Download Chapter 7: Alkenes and Alkynes – Properties and Synthesis

Chapter 7: Alkenes and Alkynes – Properties
and Synthesis. Elimination Reactions
Ch 7.1–7.4:
Olefins, Acetylenes, E–Z System,
Relative stability of alkenes, Cycloalkenes
Ch 7.5–7.6:
Dehydrohalogenation (E2), Zaitsev’s rule, Hofmann rule
Syn/Anti coplanar conformation
Ch 7.7–7.8:
Dehydration of alcohols (E2 / E1), Carbocation stability
Molecular rearrangement (1,2 shift)
Ch 7.9–7.11: Synthesis of alkynes, vic-dihalide, gem-dihalide
Terminal alkynes and their use in synthesis
Ch 7.12–7.115: Hydrogenation, Reduction, Syn/Anti addition
Dissolving metal reduction, Index of hydrogen deficiency
Alkene Diastereomers: Cis-Trans vs. E-Z
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VS.
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Overall Relative Stability of Alkenes
The cis isomer is less stable due
to greater strain from crowding
by the adjacent alkyl groups.
The greater the number of attached alkyl groups (i.e.,
the more highly substituted the carbon atoms of the
double bond), the greater is the alkene's stability.
Dehydrohalogenation: Zaitsev’s Rule
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Dehydrohalogenation: Zaitsev’s Rule
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The Stereochemistry of E2 Reactions
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Acid-Catalyzed Dehydration of Alcohols: E1 Reaction
The temperature and concentration of acid required to dehydrate an
alcohol depend on the structure of the alcohol substrate:
Primary alcohol
Tertiary alcohol
Secondary alcohol
General reactivity order
Rearrangement during Dehydration
A (80%)
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B (20%)
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Path B
B
H
CH3
H2C C
C
CH3
CH3 H
B
Path A
The formation of the more stable alkene is the general rule (Zaitsev's rule)
in the acid-catalyzed dehydration reactions of alcohols.
Hydrogenation of Alkynes to Form cis-Alkenes
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Pd/CaCO3
plus
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N
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Poisoned catalyst (P-2 and Lindlar’s catalyst) is required to
stop the hydrogenation at an alkene stage.
Anti Addition of Hydrogens to Form trans-Alkenes
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Li
Dissolving metal reduction
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Index of Hydrogen Deficiency (Degree of Unsaturatiopn)
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Alkane
.
Saturated
Hydrocarbons
(CnH2n+2)
Cycloalkane (CnH2n)
Alkene
(CnH2n)
Alkyne
(CnH2n-2)
Unsaturated
H2
H2
Aromatic
The index of hydrogen deficiency : difference in the number of
hydrogen molecules between the corresponding alkane and molecular
formula of the compound under consideration.
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IHD = 1
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Cyclohexane
(C6H12)
IHD = 2
IHD = 2
IHD = 1
Cyclohexene
(C6H10)
.
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IHD = 3
.
Chapter 8: Alkenes and Alkynes –
Addition Reactions
Ch 8.1–8.5:
Electrophilic addition reaction, Markovnikov’s rule,
Regioselective reaction, hydration
Ch 8.6–8.11: Oxymercuration–demercuration, Hydroboration–Oxydation
Anti-Markovnikov addition, Steric factors
Ch 8.12–8.15: Anti-addition of halogens, Bromonium ion,
Ionic mechanism, Stereospecific reaction, Halohydrin
Carbene, α-Elimination
Ch 8.16–8.21: Oxidation of alkenes, 1,2-Diols(glycols), syn-dihydoxylation
Oxidative cleavage, Ozonolysis, Stereoselctive reaction
Synthon, Synthetic equivalent
Addition of HX to Alkenes: Markovnikov’s Rule
Stereochemistry of the Addition of HX to Alkenes
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Alcohols from Alkenes: Oxymercuration–Demercuration
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Alcohols from Alkenes: Hydroboration–Oxidation
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Syn Addition
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Stereospecific Reactions
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Reaction 1
S
S
+ (R,R)-isomer
Stereospecific Reactions
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Reaction 2
S
R
= (R,S)-isomer (meso)
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Structure and Reactions of Methylene
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Oxidation of Alkenes: Syn-1,2-Dihydroxylation
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Oxidative Cleavage Alkenes: Ozonolysis
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H
Zn
HOAc
O
H
+
+
O
Zn(OAc)2
O
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Synthetic Strategy for Multi-step Synthesis
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In planning a synthesis we often have to consider four interrelated aspects:
1.
2.
3.
4.
Construction of the carbon skeleton
Functional group interconversions
Control of regiochemistry
Control of stereochemistry
Retrosynthesis
Synthesis
Chapter 11. Alcohols and Ethers
Ch 11.1–11.10: Structure and nomenclature
Synthesis of alcohols and ethers
Reactions of alcohols (as an acid, to alkyl halides with
PBr3 and SOCl2, to sulfonates)
Ch 11.11–11.12: Synthesis of Ethers–Williamson ether synthesis
Alkoxymercuration–Demercuration
Protection groups (tert-Butyl ether, Silyl ether)
Ch 11.13–11.15: Epoxides (their synthesis and reactions)
Anti-1,2-dihydroxylation of alkenes via epoxides
Crown ethers
Synthesis of Alcohols from Alkenes
Acid-Catalyzed Hydration of alkenes
Oxymercuration-Demercuration
Hydroboration-oxidation
Alkyl Halide from ROH: with HX, PBr3 and SOCl2
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Synthesis of Ethers: The Williamson Synthesis
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Synthesis of Ethers
Alkoxymercuration-Demercuration
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R
H
O
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R
R
O
H
NaBH4
H
Hg
OAc
O
H
H
H
H
AcOHg
H
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Epoxides (Oxiranes)
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Anti 1,2-Dihydroxylation of Alkenes via Epoxides
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OH
H
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OH
H
trans-1,2-Cyclopentadiol
(R,R and S,S)
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Chapter 12. Alcohols from Carbonyl Compounds
Ch 12.1–12.4: Structure and reactivity of carbonyl group
Oxidation–reduction reactions (oxidation of alcohols
to carbonyl groups and their reduction to alcohols)
Ch 12.5–12.8: Organometallic compounds
Organolithium and Organomagnesium compounds
Grignard reaction. Alcohols from Grignard reagents
Ch 12.9:
Organocopper reagents
Lithium dialkylcuprate
Alcohols by Reduction of Carbonyl Compounds
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Lithium Aluminum Hydride (LiAlH4)
Sodium Borohydride (NaBH4)
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Oxidation of Primary Alcohols to Aldehydes
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PCC
Jones reagent
Oxidation
H2CrO4
R
R
O
OH
HO
Oxidation
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Organometallic Compounds: Grignard Reagent
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Victor Grignard
Novel Prize (1912)
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Alcohols from Carbonyls and Grignard Reagents
Grignard reagents react with aldehydes to give secondary alcohols
Grignard reagents react with ketones to give tertiary alcohols
Esters react with two molecules of Grignard reagents to form tert-alcohols
Summary: Synthetic Connection between Alcohols and Carbonyls
Synthetic Plans Based on Grignard Reaction
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Retrosynthetic Analysis
Synthesis
Chapter 13. Conjugated Unsaturated System
Ch 13.1–15.4:
Allylic substitution, Allyl radical, Allylic chlorination
Allylic bromination, N-Bromosuccinimide
MO of allyl radical and allyl cation
Rules for writing resonance structures
Ch 13.6–13.8:
Polyunsaturated hydrocarbons, 1,3-Butadiene
(electron delocalization, conformation, MO)
Ch 13.10–13.11 Electrophilic attack on conjugated dienes
Kinetic vs. thermodynamic control
Diels-Alder reaction (factor favoring for D-A,
Stereochemistry of D-A, MO consideration,
Endo/Exo-transition state, Intramolecular D-A)
Introduction to Conjugated Unsaturated Systems
A B
A
.
H
.
B
A X
Addition
+
A H
Allylic Substitution
.
X
Systems that have a p orbital on an atom adjacent to a double bond–
with delocalized π bonds–are called conjugated unsaturated
systems. This general phenomenon is called conjugation.
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+
Cycloaddition (Diels-Alder)
Allylic Bromination with N-Bromosuccinimide
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The Stability of Allyl Radical: MO Description
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energy
Node
+
+
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Antibonding MO
H
H
C
C
H
.
C
H
Three isolated
p orbitals
Node
+
H
Allyl Radical
Nonbonding MO
+
Bonding MO
The Stability of Allyl Radical: Resonance Structures
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The Allyl Cation
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e-
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energy
Node
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Antibonding MO
H
H
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C
H
C
C
H
Three isolated
p orbitals
Node
H
Allyl Cation
Nonbonding MO
Bonding MO
The Allyl Anion
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+ e-
-
e-
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energy
Node
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Antibonding MO
H
H
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C
H
C
C
H
Three isolated
p orbitals
Node
H
Allyl Anion
Nonbonding MO
Bonding MO
Molecular Orbital of 1,3-Butadiene
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Node
energy
Antibonding MO
LUMO
p orbital
Node
HOMO
Bonding MO
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Kinetic vs. Thermodynamic Control of a Chemical Reaction
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The Diels–Alder Reaction: 1,4-Cycloaddition of Dienes
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In 1928 Otto Diels and Kurt Alder developed a 1,4-cycloaddition reaction of dienes
that has since come to bear their names. The reaction proved to be one of such great
versatility and synthetic utility that Diels and Alder were awarded the Nobel Prize in
Chemistry in 1950.
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Factors Favoring the Diels–Alder Reaction
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Diels-Alder reaction is favored by the presence of electron-withdrawing
groups in the dienophile and by electron-releasing groups in the diene.
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Stereochemistry of the Diels–Alder Reaction
1. The Diels-Alder reaction is highly stereospecific: The reaction is a syn
addition, and the configuration of the dienophile is retained in in product.
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Stereochemistry of the Diels–Alder Reaction
3. The Diels-Alder reaction occurs primarily in an endo rather than an
exo fashion when the reaction is kinetically controlled.
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Stereochemistry of the Diels–Alder Reaction
3. The Diels-Alder reaction occurs primarily in an endo rather than an
exo fashion when the reaction is kinetically controlled.
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LUMO
LUMO
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HOMO
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H
H
HOMO
Secondary
Orbital
Interaction
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O
Primary
Orbital
Interactions
O
Diene
Dienophile
O
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