Chapter 5 Structure and Preparation of Alkenes: Elimination Reactions

1
Chapter 5Structure and Preparation of
AlkenesElimination Reactions
2
5.1Alkene Nomenclature
3
Alkenes

• Alkenes are hydrocarbons that contain a
  carbon-carbon double bond
• also called "olefins"
• characterized by molecular formula CnH2n
• said to be "unsaturated"

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 number. 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

• 4) If a substituent is present, identify its
  position by number. 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.

10
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.

11
5.2Structure and Bonding in Alkenes
12
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

15
Bonding in Ethylene

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

16
5.3Isomerism in Alkenes
17
Isomers
Isomers are different compounds thathave the
same molecular formula.
18
Isomers
Constitutional isomers
Stereoisomers
19
Isomers
Constitutional isomers
Stereoisomers
consider the isomeric alkenes of molecular
formula C4H8
20
1-Butene
2-Methylpropene
trans-2-Butene
cis-2-Butene
21
1-Butene
2-Methylpropene
Constitutional isomers
cis-2-Butene
22
1-Butene
2-Methylpropene
Constitutional isomers
trans-2-Butene
23
Stereoisomers
trans-2-Butene
cis-2-Butene
24
Stereochemical Notation

• trans (identical or analogous substituents
  on opposite sides)

cis (identical or analogous substitutents on
same side)
25
Figure 5.2
Interconversion of stereoisomericalkenes does
not normally occur.Requires that ??component of
doublebond be broken.
cis
trans
26
Figure 5.2
cis
trans
27
5.4Naming Steroisomeric Alkenesby the E-Z
Notational System
28
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

29
Example

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

30
The E-Z Notational System

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

higher
lower
31
The E-Z Notational System

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

lower
higher
32
The E-Z Notational System

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

higher
lower
higher
lower
Entgegen
33
The E-Z Notational System
Question How are substituents ranked?

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

higher
lower
higher
higher
higher
lower
lower
lower
Entgegen
Zusammen
34
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.

35
Table 5.1 CIP Rules

• (1) Higher atomic number outranks lower atomic
  number

Br gt F Cl gt H
36
Table 5.1 CIP Rules

• (1) Higher atomic number outranks lower atomic
  number

Br gt F Cl gt H
(Z )-1-Bromo-2-chloro-1-fluoroethene
37
Table 5.1 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
38
Table 5.1 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)
39
Table 5.1 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)
40
Table 5.1 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)
41
Table 5.1 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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5.5Physical Properties of Alkenes
43
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
44
Dipole moments

• Chlorine is electronegative and attracts
  electrons.

? 0 D
45
Dipole moments

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

46
Dipole moments

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

? 1.7 D
47
Alkyl groups stabilize sp2 hybridizedcarbon by
releasing electrons
48
5.6Relative Stabilities of Alkenes
49
Double bonds are classified according tothe
number of carbons attached to them.
monosubstituted
R'
R
R
H
R'
H
H
H
disubstituted
disubstituted
50
Double bonds are classified according tothe
number of carbons attached to them.
51
Substituent Effects on Alkene Stability

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

52
Figure 5.4 Heats of combustion of C4H8isomers.
2717 kJ/mol
6O2
2710 kJ/mol
2707 kJ/mol
2700 kJ/mol
4CO2 8H2O
53
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.

54
Problem 5.8

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

55
Substituent Effects on Alkene Stability
Steric

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

56
Figure 5.5 cis and trans-2-Butene
cis-2-butene
trans-2-butene
57
Figure 5.5 cis and trans-2-Butene
van der Waals straindue to crowding
ofcis-methyl groups
cis-2-butene
trans-2-butene
58
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

59
5.7Cycloalkenes
60
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.

61
Stereoisomeric cycloalkenes

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

cis-Cyclooctene
trans-Cyclooctene
62
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
63
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.
64
5.8 Preparation of AlkenesElimination Reactions
65
?-Elimination Reactions Overview

• dehydrogenation of alkanes X Y H
• dehydration of alcohols X H Y OH
• dehydrohalogenation of alkyl halides X H Y
  Br, etc.

Y
X
?
?
66
Dehydrogenation

• limited to industrial syntheses of ethylene,
  propene, 1,3-butadiene, and styrene
• important economically, but rarely used in
  laboratory-scale syntheses

750C
CH3CH3
750C
CH3CH2CH3
67
5.9Dehydration of Alcohols
68
Dehydration of Alcohols
69
Relative Reactivity
70
5.10Regioselectivity in Alcohol DehydrationThe
Zaitsev Rule
71
Regioselectivity

90
10

• A reaction that can proceed in more than one
  direction, but in which one direction
  predominates, is said to be regioselective.

72
Regioselectivity

16
84

• A reaction that can proceed in more than one
  direction, but in which one direction
  predominates, is said to be regioselective.

73
The Zaitsev Rule

• When elimination can occur in more than one
  direction, the principal alkene is the one
  formed by loss of H from the ? carbon having
  thefewest hydrogens.

three protons on this ? carbon
74
The Zaitsev Rule

• When elimination can occur in more than one
  direction, the principal alkene is the one
  formed by loss of H from the ? carbon having
  thefewest hydrogens.

two protons on this ? carbon
75
The Zaitsev Rule

• When elimination can occur in more than one
  direction, the principal alkene is the one
  formed by loss of H from the ? carbon having
  thefewest hydrogens.

only one proton on this ? carbon
76
5.11Stereoselectivity in Alcohol Dehydration
77
Stereoselectivity

• A stereoselective reaction is one in which a
  single starting material can yield two or more
  stereoisomeric products, but gives one of them
  in greater amounts than any other.

78
5.12The E1 and E2 Mechanismsof Alcohol
Dehydration
79
A connecting point...

• The dehydration of alcohols and the reaction of
  alcohols with hydrogen halides share
  thefollowing common features
• 1) Both reactions are promoted by acids
• 2) The relative reactivity decreases in
  the order tertiary gt secondary gt primary
• These similarities suggest that carbocations
  areintermediates in the acid-catalyzed
  dehydration ofalcohols, just as they are in the
  reaction of alcoholswith hydrogen halides.

80
Dehydration of tert-Butyl Alcohol

H2O

• first two steps of mechanism are identical
  tothose for the reaction of tert-butyl alcohol
  withhydrogen halides

81
Mechanism
Step 1 Proton transfer to tert-butyl alcohol
..



O

• (CH3)3C

H
82
Mechanism
Step 2 Dissociation of tert-butyloxonium
ion to carbocation
Because rate-determiningstep is unimolecular,
thisis called the E1 mechanism.
83
Mechanism
Step 3 Deprotonation of tert-butyl cation.

84
Carbocations

• are intermediates in the acid-catalyzed
  dehydration of tertiary and secondary alcohols
• carbocations can
• react with nucleophileslose a ?-proton to form
  an alkene

85
Dehydration of Primary Alcohols
H2SO4

H2O
CH3CH2OH
160C

• avoids carbocation because primary carbocations
  are too unstable
• oxonium ion loses water and a proton in
  abimolecular step

86
Mechanism
Step 1 Proton transfer from acid to ethanol
..


CH3CH2
O
H
87
Mechanism
Step 2 Oxonium ion loses both a proton and a
water molecule in the same step.

88
Mechanism
Because rate-determiningstep is bimolecular,
thisis called the E2 mechanism.
89
5.13Rearrangements in Alcohol Dehydration

• Sometimes the alkene product does not have the
  same carbon skeleton as the starting alcohol.

90
Example
OH
H3PO4, heat
3
91
Rearrangement involves alkyl group migration

• carbocation can lose a proton as shown
• or it can undergo a methyl migration
• CH3 group migrates with its pair of electrons to
  adjacent positively charged carbon

3
92
Rearrangement involves alkyl group migration

CH3
CH3

• tertiary carbocation more stable

3
93
Rearrangement involves alkyl group migration

CH3
CH3
3
94
Another rearrangement

• CH3CH2CH2CH2OH

H3PO4, heat
95
Rearrangement involves hydride shift

• oxonium ion can losewater and a proton(from
  C-2) to give1-butene
• doesn't give a carbocation directlybecause
  primarycarbocations are toounstable

96
Rearrangement involves hydride shift
CH3CH2CHCH3

• hydrogen migrates with its pair of electrons
  from C-2 to C-1 as water is lost
• carbocation formed by hydride shift is
  secondary

97
Rearrangement involves hydride shift
CH3CH2CHCH3

mixture of cisand trans-2-butene
98
Hydride Shift
H
99
Carbocations can...

• react with nucleophiles
• lose a proton from the ?-carbon to form an alkene
  
• rearrange (less stable to more stable)

100
5.14 Dehydrohalogenation of Alkyl Halides
101
?-Elimination Reactions Overview

• dehydrogenation of alkanes X Y H
• dehydration of alcohols X H Y OH
• dehydrohalogenation of alkyl halides X H Y
  Br, etc.

Y
X
?
?
102
?-Elimination Reactions Overview

• dehydrogenation of alkanes industrial process
  not regioselective
• dehydration of alcohols acid-catalyzed
• dehydrohalogenation of alkyl halides consumes
  base

Y
X
?
?
103
Dehydrohalogenation

• is a useful method for the preparation of alkenes

NaOCH2CH3
ethanol, 55C
(100 )
likewise, NaOCH3 in methanol, or KOH in ethanol
104
Dehydrohalogenation

• When the alkyl halide is primary,
  potassiumtert-butoxide in dimethyl sulfoxide is
  the base/solvent system that is normally used.

CH3(CH2)15CH2CH2Cl
105
Regioselectivity

71
29

• follows Zaitsev's rule
• more highly substituted double bond predominates

106
Stereoselectivity
Br

(23)
(77)

• more stable configurationof double bond
  predominates

107
Stereoselectivity

(85)
(15)

• more stable configurationof double bond
  predominates

108
5.15Mechanism of theDehydrohalogenation of
Alkyl HalidesThe E2 Mechanism
109
Facts

• (1) Dehydrohalogenation of alkyl halides
  exhibits second-order kinetics
• first order in alkyl halide first order in
  base rate kalkyl halidebase
• implies that rate-determining step involves
  both base and alkyl halide i.e., it is
  bimolecular

110
Facts

• (2) Rate of elimination depends on halogen
• weaker CX bond faster rate rate RI gt
  RBr gt RCl gt RF
• implies that carbon-halogen bond breaks in the
  rate-determining step

111
The E2 Mechanism

• concerted (one-step) bimolecular process
• single transition state
• CH bond breaks
• ? component of double bond forms
• CX bond breaks

112
The E2 Mechanism

O
Reactants
113
The E2 Mechanism
?
..
H
O
R
..
Transition state
C C
?
114
The E2 Mechanism
..
H
O
R
..
C C
Products
115
5.16Anti Elimination in E2 Reactions

• Stereoelectronic Effects

116
Stereoelectronic effect
KOC(CH3)3(CH3)3COH
cis-1-Bromo-4-tert- butylcyclohexane
117
Stereoelectronic effect
trans-1-Bromo-4-tert- butylcyclohexane
KOC(CH3)3(CH3)3COH
118
Stereoelectronic effect
cis
KOC(CH3)3(CH3)3COH

• Rate constant for dehydrohalogenation of cis is
  500 times greater than that of trans

KOC(CH3)3(CH3)3COH
trans
119
Stereoelectronic effect
cis
KOC(CH3)3(CH3)3COH
H
H

• H that is removed by base must be anti coplanar
  to Br
• Two anti coplanar H atoms in cis stereoisomer

120
Stereoelectronic effect
trans
KOC(CH3)3(CH3)3COH

• H that is removed by base must be anti coplanar
  to Br
• No anti coplanar H atoms in trans stereoisomer
  all vicinal H atoms are gauche to Br

121
Stereoelectronic effect
cis
more reactive
trans
less reactive
122
Stereoelectronic effect

• An effect on reactivity that has its origin in
  the spatial arrangement of orbitals or bonds is
  called a stereoelectronic effect.
• The preference for an anti coplanar arrangement
  of H and Br in the transition state for E2
  dehydrohalogenation is an example of a
  stereoelectronic effect.

123
5.17Isotopes Effects And The E2 Mechanism
124
The Isotope Effect

• A C-D bond is ?12 kJ/mol stronger than a C-H
  bond.
• The activation energy for breaking a C-D bond is
  greater than for breaking a C-H bond.
• The rate constant k for an elementary step where
  C-D breaks is smaller than for a C-H bond.
• The difference in rate is expressed as a ratio
  kH/kD, and is a kinetic isotope effect.
• Because it compares 2H to 1H, it is called a
  deuterium isotope effect.

125
The Isotope Effect

• In the rate determining step of the E2 mechanism,
  a base removes a proton from a ? carbon.
• The mechanism should exhibit a deuterium isotope
  effect.

NaOCH2CH3
D2CCHCD3
D3CCHCD3
CH3CH2OH
Br
126
5.18The E1 Mechanism ofDehydrohalogenation of
Alkyl Halides
127
Example
CH3
CH2CH3
CH3
Br
128
The E1 Mechanism

• 1. Alkyl halides can undergo elimination in
  absence of base.
• 2. Carbocation is intermediate
• 3. Rate-determining step is unimolecular
  ionization of alkyl halide.

129
Step 1
130
Step 2