Chapter 3 Conformations of Alkanes and Cycloalkanes Conformations or Conformers or Rotamers; Different spatial arrangements of a molecule that are generated by rotation about single bonds.

eclipsed conformation Ethane

Ethane staggered conformation

Projection Formulas of the Staggered Conformation of Ethane NewmanSawhorseHH HH HH HH H H H H

H H HH HHHH H H H H 180° Anti 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.

H H HH HHHH H H H H 60° Gauche 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° 60° 120° 180° 240°300°360° 12 kJ/mol

The eclipsed conformation of ethane is 12 kJ/mol less stable than the staggered. The eclipsed conformation is destabilized by torsional strain. Torsional strain is the destabilization that results from eclipsed bonds. Torsional strain

0° 60° 120° 180° 240°300°360° 3 kJ/mol 14 kJ/mol

The gauche conformation of butane is 3 kJ/mol less stable than the anti. The gauche conformation is destabilized by van 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 strain

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

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

Bond Angle for Cycloalkanes if they are Planar Bond Angle =(n-2)/n*180° Cyclopropane60° Cyclobutane90° Cyclopentane108° Cyclohexane120°

Torsional strain strain that results from eclipsed bondsTorsional strain strain that results from eclipsed bonds van der Waals strain (steric strain) strain that results from atoms being too close togethervan der Waals strain (steric strain) strain that results from atoms being too close together angle strain strain that results from distortion of bond angles from normal valuesangle strain strain that results from distortion of bond angles from normal values Types of Strain in Cycloalkanes

Experimentally Measuring Strain in Cycloalkanes Experimentally Measuring Strain in Cycloalkanes Heats of combustion can be used to compare stabilities of isomers.Heats of combustion can be used to compare stabilities of isomers. Since cycloalkanes are not isomers, divideSince cycloalkanes are not isomers, divide  H by the number of C in ring  H by the number of C in ring

CycloalkanekJ/molPer CH 2 Cyclopropane2, Cyclobutane2, Cyclopentane3, Cyclohexane3, Cycloheptane4, Cyclooctane5, Cyclononane5, Cyclodecane6, Heats of Combustion in Cycloalkanes

If rings were planar,cyclopentane should have less angle strain than cyclohexane. Cyclopentane658 Cyclohexane653 Therefore, cyclohexane has less strain than cyclopentane. Heats of Combustion in Cycloalkanes

sources of strain torsional strain angle strain Small Rings; Cyclopropane

nonplanar conformation relieves some torsional strain angle strain present Small Rings; Cyclobutane

all bonds are eclipsed in planar conformation planar conformation destabilized by torsional strain Cyclopentane

EnvelopeHalf-chair Relieve some, but not all, of the torsional strain. Envelope and half-chair are of similar stability and interconvert rapidly. Nonplanar Conformations of Cyclopentane

heat of combustion suggests that angle strain is unimportant in cyclohexane tetrahedral bond angles require nonplanar geometries 3.7 Conformations of Cyclohexane

All of the bonds are staggered and the bond angles at carbon are close to tetrahedral. Chair is the most stable conformation of cyclohexane

All of the bond angles are close to tetrahedral but close contact between flagpole hydrogens causes van der Waals strain in boat. 180 pm Boat conformation is less stable than the chair

Eclipsed bonds bonds gives torsional strain to boat. Boat conformation is less stable than the chair

Less van der Waals strain and less torsional strain in skew boat. Boat Skew boat Skew boat is slightly more stable than boat

the chair conformation of cyclohexane is the most stable conformation and derivatives of cyclohexane almost always exist in the chair conformation Generalization

The 12 bonds to the ring can be divided into two sets of Axial and Equatorial Bonds in the Chair Conformation of Cyclohexane

Axial bonds point "north and south"Axial bonds point "north and south" 6 Bonds are axial 6 Bonds are equatorial Equatorial bonds lie along the equatorEquatorial bonds lie along the equator

chair-chair interconversion (ring-flipping) rapid process (activation energy = 45 kJ/mol) all axial bonds become equatorial and vice versa Conformational Inversion (Ring Flipping) in Cyclohexane (chair conformation)

Half- chair Skew boat

45 kJ/mol 23 kJ/mol