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©2014 Gregory R Cook
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0
Chapter 04Reactions of Alkenes and
Alkynes
CHEM 240: Spring 2014
Prof. Greg Cook
cook.chem.ndsu.nodak.edu/chem240
Thursday, September 18, 14
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0Electrophilic Addition of HX
2
C CH
H
H
H+ HBr C C
H
BrHH
H
H
Thursday, September 18, 14
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0Electrophilic Addition of HX
3
C CH
H
H
H+ HBr C C
HHH
H
H
E
reaction progress
transition state ‡
ΔG°
transition state ‡
C CH
H
H
H+ HBr
C C
H
Br
HH
H
H
C CH
HH
H
H
activationenergyΔG‡
C CH
BrHH
H
HBrRDS(slow step)
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0Regioselectivity
4
HBr
Br
HH
Br+
80% yield not formed
HBr HBr
90% yield
HBr
100% yield
Br
H
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0Markovnikov’s Rule
5
• “When an unsymmetrically substituted alkene reacts with HX, the hydrogen adds to the carbon that has the greater number of hydrogens, and the halogen adds to the carbon having fewer hydrogens.”
Thursday, September 18, 14
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0Carbocation Structure
6
• Carbocations are sp2 hybridized with a trigonal planar geometry (empty p orbital)
C
CC
CCH
HH
H
HH
HHH
C CCHH
H
H
H
H
H HH
tert-butyl
Thursday, September 18, 14
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0Carbocation Stability
7
• More substituted carbocations are more stable (lower in energy)
C
C
H
HHC
H
HCH
HH
C
H
CCH
HH
H
H
HC
C
CCH
HH
H
H
H
HHH
CHHH C
HHC
HH
HCH
CCHH
H
H
H
HC CC
HH
H
H
H
H
H HH
methyl ethyl isopropyl tert-butyl
stability (lower energy)
Thursday, September 18, 14
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0Carbocation Stability
8
• Spread out the positive charge (more stable)
• inductive effects and hypercongujugation
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0Carbocation Stability
9
• Spread out the positive charge (more stable)
• inductive effects and hypercongujugation
Thursday, September 18, 14
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0Carbocation Stability
10
• Spread out the positive charge (more stable)
• inductive effects and hypercongujugation
Thursday, September 18, 14
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0Mechanistic basis for Regioselectivity
11
Thursday, September 18, 14
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0Markovnikov’s Rule - Restated
12
• When an unsymmetrically substituted alkene reacts with HX, protonation occurs to form the more stable carbocation.
Thursday, September 18, 14
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0Addition of H2O - Hydration of Alkenes
13
H3PO4 cat.OH
250 °C+ H2O
Thursday, September 18, 14
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0Halogenation of Alkenes
14
+C C C CX
XX X
+ Br BrBr
BrCHCl3
anti addition
Thursday, September 18, 14
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0Mechanism of Electrophilic Halogenation
15
Br Br
Br
Br
Br
Br
A bromonium ion
H
OH
O
Br
H
HOH
Br
-H+
a halohydrin
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0Generalities of Electrophilic Addition
16
A+ B-
A
orA
A
B
A+ B-overall equivalent
reagents notes
HX H+ X- MarkovnikovX2 X+ X- bridged, anti addnX2/H2O X+ HO- bridged, anti addn, Markovnikov
H2O (H+ cat) H+ HO- MarkovnikovBH3 OH+ H- syn addn, anti-Markovnikovthen H2O2/OH-
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0Addition of H2 - Hydrogenation
17
C C
H
H
H
H
H
HC C
H
H H
H
+ H Hcatalyst
ΔH° = -136 kJ/mol (-32.6 kcal/mol)
carbon
Pd H H
carbon
Pd
H H
Hydrogen is activated
C C
H
H
H
H
C C
H
H
H
H
H
H
The catalyst activates hydrogen gas by splitting the H-H bond. It then transfers the H's to the alkene and this regenerates the metal catalyst to activate more hydrogen
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0Heat of Hydrogenation
18
Thursday, September 18, 14
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0Hydrogenation Stereochemistry
19
CO2CH3
CO2CH3
H2Pt
CO2CH3
CO2CH3
H
H
syn addition of H2
CO2CH3
H
H
CO2CH3
only product
H
H
H
H
H
H
H
C C
H
H
H
H
H
H
H
C C H
H
H
H
H
H
CCH
CCH
H
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0Hydrogenation Stereochemistry
20
H
HH3C
H
H3C CH3H
HH
CH3
H3C CH3
H
H3C
H3C CH3
α-pinene cis-pinene trans-pinene
H2Ni
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0Oxidation-Reduction
21
• Oxidation of carbon - increase the number of bonds to oxygen (more electronegative atom)
• Reduction of carbon - decrease the number of bonds to oxygen (more electronegative atom)
C
H
H H
H
C OOoxidation
C-4 C+4
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0Oxidation-Reduction
22
CH
HH
H CH
HH
OH
CH
HH
Cl
CHH
O
CH
HCl
Cl
CHOH
O
CH
ClCl
Cl
CO
CCl
ClCl
Cl
O
Lower Oxidation State
Higher Oxidation State
CO
HO OH
CH
HH
Cl + 2 Li CH
HH
Li + LiCl
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0Epoxides
23
water
HO
H RO
H RO
Ralcohol ether
C C
O
Thursday, September 18, 14
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0Expoxidation of Alkenes
24
peroxy acid
C C
O
+C C R O
OO
H
epoxide
Thursday, September 18, 14
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0Formation of 1,2-diols
25
mCPBAO
H3O+ OH
OH
OH
OH
Thursday, September 18, 14
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0Oxidation of Alkenes
26
OH
OH
+ KMnO4
H2ONaOH
O
O
+ KMnO4
H3O+
Thursday, September 18, 14
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0Reactions of Alkenes with Alkenes - Polymers
27
polymerization
polyethylene
polymerization
polystyrene
Ph Ph Ph Ph Ph Ph Ph Ph Ph Ph Ph Ph Ph Ph
Ph Ph Ph Ph Ph Ph Ph Ph Ph Ph Ph Ph Ph Ph
Ph =
Cl Cl Cl Cl Cl Cl Cl Cl
nPVC (polyvinylchloride)
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0Free Radical Polymerization Mechanism
28
• Initiation - A stable molecule is activated to generate free radical species
• Step 1 - Benzoyl peroxide breaks apart to form two radicals
OO
O
OO
O
benzoyl peroxide BzO•
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0Free Radical Polymerization Mechanism
29
• Chain Propogation - a stable molecule (alkene) reacts with a radical to produce a new stable molecule and new radical
• Step 2 - Addition of BzO• to alkene
O
O
BzO•
+ BzO
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0Free Radical Polymerization Mechanism
30
• Chain Propogation
• Step 3 - Alkyl radical reacts with another alkene
+ BzOBzO
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0Free Radical Polymerization Mechanism
31
• Termination - two radicals combine to form a stable molecule. This stops chain propogation
+BzOR BzO
R
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0Free Radical Polymerization Mechanism
32
+ BzOBzO
O
O
BzO•
+ BzO
OO
O
OO
O
benzoyl peroxide BzO•
+ BzOBzO R R
Initiation
Propagation
Termination
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0Conjugation
• Conjugation - conjugare (Latin) to link together - when pi bonds are extended beyond 2 carbons
33
conjugated not conjugated
CH
H
β-Carotene - orange colored constituent of carrots
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0Conjugation and Color
34
lycopene (red color of tomatos)
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0Conjugation and Color
35
O
OHR
R
OROH
HO NaOH O
OR
R
OROH
O
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0Electrophilic Addition to Dienes
36
HBr
Br
Br+H H
71% 29%
Br2
Br
Br+Br Br
55% 45%
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0Allylic Conjugation
37
allylic carbocation allylic free radical allylic carbanion
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0Allylic and Vinylic Positions
38
allylic alcohol
OH
allylic bromide
Br
CC
C
H
H
H
H
HH
allylic hydrogens
vinylic hydrogens
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0 ResonanceSection 4.9-10
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0Allylic Carbocations
40
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0Allylic Carbocations
41
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0Resonance
• Some molecules may have more than one correct Lewis structure
• These are NOT isomers. They are resonance forms.
42
OO
O OO
O
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0General Rules for Resonance Structures
• Individual resonance forms DO NOT EXIST - they represent the extremes. The actual chemical species is a hybrid of all the resonance forms.
• Resonance forms differ only in the distribution of pi or non-bonding electrons. Atoms do not move.
• Individual resonance forms must obey normal valency rules. You cannot break the octet rule.
• The actual hybrid is more stable than the individual resonance forms and the individual structures do not have to be equal in energy.
43
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0
• How many resonance forms are there?
Example HNO3
44
H3C C
O
O
O N
O
O
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0Example Benzene
45
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0Not all Resonance Forms are Equal
• Formamide
46
HC
NH
H
O
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0 Reactions of AlkynesSection 4.11
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0Hydrocarbons
• Alkanes: C-C single bondsAlkenes: C-C double bondsAlkynes: C-C triple bonds
• Aromatic hydrocarbons
48
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0Naming Alkynes
• Alkynes are named similar to alkenes
• They have equal priority
49
oct-1-ene-6-yne
propyne 1-butyne 2-butyne
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0Structure of Alkynes
50
C CH H
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0Hydrogenation of Alkynes
• Similar to alkenes, alkynes can be reduced by catalytic hydrogenation
51
H2Pd/C
H H H2Pd/C
H H
H H
H HH2
Lindlar's Catalyststops at alkene
Lindlar's CatalystPd / Pb(OAc)4
quinolineon CaCO3
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0Addition of HX to Alkynes
52
HBr1 equiv
H
Br
HBr1 equiv
H H
Br Br
rate = k [alkyne] [HBr]2
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0Halogenation of Alkynes
53
Br21 equiv
Br
Br
Br21 equiv
Br Br
Br Br
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0Hydration of Alkynes
54
an enol
OHH+, H2O O
a ketone
cat. HgSO4H2O
OH
Hg+
H2SO4OH
Henol
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0Acidity of Terminal Alkynes
55
C CH
HH
H
HH C C
H
H
H
HC CH H
pKa 62 45 26
base base base
C CH
HH
HH C C
H
H HC CH
carbanion carbanion carbanion
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0Deprotonation of Alkynes
56
C CH HpKa 26
C CH+ H O H + H O H
H
pKa -1.7
C CH HpKa 26
C CH+ O H + H O HpKa 15.7
C CH HpKa 26
C CH+ N H +pKa 36
H H N H
H
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0Alkylation of Alkynes
• Terminal alkynes can be alkylated to make new carbon-carbon bonds. Reaction proceeds via a SN2 substitution.
57
NaNH2H NaH3C Br
δ+ δ-
CH3
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