chapter 7: alkenes and alkynes – properties and synthesis ... · pdf filechapter 7:...
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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 ruleSyn/Anti coplanar conformation
Ch 7.7–7.8: Dehydration of alcohols (E2 / E1), Carbocation stabilityMolecular rearrangement (1,2 shift)
Ch 7.9–7.11: Synthesis of alkynes, vic-dihalide, gem-dihalideTerminal alkynes and their use in synthesis
Ch 7.12–7.115: Hydrogenation, Reduction, Syn/Anti additionDissolving metal reduction, Index of hydrogen deficiency
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Alkene Diastereomers: Cis-Trans vs. E-Z
VS..
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Overall Relative Stability of Alkenes
The greater the number of attached alkyl groups (i.e.,the more highly substituted the carbon atoms of thedouble bond), the greater is the alkene's stability.
The cis isomer is less stable due to greater strain from crowding by the adjacent alkyl groups.
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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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The temperature and concentration of acid required to dehydrate an alcohol depend on the structure of the alcohol substrate:
Primary alcohol Secondary alcohol
Tertiary alcohol General reactivity order
Acid-Catalyzed Dehydration of Alcohols: E1 Reaction
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Rearrangement during Dehydration
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C
CH3
H2C C CH3
CH3
H
H
B
B
Path A
Path B
A B
The formation of the more stable alkene is the general rule (Zaitsev's rule) in the acid-catalyzed dehydration reactions of alcohols.
(80%) (20%)
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Hydrogenation of Alkynes to Form cis-Alkenes
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Pd/CaCO3plus
N
Poisoned catalyst (P-2 and Lindlar’s catalyst) is required to stop the hydrogenation at an alkene stage.
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Anti Addition of Hydrogens to Form trans-Alkenes
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Li
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Dissolving metal reduction
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Index of Hydrogen Deficiency (Degree of Unsaturatiopn)
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Hydrocarbons
Saturated
Unsaturated
Alkane
Cycloalkane
Alkene
AlkyneAromatic
(CnH2n+2)
(CnH2n)
(CnH2n)
(CnH2n-2)
H2
H2
The index of hydrogen deficiency : difference in the number of hydrogen molecules between the corresponding alkane and molecular formula of the compound under consideration.
Cyclohexene (C6H10)
Cyclohexane (C6H12)
IHD = 1
IHD = 1
IHD = 2
IHD = 2
IHD = 3
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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–OxydationAnti-Markovnikov addition, Steric factors
Ch 8.12–8.15: Anti-addition of halogens, Bromonium ion, Ionic mechanism, Stereospecific reaction, HalohydrinCarbene, α-Elimination
Ch 8.16–8.21: Oxidation of alkenes, 1,2-Diols(glycols), syn-dihydoxylationOxidative cleavage, Ozonolysis, Stereoselctive reactionSynthon, Synthetic equivalent
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Addition of HX to Alkenes: Markovnikov’s Rule
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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
Reaction 1
S S
+ (R,R)-isomer
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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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O O+
OH H
Zn
HOAcZn(OAc)2+
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Synthetic Strategy for Multi-step Synthesis In planning a synthesis we often have to consider four interrelated aspects:1. Construction of the carbon skeleton2. Functional group interconversions3. Control of regiochemistry4. Control of stereochemistry
Retrosynthesis Synthesis
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Chapter 11. Alcohols and Ethers
Ch 11.1–11.10: Structure and nomenclatureSynthesis of alcohols and ethersReactions of alcohols (as an acid, to alkyl halides withPBr3 and SOCl2, to sulfonates)
Ch 11.11–11.12: Synthesis of Ethers–Williamson ether synthesisAlkoxymercuration–DemercurationProtection groups (tert-Butyl ether, Silyl ether)
Ch 11.13–11.15: Epoxides (their synthesis and reactions)Anti-1,2-dihydroxylation of alkenes via epoxidesCrown ethers
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Synthesis of Alcohols from AlkenesAcid-Catalyzed Hydration of alkenes
Oxymercuration-Demercuration
Hydroboration-oxidation
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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
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Alkoxymercuration-Demercuration
HgOAc
HH
HO
R
AcOHgH
H
OR
NaBH4
HH
H
OR
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Epoxides (Oxiranes).
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Anti 1,2-Dihydroxylation of Alkenes via Epoxides
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H
OH
OH
H
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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 groupOxidation–reduction reactions (oxidation of alcoholsto carbonyl groups and their reduction to alcohols)
Ch 12.5–12.8: Organometallic compoundsOrganolithium and Organomagnesium compoundsGrignard reaction. Alcohols from Grignard reagents
Ch 12.9: Organocopper reagentsLithium dialkylcuprate
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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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Oxidation
Oxidation
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Jones reagent
PCC
ROH
RO
H2CrO4
HO
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Organometallic Compounds: Grignard Reagent.
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Victor GrignardNovel Prize (1912)
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Alcohols from Carbonyls and Grignard Reagents
Esters react with two molecules of Grignard reagents to form tert-alcohols
Grignard reagents react with ketones to give tertiary alcohols
Grignard reagents react with aldehydes to give secondary alcohols
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Summary: Synthetic Connection between Alcohols and Carbonyls
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Retrosynthetic Analysis
Synthesis
Synthetic Plans Based on Grignard Reaction
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Chapter 13. Conjugated Unsaturated System
Ch 13.1–15.4: Allylic substitution, Allyl radical, Allylic chlorinationAllylic bromination, N-BromosuccinimideMO of allyl radical and allyl cationRules 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 dienesKinetic vs. thermodynamic controlDiels-Alder reaction (factor favoring for D-A,Stereochemistry of D-A, MO consideration,Endo/Exo-transition state, Intramolecular D-A)
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Introduction to Conjugated Unsaturated Systems
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A BA B Addition
A X
H X+ A H Allylic Substitution
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.
Cycloaddition (Diels-Alder)+
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Allylic Bromination with N-Bromosuccinimide.
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The Stability of Allyl Radical: MO Description.
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CC
C
HH
H
H
HAllyl Radical
energy
Three isolated p orbitals
Bonding MO
Nonbonding MO
Antibonding MO
Node
Node
+
+
+ +
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The Stability of Allyl Radical: Resonance Structures
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The Allyl Cation
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e-
CC
C
HH
H
H
HAllyl Cation
energy
Three isolated p orbitals
Bonding MO
Nonbonding MO
Antibonding MO
Node
Node
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The Allyl Anion
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e-e-+
CC
C
HH
H
H
HAllyl Anion
energy
Three isolated p orbitals
Bonding MO
Nonbonding MO
Antibonding MO
Node
Node
- -
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Molecular Orbital of 1,3-Butadiene.
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energy
p orbital
Bonding MO
Antibonding MO
HOMO
LUMO
Node
Node
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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 dienesthat 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 groups in the dienophile
the presence of electron-withdrawing and by electron-releasing groups in the diene.
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Stereochemistry of the Diels–Alder Reaction
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1. The Diels-Alder reaction is highly stereospecific: The reaction is a synaddition, and the configuration of the dienophile is retained in in product.
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Stereochemistry of the Diels–Alder Reaction
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3. The Diels-Alder reaction occurs primarily in an endo rather than anexo fashion when the reaction is kinetically controlled.
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Stereochemistry of the Diels–Alder Reaction
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3. The Diels-Alder reaction occurs primarily in an endo rather than anexo fashion when the reaction is kinetically controlled.
HOMO
LUMO
HOMO
LUMO
Diene Dienophile
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H
H
O
O
O
PrimaryOrbitalInteractions
SecondaryOrbitalInteraction