Chapter 3 Conformations of Alkanes and Cycloalkanes

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• Chapter 3Conformations of Alkanes and
  Cycloalkanes

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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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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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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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• The 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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Small 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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Cyclopentane
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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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Conformations 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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Axial 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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Conformational 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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(No Transcript)
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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
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Conformational Analysis ofMonosubstituted
Cyclohexanes

• most stable conformation is chair
• substituent is more stable when equatorial

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Methylcyclohexane
5
95

• Chair chair interconversion occurs, but at any
  instant 95 of the molecules have their methyl
  group equatorial.
• Axial methyl group is more crowded than an
  equatorial one.

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Methylcyclohexane
5
95

• Source of crowding is close approach to axial
  hydrogens on same side of ring.
• Crowding is called a "1,3-diaxial repulsion" and
  is a type of van der Waals strain.

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Fluorocyclohexane
40
60

• Crowding is less pronounced with a "small"
  substituent such as fluorine.
• Size of substituent is related to its branching.

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tert-Butylcyclohexane
Less than 0.01
Greater than 99.99

• Crowding is more pronounced with a "bulky"
  substituent such as tert-butyl.
• tert-Butyl is highly branched.

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tert-Butylcyclohexane
van der Waalsstrain due to1,3-diaxialrepulsions
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Disubstituted CycloalkanesStereoisomers

• Stereoisomers are isomers that have same
  constitution but different arrangement of atoms
  in space

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Isomers
Constitutional isomers
Stereoisomers
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1,2-Dimethylcyclopropane

• There are two stereoisomers of 1,2-dimethylcyclop
  ropane.
• They differ in spatial arrangement of atoms.

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1,2-Dimethylcyclopropane

• cis-1,2-Dimethylcyclopropane has methyl groupson
  same side of ring.
• trans-1,2-Dimethylcyclopropane has methyl
  groupson opposite sides.

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Relative stabilities of stereoisomers may
bedetermined from heats of combustion.
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van der Waals strain makes cisstereoisomer less
stable than trans
3371 kJ/mol
3366 kJ/mol
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Conformational Analysis ofDisubstituted
Cyclohexanes
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1,4-Dimethylcyclohexane stereoisomers
CH3
cis
trans
5219 kJ/mol
5212 kJ/mol
less stable
more stable

• Trans stereoisomer is more stable than cis, but
  methyl groups are too far apart to crowd each
  other.

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Conformational analysis of cis-1,4-dimethylcycloh
exane
CH3

Two equivalent conformations each has one axial
methyl group and one equatorial methyl group
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Conformational analysis of trans-1,4-dimethylcycl
ohexane

Two conformations are not equivalent most
stableconformation has both methyl groups
equatorial.
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1,2-Dimethylcyclohexane stereoisomers
cis
trans
5223 kJ/mol
5217 kJ/mol
less stable
more stable

• Analogous to 1,4 in that trans is more
  stablethan cis.

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Conformational analysis of cis-1,2-dimethylcycloh
exane
Two equivalent conformations each has one axial
methyl group and one equatorial methyl group
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CH3
Conformational analysis of trans-1,2-dimethylcycl
ohexane
H
H3C
H
CH3


H
H
CH3
H
H3C
CH3
H
Two conformations are not equivalent most
stableconformation has both methyl groups
equatorial.
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1,3-Dimethylcyclohexane stereoisomers
CH3
H
H3C
H
cis
trans
5212 kJ/mol
5219 kJ/mol
more stable
less stable

• Unlike 1,2 and 1,4 cis-1,3 is more stable than
  trans.

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Conformational analysis of cis-1,3-dimethylcycloh
exane
Two conformations are not equivalent most
stableconformation has both methyl groups
equatorial.
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CH3
Conformational analysis of trans-1,3-dimethylcycl
ohexane
H3C
H
H

CH3


H
H
H
CH3
H3C
H3C
H
Two equivalent conformations each has one
axialand one equatorial methyl group.
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Table 3.2 Heats of Combustion of Isomeric
Dimethylcyclohexanes

• Compound Orientation -DH
• cis-1,2-dimethyl ax-eq 5223trans-1,2-dimethyl eq-
  eq 5217
• cis-1,3-dimethyl eq-eq 5212trans-1,3-dimethyl ax
  -eq 5219
• cis-1,4-dimethyl ax-eq 5219trans-1,4-dimethyl eq-
  eq 5212

more stable stereoisomer of pair
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Medium and Large Rings
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Cycloheptane and Larger Rings

• More complicated than cyclohexane.
• Common for several conformations to be of
  similar energy.
• Principles are the same, however.Minimize total
  strain.

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Polycyclic Ring Systems

• Contain more than one ring..
• bicyclic, tricyclic, tetracyclic, etc.

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Number of rings

• equals minimum number of bond disconnectionsrequi
  red to give a noncyclic species

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Monocyclic

• requires one bond disconnection

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Bicyclic

• requires two bond disconnections

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Bicyclic

• requires two bond disconnections

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Types of ring systems

• spirocyclic
• fused ring
• bridged ring

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Spirocyclic

• one atom common to two rings

Spiro4.5decane
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Fused ring

• adjacent atoms common to two rings
• two rings share a common side

Bicyclo4.3.0nonane
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Bridged ring

• nonadjacent atoms common to two rings
• Bicyclo3.2.1octane

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Steroids

• carbon skeleton is tetracyclic

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Heterocyclic Compounds
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Heterocyclic Compound

• a cyclic compound that contains an atom other
  than carbon in the ring
• (such atoms are called heteroatoms)
• typical heteroatoms are N, O, and S

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Oxygen-containing heterocycles
O
Tetrahydropyran
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Nitrogen-containing heterocycles
Piperidine
Pyrrolidine
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Sulfur-containing heterocycles
Lipoic acid
Lenthionine
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End of Chapter 3