Chapter 3 Alkanes and Cycloalkanes: Conformations and cis-trans Stereoisomers

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Chapter 3Alkanes and Cycloalkanes Conformations
and cis-trans Stereoisomers
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3.1Conformational 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 Staggered Conformation
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 apply only to bonds (or
  groups) on adjacent carbons, and only to
  staggered conformations.

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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/mol
  less stable than the staggered.
• The eclipsed conformation is destabilized
  bytorsional strain.
• Torsional strain is the destabilization that
  resultsfrom eclipsed or partially eclipsed bonds.

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3.2Conformational Analysis of Butane
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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

• 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.

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van der Waals Strain

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

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3.3Conformations of Higher Alkanes
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Unbranched Alkanes
Hexane

• The most stable conformation of
  unbranchedalkanes has anti relationships between
  carbons.

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3.4The Shapes of CycloalkanesPlanar or
Nonplanar?
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Adolf von Baeyer (19th century)

• Baeyer assumed cycloalkanes are planar polygons,
• and that 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 close
  together
• angle strain strain that results from
  distortion of bond angles 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 in 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 in 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 in planar conformation
• 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 angle strain 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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Generalization

• 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 Bonds in Cyclohexane
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The 12 bonds to the ring can be divided into two
sets of 6.
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6 Bonds are axial
Axial bonds point "north and south"
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The 12 bonds to the ring can be divided into two
sets of 6.
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6 Bonds are equatorial
Equatorial bonds lie along the equator.
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3.9Conformational Inversion 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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45 kJ/mol
23 kJ/mol
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3.10Conformational Analysis of Monosubstituted
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
F
F
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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3.11Disubstituted Cycloalkanescis-trans
Stereoisomers

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

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Isomers
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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 cis stereoisomer less
stable than trans.
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3.12Conformational Analysisof Disubstituted
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 ofcis-1,4-dimethylcyclohe
xane
CH3
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Conformational analysis oftrans-1,4-dimethylcyclo
hexane
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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 ofcis-1,2-dimethylcyclohe
xane
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Conformational analysis oftrans-1,2-dimethylcyclo
hexane
CH3
H
H3C
H
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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 ofcis-1,3-dimethylcyclohe
xane
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Conformational analysis oftrans-1,3-dimethylcyclo
hexane
CH3
H3C
H
H
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Table 3.2 Heats of Combustion ofIsomeric
Dimethylcyclohexanes

• Compound Orientation -?H (kJ/mol)
• 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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3.13Medium 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, howeverminimize total
  strain.

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3.14Polycyclic Ring Systems

• contain more than one ring
• bicyclic
• tricyclic
• tetracyclic
• etc

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Types of Ring Systems

• spirocyclic
• fused ring
• bridged ring

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Adamantane A Tricyclic Compound
Three bond cleavages are needed to create an
open-chain structure.
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Spirocyclic

• one atom common to two rings

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Fused Ring

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

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Bridged Ring

• nonadjacent atoms common to two rings

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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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Bridged Bicyclic

• requires two bond disconnections

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Steroids

• carbon skeleton is tetracyclic

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3.15Heterocyclic 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
Ethylene oxide
Tetrahydrofuran
Tetrahydropyran
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Nitrogen-containing Heterocycles
Piperidine
Pyrrolidine
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Sulfur-containing Heterocycles
Lipoic acid
Lenthionine