Chapter 5 Structure and Preparation of Alkenes: Elimination Reactions

1
Chapter 5Structure and Preparation of
AlkenesElimination Reactions
2
Alkene Nomenclature
3
Alkenes

• Alkenes are hydrocarbons that contain a
  carbon-carbon double bond
• Olefins is a common name for alkenes
• Molecular formula CnH2n
• Same as that for a monocyclic alkane
• Formula is correct for linear alkenes with 1 CC
• Are called "unsaturated hydrocarbons

4
Alkene Nomenclature
Ethene or Ethylene(both are acceptableIUPAC
names)
Propene (Propylene issometimes used but is not
an acceptableIUPAC name)
5
Alkene Nomenclature
1-Butene

• 1) Find the longest continuous chain that
  includes the double bond.
• 2) Replace the -ane ending of the unbranched
  alkane having the same number of carbons by -ene.
• 3) Number the chain in the direction that gives
  the lowest number to the doubly bonded carbon.

6
Alkene Nomenclature

• 4) If a substituent is present, identify its
  position by a locator. The double bond takes
  precedence over alkyl groups and halogens when
  the chain is numbered.
• The compound shown above is4-bromo-3-methyl-1-bu
  tene.

7
Alkene Nomenclature

• 5) If a substituent is present, identify its
  position by a locator. Hydroxyl groups take
  precedence over the double bond when the chain is
  numbered.
• The compound shown above is2-methyl-3-buten-1-ol
  .

8
Alkenyl Groups

• methylene
• vinyl
• allyl
• isopropenyl

CH
H2C
CHCH2
9
Cycloalkene Nomenclature
Cyclohexene

• 1) Replace the -ane ending of the cycloalkane
  having the same number of carbons by -ene.

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Cycloalkene Nomenclature
6-Ethyl-1-methylcyclohexene

• 1) Replace the -ane ending of the cycloalkane
  having the same number of carbons by -ene.
• 2) Number through the double bond in
  thedirection that gives the lower number to the
  first-appearing substituent.

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Structure and Bonding in Alkenes
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Structure of Ethylene

• bond angles H-C-H 117
• H-C-C 121
• bond distances CH 110 pm
• CC 134 pm

planar
13
Bonding in Ethylene
?
?
?
?
?

• Framework of ? bonds
• Each carbon is sp2 hybridized

14
Bonding in Ethylene

• Each carbon has a half-filled p orbital

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Bonding in Ethylene

• Side-by-side overlap of half-filled p orbitals
  gives a ? bond

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Isomerism in Alkenes
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Isomers
Isomers are different compounds thathave the
same molecular formula.
18
Recall
Isomers
Constitutional isomers
Stereoisomers
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C4H8 Alkene Isomers
2-Methylpropene
1-Butene
trans-2-Butene
cis-2-Butene
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1-Butene
2-Methylpropene
Constitutional isomers
cis-2-Butene
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1-Butene
2-Methylpropene
Constitutional isomers
trans-2-Butene
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Stereoisomers
trans-2-Butene
cis-2-Butene
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Stereochemical Notation

• trans (identical or analogous substituents
  on opposite sides)

cis (identical or analogous substitutents on
same side)
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Figure 5.2
cis
trans
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Naming Steroisomeric Alkenesby the E-Z
Notational System
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Stereochemical Notation
CH2(CH2)6CO2H
CH3(CH2)6CH2
Oleic acid
H
H

• cis and trans are useful when substituents are
  identical or analogous (oleic acid has a cis
  double bond)
• cis and trans are ambiguous when analogies are
  not obvious

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Example

• What is needed1) systematic body of rules
  for ranking substituents
• 2) new set of stereochemical symbols
  other than cis and trans

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The E-Z Notational System

• E higher ranked substituents on opposite sides
• Z higher ranked substituents on same side

higher
lower
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The E-Z Notational System

• E higher ranked substituents on opposite sides
• Z higher ranked substituents on same side

lower
higher
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The E-Z Notational System

• E higher ranked substituents on opposite sides
• Z higher ranked substituents on same side

higher
lower
higher
lower
Entgegen
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The E-Z Notational System
Question How are substituents ranked?

• Answer They are ranked in order of
  decreasing atomic number.

higher
lower
higher
higher
higher
lower
lower
lower
Entgegen
Zusammen
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The Cahn-Ingold-Prelog (CIP) System

• The system that we use was devised by R. S.
  Cahn Sir Christopher Ingold Vladimir Prelog
• Their rules for ranking groups were devised in
  connection with a different kind of
  stereochemistryone that we will discuss in
  Chapter 7but have been adapted to alkene
  stereochemistry.

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CIP Rules

• (1) Higher atomic number outranks lower atomic
  number

Br gt F Cl gt H
(Z )-1-Bromo-2-chloro-1-fluoroethene
34
CIP Rules

• (2) When two atoms are identical, compare the
  atoms attached to them on the basis of their
  atomic numbers. Precedence is established at
  the first point of difference.

CH2CH3 outranks CH3
35
CIP Rules

• (3) Work outward from the point of attachment,
  comparing all the atoms attached to a
  particular atom before proceeding
  further along the chain.

CH(CH3)2 outranks CH2CH2OH
C(C,H,H)
C(C,C,H)
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CIP Rules

• (4) Evaluate substituents one by one. Don't
  add atomic numbers within groups.

CH2OH outranks C(CH3)3
C(O,H,H)
C(C,C,C)
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CIP Rules

• (5) An atom that is multiply bonded to another
  atom is considered to be replicated as a
  substituent on that atom.

CHO outranks CH2OH
C(O,O,H)
C(O,H,H)
38
CIP Rules

• A table of commonly encountered substituents
  ranked according to precedence is given on the
  inside back cover of the text.

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Physical Properties of Alkenes
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Dipole moments

• What is direction of dipole moment?
• Does a methyl group donate electrons to the
  double bond, or does it withdraw them?

? 0 D
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Dipole moments

• Chlorine is electronegative and attracts
  electrons.

? 0 D
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Dipole moments

• Dipole moment of 1-chloropropene is equal to the
  sum of the dipole moments of vinyl chloride and
  propene.

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Dipole moments

• Therefore, a methyl group donates electrons to
  the double bond.

? 1.7 D
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Alkyl groups stabilize sp2 hybridizedcarbon by
releasing electrons
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Relative Stabilities of Alkenes
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Double bonds are classified according tothe
number of carbons attached to them.
monosubstituted
R'
R
R
H
R'
H
H
H
disubstituted
disubstituted
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Double bonds are classified according tothe
number of carbons attached to them.
48
Substituent Effects on Alkene Stability

• Electronic
• disubstituted alkenes are more stable than
  monosubstituted alkenes
• Steric
• trans alkenes are more stable than cis alkenes

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Figure 5.4 Heats of combustion of C4H8isomers.
2717 kJ/mol
6O2
2710 kJ/mol
2707 kJ/mol
2700 kJ/mol
4CO2 8H2O
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Substituent Effects on Alkene Stability
Electronic

• Alkyl groups stabilize double bonds more than H
• more highly substituted double bonds are
  morestable than less highly substituted ones.

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Problem 5.8

• Give the structure or make a molecular model of
  the most stable C6H12 alkene.

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Substituent Effects on Alkene Stability
Steric

• trans alkenes are more stable than cis alkenes
• cis alkenes are destabilized by van der
  Waalsstrain

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Figure 5.5 cis and trans-2-Butene
cis-2-butene
trans-2-butene
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Figure 5.5 cis and trans-2-Butene
van der Waals straindue to crowding
ofcis-methyl groups
cis-2-butene
trans-2-butene
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van der Waals Strain

• Steric effect causes a large difference in
  stabilitybetween cis and trans-(CH3)3CCHCHC(CH3)
  3
• cis is 44 kJ/mol less stable than trans

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Cycloalkenes
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Cycloalkenes

• Cyclopropene and cyclobutene have angle strain.
• Larger cycloalkenes, such as cyclopenteneand
  cyclohexene, can incorporate a double bond into
  the ring with little or no angle strain.

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Stereoisomeric cycloalkenes

• cis-cyclooctene and trans-cycloocteneare
  stereoisomers
• cis-cyclooctene is 39 kJ/ molmore stablethan
  trans-cyclooctene

cis-Cyclooctene
trans-Cyclooctene
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Stereoisomeric cycloalkenes

• trans-cyclooctene is smallest trans-cycloalkene
  that is stable at room temperature
• cis stereoisomer is more stable than trans
  through C11 cycloalkenes

trans-Cyclooctene
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Stereoisomeric cycloalkenes

• cis and trans-cyclododeceneare approximately
  equal instability

trans-Cyclododecene
cis-Cyclododecene
When there are more than 12 carbons in thering,
trans-cycloalkenes are more stable than cis.The
ring is large enough so the cycloalkene behaves
much like a noncyclic one.