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University of Illinois at ChicagoUICCHEM 494
Special Topics in Chemistry
Prof. Duncan WardropOctober 22, 2012
CHEM 494 - Lecture 7
University of Illinois at ChicagoUICCHEM 494
Special Topics in Chemistry
Chapter 19
Preparation of Alkenes Elimination
UICUniversity of Illinois at Chicago CHEM 494, Spring 2010 Slide
Lecture 8: November 5
Dehydration can be “Coupled” with Other Chemical Transformation
3
Two-step, one-pot transformation involves a Friedel-Crafts reaction (see, Chapter 12) and dehydration of the resulting 3° alcohol
N
Cl
O
NMe
HFN
NMe
ClHF
N
NMe
ClOH
Loratidine (Claritin)
UICUniversity of Illinois at Chicago CHEM 494, Spring 2010 Slide
Lecture 8: November 5
Rate of Alcohol DehydrationMirrors Ease of Carbocation Formation
4
rate of dehydration = 3º > 2º > 1º alcohol
tertiaryalcohol (3º)
secondaryalcohol (2º)
primaryalcohol (1º) OH
OH
OH
H
H
H
tertiarycation (3º)
secondarycation (2°)
primarycation (1°)
Reac
tivity
Stab
ility
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012 Slide
Lecture 8: November 5
Predict the product for the following reaction scheme.
Self Test Question
5
A. B.
C. D.
E. no reaction
OHβHβ
HβHβ
HH
HβHβ
HβHβ
-H2O
-Hβ+
OH H2SO4
140 ºC?
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012 Slide
Lecture 8: November 5
Predict the product for the following reaction scheme.
Self Test Question
6
A. B.
C. D.
E. no reaction
H2SO4
140 ºC?
OH
-H2O
-Hβ+ or -Hβ+
OH
H HβHβ
Hβ
Hβ
HβHβ Hβ
University of Illinois at ChicagoUICCHEM 494
Special Topics in Chemistry
Chapter 19
Regioselectivity & Stereoselectivity of Dehydration
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012 Slide
Lecture 8: November 5
A.
B.
C.
D.
E.
What is the product(s) of the following reaction?
Self Test Question
8
HO
H2SO4
80 ºC?
O2S
UICUniversity of Illinois at Chicago CHEM 494, Spring 2010 Slide
Lecture 8: November 5
Types of Selectivity in Organic Chemistry
9
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.
UICUniversity of Illinois at Chicago CHEM 494, Spring 2010 Slide
Lecture 8: November 5
Regioselectivity of Elimination
10
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
+ +HO
H2SO4
80 ºC
10% (identical) 90%
CH3H3C
CH3HO
H H
UICUniversity of Illinois at Chicago CHEM 494, Spring 2010 Slide
Lecture 8: November 5
Regioselectivity of Elimination
11
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
H3PO4
heat
CH3 CH3 CH3
84% 16%
OHCH3
HβHβ
Hβ
2 different leaving group/Hβ relationships
UICUniversity of Illinois at Chicago CHEM 494, Spring 2010 Slide
Lecture 8: November 5
Greek Lettering & Elimination Reactions
12
NomenclatureThe 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 α-carbon
Cl
CH2
CH2
CH3H2C
H3C
Hα
β
γ
δ
β'
γ'
UICUniversity of Illinois at Chicago CHEM 494, Spring 2010 Slide
Lecture 8: November 5
Regioselectivity of EliminationZaitsev Rule
13
1 Hs on this β carbon
2 Hs on this β carbon
3 Hs on this β carbon
Zaitsev RuleWhen 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
CH3HOCH3 CH3
CH3
CH3 CH3
CH3KHSO4
heat87% 13% 0%
+ +
CH3HOH
CH3
HH β β
β
α
OSO
Na+ -O OH
hydrosulfate
UICUniversity of Illinois at Chicago CHEM 494, Spring 2010 Slide
Lecture 8: November 5
Considering Stereo & Regioselectivity
14
Combine Zaitsev’s Rule and observations about stereoselectivity to predict the major products of dehydration (elimination)
trisubstituted trisubstituted disubstituted disubstituted
most stable alkenes have largest groups on each carbon trans to each other
major product
OHH2SO4
80 ºC
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012 Slide
Lecture 8: November 5
What is the major product expected for the reaction scheme below?
Self Test Question
15
A.
B.
C.
D.
E.
HO
H2SO4
80 ºC?
University of Illinois at ChicagoUICCHEM 494
Special Topics in Chemistry
Chapter 19
E1 & E2 Mechanisms of Alcohol Dehydration
UICUniversity of Illinois at Chicago CHEM 494, Spring 2010 Slide
Lecture 8: November 5
Organic Mechanisms (SN1)
17
OH
OH
HClH Cl
fast & reversible
alkyloxonium ion
MeMe
Me
H2Ocarbocation(t-butyl cation)
slow
t-butyl chloride
fastCl Cl
UICUniversity of Illinois at Chicago CHEM 494, Spring 2010 Slide
Lecture 8: November 5
Remember Curved Arrow Notation?
18
curved arrows show the movement of
electrons; never atoms electronsatoms
resonance: electrons in a covalent bond moving
out to an atom
resonance: lone pair of electrons moving in between two atoms to
form a new covalent bond
bond making: lone pair of electrons
forming a new bond to another atom
bond breaking: electrons in a bond
leaving to most electronegative atom
H3C N
OCH3
CH3
H3C N
OCH3
CH3H
OH H H3O+
H3C O
OH
H3C O
O+ H
UICUniversity of Illinois at Chicago CHEM 494, Spring 2010 Slide
Lecture 8: November 5
Mechanism of Dehydration (E1)
19
Step OneProton Transfer (Protonation)
pKa = -3.0
OH
OH
H
fast & reversible
alkyloxonium ion
OSO
O OHHOSO
O OH
UICUniversity of Illinois at Chicago CHEM 494, Spring 2010 Slide
Lecture 8: November 5
Mechanism of Dehydration (E1)
20
Step TwoDissociation
OH
H MeMe
MeH2O
carbocation(t-butyl cation)
slow
UICUniversity of Illinois at Chicago CHEM 494, Spring 2010 Slide
Lecture 8: November 5
Mechanism of Dehydration (E1)
21
Step ThreeCarbocation Capture β-Deprotonation!
CH2Me
Me
carbocation(t-butyl cation)
alkene(2-methylpropene)
OSO
O OHH
OSO
O OH
H
Me Me
sulfuric acid(regenerated)
fast
OSO
O OHalkyl hydrogen
sulfate(product of SN1)
UICUniversity of Illinois at Chicago CHEM 494, Spring 2010 Slide
Lecture 8: November 5
Hughes-Ingold Nomenclature
22
E1
overall reaction = β-Eliminationrate determining step (RDS) involves on species = unimolecular
rate = k[alkyl oxonium ion] = %rst order
eliminationunimolecular
OH
H MeMe
MeH2O
carbocation(t-butyl cation)
slow
UICUniversity of Illinois at Chicago CHEM 494, Spring 2010 Slide
Lecture 8: November 5
Each Step of E1 Mechanism is Reversible
23
OH
OH
H
fast &reversible
CH2Me
Me
H2O
slow &reversible
OSO
O OHHOSO
O OH
OSO
O OH
H
Me Me
fast &reversible
If all steps in E1 are reversible, what drives the reaction forward?
UICUniversity of Illinois at Chicago CHEM 494, Spring 2010 Slide
Lecture 8: November 5
Alkenes Isolated from Dehydration Reactions by Distillation
24
• alkenes have much lower boiling points than alcohols
• alcohols have higher boiling points (b.p.) because of larger van der Waals forces, including strong hydrogen-bonding
• by removing alkenes through distillation (boiling), equilibrium is shifted toward products (LeChatlier Principle) until no more reactants remain
OH
4-methyl-2-pentanolbp = 132 ºC
trans-4-methyl-2-pentenebp = 59 ºC
H2SO4
2-methyl-1-pentenebp = 62 ºC
cis-4-methyl-2-pentenebp = 58 ºC
+ + + +
2-methyl-2-pentenebp = 67 ºC
4-methyl-1-pentenebp = 54 ºC
UICUniversity of Illinois at Chicago CHEM 494, Spring 2010 Slide
Lecture 8: November 5
Why Can’t Hydrogen Halides Be Used for Elimination Reactions?
OH
OH
H
fast &reversible
CH2Me
Me
slow &reversible
ClH
HO
HMe
Me
fast &reversible
Clalkyl chloride
(product of SN1)
Cl fast &irreversible
H
Cl
ClH
nucleophilicaddition
N u c l e o p h i l i c a d d i t i o n o f chloride (Cl–) to a carbocation is not reversible
UICUniversity of Illinois at Chicago CHEM 494, Spring 2010 Slide
Lecture 8: November 5
Reactivity Explained
26
R1 OH
R3R2
H3O+
R1 OH2
R3R2
H2O
R1 R3
R2 R1 = CR2 = CR3 = C
R2 R3
R2 R1 = CR2 = CR3 = H
3º Carbocation
2º Carbocation
• 3º carbocations are more stable than 2º = 3º lower in energy
• smaller activation energy leading to 3º carbocation results in faster reaction
UICUniversity of Illinois at Chicago CHEM 494, Spring 2010 Slide
Lecture 8: November 5
Bimolecular Substitution - SN2 Mechanism(from Lecture 8)
27
• 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
H3C OH
H Br
Step 1Protonation
H3C OH
BrH
HO
H+
H3C Br
H3C H
H
‡CH3
C
H H
OBrH
Hδ+
δ-
Step 2Nucleophilic Attack
fast
slow (rate-determining)
UICUniversity of Illinois at Chicago CHEM 494, Spring 2010 Slide
Lecture 8: November 5
Dehydration of Primary AlcoholsProceeds via E2 Mechanism
28
• 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
OH3C
H
H
fast &reversibleO
SO
O OHHOSO
O OH
H H
slow
H3C O
H
H3C CH
H
1° Cation
HHH2C
O
OSO
OHO
Hβ
HH
Step 1Protonation
Step 2β-Deprotonation(elimination)
University of Illinois at ChicagoUICCHEM 494
Special Topics in Chemistry
Regioselectivity & Stereoselectivity of Dehydration
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012 Slide
Lecture 8: November 5
A.
B.
C.
D.
E.
What is the product(s) of the following reaction?
Self Test Question
30
HO
H2SO4
80 ºC?
O2S
UICUniversity of Illinois at Chicago CHEM 494, Spring 2010 Slide
Lecture 8: November 5
Types of Selectivity in Organic Chemistry
31
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.
UICUniversity of Illinois at Chicago CHEM 494, Spring 2010 Slide
Lecture 8: November 5
Regioselectivity of Elimination
32
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
+ +HO
H2SO4
80 ºC
10% (identical) 90%
CH3H3C
CH3HO
H H
UICUniversity of Illinois at Chicago CHEM 494, Spring 2010 Slide
Lecture 8: November 5
Regioselectivity of Elimination
33
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
H3PO4
heat
CH3 CH3 CH3
84% 16%
OHCH3
HβHβ
Hβ
2 different leaving group/Hβ relationships
UICUniversity of Illinois at Chicago CHEM 494, Spring 2010 Slide
Lecture 8: November 5
Greek Lettering & Elimination Reactions
34
NomenclatureThe 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 α-carbon
Cl
CH2
CH2
CH3H2C
H3C
Hα
β
γ
δ
β'
γ'
UICUniversity of Illinois at Chicago CHEM 494, Spring 2010 Slide
Lecture 8: November 5
Regioselectivity of EliminationZaitsev Rule
35
1 Hs on this β carbon
2 Hs on this β carbon
3 Hs on this β carbon
Zaitsev RuleWhen 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
CH3HOCH3 CH3
CH3
CH3 CH3
CH3KHSO4
heat87% 13% 0%
+ +
CH3HOH
CH3
HH β β
β
α
OSO
Na+ -O OH
hydrosulfate
UICUniversity of Illinois at Chicago CHEM 494, Spring 2010 Slide
Lecture 8: November 5
Considering Stereo & Regioselectivity
36
Combine Zaitsev’s Rule and observations about stereoselectivity to predict the major products of dehydration (elimination)
trisubstituted trisubstituted disubstituted disubstituted
most stable alkenes have largest groups on each carbon trans to each other
major product
OHH2SO4
80 ºC
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012 Slide
Lecture 8: November 5
What is the major product expected for the reaction scheme below?
Self Test Question
37
A.
B.
C.
D.
E.
HO
H2SO4
80 ºC?
University of Illinois at ChicagoUICCHEM 494
Special Topics in Chemistry
Section: 5.12
E1 & E2 Mechanisms of Alcohol Dehydration
UICUniversity of Illinois at Chicago CHEM 494, Spring 2010 Slide
Lecture 8: November 5
Organic Mechanisms (SN1)
39
OH
OH
HClH Cl
fast & reversible
alkyloxonium ion
MeMe
Me
H2Ocarbocation(t-butyl cation)
slow
t-butyl chloride
fastCl Cl
UICUniversity of Illinois at Chicago CHEM 494, Spring 2010 Slide
Lecture 8: November 5
Remember Curved Arrow Notation?
40
curved arrows show the movement of
electrons; never atoms electronsatoms
resonance: electrons in a covalent bond moving
out to an atom
resonance: lone pair of electrons moving in between two atoms to
form a new covalent bond
bond making: lone pair of electrons
forming a new bond to another atom
bond breaking: electrons in a bond
leaving to most electronegative atom
H3C N
OCH3
CH3
H3C N
OCH3
CH3H
OH H H3O+
H3C O
OH
H3C O
O+ H
UICUniversity of Illinois at Chicago CHEM 494, Spring 2010 Slide
Lecture 8: November 5
Mechanism of Dehydration (E1)
41
Step OneProton Transfer (Protonation)
pKa = -3.0
OH
OH
H
fast & reversible
alkyloxonium ion
OSO
O OHHOSO
O OH
UICUniversity of Illinois at Chicago CHEM 494, Spring 2010 Slide
Lecture 8: November 5
Mechanism of Dehydration (E1)
42
Step TwoDissociation
OH
H MeMe
MeH2O
carbocation(t-butyl cation)
slow
UICUniversity of Illinois at Chicago CHEM 494, Spring 2010 Slide
Lecture 8: November 5
Mechanism of Dehydration (E1)
43
Step ThreeCarbocation Capture β-Deprotonation!
CH2Me
Me
carbocation(t-butyl cation)
alkene(2-methylpropene)
OSO
O OHH
OSO
O OH
H
Me Me
sulfuric acid(regenerated)
fast
OSO
O OHalkyl hydrogen
sulfate(product of SN1)
UICUniversity of Illinois at Chicago CHEM 494, Spring 2010 Slide
Lecture 8: November 5
Hughes-Ingold Nomenclature
44
E1
overall reaction = β-Eliminationrate determining step (RDS) involves on species = unimolecular
rate = k[alkyl oxonium ion] = %rst order
eliminationunimolecular
OH
H MeMe
MeH2O
carbocation(t-butyl cation)
slow
UICUniversity of Illinois at Chicago CHEM 494, Spring 2010 Slide
Lecture 8: November 5
Each Step of E1 Mechanism is Reversible
45
OH
OH
H
fast &reversible
CH2Me
Me
H2O
slow &reversible
OSO
O OHHOSO
O OH
OSO
O OH
H
Me Me
fast &reversible
If all steps in E1 are reversible, what drives the reaction forward?
UICUniversity of Illinois at Chicago CHEM 494, Spring 2010 Slide
Lecture 8: November 5
Alkenes Isolated from Dehydration Reactions by Distillation
46
• alkenes have much lower boiling points than alcohols
• alcohols have higher boiling points (b.p.) because of larger van der Waals forces, including strong hydrogen-bonding
• by removing alkenes through distillation (boiling), equilibrium is shifted toward products (LeChatlier Principle) until no more reactants remain
OH
4-methyl-2-pentanolbp = 132 ºC
trans-4-methyl-2-pentenebp = 59 ºC
H2SO4
2-methyl-1-pentenebp = 62 ºC
cis-4-methyl-2-pentenebp = 58 ºC
+ + + +
2-methyl-2-pentenebp = 67 ºC
4-methyl-1-pentenebp = 54 ºC
UICUniversity of Illinois at Chicago CHEM 494, Spring 2010 Slide
Lecture 8: November 5
Why Can’t Hydrogen Halides Be Used for Elimination Reactions?
OH
OH
H
fast &reversible
CH2Me
Me
slow &reversible
ClH
HO
HMe
Me
fast &reversible
Clalkyl chloride
(product of SN1)
Cl fast &irreversible
H
Cl
ClH
nucleophilicaddition
N u c l e o p h i l i c a d d i t i o n o f chloride (Cl–) to a carbocation is not reversible
UICUniversity of Illinois at Chicago CHEM 494, Spring 2010 Slide
Lecture 8: November 5
Reactivity Explained
48
R1 OH
R3R2
H3O+
R1 OH2
R3R2
H2O
R1 R3
R2 R1 = CR2 = CR3 = C
R2 R3
R2 R1 = CR2 = CR3 = H
3º Carbocation
2º Carbocation
• 3º carbocations are more stable than 2º = 3º lower in energy
• smaller activation energy leading to 3º carbocation results in faster reaction
UICUniversity of Illinois at Chicago CHEM 494, Spring 2010 Slide
Lecture 8: November 5
Bimolecular Substitution - SN2 Mechanism(from Lecture 8)
49
• 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
H3C OH
H Br
Step 1Protonation
H3C OH
BrH
HO
H+
H3C Br
H3C H
H
‡CH3
C
H H
OBrH
Hδ+
δ-
Step 2Nucleophilic Attack
fast
slow (rate-determining)
UICUniversity of Illinois at Chicago CHEM 494, Spring 2010 Slide
Lecture 8: November 5
Dehydration of Primary AlcoholsProceeds via E2 Mechanism
50
• 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
OH3C
H
H
fast &reversibleO
SO
O OHHOSO
O OH
H H
slow
H3C O
H
H3C CH
H
1° Cation
HHH2C
O
OSO
OHO
Hβ
HH
Step 1Protonation
Step 2β-Deprotonation(elimination)
University of Illinois at ChicagoUICCHEM 494
Special Topics in Chemistry
Addition Reactions of Alkenes
UICUniversity of Illinois at Chicago CHEM 494, Spring 2010 Slide
Lecture 8: November 5
Addition Reactions of Alkenes
52
C C X Y+ additionC CX Y
C C H H+ hydrogenationC CH H
C C H X+hydrogen
halide additionC CH X
C C H Br+free radical
bromine addition C CH Br
C C +sulfuric acid
addition C CH OSO3H
C C H OH+ hydrationC CH OH
O SO
OOHH
UICUniversity of Illinois at Chicago CHEM 494, Spring 2010 Slide
Lecture 8: November 5
Hydrogenation
53
Pd/C, CH3CH2OH
• exothermic reaction (-∆Hº), but high Eact - catalyst required
• catalysts are heterogeneous transition metals (Pd, R
• solvent is typically an alcohol (e.g. ethanol, CH3CH2OH)
• metals are insoluble (heterogeneous mixture)
• heat of hydrogenation = -∆Hº
UICUniversity of Illinois at Chicago CHEM 494, Spring 2010 Slide
Lecture 8: November 5
Heat of Hydrogenation (-∆Hº)
54
UICUniversity of Illinois at Chicago CHEM 494, Spring 2010 Slide
Lecture 8: November 5
Heat of Hydrogenation (-∆Hº)
55
H
H H
H
Alkene -∆Hº (kJ/mol) example
ethylene 136
monosubstituted 126
cis-disubstituted 119
terminally disubstituted 117
trisubstituted 112
tetrasubstituted 110
Since only the double bond is undergoing the reaction, heat of hydrogenation is independent of the number of carbon atoms in the molecule
UICUniversity of Illinois at Chicago CHEM 494, Spring 2010 Slide
Lecture 8: November 5
General Mechanism forHeterogenious Hydrogenation
56
catalyst surface
H H
catalyst surfaceMII
HH
catalyst surfaceMII
HH
oxidativeaddition
d
σ∗
catalyst surfaceMII
H H
coordinationinsertion
reductiveelimination
H H
M0
reaction takes places at the surface of the catalyst (many metal atoms combined)
UICUniversity of Illinois at Chicago CHEM 494, Spring 2010 Slide
Lecture 8: November 5
Step 1: Oxidative Addition
57
catalyst surface
H H
M0
catalyst surfaceMII
H H
oxidativeaddition
d
σ∗
• hydrogen (H2) is added to metal• metal is oxidized from M0 to MII
Pd0
H H
H HPdII
UICUniversity of Illinois at Chicago CHEM 494, Spring 2010 Slide
Lecture 8: November 5
Step 2: Coordination
58
catalyst surfaceMII
H H
catalyst surfaceMII
H H coordination
• metal is a Lewis acid (electron acceptor)• π-bond is a lewis base (electron donor)• coordination = Lewis acid/base complex
H HPdIIH HPdII
UICUniversity of Illinois at Chicago CHEM 494, Spring 2010 Slide
Lecture 8: November 5
Step 3: Insertion
59
catalyst surfaceMII
H H
catalyst surfaceMII
H H
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 HPdIIH
PdIIH
UICUniversity of Illinois at Chicago CHEM 494, Spring 2010 Slide
Lecture 8: November 5
Step 4: Reductive Elimination
60
catalyst surface
H H
M0
d
σ∗
catalyst surfaceMII
H H
reductiveelimination
H H
• 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
PdIIH H H
UICUniversity of Illinois at Chicago CHEM 494, Spring 2010 Slide
Lecture 8: November 5
Complete Mechanism
61
oxidativeaddition
coordinationinsertion
reductiveelimination
H H
Pd0
H H
H HPdII
H HPdII
H
PdIIH
UICUniversity of Illinois at Chicago CHEM 494, Spring 2010 Slide
Lecture 8: November 5
Syn Addition of Hydrogen
62
• as a consequence of mechanism, both hydrogens are added to the same face of the π-bond: syn addition
• no anti-addition products are formed (addition of hydrogen to opposite faces)
UICUniversity of Illinois at Chicago CHEM 494, Spring 2010 Slide
Lecture 8: November 5
Hydrogenation is Stereoselective
63
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 alkene
• stereoselective: preference for one stereoisomer when two or more are possible
UICUniversity of Illinois at Chicago CHEM 494, Spring 2010 Slide
Lecture 8: November 5
Example of Syn Addition
64
C8H12
CH3 CH3
HO
HH
H
C8H12CH3CH3
HOH H
C8H12CH3CH3
HOH H
C8H12
CH3 CH3
HO
C8H12
CH3 CH3
HO
H HH
C8H12
CH3 CH3
HO
H H
2. Hydrogen absorbed on to catalyst surface. H-H bond cleaved.
3. π−System of alkene coordinates catalyst surface.
4. One hydrogen atom is transferred to alkene, forming C-catalyst bond.
5. Second hydrogen atom is transferred, breaking substrate-catalyst bond. Alkane diffuses away from catalyst.
1. Diatomic hydrogen and alkene are present in solution phase. No reaction occurs.
6. Steps 1-5 are repeated.
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012 Slide
Lecture 8: November 5
A.
B.
C.
D.
What is the major product of the following hydrogenation reaction?
Self Test Question
65
H2, Pd/C
CH3CH2OH
D
H
DH
D
HH
H
D
H
H
H
D
H
H
H
H
H
H
H
H
D
catalyst surfaceMIID
catalyst surfaceMII
UICUniversity of Illinois at Chicago CHEM 494, Spring 2010 Slide
Lecture 8: November 5
Addition of Electrophiles to Alkene
66
C C H H+ hydrogenationC CH H
C C H X+hydrogen
halide additionC CH X
C C H Br+free radical
bromine addition C CH Br
C C +sulfuric acid
addition C CH OSO3H
C C H OH+ hydrationC CH OH
O SO
OOHH
UICUniversity of Illinois at Chicago CHEM 494, Spring 2010 Slide
Lecture 8: November 5
Electrophilic Addition of HX
67
δ+ δ–
electrophilenucleophile
C C H X+hydrogen
halide additionC CH X
UICUniversity of Illinois at Chicago CHEM 494, Spring 2010 Slide
Lecture 8: November 5
Reaction Conditions
68
• hydrogen halide: HX• common solvents: chloroform
(CHCl3) ,dichloromethane (CH2Cl2), pentane, acetic acid
• generally performed at low temperature (below 0 °C)
• generally a fast reaction
HHBr
CHCl3, -30 ºCH
Br
H H
H
UICUniversity of Illinois at Chicago CHEM 494, Spring 2010 Slide
Lecture 8: November 5
Electrophilic Addition (AdE) Mechanism
69
• electrophilic addition: AdE• RDS = protonation of carbon• rate = k[alkene][hydrogen halide]• unlike oxygen and nitrogen, protonation of
carbon is slow• proceeds through carbocation intermediate
H Br
H
H H H
HBr
Br
H H
HProtonation(slow)
CationCapture(fast)
UICUniversity of Illinois at Chicago CHEM 494, Spring 2010 Slide
Lecture 8: November 5
HX Addition is Regioselective
70
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
H
H X R
HH
H
HX
R
R
H
H
H X R
HR
H
HX
R
R
H
R
H X R
HR
H
RX
R
XH
H
HH
R
XR
H
HH
R
XR
H
RH
rather than
rather than
rather than
UICUniversity of Illinois at Chicago CHEM 494, Spring 2010 Slide
Lecture 8: November 5
Markovnikov’s Rule
71
3º 2º
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
CH3HBr
CH2Cl2-40 ºC
H3C BrH H
H
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012 Slide
Lecture 8: November 5
A. 3-chloro-2,4-dimethylpentane
B. 2-chloroohexane
C. 2,3-dichloro-2,4-dimethylpentane
D. 2-chloro-2,4-dimethylpentane
E. 1-chloro-2,4-dimethylpentane
Predict the product when 2,4-dimethyl-2-pentene is treated with HCl?
Self Test Question
72
HCl Cl
UICUniversity of Illinois at Chicago CHEM 494, Spring 2010 Slide
Lecture 8: November 5
Mechanistic Basis for Markovnikov’s Rule
73
H X
H X
H
H
X
+
X
X
3º carbocation
2º carbocation
3º alkyl halide
2º alkyl halide
X
H
H
• curved arrows do not indicate which carbon is protonated
• fastest protonation leads to more stable (more substituted) carbocation
• more substituted carbocation = more substituted alkyl halide
UICUniversity of Illinois at Chicago CHEM 494, Spring 2010 Slide
Lecture 8: November 5
Mechanistic Basis for Markovnikov’s Rule
74
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