chapter 8 - wade_cps_students

8/13/2019 Chapter 8 - Wade_CPS_Students

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Chapter 8

Reactions of Alkenes

Department of Chemistry

Introduct ion to General and

Organic Chemis try III


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Chapter 8 2

Reactivity of C=C

Electrons in pi bond are loosely held.

Electrophiles are attracted to the pi

electrons. Carbocation intermediate forms.

Nucleophile adds to the carbocation.

Net result is additionto the double bond.=>


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Chapter 8 3

Electrophilic Addition


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Chapter 8 4

Carbocation Formation


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Chapter 8 5

Types of Additions


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Chapter 8 6

Addition of HBr (1)

Protonation of double bond yields the most stablecarbocation. Positive charge goes to the carbon

that was notprotonated.


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Chapter 8 7

Addition of HBr (2)


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Chapter 8 8

Regiospecificity

Markovnikovs Rule: The proton of an

acid adds to the carbon of the double

bond that already has the most Hs. Rich

get richer.

More general Markovnikovs Rule: In an

electrophilic addition to an alkene, the

electrophile adds in such a way as to

form the most stable intermediate.

HCl, HBr, and HI add to alkenes to form

Markovnikov products.


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Chapter 8 9

Examples:


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Chapter 8 10

Cl

HCl

In some cases, product(s) resulting

From rearrangementof the initially

Formed carbocation is observed!

2oR+


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Chapter 8 12

Free-Radical

Addition of HBr

In the presence of peroxides (ROOR),

HBradds to an alkene to form the anti-

Markovnikov product.

OnlyHBr has the right bond energy.

HCl bond is too strong.

HI bond tends to break heterolytically toform ions.

=>


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Chapter 8 13

Mechanism

Peroxide O-O bond breaks easily to

form free radicals.

Hydrogen is abstracted from HBr.

Initiation:


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Chapter 8

Bromine adds to the double bond.

Hydrogen is abstracted from HBr.

Propagation Steps:


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Chapter 8 15

Why Anti-Markovnikov ?

Tertiary radical is more stable, so thatintermediate is formed faster.

CH3 C

CH3

CH CH3 Br+


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Chapter 8 16

Hydration of Alkenes

Reverse of dehydration of alcohols.

Use very dilute solutions of H2SO4or

H3PO4to drive equilibrium towardhydration.

=>

C C + H2OH+

C

H

C

OH

alkene

alcohol


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17

Mechanism of Hydration


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Chapter 8 18

Orientation for Hydration

Markovnikov product is formed.

CH3 C

CH3

CH CH3 OH H

H

++


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Chapter 8 19

Example:


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Chapter 8 20

Indirect Hydration

Oxymercuration-DemercurationMarkovnikov product formed

Anti addition of H-OH

No rearrangements

Hydroboration-OxidationAnti-Markovnikov product formed

Syn addition of H-OH


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Chapter 8 21

Oxymercuration (1)

Reagent is mercury(II) acetateHg(OAc)2whichdissociates slightly to form +Hg(OAc).

+Hg(OAc)is the electrophile that attacks thepi bond.

The intermediate is a cyclic mercurinium ion,a three-membered ring system with a positivecharge.

Water approaches the mercurinium ion from

the side opposite the ring (anti addition). Water adds to the more substituted carbon to

form the Markovnikov product.


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Chapter 8 22

Oxymercuration (2)


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Chapter 8 23

Demercuration

Sodium borohydride, NaBH4, a reducing agent,replaces the mercury with hydrogen.

C

O

C

Hg

H

OAc

4 4 C

O

C

H

H

+ NaBH4 + 4 OH

_+ NaB(OH)4

+ 4 Hg + 4 OAc_

=>


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Chapter 8 24

Example:


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Chapter 8 26

Alkoxymercuration -

Demercuration

If the nucleophile is an alcohol, ROH, instead ofwater, HOH, the product is an ether.

C C

(1) Hg(OAc)2,

CH3OH

C

O

C

Hg(OAc)

H3C

(2) NaBH4C

O

C

H3C

H

=>


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Chapter 8 27

CH3 CH3

OEt

H

CH3

OEt

1) Hg(OAc)2,EtOH

2) NaBH4

CH3

OEt

Example:

Not Formed !!


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Chapter 8 28

Hydroboration

Borane, BH3, adds a hydrogen to themost substituted carbonin the doublebond.

The alkylborane is then oxidized to the

alcohol which is the anti-Mkvproduct.

C C

(1) BH3

CH

CBH2

(2) H2O2, OH-

CH

COH

=>


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Chapter 8 29

Borane Reagent

Borane exists as a dimer, B2H6, in equilibriumwith its monomer.

Borane is a toxic, flammable, explosive gas.

Safe when complexed (Lewis acid-base) with

tetrahydrofuran, THF.

THF THF .BH3

O B2H6 O+ B-

H

H

H

+2 2 =>


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Chapter 8 30

Mechanism The electron-deficient borane (remember Boron does

not have a filled octed) adds tothe least-substituted carbon. The other carbon acquires a positive charge. H adds to adjacent C on the same side (syn).

=>


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Chapter 8 31

Note: Borane prefers the least-substituted carbondue to steric hindrance as well as chargedistribution.

C C

H3C

H3C

H

H + BH3 B

CC H

CH3

H3C

H

H

C

CH

HH

CH3

CH3

C

C

H

H

H3C

CH3

H

3

Note: The Borane molecule actually reacts

with three equivalents of the alkene forming a

trialkyl borane.


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Chapter 8 32

Oxidation to Alcohol

Oxidation of the alkyl borane with basichydrogen peroxide produces the alcohol.

Orientation is anti-Markovnikov.

CH3 C

CH3

H

C

H

H

B

H2O2, NaOH

H2O

CH3 C

CH3

H

C

H

H

OH


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Chapter 8 33

Example:

Predict the product when the given alkene

reacts with borane in THF, followed by

oxidation with basic hydrogen peroxide.

CH3

D

(1)

(2)

BH3, THF

H2O2, OH-


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Chapter 8 34

Hydrogenation

Alkene + H2

Alkane Catalyst required, usually Pt, Pd, orNi.

Finely divided metal, heterogeneous

Syn addition


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Chapter 8 35

Addition of Halogens

Cl2, Br2, and sometimes I2add to adouble bond to form a vicinal dihalide.

Anti additionobserved, so reaction isstereospecific.

CC + Br2 C C

Br

Br

=>


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Chapter 8 36

Mechanism


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Chapter 8 37

Example:


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Chapter 8 38

Stereospecific Reactions

A reaction is stereospecific when a

Specific stereoisomer reacts to give

Only one stereoisomer of the product.


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Chapter 8 39

Examples:

=>


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Chapter 8 40

Test for Unsaturation

Add Br2in CCl4(dark, red-brown color) to

an alkene (in the presence of light).

The colorquickly disappearsas the

bromine adds to the double bond.

Decolorizing bromineis a chemical test

for the presence of a double bond.

=>


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Chapter 8 41

Formation of Halohydrin

If a halogen is added in the presence ofwater, a halohydrin is formed.

Water is the nucleophile, instead of

halide. Product is Markovnikov and anti.

CC

Br

H2O

CC

Br

OH H

H2O

CC

Br

OH

+ H3O+

=>


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Chapter 8 42

General Mechanism:

R i ifi it


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Chapter 8 43

Regiospecificity

The most highly substituted carbon has

the most positive charge, so nucleophile

attacks there.


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Chapter 8 44

Example:

Predict the product when the given alkenereacts with chlorine in water.

CH3

D

Cl2, H2O

=>

OHCH3

D

Cl


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Chapter 8 45

BrOH

Br

OH

OH

Br

Br2H2O

Not Formed !!

Example:


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Chapter 8 46

Epoxidation

Alkenes react with a peroxyacid to forman epoxide (also called oxirane).

Usual reagent is peroxybenzoic acid.

CC + R C

O

O O H CC

O

R C

O

O H+

O

OO

H GeneralPeroxyacid:

RCO3H

Epoxide


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Chapter 8 47

Mechanism

Reaction takes place in a single step(concerted). Bond cleavage and bond

formation occurring simultaneously.

OC

O

R

H

C

C

OO

H

OC

O

RC

C

+

E id


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Chapter 8 48

Epoxide

Stereochemistry

Since there is no opportunity for rotation

around the double-bonded carbons, cis

or trans stereochemistry is maintained.

CCCH3 CH3

H H Ph C

O

O O H

CCCH3 CH3

H HO

Epoxidation is Stereospecific!!


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Chapter 8 49

Examples:

H

H

RCO3H

CHCl 3O

H

H

CH3

H

RCO3H

CHCl 3O

H

CH3

O

H

CH3

+

Enantiomers


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Chapter 8 50

Opening the Epoxide Ring

Acid catalyzed.

Water attacks the protonated epoxide.

Trans diol(Glycol) is formed.


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Chapter 8 51

One-Step Synthesis of Diols

To synthesize the glycol withoutisolating the epoxide, use aqueous

peroxyacetic acid or peroxyformic acid.

The reaction is stereospecific.

CH3COOH

O

OH

H

OH

H

=>H2O

S H d l ti


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Chapter 8 52

Syn Hydroxylation

of Alkenes

Alkene is converted to a cis-1,2-diol.

Two reagents commonly used:

Osmium tetroxide(OsO4expensive!),followed by hydrogen peroxide or

Cold, dilute aqueous Potassium

permanganate ( KMnO4), followed by

hydrolysis with base

=>

Example:


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Chapter 8 53

Example:

H

H

OH

OHH

HKMnO4

NaOH

0oC

OH

OH

NOT FORMED !!


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Chapter 8 54

Mechanism with OsO4

Concertedsynaddition of two oxygens toform a cyclic ester.

C

COs

O O

OO

C

C

O O

OO

Os

C

C

OH

OH + OsO4

H2O2

=>


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Chapter 8 55

Stereospecificity

If a chiral carbon is formed, only onestereoisomer will be produced (or a pair

of enantiomers).

C

C

CH2CH3

H CH2CH3

C

C

CH2CH3

CH2CH3

OH

OH

H

HH2O2

H

(2)

(1)OsO4

cis-3-hexene meso-3,4-hexanedi


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Chapter 8 56

H CH3

CH3HKMnO4

NaOH

0oC

HCH3

OH

CH3

H OH

H3C H

CH3HKMnO4

NaOH

0oC

H

CH3

OH

CH3

HO H

H

CH3

HO

CH3

OHH+

The 2-Butenes

Rationale


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Chapter 8 57

H3C

H

CH3

HKMnO4

NaOH

0oC

CH3

H

H3C

H

OH

OH

HHO

H

OH

CH3

CH3

H

CH3

OH

CH3

HO H

Rationale:

Rotation


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Chapter 8 58

Oxidative Cleavage

Both the pi and sigma bonds break. C=C becomes two C=O.

Ketones, carboxylic acid or even CO2

may be obtained. Two methods:

Warm or concentrated or acidic KMnO4.

Ozonolysis

Used to determine the position of adouble bond in an unknown.


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Chapter 8 59

Cleavage with KMnO4

Permanganate is a strong oxidizing

agent.

Glycol initially formed is further oxidized.

Disubstituted carbons become ketones.

Monosubstituted carbons become

carboxylic acids.

Terminal =CH2becomes CO2.

=>


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Chapter 8 60

Permanganate Cleavage

E l


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Chapter 8 61

Example

CCCH3 CH3

H CH3 KMnO4

(warm, conc.)C C

CH3

CH3

OHOH

H3C

H

C

O

H3C

H

C

CH3

CH3

O

C

O

H3COH

+


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Chapter 8 62

HO2C CO2H

1) KMnO4, Conc

2) H3O+

CH3

1)KMnO4, NaOH heat

2) H3O+ CO2H

O

Examples:


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Chapter 8 63

Ozonolysis

Reaction with ozone forms an ozonide.

Ozonides are not isolated, but are

treated with a mild reducing agent like

Znor dimethyl sulfide, (Me2S).

Milder oxidation than permanganate.

Products formed are ketones or

aldehydes.=>

Mechanism


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Chapter 8 64

Mechanism


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Chapter 8 65

Example

CCCH3 CH3

H CH3 O3C

H3C

H

O O

CCH3

CH3

O

Ozonide

+

(CH3)2S

CH3C

H

O C

CH3

CH3O CH3 S

O

CH3

DMSO


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Chapter 8 66

CH3 CHO

O1) O3, CH2Cl2

2) Zn, H2

O

1) O3, CH2Cl2

2) Zn, H2O

O

HCHO+

Example

Question


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Chapter 8 67

An alkene adds hydrogen in the presence of a catalyst

to give 3,4-dimethylhexane. Ozonolysis of the alkene

followed by treatment with zinc and acetic acid gives asingle organic product. The alkene is:

A.

B.

C.

D.

Q


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End of Chapter 8

chapter 8 - wade_cps_students

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