Chapter 3 Conformations of Alkanes and Cycloalkanes

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• Chapter 3Conformations of Alkanes and
  Cycloalkanes

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3.1 Conformational Analysis of Ethane
Conformations are different spatial arrangements
of a molecule that are generated by rotation
about single bonds.
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Ethane
Eclipsed conformation
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Ethane
Eclipsed conformation
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Ethane
Staggered conformation
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Ethane
Staggered conformation
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Projection formulas of the staggeredconformation
of ethane
Newman
Sawhorse
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Anti relationships
180
Two bonds are anti when the angle between them is
180.
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Gauche relationships
H
60
H
H
H
H
H
H
H
H
H
H
H
Two bonds are gauche when the angle between them
is 60.
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An important point The terms anti and gauche
applyonly to bonds (or groups) on
adjacentcarbons, and only to staggeredconformati
ons.
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12 kJ/mol
0 60 120 180 240 300 360
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Torsional strain

• The eclipsed conformation of ethane is 12
  kJ/molless stable (higher energy) than the
  staggered.
• The eclipsed conformation is destabilized
  bytorsional strain.
• Torsional strain is the destabilization that
  resultsfrom eclipsed bonds.

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3.2 Conformational Analysis of Butane
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Conformational Analysis of Butane C2-C3 Rotation
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14 kJ/mol
3 kJ/mol
0 60 120 180 240 300 360
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van der Waals strain
gauche
anti

• 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)
• which results from atoms being too close
  together.

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van der Waals strain
eclipsed

• 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.

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3.3 Conformational Analysis of Higher Alkanes
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• The most stable conformation of
  unbranchedalkanes has anti relationships between
  carbons

Hexane
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• 3.4The Shapes of CycloalkanesPlanar or
  Nonplanar?

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Adolf von Baeyer (19th century)

• assumed cycloalkanes are planar polygons
• distortion of bond angles from 109.5 givesangle
  strain to cycloalkanes with rings eithersmaller
  or larger than cyclopentane
• Baeyer deserves credit for advancing the ideaof
  angle strain as a destabilizing factor.
• But Baeyer was incorrect in his belief that
  cycloalkanes were planar.

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Types of Strain

• Torsional strain
• strain that results from eclipsed bonds
• van der Waals strain (steric strain)
• strain that results from atoms being too
  closetogether
• angle strain
• strain that results from distortion of
  bondangles from normal values

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Measuring Strain in Cycloalkanes

• Heats of combustion can be used to
  comparestabilities of isomers.
• But cyclopropane, cyclobutane, etc. are not
  isomers.
• All heats of combustion increase as the numberof
  carbon atoms increase.

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Measuring Strain in Cycloalkanes

• Therefore, divide heats of combustion by number
  of carbons and compare heats of combustion on a
  "per CH2 group" basis.

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Heats of Combustion of Cycloalkanes

• Cycloalkane kJ/mol Per CH2
• Cyclopropane 2,091 697
• Cyclobutane 2,721 681
• Cyclopentane 3,291 658
• Cyclohexane 3,920 653
• Cycloheptane 4,599 657
• Cyclooctane 5,267 658
• Cyclononane 5,933 659
• Cyclodecane 6,587 659

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Heats of Combustion of Cycloalkanes

• Cycloalkane kJ/mol Per CH2
• According to Baeyer, cyclopentane should
• have less angle strain than cyclohexane.
• Cyclopentane 3,291 658
• Cyclohexane 3,920 653
• The heat of combustion per CH2 group is
• less for cyclohexane than for cyclopentane.
• Therefore, cyclohexane has less strain than
• cyclopentane.

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Adolf von Baeyer (19th century)

• assumed cycloalkanes are planar polygons
• distortion of bond angles from 109.5 givesangle
  strain to cycloalkanes with rings eithersmaller
  or larger than cyclopentane
• Baeyer deserves credit for advancing the ideaof
  angle strain as a destabilizing factor.
• But Baeyer was incorrect in his belief that
  cycloalkanes were planar.

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3.5Small Rings

• Cyclopropane
• Cyclobutane

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Cyclopropane

• sources of strain
• torsional strain
• angle strain

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Cyclobutane

• nonplanar conformation relieves some torsional
  strain
• angle strain present

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3.6Cyclopentane
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Cyclopentane

• all bonds are eclipsed
• planar conformation destabilizedby torsional
  strain

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Nonplanar conformations of cyclopentane
Envelope
Half-chair

• Relieve some, but not all, of the torsional
  strain.
• Envelope and half-chair are of similar
  stabilityand interconvert rapidly.

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3.7Conformations of Cyclohexane

• heat of combustion suggests that anglestrain is
  unimportant in cyclohexane
• tetrahedral bond angles require nonplanar
  geometries

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Chair is the most stable conformation of
cyclohexane

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

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Boat conformation is less stable than the chair
180 pm

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

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Boat conformation is less stable than the chair

• Eclipsed bonds bonds gives torsional strain
  toboat.

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Skew boat is slightly more stable than boat
Skew boat
Boat

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

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• The chair conformation of cyclohexane is themost
  stable conformation and derivativesof
  cyclohexane almost always exist in the chair
  conformation

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3.8Axial and Equatorial Bondsin Cyclohexane
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The 12 bonds to the ring can be divided intotwo
sets of 6.
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6 bonds are axial
Axial bonds point "north and south"
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6 bonds are equatorial
Equatorial bonds lie along the equator
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3.9Conformational Inversion (Ring-Flipping) in
Cyclohexane
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Conformational Inversion

• chair-chair interconversion (ring-flipping)
• rapid process (activation energy 45 kJ/mol)
• all axial bonds become equatorial and vice versa

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Half-chair
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Half-chair
Skewboat
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Half-chair
Skewboat
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Half-chair
Skewboat
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45 kJ/mol
23 kJ/mol