conformation

EthaneEthane

EthaneEthane

staggered conformation

Projection Formulas of the Staggered Projection Formulas of the Staggered Conformation of EthaneConformation of Ethane

NewmanNewman SawhorseSawhorse

HH

HHHH HH

HH HH

HH

HH HHHH

HH

HH

HH

HHHH HH

HH HH

HH

HH HHHH

HH

HH180°180°

Anti RelationshipsAnti Relationships

Two bonds are anti when the angle between them is 180°.

The terms anti and gauche apply only to bonds (or groups) on adjacent carbons, and only to staggered conformations.

HH

HHHH HH

HH HH

HH

HH HHHH

HH

HH

60°60°

Gauche RelationshipsGauche Relationships

Two bonds are gauche when the angle between them is 60°.

The terms anti and gauche apply only to bonds (or groups) on adjacent carbons, and only to staggered conformations.

0°0° 60° 60° 120° 120° 180° 240° 180° 240° 300°300° 360°360°

12 kJ/mol12 kJ/mol

The eclipsed conformation of ethane is 12 kJ/mol less stable than the staggered.The eclipsed conformation is destabilized bytorsional strain.Torsional strain is the destabilization that resultsfrom eclipsed bonds.

Torsional strainTorsional strain

0°0° 60° 60° 120° 120° 180° 240° 180° 240° 300°300° 360°360°

3 kJ/mol3 kJ/mol

14 kJ/mol14 kJ/mol

•The gauche conformation of butane is 3 kJ/molless stable than the anti.•The gauche conformation is destabilized byvan der Waals strain (also called steric strain).•van der Waals strain is the destabilization that results from atoms being too close together.

van der Waals strainvan der Waals strain

•The conformation of butane in which the twomethyl groups are eclipsed with each other isis the least stable of all the conformations.•It is destabilized by both torsional strain(eclipsed bonds) and van der Waals strain.

van der Waals strainvan der Waals strain

Line Notation is Obtained from the most Line Notation is Obtained from the most stable conformer (anti-staggered stable conformer (anti-staggered

conformation)conformation)

Bond Angle for CycloalkanesBond Angle for Cycloalkanes ifif they are they are PlanarPlanarBond Angle =(n-2)/n*180°Bond Angle =(n-2)/n*180°

Cyclopropane 60°

Cyclobutane 90°

Cyclopentane 108°

Cyclohexane 120°

•• Torsional strainTorsional strain strain that results from eclipsed bonds strain that results from eclipsed bonds

•• van der Waals strain (steric strain)van der Waals strain (steric strain)strain that results from atoms being too strain that results from atoms being too close togetherclose together

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

Types of Strain in CycloalkanesTypes of Strain in Cycloalkanes

Experimentally Measuring Strain in CycloalkanesExperimentally Measuring Strain in Cycloalkanes

•Heats of combustion can be used to compareHeats of combustion can be used to comparestabilities of isomers.stabilities of isomers.

•Since cycloalkanes are not isomers, divideSince cycloalkanes are not isomers, divide

H by the number of C in ringH by the number of C in ring

CycloalkaneCycloalkane kJ/molkJ/mol Per CHPer CH22

CyclopropaneCyclopropane 2,0912,091 697697CyclobutaneCyclobutane 2,7212,721 681681CyclopentaneCyclopentane 3,2913,291 658658CyclohexaneCyclohexane 3,9203,920 653653CycloheptaneCycloheptane 4,5994,599 657657CyclooctaneCyclooctane 5,2675,267 658658CyclononaneCyclononane 5,9335,933 659659CyclodecaneCyclodecane 6,5876,587 659659

Heats of Combustion in CycloalkanesHeats of Combustion in Cycloalkanes

If rings were planar,cyclopentane should If rings were planar,cyclopentane should have less angle strain than cyclohexane.have less angle strain than cyclohexane.

CyclopentaneCyclopentane 658658CyclohexaneCyclohexane 653653

Therefore, cyclohexane has less strain Therefore, cyclohexane has less strain thanthancyclopentane.cyclopentane.

Heats of Combustion in CycloalkanesHeats of Combustion in Cycloalkanes

sources of strainsources of strain

torsional straintorsional strain

angle strainangle strain

Small Rings; CyclopropaneSmall Rings; Cyclopropane

nonplanar conformation relieves some torsional strainnonplanar conformation relieves some torsional strain

angle strain presentangle strain present

Small Rings; CyclobutaneSmall Rings; Cyclobutane

all bonds are eclipsed in planar conformationall bonds are eclipsed in planar conformation

planar conformation destabilizedplanar conformation destabilizedby torsional strainby torsional strain

CyclopentaneCyclopentane

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.

Nonplanar Conformations of CyclopentaneNonplanar Conformations of Cyclopentane

heat of combustion suggests that angle strain heat of combustion suggests that angle strain is unimportant in cyclohexaneis unimportant in cyclohexane

tetrahedral bond angles require nonplanar tetrahedral bond angles require nonplanar geometriesgeometries

3.73.7Conformations of CyclohexaneConformations of Cyclohexane

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

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

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 causes van der Waalsvan der Waals strainstrain in boat. in boat.

180 pm180 pm

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

Eclipsed bonds bonds gives Eclipsed bonds bonds gives torsional straintorsional strain to toboat.boat.

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

Less van der Waals strain and less torsional Less van der Waals strain and less torsional strain in skew boat.strain in skew boat.

BoatBoat Skew boatSkew boat

Skew boat is slightly more stable Skew boat is slightly more stable than boatthan boat

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

GeneralizationGeneralization

The 12 bonds to the ring can be The 12 bonds to the ring can be divided into two sets of 6.divided into two sets of 6.

3.83.8Axial and Equatorial Bonds in Axial and Equatorial Bonds in

the Chair Conformation ofthe Chair Conformation ofCyclohexaneCyclohexane

•Axial bonds point "north and south"Axial bonds point "north and south"

6 Bonds are axial6 Bonds are axial 6 Bonds are 6 Bonds are equatorialequatorial

•Equatorial bonds lie along the equatorEquatorial bonds lie along the equator

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

Conformational Inversion (Ring Flipping)Conformational Inversion (Ring Flipping)in Cyclohexane (chair conformation)in Cyclohexane (chair conformation)

Half-Half-chairchair

SkewSkewboatboat

45 45 kJ/molkJ/mol

45 45 kJ/molkJ/mol

23 23 kJ/molkJ/mol