Transcript CHEM 494 Lecture 8 - UIC Department of Chemistry

CHEM 494
Special Topics in Chemistry
University of
Illinois at Chicago
UIC
CHEM 494 - Lecture 7
Prof. Duncan Wardrop
October 22, 2012
CHEM 494
Special Topics in Chemistry
University of
Illinois at Chicago
UIC
Preparation of Alkenes
Elimination
Chapter 19
Dehydration can be “Coupled” with Other
Chemical Transformation
Cl
O
N
OH
HF
Cl
Cl
HF
N
N
N
N
N
Me
Me
Me
Loratidine (Claritin
Two-step, one-pot transformation involves a Friedel-Crafts
reaction (see, Chapter 12) and dehydration of the resulting 3°
alcohol
University of
Illinois at Chicago
UIC
CHEM 494, Spring 2010
Slide 3
Lecture 8: November 5
Rate of Alcohol Dehydration
Mirrors Ease of Carbocation Formation
rate of dehydration = 3º > 2º > 1º alcohol
primary
alcohol (1º)
University of
Illinois at Chicago
UIC
OH
Stability
secondary
alcohol (2º)
tertiary
cation (3º)
Reactivity
tertiary
alcohol (3º)
OH
H
secondary
cation (2°)
H
OH
H
CHEM 494, Spring 2010
primary
cation (1°)
Slide 4
Lecture 8: November 5
Self Test Question
Predict the product for the following reaction scheme.
OH
H2SO4
?
A.
B.
C.
D.
140 ºC
-H
H
O H
H
H
H
H
University of
Illinois at Chicago
UIC
-H2O
H
H
H
H
CHEM 494, Fall 2012
E. no reaction
Slide 5
Lecture 8: November 5
Self Test Question
Predict the product for the following reaction scheme.
OH
H2SO4
?
A.
B.
C.
D.
140 ºC
-Hor -H
H H
H
H
H O
-H2O
H
H
H
H
E. no reaction
University of
Illinois at Chicago
UIC
CHEM 494, Fall 2012
Slide 6
Lecture 8: November 5
CHEM 494
Special Topics in Chemistry
University of
Illinois at Chicago
Regioselectivity &
Stereoselectivity of
Dehydration
Chapter 19
UIC
Self Test Question
What is the product(s) of the following reaction?
A.
B.
H2SO4
HO
?
80 ºC
C.
D.
E.
University of
Illinois at Chicago
UIC
CHEM 494, Fall 2012
O2S
Slide 8
Lecture 8: November 5
Types of Selectivity in Organic Chemistry
There are three forms of selectivity to consider . . .
.
Chemoselectivity: which functional group will react
•
Regioselectivity: where it will react
•
Stereoselectivity: how it will react with regards to
stereochemical outcome
. . . for each transformation, always question which
of these are factors are at play.
University of
Illinois at Chicago
UIC
CHEM 494, Spring 2010
Slide 9
Lecture 8: November 5
Regioselectivity of Elimination
Regioselectivity: Where Will It React?
Preferential reaction at one site of a single functional group over other sites that
could undergo the same reaction
CHEM 232 Definition, 2009
H2SO4
HO
+
80 ºC
H H
H3C
HO
+
10% (identical)
90%
CH3
CH3
University of
Illinois at Chicago
UIC
CHEM 494, Spring 2010
Slide 10
Lecture 8: November 5
Regioselectivity of Elimination
Regioselectivity: Where Will It React?
Preferential reaction at one site of a single functional group over other sites that
could undergo the same reaction
CHEM 232 Definition, 2009
OH
CH3
CH3
H3PO4
CH3
+
heat
84%
OH
H
CH3
H
University of
Illinois at Chicago
16%
H
UIC
2 different leaving group/Hrelationships
CHEM 494, Spring 2010
Slide 11
Lecture 8: November 5
Greek Lettering & Elimination Reactions
Nomenclature
The α-carbon is the one to
which the leaving group is
initially bonded, and the
carbon chain from this may
be labelled β (beta), γ
(gamma), δ (delta) etc,
following Greek alphabet.
Use primed letters for
chains branching at αcarbon
University of
Illinois at Chicago
UIC

Cl
H
2

C
H3C
C
H2

CHEM 494, Spring 2010

H
C
H2
'
CH3
'
Slide 12
Lecture 8: November 5
Regioselectivity of Elimination
Zaitsev Rule
HO
CH3
CH3
KHSO4
CH3
CH3
CH3
CH3
+
CH3
+
heat
87%
O
13%
0%
Na+ -O S OH
O
hydrosulfate
3 Hs on this
β carbon

HO
H

H 
CH3
H
 CH3
1 Hs on this
β carbon
2 Hs on this
β carbon
University of
Illinois at Chicago
UIC
Zaitsev Rule
When elimination can
occur in more than one
direction, the major
alkene is the one formed
by loss of a H atom from
the β carbon having the
fewest hydrogens
CHEM 494, Spring 2010
Slide 13
Lecture 8: November 5
Considering Stereo & Regioselectivity
Combine Zaitsev’s Rule and observations about
stereoselectivity to predict the major products of dehydration
(elimination)
OH
H2SO4
80 ºC
major product
trisubstituted
University of
Illinois at Chicago
UIC
trisubstituted
disubstituted
disubstituted
most stable alkenes have largest
groups on each carbon trans to each
other
CHEM 494, Spring 2010
Slide 14
Lecture 8: November 5
Self Test Question
What is the major product expected for
the reaction scheme below?
A.
B.
H2SO4
HO
?
C.
80 ºC
D.
E.
University of
Illinois at Chicago
UIC
CHEM 494, Fall 2012
Slide 15
Lecture 8: November 5
CHEM 494
Special Topics in Chemistry
University of
Illinois at Chicago
UIC
E1 & E2 Mechanisms of
Alcohol Dehydration
Chapter 19
Organic Mechanisms (SN1)
fast & reversible
H
O
H Cl
O
H
Cl
H
alkyloxonium
ion
slow
Me
Me
fast
Cl
Cl
Me
carbocation
(t-butyl cation)
University of
Illinois at Chicago
UIC
H2O
CHEM 494, Spring 2010
t-butyl chloride
Slide 17
Lecture 8: November 5
Remember Curved Arrow Notation?
curved arrows show
the movement of
electrons; never
atoms
O
N
H3C
CH3
atoms
electrons
resonance: electrons in
a covalent bond moving
out to an atom
H
O
H
bond making: lone
pair of electrons
H3O+ forming a new bond
to another atom
H
CH3
O
H3C
N
CH3
CH3
University of
Illinois at Chicago
resonance: lone pair
of electrons moving in
between two atoms to
form a new covalent
bond
UIC
O
H3C
O
O
H
H3C
CHEM 494, Spring 2010
O
bond breaking:
electrons in a bond
+ H
leaving to most
electronegative atom
Slide 18
Lecture 8: November 5
Mechanism of Dehydration (E1)
Step One
Proton Transfer (Protonation)
O
fast & reversible
H
H O S OH
O
H
O
pKa = -3.0
University of
Illinois at Chicago
O
UIC
O
O S OH
H
O
alkyloxonium
ion
CHEM 494, Spring 2010
Slide 19
Lecture 8: November 5
Mechanism of Dehydration (E1)
Step Two
Dissociation
H
slow
Me
Me
O
H2O
H
Me
carbocation
(t-butyl cation)
University of
Illinois at Chicago
UIC
CHEM 494, Spring 2010
Slide 20
Lecture 8: November 5
Mechanism of Dehydration (E1)
Step Three
Carbocation Capture βDeprotonation!
H
O
CH2
Me
O S OH
O
Me
carbocation
(t-butyl cation)
fast
O
H O S OH
Me
Me
alkene
(2-methylpropene)
O
O S OH
O
University of
Illinois at Chicago
UIC
CHEM 494, Spring 2010
O
sulfuric acid
(regenerated)
alkyl hydrogen
sulfate
(product of SN1)
Slide 21
Lecture 8: November 5
Hughes-Ingold Nomenclature
H
slow
Me
Me
O
H2O
H
Me
carbocation
(t-butyl cation)
E1
elimination
unimolecular
overall reaction = β-Elimination
rate determining step (RDS) involves on species = unimolecular
rate = k[alkyl oxonium ion] = first order
University of
Illinois at Chicago
UIC
CHEM 494, Spring 2010
Slide 22
Lecture 8: November 5
Each Step of E1 Mechanism is Reversible
fast &
reversible
O
H O S OH
O
H
O
O
H
slow &
reversible
CH2
Me
O
O S OH
UIC
O
fast &
reversible
Me
University of
Illinois at Chicago
O S OH
H
O
H2O
O
H
Me
Me
If all steps in E1 are reversible,
what drives the reaction
forward?
CHEM 494, Spring 2010
Slide 23
Lecture 8: November 5
Alkenes Isolated from Dehydration
Reactions by Distillation
H2SO4
OH
+
+
4-methyl-2-pentanol
bp = 132 ºC
2-methyl-1-pentene
bp = 62 ºC
2-methyl-2-pentene
bp = 67 ºC
+
trans-4-methyl2-pentene
bp = 59 ºC
+
cis-4-methyl2-pentene
bp = 58 ºC
4-methyl-1-pentene
bp = 54 ºC
•
alkenes have much lower boiling points than alcohols
•
alcohols have higher boiling points (b.p.) because of
larger van der Waals forces, including strong hydrogenbonding
•
by removing alkenes through distillation (boiling),
equilibrium is shifted toward products (LeChatlier
Principle) until no more reactants remain
University of
Illinois at Chicago
UIC
CHEM 494, Spring 2010
Slide 24
Lecture 8: November 5
Why Can’t Hydrogen Halides Be Used for
Elimination Reactions?
fast &
reversible
H
H Cl
O
H
H
H
slow &
reversible
O H
CH2
Me
fast &
reversible
fast &
irreversible
Cl
University of
Illinois at Chicago
UIC
Me
Me
Me
nucleophilic
addition
Nucleophilic
addition
of
chloride (Cl–) to
a carbocation is
not reversible
H Cl
H
Cl
Cl
O
alkyl chloride
(product of SN1)
CHEM 494, Spring 2010
Slide
Lecture 8: November 5
Reactivity Explained
R2
R1 = C
R2 = C
R3 3
R2
R =H
2º Carbocation
R2
R1 = C
R2 = C
3
1
R
R
R3 = C
3º Carbocation
R2
R1
•
3º carbocations are more
stable than 2º = 3º lower in
energy
•
smaller activation energy
leading to 3º carbocation
results in faster reaction
R3
OH
H3O+
R2
R3
R1
University of
Illinois at Chicago
UIC
OH2
H2O
CHEM 494, Spring 2010
Slide 26
Lecture 8: November 5
Bimolecular Substitution - SN2 Mechanism
(from Lecture 8)
H3C
O
H
fast
H
H3C
O
H
Br
H3C
H
H
H
Br
slow (rate-determining)
Step 1
Protonation
Step 2
Nucleophilic Attack
CH3
-
Br
C
H
H
H
‡
O +
H
H3C
Br
+
H
O
H
• C-O bond breaks at the same time the nucleophile (Br) forms the C-X bond
• RDS is nucleophilic attack; bimolecular, therefore Ingold notation = SN2
• fewer steps does not mean faster reaction
University of
Illinois at Chicago
UIC
CHEM 494, Spring 2010
Slide 27
Lecture 8: November 5
Dehydration of Primary Alcohols
Proceeds via E2 Mechanism
H
H3C C
fast &
reversible
O
H3C
O
Step 1
Protonation
Step 2
-Deprotonation
(elimination)
H
O
O
H O S OH
H
H
1° Cation
H3C
O
O
HO S O
O S OH
H
O
H
slow
H2C
O
H
O
H
H
H
H
H
• C-O bond breaks at the same time the nucleophile (Br) forms the C-X bond
• RDS is nucleophilic attack; bimolecular, therefore Ingold notation = SN2
• fewer steps does not mean faster reaction
University of
Illinois at Chicago
UIC
CHEM 494, Spring 2010
Slide 28
Lecture 8: November 5
CHEM 494
Special Topics in Chemistry
University of
Illinois at Chicago
Regioselectivity &
Stereoselectivity of
Dehydration
UIC
Self Test Question
What is the product(s) of the following reaction?
A.
B.
H2SO4
HO
?
80 ºC
C.
D.
E.
University of
Illinois at Chicago
UIC
CHEM 494, Fall 2012
O2S
Slide 30
Lecture 8: November 5
Types of Selectivity in Organic Chemistry
There are three forms of selectivity to consider . . .
.
Chemoselectivity: which functional group will react
•
Regioselectivity: where it will react
•
Stereoselectivity: how it will react with regards to
stereochemical outcome
. . . for each transformation, always question which
of these are factors are at play.
University of
Illinois at Chicago
UIC
CHEM 494, Spring 2010
Slide 31
Lecture 8: November 5
Regioselectivity of Elimination
Regioselectivity: Where Will It React?
Preferential reaction at one site of a single functional group over other sites that
could undergo the same reaction
CHEM 232 Definition, 2009
H2SO4
HO
+
80 ºC
H H
H3C
HO
+
10% (identical)
90%
CH3
CH3
University of
Illinois at Chicago
UIC
CHEM 494, Spring 2010
Slide 32
Lecture 8: November 5
Regioselectivity of Elimination
Regioselectivity: Where Will It React?
Preferential reaction at one site of a single functional group over other sites that
could undergo the same reaction
CHEM 232 Definition, 2009
OH
CH3
CH3
H3PO4
CH3
+
heat
84%
OH
H
CH3
H
University of
Illinois at Chicago
16%
H
UIC
2 different leaving group/Hrelationships
CHEM 494, Spring 2010
Slide 33
Lecture 8: November 5
Greek Lettering & Elimination Reactions
Nomenclature
The α-carbon is the one to
which the leaving group is
initially bonded, and the
carbon chain from this may
be labelled β (beta), γ
(gamma), δ (delta) etc,
following Greek alphabet.
Use primed letters for
chains branching at αcarbon
University of
Illinois at Chicago
UIC

Cl
H
2

C
H3C
C
H2

CHEM 494, Spring 2010

H
C
H2
'
CH3
'
Slide 34
Lecture 8: November 5
Regioselectivity of Elimination
Zaitsev Rule
HO
CH3
CH3
KHSO4
CH3
CH3
CH3
CH3
+
CH3
+
heat
87%
O
13%
0%
Na+ -O S OH
O
hydrosulfate
3 Hs on this
β carbon

HO
H

H 
CH3
H
 CH3
1 Hs on this
β carbon
2 Hs on this
β carbon
University of
Illinois at Chicago
UIC
Zaitsev Rule
When elimination can
occur in more than one
direction, the major
alkene is the one formed
by loss of a H atom from
the β carbon having the
fewest hydrogens
CHEM 494, Spring 2010
Slide 35
Lecture 8: November 5
Considering Stereo & Regioselectivity
Combine Zaitsev’s Rule and observations about
stereoselectivity to predict the major products of dehydration
(elimination)
OH
H2SO4
80 ºC
major product
trisubstituted
University of
Illinois at Chicago
UIC
trisubstituted
disubstituted
disubstituted
most stable alkenes have largest
groups on each carbon trans to each
other
CHEM 494, Spring 2010
Slide 36
Lecture 8: November 5
Self Test Question
What is the major product expected for
the reaction scheme below?
A.
B.
H2SO4
HO
?
C.
80 ºC
D.
E.
University of
Illinois at Chicago
UIC
CHEM 494, Fall 2012
Slide 37
Lecture 8: November 5
CHEM 494
Special Topics in Chemistry
University of
Illinois at Chicago
UIC
E1 & E2 Mechanisms of
Alcohol Dehydration
Section: 5.12
Organic Mechanisms (SN1)
fast & reversible
H
O
H Cl
O
H
Cl
H
alkyloxonium
ion
slow
Me
Me
fast
Cl
Cl
Me
carbocation
(t-butyl cation)
University of
Illinois at Chicago
UIC
H2O
CHEM 494, Spring 2010
t-butyl chloride
Slide 39
Lecture 8: November 5
Remember Curved Arrow Notation?
curved arrows show
the movement of
electrons; never
atoms
O
N
H3C
CH3
atoms
electrons
resonance: electrons in
a covalent bond moving
out to an atom
H
O
H
bond making: lone
pair of electrons
H3O+ forming a new bond
to another atom
H
CH3
O
H3C
N
CH3
CH3
University of
Illinois at Chicago
resonance: lone pair
of electrons moving in
between two atoms to
form a new covalent
bond
UIC
O
H3C
O
O
H
H3C
CHEM 494, Spring 2010
O
bond breaking:
electrons in a bond
+ H
leaving to most
electronegative atom
Slide 40
Lecture 8: November 5
Mechanism of Dehydration (E1)
Step One
Proton Transfer (Protonation)
O
fast & reversible
H
H O S OH
O
H
O
pKa = -3.0
University of
Illinois at Chicago
O
UIC
O
O S OH
H
O
alkyloxonium
ion
CHEM 494, Spring 2010
Slide 41
Lecture 8: November 5
Mechanism of Dehydration (E1)
Step Two
Dissociation
H
slow
Me
Me
O
H2O
H
Me
carbocation
(t-butyl cation)
University of
Illinois at Chicago
UIC
CHEM 494, Spring 2010
Slide 42
Lecture 8: November 5
Mechanism of Dehydration (E1)
Step Three
Carbocation Capture βDeprotonation!
H
O
CH2
Me
O S OH
O
Me
carbocation
(t-butyl cation)
fast
O
H O S OH
Me
Me
alkene
(2-methylpropene)
O
O S OH
O
University of
Illinois at Chicago
UIC
CHEM 494, Spring 2010
O
sulfuric acid
(regenerated)
alkyl hydrogen
sulfate
(product of SN1)
Slide 43
Lecture 8: November 5
Hughes-Ingold Nomenclature
H
slow
Me
Me
O
H2O
H
Me
carbocation
(t-butyl cation)
E1
elimination
unimolecular
overall reaction = β-Elimination
rate determining step (RDS) involves on species = unimolecular
rate = k[alkyl oxonium ion] = first order
University of
Illinois at Chicago
UIC
CHEM 494, Spring 2010
Slide 44
Lecture 8: November 5
Each Step of E1 Mechanism is Reversible
fast &
reversible
O
H O S OH
O
H
O
O
H
slow &
reversible
CH2
Me
O
O S OH
UIC
O
fast &
reversible
Me
University of
Illinois at Chicago
O S OH
H
O
H2O
O
H
Me
Me
If all steps in E1 are reversible,
what drives the reaction
forward?
CHEM 494, Spring 2010
Slide 45
Lecture 8: November 5
Alkenes Isolated from Dehydration
Reactions by Distillation
H2SO4
OH
+
+
4-methyl-2-pentanol
bp = 132 ºC
2-methyl-1-pentene
bp = 62 ºC
2-methyl-2-pentene
bp = 67 ºC
+
trans-4-methyl2-pentene
bp = 59 ºC
+
cis-4-methyl2-pentene
bp = 58 ºC
4-methyl-1-pentene
bp = 54 ºC
•
alkenes have much lower boiling points than alcohols
•
alcohols have higher boiling points (b.p.) because of
larger van der Waals forces, including strong hydrogenbonding
•
by removing alkenes through distillation (boiling),
equilibrium is shifted toward products (LeChatlier
Principle) until no more reactants remain
University of
Illinois at Chicago
UIC
CHEM 494, Spring 2010
Slide 46
Lecture 8: November 5
Why Can’t Hydrogen Halides Be Used for
Elimination Reactions?
fast &
reversible
H
H Cl
O
H
H
H
slow &
reversible
O H
CH2
Me
fast &
reversible
fast &
irreversible
Cl
University of
Illinois at Chicago
UIC
Me
Me
Me
nucleophilic
addition
Nucleophilic
addition
of
chloride (Cl–) to
a carbocation is
not reversible
H Cl
H
Cl
Cl
O
alkyl chloride
(product of SN1)
CHEM 494, Spring 2010
Slide
Lecture 8: November 5
Reactivity Explained
R2
R1 = C
R2 = C
R3 3
R2
R =H
2º Carbocation
R2
R1 = C
R2 = C
3
1
R
R
R3 = C
3º Carbocation
R2
R1
•
3º carbocations are more
stable than 2º = 3º lower in
energy
•
smaller activation energy
leading to 3º carbocation
results in faster reaction
R3
OH
H3O+
R2
R3
R1
University of
Illinois at Chicago
UIC
OH2
H2O
CHEM 494, Spring 2010
Slide 48
Lecture 8: November 5
Bimolecular Substitution - SN2 Mechanism
(from Lecture 8)
H3C
O
H
fast
H
H3C
O
H
Br
H3C
H
H
H
Br
slow (rate-determining)
Step 1
Protonation
Step 2
Nucleophilic Attack
CH3
-
Br
C
H
H
H
‡
O +
H
H3C
Br
+
H
O
H
• C-O bond breaks at the same time the nucleophile (Br) forms the C-X bond
• RDS is nucleophilic attack; bimolecular, therefore Ingold notation = SN2
• fewer steps does not mean faster reaction
University of
Illinois at Chicago
UIC
CHEM 494, Spring 2010
Slide 49
Lecture 8: November 5
Dehydration of Primary Alcohols
Proceeds via E2 Mechanism
H
H3C C
fast &
reversible
O
H3C
O
Step 1
Protonation
Step 2
-Deprotonation
(elimination)
H
O
O
H O S OH
H
H
1° Cation
H3C
O
O
HO S O
O S OH
H
O
H
slow
H2C
O
H
O
H
H
H
H
H
• C-O bond breaks at the same time the nucleophile (Br) forms the C-X bond
• RDS is nucleophilic attack; bimolecular, therefore Ingold notation = SN2
• fewer steps does not mean faster reaction
University of
Illinois at Chicago
UIC
CHEM 494, Spring 2010
Slide 50
Lecture 8: November 5
CHEM 494
Special Topics in Chemistry
University of
Illinois at Chicago
Addition Reactions of
Alkenes
UIC
Addition Reactions of Alkenes
C C
+
X Y
C C
+
H H
C C
C C
+
+
H X
H Br
O
C C
+
H O S OH
addition
hydrogenation
hydrogen
halide addition
free radical
bromine addition
sulfuric acid
addition
X C C Y
H C C H
H C C X
H C C Br
H C C OSO3H
O
C C
University of
Illinois at Chicago
UIC
+
H OH
hydration
CHEM 494, Spring 2010
H C C OH
Slide 52
Lecture 8: November 5
Hydrogenation
Pd/C, CH3CH2OH
University of
Illinois at Chicago
•
exothermic reaction (-∆Hº), but high Eact - catalyst
required
•
catalysts are heterogeneous transition metals (Pd,
Rsolvent is typically an alcohol (e.g. ethanol,
CH3CH2OH)metals are insoluble (heterogeneous
mixture)heat of hydrogenation = -∆Hº
UIC
CHEM 494, Spring 2010
Slide 53
Lecture 8: November 5
Heat of Hydrogenation (-∆Hº)
University of
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UIC
CHEM 494, Spring 2010
Slide 54
Lecture 8: November 5
Heat of Hydrogenation (-∆Hº)
Since only the double bond is undergoing the reaction, heat of hydrogenation
is independent of the number of carbon atoms in the molecule
Alkene
-∆Hº (kJ/mol)
ethylene
136
monosubstituted
126
cis-disubstituted
119
terminally disubstituted
117
trisubstituted
112
tetrasubstituted
110
UIC
CHEM 494, Spring 2010
University of
Illinois at Chicago
example
Slide 55
Lecture 8: November 5
General Mechanism for
Heterogenious Hydrogenation

H
H
H
H
d
catalyst surface M0
oxidative
addition
reductive
elimination
H
H
H
catalyst surface MII
catalyst surface MII
insertion
H
H
H
reaction takes
places at the
surface of the
catalyst (many
metal atoms
combined)
coordination
catalyst surface MII
University of
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UIC
CHEM 494, Spring 2010
Slide 56
Lecture 8: November 5
Step 1: Oxidative Addition

H
H
d
catalyst surface M0
oxidative
addition
•
•
hydrogen (H2) is added to metal
metal is oxidized from M0 to MII
H
H
catalyst surface MII
H
H
Pd0
University of
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H
PdII H
CHEM 494, Spring 2010
Slide 57
Lecture 8: November 5
Step 2: Coordination
• metal is a Lewis acid (electron acceptor)
• π-bond is a lewis base (electron donor)
• coordination = Lewis acid/base complex
H
PdII H
PdII H
H
H
H
catalyst surface MII
H
H
coordination
catalyst surface MII
University of
Illinois at Chicago
UIC
CHEM 494, Spring 2010
Slide 58
Lecture 8: November 5
Step 3: Insertion
• two carbon atoms inserted between Pd-H
• formation of a weak metal-carbon σ-bond
• formation of a strong C-H σ-bond
• break a weak metal-H σ-bond
H
H
H
H
PdII H
H
PdII
catalyst surface MII
insertion
H
H
catalyst surface MII
University of
Illinois at Chicago
UIC
CHEM 494, Spring 2010
Slide 59
Lecture 8: November 5
Step 4: Reductive Elimination

H
H
H
d
H
catalyst surface M0
reductive
elimination
H
H
catalyst surface MII
• metal is reduced from MII to M0
• last σ C-H -bond is formed from the same
•
face of alkene as previous
metal is a catalyst; it is regenerated
H
H
University of
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UIC
PdII
CHEM 494, Spring 2010
H
H
Slide 60
Lecture 8: November 5
Complete Mechanism
H
H
H
H
oxidative
addition
Pd0
reductive
elimination
H
H
H
PdII
PdII H
insertion
coordination
H
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PdII H
CHEM 494, Spring 2010
Slide 61
Lecture 8: November 5
Syn Addition of Hydrogen
•
University of
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as a consequence of mechanism, both hydrogens
are added to the same face of the π-bond: syn
additionno anti-addition products are formed
(addition of hydrogen to opposite faces)
UIC
CHEM 494, Spring 2010
Slide 62
Lecture 8: November 5
Hydrogenation is Stereoselective
This methyl group sterically hinders hydrogen
from approaching the π-bond from the top face
•
both products are arise from syn additions of hydrogen to alkenestereoselecti
preference for one stereoisomer when two or more are possible
University of
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UIC
CHEM 494, Spring 2010
Slide 63
Lecture 8: November 5
Example of Syn Addition
CH3
CH3
C8H12
HO
CH3
CH3
H
CH3
C8H12
HO
C8H12
HO
H
H
H
H
1. Diatomic hydrogen and alkene are
present in solution phase.
No reaction occurs.
CH3
CH3
CH3 C H
8 12
H
H
4. One hydrogen atom is transferred
to alkene, forming C-catalyst bond.
University of
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H
3. System of alkene
coordinates catalyst surface.
2. Hydrogen absorbed
on to catalyst surface.
H-H bond cleaved.
CH3
CH3 C H
8 12
CH3
C8H12
HO
HO
H
HO
CH3
H
5. Second hydrogen atom is transferred,
breaking substrate-catalyst bond.
Alkane diffuses away from catalyst.
CHEM 494, Spring 2010
H
H
6. Steps 1-5 are repeated.
Slide 64
Lecture 8: November 5
Self Test Question
What is the major product of the following
hydrogenation reaction?
D
A.
H2, Pd/C
H
H
D
H
H
CH3CH2OH
B.
H
H
C.
UIC
CHEM 494, Fall 2012
H
H
H
D
D.
University of
Illinois at Chicago
D
catalyst surface
D
catalyst surface MII
D
H
H
MII
H
H
H
H
D
H
Slide 65
Lecture 8: November 5
Addition of Electrophiles to Alkene
C C
C C
C C
+
+
+
H H
H X
H Br
O
C C
+
H O S OH
hydrogenation
hydrogen
halide addition
free radical
bromine addition
sulfuric acid
addition
H C C H
H C C X
H C C Br
H C C OSO3H
O
C C
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+
H OH
hydration
CHEM 494, Spring 2010
H C C OH
Slide 66
Lecture 8: November 5
Electrophilic Addition of HX
C C
+
nucleophile
University of
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δ+
δ–
H X
hydrogen
halide addition
H C C X
electrophile
CHEM 494, Spring 2010
Slide 67
Lecture 8: November 5
Reaction Conditions
H
H
•
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HBr
H
CHCl3, -30 ºC
H H
Br
hydrogen halide: HXcommon solvents:
chloroform (CHCl3) ,dichloromethane
(CH2Cl2), pentane, acetic acidgenerally
performed at low temperature (below 0
°C)generally a fast reaction
CHEM 494, Spring 2010
Slide 68
Lecture 8: November 5
Electrophilic Addition (AdE) Mechanism
H Br
Br
Protonation
(slow)
H
H H
H
•
University of
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H
UIC
H
Cation
Capture
(fast)
Br
H H
electrophilic addition: AdERDS =
protonation of carbonrate =
k[alkene][hydrogen halide]unlike oxygen
and nitrogen, protonation of carbon is
slowproceeds through carbocation
intermediate
CHEM 494, Spring 2010
Slide 69
Lecture 8: November 5
HX Addition is Regioselective
Regioselectivity
Preferential reaction at one site of a single functional group over other sites that
could undergo the same reaction
CHEM 232 Definition, 2010
R
H
H
R
H
R
H
R
H
R
University of
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H
UIC
R
H X
R
H
H X
R
R
H X
X
X
R
R
X
R
H
H
H
rather than
H
rather than
H
R
H
R
H
R
H
R
H
H
H
rather than
CHEM 494, Spring 2010
R
H
H
X
H
H
X
H
H
X
R
Slide 70
Lecture 8: November 5
Markovnikov’s Rule
3º
2º
CH3
H3C Br
H
HBr
H
H
CH2Cl2
-40 ºC
addition of HX to an unsymmetrically substituted alkene
proceeds so that hydrogen (H) adds to the least
substituted carbon and the halide (X) adds to the most
substituted carbon atom
University of
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UIC
CHEM 494, Spring 2010
Slide 71
Lecture 8: November 5
Self Test Question
Predict the product when
2,4-dimethyl-2-pentene is
treated with HCl?
A. 3-chloro-2,4-dimethylpentane
B. 2-chloroohexane
HCl
Cl
C. 2,3-dichloro-2,4dimethylpentane
D. 2-chloro-2,4-dimethylpentane
E. 1-chloro-2,4-dimethylpentane
University of
Illinois at Chicago
UIC
CHEM 494, Fall 2012
Slide 72
Lecture 8: November 5
Mechanistic Basis for Markovnikov’s Rule
X
X
H X
H
H
3º alkyl halide
3º carbocation
X
H X
H
+
H
X
2º carbocation
curved arrows do
not indicate which
carbon is
protonated
University of
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fastest protonation
leads to more stable
(more substituted)
carbocation
CHEM 494, Spring 2010
2º alkyl halide
more substituted
carbocation = more
substituted alkyl
halide
Slide 73
Lecture 8: November 5
Mechanistic Basis for Markovnikov’s Rule
Hammond Postulate:
transition state structure
resembles closest energy
intermediate
•transition state resembles
carbocation for endothermic
RDS (late transition state)
•what stabilizes carbocation
also stabilizes transition state
•lowest energy transition
state leads to more
substituted carbocation
•more substituted
carbocation provides more
substituted alkyl halide
University of
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UIC
CHEM 494, Spring 2010
Slide 74
Lecture 8: November 5