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Chem
istry CHAPTER 05
REACTIONS OF ALKENES AND ALKYNES
CHEM 240: Fall 2019
Prof. Greg Cook
cook.chem.ndsu.nodak.edu/chem240
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Chem
istry
Radical and Polar Reactions
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A B A B+ free radicals (radical reactions)
A B A B+ Ions (polar reactions)
A BA B+
electrophile(e- poor)
nucleophile(e- rich)
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Chem
istry
General Reaction Classes
• Addition Reactions - two or more molecules combine to form a new product
• Elimination Reactions - a molecule breaks apart into two or more molecules
• Substitution Reactions - part of a molecule is substituted for another group
• Rearrangement Reactions - A molecule does not gain or lose atoms, but reorganizes its bonds or groups. Also called isomerization.
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Chem
istry
Structure and Bonding
• A double bond is sp2 hybridized (pi bond)
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Chem
istry
Characteristic Reactions of Alkenes
• Addition Reactions - two or more molecules combine to form a new product
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Chem
istry
Electrophilic Addition of HX
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C CH
H
H
H+ HBr C C
H
Br
HH
H
H
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Chem
istry
Electrophilic Addition of HX
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C CH
H
H
H+ HBr C C
H
HH
H
H
C C
H
Br
HH
H
HBr
a carbocationintermediate
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Chem
istry
Reaction Energy Diagrams
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Chem
istry
Kinetics
• A multistepreaction path
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Chem
istry
Common steps in reaction mechanism
• Protonation
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Chem
istry
Common steps in reaction mechanism
• Deprotonation
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Chem
istry
Common steps in reaction mechanism
• Electrophile plus Nucleophile
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Chem
istry
Common steps in reaction mechanism
• Rearrangements of bonds
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Chem
istry
Common steps in reaction mechanism
• Bond breaking to make relatively stable ions
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Chem
istry
Electrophilic Addition of HX
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C CH
H
H
H+ HBr C C
H
Br
HH
H
H
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Chem
istry
Electrophilic Addition of HX
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C CH
H
H
H+ HBr C C
H
HH
H
H
E
reaction progress
transition state ‡
ΔG°
transition state ‡
C CH
H
H
H+ HBr
C C
H
Br
HH
H
H
C C
H
HH
H
H
activationenergyΔG‡
C CH
Br
HH
H
HBrRDS(slow step)
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Chem
istry
Regioselectivity
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HBr
Br
HH
Br+
80% yield not formed
HBr HBr
90% yield
HBr
100% yield
Br
H
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Chem
istry
Markovnikov’s Rule
• “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.”
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Chem
istry
Carbocation Structure
• Carbocations are sp2 hybridized with a trigonal planar geometry (empty p orbital)
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C
C
C
CCH
HH
H
H
H
HHH
C CCHH
H
H
H
H
H HH
tert-butyl
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Chem
istry
Carbocation Stability
• More substituted carbocations are more stable (lower in energy)
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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)
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Chem
istry
Carbocation Stability
• Spread out the positive charge (more stable)
• inductive effects and hypercongujugation
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Chem
istry
Mechanistic basis for Regioselectivity
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Chem
istry
Markovnikov’s Rule - Restated
• When an unsymmetrically substituted alkene reacts with HX, protonation occurs to form the more stable carbocation.
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Chem
istry
Addition of H2O - Hydration of Alkenes
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H3PO4 cat.OH
250 °C+ H2O
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Chem
istry
Halogenation of Alkenes
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+C C C CX
XX X
+ Br BrBr
BrCHCl3
anti addition
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Chem
istry
Mechanism of Electrophilic Halogenation
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Br Br
Br
Br
Br
Br
A bromonium ion
H
OH
O
Br
H
HOH
Br
-H+
a halohydrin
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Chem
istry
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Chem
istry
Carbocation Rearrangements
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Chem
istry H+ catalyst OH
OHnot
R2B H
HBR2
OH-H2O2
Anti-Markovnikov Selectivity?
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Chem
istry
H
syn addition
HOH
H3C
H2O2 / NaOHH
BR2
anti-Markovnikovhydration
H
H BR2
H3C
H2O2 / NaOH
H
H OH
H3C
BR2H
Hydroboration/Oxidation Stereochemistry
30
BH3
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Chem
istry
Hydroboration Borohydride Sources
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BH3•THF
B OH
HH
B2H6 in diglyme
BH
BH
H
H H
H
OO
O
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Chem
istry
Mechanism of Hydroboration
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Chem
istry
Generalities of Electrophilic Addition
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A+ B-
A
orA
A
B
A+ B-overall equivalent
reagents notes
HX H+ X- MarkovnikovX2 X+ X- bridged, anti addn
X2/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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Chem
istry
Addition of H2 - Hydrogenation
34
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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Chem
istry
Catalysis
• A catalyst increases the rate of a reaction but is not changed in a reaction. It lowers the overall energy barrier.
35
E
reaction progress
transition state ‡
G°
A
B
energybarrierto climb
catalyst - lowersoverall energy barrier
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Chem
istry
Hydrogenation Stereochemistry
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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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Chem
istry
Hydrogenation Stereochemistry
37
H
HH3C
H
H3C CH3H
HH
CH3
H3C CH3
H
H3C
H3C CH3
α-pinene cis-pinene trans-pinene
H2Ni
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Chem
istry
Heat of Hydrogenation
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Chem
istry
Organic Synthesis
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• the preparation of complex molecules from commercially available starting materials via a multistep sequence of transformations
Starting Materials
Target Molecule
How do we design this?
Retrosynthetic Analysis
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Chem
istry
Generalities of Electrophilic Addition
40
A+ B-
A
orA
A
B
A+ B-overall equivalent
reagents notes
HX H+ X- MarkovnikovX2 X+ X- bridged, anti addn
X2/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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Chem
istry
Structure of Alkynes
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C CH H
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Chem
istry
Hydrogenation of Alkynes
• Similar to alkenes, alkynes can be reduced by catalytic hydrogenation
42
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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Chem
istry
Alkylation of Alkynes
• Terminal alkynes can be alkylated to make new carbon-carbon bonds. Reaction proceeds via a SN2 substitution.
43
NaNH2H NaH3C Br
δ+ δ-
CH3
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Chem
istry
Organic Synthesis
44
NaNH2H
Br
LiNH3
H HNaNH2
BrH
NaNH2
Br
1)
2)
2) 2)
1) 1)