Chapter 9 Addition Reactions of Alkenes '13 BW(1)

8/13/2019 Chapter 9 Addition Reactions of Alkenes '13 BW(1)

1/25

1

Chapter 9. Addition Reactions of Alkenes

Junha Jeon

Department of Chemistry

University of Texas at Arlington

Arlington, Texas 76019Chem 2321, Fall 123

9.1 Addition Reactions: A Pi Bond is Converted to a Sigma Bond

Various Addition Reactions Various Addition Reactions

9.2 Addition vs. Elimination: Thermodynamics

in equilibrium (thermodynamics)

Addition vs. Elimination: A Thermodynamic Perspective


2/25

2

Addition vs. Elimination: A Thermodynamic Perspective Addition vs. Elimination: A Thermodynamic Perspective

Addition vs. Elimination: A Thermodynamic Perspective Addition vs. Elimination: A Thermodynamic Perspective

Addition vs. Elimination: A Thermodynamic Perspective 9.3 Hydrohalogenation


3/25

3

Hydrohalogenation

Regiochemistry!!

9.3 Hydrohalogenation

Markovnikov Addition



Hydrogen tends to add to the carbon already bearing more H atoms.



Hydrogen tends to add to the carbon already bearing more H atoms.

Hydrohalogenation


Halogen is generally placed at the more substituted position.

Hydrohalogenation

Regioselective!




4/25

4

Hydrohalogenation

Of course,

Anti-Markovnikov Addition

in Chapter 11

Hydrohalogenation

Mechanism of Hydrohalogenation

Mechanism of Hydrohalogenation

Two Mechanistic Pathways Two Mechanistic Pathways


5/25

5

Two Mechanistic Pathways Mechanistic Pathways

Which transition state (TS) is

more stable? And why?

Mechanistic Pathways

Which transition state (TS) ismore stable? And why?Hammond Postulate

Recall the Hammond Postulate

The structure of a transition state resembles the structure of the nearest stablespecies. Transition states for endergonic steps structurally resemble products,and transition states for exergonic steps structurally resemble reactants.


Which transition state (TS) ismore stable? And why?Hammond Postulate



6/25

6

Ionic Stepwise Reaction: Markovnikov Addition (Regioselectivity)

Ea

carbocationcharacters



The take home message is that the regioselectivity of an ionic addition

reaction is determined by the preference for the reaction to proceed

through the more stable carbocation intermediate: Markovnikov addition



The take home message is that the regioselectivity of an ionic addition

reaction is determined by the preference for the reaction to proceed

through the more stable carbocation intermediate: Markovnikov addition

Markovnikov Addition:

Hydrogen tend to add to the carbon already bearing more H atoms.


Stereochemistry of Hydrohalogenation

Stereochemistry of Hydrohalogenation

Two enantiomers as a racemic mixture are produced

Stereochemistry: Carbocation Intermediate


7/25


8/25

8

Acid-Catalyzed Hydration: Markovnikov Addition Mechanism and Source of Regioselectivity

Recall: Addition vs. Elimination: A Thermodynamic Perspective Acid-Catalyzed Hydration

Le Chateliers Principle:

If a chemical system at equilibrium experiences a change in concentration,

temperature, volume, or partial pressure, thenthe equilibrium shifts to

counteract the imposed change and a new equilibrium is established.

Acid-Catalyzed Hydration: Stereochemistry Observation Recall Stereochemistry: Carbocation Intermediate


9/25


10/25


11/25

11

Oxymercuration Demercuration

OxymercurationDemercuration:

1. Markovnikov Addition

2. Not readily undergo carbocation rearrangements

9.6 HydroborationOxidation

1. Regioselectivity: Anti-Markovnikov Addition

2. Stereospecificity: Syn Addition

9.6 HydroborationOxidation



HydroborationOxidation: Boron Isoelectronic to Carbocation

CH3+

BH3

The BH3molecule is similar to a carbocation but not as reactive,

because it does not carry a formal charge.

Diborane

The BH3molecule is similar to a carbocation but not as reactive, because it does

not carry a formal charge. But,

because of their broken octet, BH3molecules is still reactive and undergo

intermolecular resonance to help fulfill their octets.

Diborane

The BH3molecule is similar to a carbocation but not as reactive, because it does

not carry a formal charge. But,

because of their broken octet, BH3molecules is still reactive and undergo

intermolecular resonance to help fulfill their octets.


12/25

12

Diborane: Three-center, Two-electron Bonds

Three-center, Two-electron Bonds

Stabilizing Borane: Diborane via Three-center, Two-electron Bonds

B

H

HHB

H

HH +

H

B

H

B

H

HH

H

Stabilizing Borane: Ate Complex HydroborationOxidation: Mechanism



HydroborationOxidation: Mechanism



HydroborationOxidation: Mechanism Electronic Considerations

1.Regioselectivity: Anti-Markovnikov Addition



13/25

13

HydroborationOxidation: Mechanism Steric Considerations



Oxidation Mechanism

Oxidation Mechanism Oxidation Mechanism

HydroborationOxidation: Stereochemistry HydroborationOxidation: Stereochemistry

to the board


14/25

14

9.7 Catalytic Hydrogenation Catalytic Hydrogenation: Stereospecificity

# The addition of H2across a C=C double bond:

# If a chirality center is formed, SYN addition is observed.

Catalytic Hydrogenation: The Role of the Catalyst Catalytic Hydrogenation: Catalysts Pt, Pd, Ni

Catalytic Hydrogenation: Catalysts Pt, Pd, Ni Yes, IRONMAN!! Pt, Pd, NiPalladium Arc Reactor


15/25

15

Pt, Pd, NiPalladium Arc ReactorAre you kidding? Molecular Hydrogen Bound to Catalyst and Breaking HH bonds

Alkene Bound to Catalyst One H Atom Tranfer: Partially Reduced Intermediate

A Second H Transfer: Fully Reduced Alkene SynAddition Homogeneous Catalysts


16/25

16

Homogeneous Catalysts Homogeneous Catalysts

Skip this section: A Chiral Phosphine Ligand Skip this section: Asymmetric Hydrogenation

NH2

OH

O

NH2

OH

O

OH

HO HO

OH

L-dopafor

Parkinson's disease

(S)catalyst

H2

Skip this section: Asymmetric Hydrogenation 9.8 Halogenation and Halohydrin Formation


17/25

17

Halogenation

# Halogenation with Cl2and Br2 is generally effective, but halogenation

with I2is too slow, and halogenation with F2is too violent.

# Halogenation occurs with ANTI addition# Given the stereospecificity, is it likely to be a concerted or a multi-step

process?

Halogenation


with I2is too slow, and halogenation with F 2is too violent.

# Halogenation occurs with ANTI addition# Given the stereospecificity, is it likely to be a concerted or a multi-step

process?

Halogenation


with I2is too slow, and halogenation with F2is too violent.

# Halogenation occurs with ANTI addition.# Given the stereospecificity, is it likely to be a concerted or a multi-step

process?

Halogenation


with I2is too slow, and halogenation with F 2is too violent.

# Halogenation occurs with ANTI addition.# Given the stereospecificity, is it likely to be a concerted or a multi-step

process?

Halogenation: London Dispersion Forces (Induced Dipole Moment) Halogenation: London Dispersion Forces (Induced Dipole Moment)


18/25

18

Halogenation Should be Anti Addition??? Halogenation: Formation of Bromonium Ion

Halogenation: AntiAddition Halogenation: AntiAddition

Stereochemistry: E and Z alkenes Stereochemistry: E and Z alkenes


19/25

19

Halohydrin: Intercepting the Bromonium Intermediate

# Halohydrins are formed when halogens (Cl2or Br2) are added to an

alkene with WATER as the solvent.

# The bromonium ion forms from Br2+ alkene, and then it is attacked by

water.

Halohydrin: Intercepting the Bromonium Intermediate

Halohydrin Halohydrin: Regioselectivity

Halohydrin: Regioselectivity Halohydrin: Regioselectivity


20/25

20

Halohydrin: Regioselectivity Stable Transition State: Partial C+ Halohydrin: Regioselectivity Stable Transition State: Steric?

Water is a small

molecule that can

easily access the

more sterically

hindered site.

9.9 AntiDihydroxylation AntiDihydroxylation

AntiDihydroxylation Formation of an Epoxide

#

Replacing the relatively unstable OO single bond is the thermodynamicdriving force for this process.


21/25

21

Peroxy Acid (RCO3H)

# Replacing the relatively unstable OO single bond is the thermodynamic

driving force for this process.

Formation of an Epoxide

Water is apoornucleophile,

so theepoxide isactivated

with an acid.

Key Intermediates 9.10 SynDihydroxylaton: Concerted Process

SynDihydroxylaton SynDihydroxylaton# Because OsO4is expensive and toxic, conditions have been developed where

the OsO4is regenerated after reacting, so only catalytic amounts are needed.


22/25

22

SynDihydroxylaton

# MnO41-is similar to OsO4but more reactive.

# SYN dihydroxylation can be achieved with KMnO4but only under mild conditions

(cold temperatures).# Diols are often further oxidized by MnO4

1-, and MnO41- is reactive toward many

other functional groups as well.

# The synthetic utility of MnO41- is limited.

9.11 Oxidative Cleavage of Alkenes by Ozonolysis

Ozone exists as a resonance hybrid of two contributors Ozone exists as a resonance hybrid of two contributors

Common reducing agents include dimethyl sulfide (Me2S) and Zn.

Ozone exists as a resonance hybrid of two contributors

Common reducing agents include dimethyl sulfide (Me2S) and Zn.

Example of Ozonolysis

1) O3

2) DMS (Me2S)

H

O

O


23/25

23


1) O3

2) DMS (Me2S)

H

O

O


1) O3

2) DMS (Me2S)

H

O

O

Predicting the Products of an Addition Reaction

1.# What are the identities of the groups being added the double bond?

2.# What is the expected regioselectivity (Markovnikovor anti-Markovnikovaddition)?

3.# What is the expected stereospecificity (syn or antiaddition)?









9.13 Synthesis Strategies: One-Step Syntheses


24/25

24

Synthesis Strategies: One-Step Syntheses Synthesis Strategies: One-Step Syntheses

Synthesis Strategies: Multi-Step Syntheses

!#Changing the position of a leaving group

What are our weapons?

1. Substitution reactions

2. Elimination reactions

3. Addition reactions




















25/25

A Choice of Reagents




Hydrohalogenation






Synthesis Strategies: Multi-Step Syntheses Example Synthesis Strategies: Multi-Step Syntheses Example


!#Changing the position of a $bond

Date post:	04-Jun-2018
Category:	Documents
Upload:	acb4039
View:	228 times
Download:	0 times

Chapter 9 Addition Reactions of Alkenes '13 BW(1)

Documents