y REACTIONS OF ALKENES CHAPTER 05cook.chem.ndsu.nodak.edu/chem240/wp-content/... · y General...

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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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Structure and Bonding

• A double bond is sp2 hybridized (pi bond)

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Characteristic Reactions of Alkenes

• Addition Reactions - two or more molecules combine to form a new product

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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

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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

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Reaction Energy Diagrams

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Kinetics

• A multistepreaction path

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Common steps in reaction mechanism

• Protonation

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Common steps in reaction mechanism

• Deprotonation

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Chem

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Common steps in reaction mechanism

• Electrophile plus Nucleophile

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Chem

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Common steps in reaction mechanism

• Rearrangements of bonds

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Chem

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Common steps in reaction mechanism

• Bond breaking to make relatively stable ions

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Chem

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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

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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

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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

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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

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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

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Carbocation Stability

• Spread out the positive charge (more stable)

• inductive effects and hypercongujugation

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Mechanistic basis for Regioselectivity

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Markovnikov’s Rule - Restated

• When an unsymmetrically substituted alkene reacts with HX, protonation occurs to form the more stable carbocation.

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Addition of H2O - Hydration of Alkenes

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H3PO4 cat.OH

250 °C+ H2O

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Halogenation of Alkenes

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+C C C CX

XX X

+ Br BrBr

BrCHCl3

anti addition

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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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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

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BH3

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Chem

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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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Mechanism of Hydroboration

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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

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Addition of H2 - Hydrogenation

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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

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Catalysis

• A catalyst increases the rate of a reaction but is not changed in a reaction. It lowers the overall energy barrier.

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E

reaction progress

transition state ‡

G°

A

B

energybarrierto climb

catalyst - lowersoverall energy barrier

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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

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Hydrogenation Stereochemistry

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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

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Heat of Hydrogenation

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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

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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

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Structure of Alkynes

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C CH H

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Hydrogenation of Alkynes

• Similar to alkenes, alkynes can be reduced by catalytic hydrogenation

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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.

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NaNH2H NaH3C Br

δ+ δ-

CH3

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Organic Synthesis

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NaNH2H

Br

LiNH3

H HNaNH2

BrH

NaNH2

Br

1)

2)

2) 2)

1) 1)