Chapter 7: Alkenes and Alkynes – Properties and Synthesis ... · PDF fileChapter 7: Alkenes and Alkynes – Properties and Synthesis. Elimination Reactions Ch 7.1–7.4: Olefins,

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

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%)

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.

Anti Addition of Hydrogens to Form trans-Alkenes

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Li

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Dissolving metal reduction

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

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

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

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

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

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

Epoxides (Oxiranes).

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Anti 1,2-Dihydroxylation of Alkenes via Epoxides

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H

OH

OH

H

??

trans-1,2-Cyclopentadiol (R,R and S,S)

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

Alcohols by Reduction of Carbonyl Compounds.

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Lithium Aluminum Hydride (LiAlH4)

Sodium Borohydride (NaBH4)

Oxidation of Primary Alcohols to Aldehydes.

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Oxidation

Oxidation

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Jones reagent

PCC

ROH

RO

H2CrO4

HO

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

Summary: Synthetic Connection between Alcohols and Carbonyls

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Retrosynthetic Analysis

Synthesis

Synthetic Plans Based on Grignard Reaction

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)

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)+

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

- -

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