ch8orgo

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Chapter 6Addition Reactions of

AlkAlkenes

• The characteristic reaction of alkenes is addition to the double bond.

• Reverse of elimination, A—B represents: H—H; hydrogenation reaction H—OH; hydration reaction H—X; hydrohalogenation reaction

Reactions of Alkenes

; y g X—X; halogenation reaction

• Also epoxidation, formation of a 3-membered ring with an O

+ A—BC C A C C B

+ H—H

H C C

H H

H H

H

C C

H

H

H

H

Hydrogenation of Ethylene

• exothermic H° = –136 kJ/mol

• catalyzed by finely divided Pt, Pd, Rh, Ni

Example

H2, Pt

CH2H3C

H3C

(73%)

CH3

HH3C

H3C

• What three alkenes yield 2-methylbutane on catalytic hydrogenation?

Problem 6.1

H2, Pt

• can be used to measure relative stability of isomeric alkenes

• correlation with structure is same as when heats of combustion are measured

Heats of Hydrogenation

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H H HC C

A

B

X

Y

Mechanism of Catalytic Hydrogenation

Interaction with the metal catalyst is critical Hydrogen bonds to the surface of the metal

H

H H


• Alkene uses its electrons to bond to the surface of the metal; the other carbon bonds to H

H HA

BXY

H HHCC

H


H HH

CC

AB

XY

H HHH

Heats of Hydrogenation of Isomers

CH3CH2CH2CH3

119115

• Ethylene 136

• Monosubstituted 125-126

• cis-Disubstituted 117-119

Heats of Hydrogenation (kJ/mol)

• trans-Disubstituted 114-115

• Terminally disubstituted 116-117

• Trisubstituted 112

• Tetrasubstituted 110

Match each alkene of Problem 6.1 with its correctheat of hydrogenation.

126 kJ/molhighest heat ofhydrogenation;least stable isomer

Problem 6.2

118 kJ/mol

112 kJ/mollowest heat ofhydrogenation;most stable isomer

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Stereochemistry of Alkene Hydrogenation

• There are two spatial stereochemical aspects of alkene hydrogenation: syn addition of both H atoms to double bond hydrogenation is stereoselective, corresponding to

addition to less crowded face of double bond

• A reaction in which a single starting material• A reaction in which a single starting materialcan give two or more stereoisomeric productsbut yields one of them in greater amounts thanthe other (or even to the exclusion of the other)is said to be stereoselective.

syn addition anti addition

syn Addition versus anti Addition

Example of syn-Addition

H2, PtCO2CH3

CO2CH3

CO2CH3

CO2CH3

H

(100%)CO2CH3

H

A single starting material can give two or morestereoisomeric products, but gives one of themin greater amounts than any other.

Stereoselective reaction

H3C CH3

H3C

H

H cat

Both products correspond to syn addition of H2.

Example of A Stereoselective Reaction

H2, cat

CH3H3C

H3C

HH

H

H3C

CH3H3C

H

H

H

But only this one is formed.

H3C CH3

H3C

H

Top face of doublebond blocked by

Example of A Stereoselective

Reaction

H2, catbond blocked bythis methyl group

H2 adds to bottom face of double bond.

HCH3

H3C

H3CH

H

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General Equation for Electrophilic Addition

• The electrons of the alkene can be used to form bonds in a reaction – it is a nucleophile.

• Alkenes “can add on” species that are electrophiles

• An hydrogen halide (acid) is an electrophilic

+ E—Y –

C C E YC C

y g ( ) pspecies – the electrons can be used to bond to H+

A Hydrogen Halide is an Electrophile

(E—Y is H—X in the reaction below)

+ H—X –

C C H XC C

CH3CH2 CH2CH3

H H

CHCl3, -30°CC CHBr

• Electrophilic addition of hydrogen halides to alkenes proceeds by rate-determining formation of a carbocationintermediate.

• Electrons flow from the

Mechanism

• Electrons flow from the system of the alkene (electron rich) toward the positively polarized proton of the hydrogen halide.

Mechanism

..

..:X:–

HC C+

XH

C C

..

..:

HC C....X:

Markovnikov’s Rule: • When an unsymmetrically substituted alkene

reacts with a hydrogen halide, the hydrogen adds to the carbon that has the greater number f h d b i d h h l dd

Regioselectivity of Hydrogen Halide Addition

of hydrogen substituents, and the halogen adds to the carbon that has the fewer hydrogen substituents.

Markovnikov's Rule

acetic acidBr

CH3CH2CHCH2HCH2CH3CH2CHHBr

(80%)

CH H

CH3

CH CC CHBr

CH3 H

CH2HCH3 C

Br (90%)

C Cacetic acid

CH3 H

(100%)HCl

CH3

CH3

Cl0ºC

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CH3CH2CH2—CH2

+

primary carbocation is less stable: not formed

Mechanistic Basis for Markovnikov's Rule

• Protonation of double bond occurs in the direction that gives more stable of two possible carbocations.

Br

CH3CH2CHCH3CH2CH3CH2CH

HBr

CH3CH2CH—CH3 + Br –+


(CH3)2CH—CH2

+



(CH3)2C—CH3 + Br –+

HBr

( 3)2 3

CH3

CH3

CH3 C

Br

C C

CH3

CH3

H

H


H H

CH3

H

+

H

secondary carbocation is less stable: not formed

HClH

CH3

CH3

Cl

CH3+ Cl–

HCl, 0°C

CH3CHCH(CH3)2

+

H2C CHCH(CH3)2

+CH CHC(CH )

H

Carbocation Rearrangements Can Occur

CH3CHCH(CH3)2 CH3CHC(CH3)2

CH3CHCH(CH3)2

Cl(40%)

CH3CH2C(CH3)2

Cl(60%)

CH3CHCH3

OSO2OH

CH3CH CH2HOSO2OH

Isopropylh d lf t

Addition of H2SO4 to Alkenes

• follows Markovnikov's rule • yields an alkyl hydrogen sulfate

hydrogen sulfate

slow

+CH3CH CH2 H..

SO2OH..O

+ ..–

Mechanism

CH3CH CH3 + SO2OH..:O

fastCH3CHCH3

OSO2OH..:

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Alkyl Hydrogen Sulfates Undergo Hydrolysis in Hot Water, Form Alcohols

H—OHCH3CHCH3

O SO2OH heat

HO SO OH+CH CHCH

+

HO—SO2OH+CH3CHCH3

O H

1. H2SO4

2. H2O, heat (75%)

OH

• not all alkenes yield alkyl hydrogen sulfateson reaction with sulfuric acid

• these do: H2C=CH2, RCH=CH2, RCH=CHR' • these don't: R2C=CH2, R2C=CHR, R2C=CR2

Reaction Selectivity

• The presence of one H on each carbon is critical to the addition of H2SO4.

• Does that mean the other alkenes cannot be hydrated to alcohols?

Acid-Catalyzed Hydration of Alkenes

H—OHC C + OHC CH

• reaction is acid catalyzed; typical hydration medium is 50% H2SO4-50% H2O

(90%)

50% H2SO4

50% H2O

H3C

H3C CH3

H

C C

OH

C CH2CH3CH3

CH3

• Regioselectivity follows Markovnikov’s rule

Follows Markovnikov's Rule

50% H2SO4

50% H2O(80%)

OH

CH3CH2

• involves a carbocation intermediate • is the reverse of acid-catalyzed dehydration of

+ H2OH+

H3C

H3C

C CH2

OH

C CH3CH3

CH3

is the reverse of acid catalyzed dehydration of alcohols to alkenes

Step (1) Protonation of double bond

H3C

C CH2+ O

+H

H

:

Mechanism

H3C Hslow

+H3C

H3C

C CH3

H

+ O

H

::

Step (2) Capture of carbocation by water

+H3C

H3C

C CH3

H

+ O

H

::

fast

Mechanism

H3C H

+

H

O

H

:C

CH3

CH3

CH3

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Step (3) Deprotonation of oxonium ion

+

H

O

H

:C

CH3

CH

CH3 O

H

::

H

+

Mechanism

HCH3Hfast

O+

H

H

H

:+

H

O:C

CH3

CH3

CH3

..

• ethylene CH2=CH2 1.0

• propene CH3CH=CH2 1.6 x 106

• 2-methylpropene (CH3)2C=CH2 2.5 x 1011

The more stable the carbocation the faster it is

Relative Rates – Acid Catalyzed Hydration

• The more stable the carbocation, the faster it is formed, and the faster the reaction rate.

+ H2OH+

H3C

H3C

C CH2

OH

C CH3CH3

CH3

Principle of Microscopic Reversibility

• In an equilibrium process, the same intermediates and transition states are encountered in the forward direction and the reverse, but in the opposite order.

• Let’s examine the kinetics and thermodynamics…

Hydration-Dehydration Equilibrium

• How do we control the position of the equilibrium and maximize the product?

Le Chatelier’s Principle

• A system at equilibrium adjusts so to minimize any stress applied to it.

• For hydration-dehydration, the key stress is water.• Adding water pushes the equilibrium toward

product (alcohol); removing water pushes toward reactant (alkene)reactant (alkene)

• At constant T and P, reactions proceed to decrease free energy (G, spontaneous reaction)

• The sign of G is always positive, but G can be positive or negative.

• G = Gproduct – Greactant; spontaneous when G < 0

Le Chatelier’s Principle

For a reversible reaction: aA + bB cC + dD The relationship between G and Go is:

At equilibrium, G = 0 and the following becomes true:

Substituting Keq into the previous equation gives: Go = - RT lnKeq

Reactions for Go positive are endergonic and forGo negative are exergonic.

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Synthesis: Suppose you wanted to prepare 1-decanol from 1-decene?

Hydroboration Oxidation of Alkenes

• Needed: a method for hydration of alkenes with a regioselectivity opposite to Markovnikov's rule.

OH

• Two-step reaction sequence called hydroboration-oxidation converts alkenes to alcohols with a regiochemistry opposite to Markovnikov's rule.

Synthesis

1 h droboration1. hydroboration

2. oxidation

OH

Hydroboration Step

+ H—BH2C C H BH2C C

• Hydroboration can be viewed as the addition of borane (BH3) to the double bond.

• But BH3 is not the reagent actually used.

Hydroboration Step

Hydroboration reagents:

H2B

H

H

BH2

Diborane (B2H6)normally used in an ether-like solventcalled “diglyme”g y

Borane-tetrahydrofurancomplex (H3B-THF)+O

BH3–

••

Oxidation Step

H2O2, HO–

H BH2C C H OHC C

Organoborane formed in the hydroborationstep is oxidized with hydrogen peroxide.

Examples

1. B2H6, diglyme

2. H2O2, HO–

(93%)OH

( %)

1. B2H6, diglyme

2. H2O2, HO–

(82%)

OH

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• hydration of alkenes • regioselectivity opposite to Markovnikov's rule • no rearrangement • stereospecific syn addition

Features of Hydroboration-Oxidation

(98%)

H3C

H3C

CH3

H

C C1. H3B-THF

2. H2O2, HO–

H

C CCH3

CH3

CH3

H OH

• H and OH become attached to same face of double bond

syn Addition

1. B2H6, diglymeCH3CH3

H

only product is trans-2-methylcyclopentanol(86%) yield

2. H2O2, NaOH

H HHO

Mechanism: 1-Methylcyclopentene + BH3

syn addition of H and B to double bond B adds to less substituted carbon

Organoborane Intermediate

Add hydrogen peroxide; OH replaces B on same side

Trans-2-methylcyclohexanol

+ X2 X XC CC C

Addition of Halogens to Alkenes

• Electrophilic addition to double bond forms a ect op c add t o to doub e bo d o s avicinal dihalide.

• limited to Cl2 and Br2

• F2 addition proceeds with explosive violence.• I2 addition is endothermic: vicinal diiodides

dissociate to an alkene and I2.

Examples

CHCl30ºC

CHCH(CH3)2CH3CHBr2

CH3CHCHCH(CH3)2

(100%)Br Br

The stereochemistry of halogen addition is anti.

H

trans-1,2-Dibromocyclopentane80% yield; only product

Br2H

H

Br

Br

H

H

• A halonium ion is the key intermediate in the halogenation of alkenes

• Br2 is not polar, but it is polarizable. • two steps

(1) formation of bromonium ion

Mechanism is electrophilic addition

(1) formation of bromonium ion(2) nucleophilic attack on bromonium ion by bromide

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ethylene H2C=CH2 1

propene CH3CH=CH2 61

2-methylpropene (CH ) C=CH 5400

Relative Rates of Bromination

2-methylpropene (CH3)2C=CH2 5400

2,3-dimethyl-2-butene (CH3)2C=C(CH3)2 920,000

More highly substituted double bonds react faster.Alkyl groups on the double bond make it more “electron rich.”

H2C CH2 BrCH2CH2Br+ Br2

?

+C C+ ..

: :Br–

Mechanism?

C C

Br: :..

: :..Br

• No obvious explanation for anti addition provided by this mechanism.

H2C CH2 BrCH2CH2Br+ Br2

+C C..

: :Br–

Mechanism

Cyclic bromonium ion

+C C

Br: :+

: :..Br Mutual polarization of

BrBr

Electrons flow from

BrBr

–

++

Formation of Bromonium Ion

Mutual polarization of electron distributions of Br2 and alkene

Electrons flow from alkene toward Br2.

electrons of alkene displace Br– from Br.

Br–

Br+

Stereochemistry of Anti Addition

+

–Br: :..

..

Br..

..

..

Br

Br..

:..

..::

• Attack of Br– from side opposite C—Br bond of bromonium ion gives anti addition.

Cyclopentene +Br2

B i i trans-Stereochemistry–Bromonium ion

––

––Bromide ion attacks the bromonium ion from side opposite carbon-bromine bond; anti addition – trans-1,2-dibromo-cyclopentane is the only product

trans-Stereochemistry in vicinal dibromide

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Anti Addition of Bromine to 2-Butene

Br2R

S R

Smeso

• Anti addition to trans 2 butene gives the meso

Br2R

R

50%

+

S

S

50%

• Anti addition to trans-2-butene gives the mesocompound; cis-2-butene gives a racemic mixture

+ X2 X XC CC C

• Alkenes react with X2 to form vicinal dihalides. • Alkenes react with X2 in water to give vicinal

Vicinal Halohydrins from Alkenes

+ H2O OH

+ H—X

+ X2 X C CC C

2 ghalohydrins.

Examples

H2C CH2 BrCH2CH2OH+ Br2

H2O

(70%)

Cl2H OH

H

trans-2-chlorocyclopentanol anti addition: only product

H2OH

H

OH

Cl H

Mechanism

+Br..

..

:..O

Br..

:..

..O

+

• Bromonium ion is the intermediate• Water is nucleophile that attacks

bromonium ion.

Cyclopentene + Cl2

Chloronium ion

Water attacks chloronium ion from side opposite carbon-chlorine bond.

trans-Stereochemistry in oxonium ion

(77%)

H3C

C CH2

H3C

CH3

OH

C CH2BrCH3

Br2

H2O

Regioselectivity

• Markovnikov's rule is applied to halohydrin formation: the halogen adds to the carbon having the greater number of hydrogens.

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H

OH ..

H

O H..

H3C CH2

H3C C H3C CH2

H3C

C

Explanation

• Transition state for attack of water on bromonium ion has carbocation character;

• more stable transition state (left) has positive charge on more highly substituted carbon.

Br: :

Br: :

acetic acidBr

CH3CH2CHCH3CH2CH3CH2CHHBr

(80%)

Markovnikov's Rule

• Can HBr addition take place such that Br is bonded to C1 in this molecule? Yesbonded to C1 in this molecule? Yes.

• When HBr is added to an alkene in the presence of peroxides, an anti-Markovnikov addition is observed – called the “peroxide effect”

• The reaction occurs via the free radical mechanism…

C C C C

CH2CH3CH2CH

HBr

Addition of HBr to 1-Butene

CH3CH2CH2CH2Br

only product inabsence of peroxides

only product when peroxides added to reaction mixture

Br

CH3CH2CHCH3 C C C C

CH2CH3CH2CH

HBraddition opposite to Markovnikov's rule occurs with

Addition of HBr to 1-Butene

CH3CH2CH2CH2Bronly product when peroxides added to reaction mixture

rule – occurs with HBr (not HCl or HI)

• Addition of HBr with a regiochemistry opposite to

+ HBrh

CH2

(60%)

CH2Br

H

Photochemical Addition of HBr

Markovnikov's rule can also occur when initiated with light with or without added peroxides.

• Addition of HBr opposite to Markovnikov's rule proceeds by a free-radical chain mechanism.

• Initiation steps:

Mechanism

..

..O RR O....

O .R ....

..

..O R.+

+O .R ....

+OR ....

H . Br..

..:..BrH

..:

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Propagation steps:

Mechanism

.

..CH3CH2CH CH2 Br:..

+CH3CH2CH CH2 ... Br

..:

..

..

+....

.CH3CH2CH CH2 Br:..

..

H Br:

....CH3CH2CH2CH2Br:

. Br:

H2C CH2 H2C CHCH3

Epoxides

• are examples of heterocyclic compounds • three-membered rings that contain oxygen

ethylene oxide propylene oxide

O O

H3C

Epoxide Nomenclature

• Substitutive nomenclature: named as epoxy-substituted alkanes.

• “epoxy” precedes name of alkane • 1,2-epoxypropane 2-methyl-2,3-epoxybutane

H2C CHCH3

O

CHCH3

O

C

3C

H3C1

2 3 4

O

RCOOH

Epoxidation of Alkenes

peroxy acid

C C +

O

+ CH3COOH

(52%)+ CH3COH

O

O

Stereochemistry of Epoxidation

C C +

syn addition

RCOOH

O

CC

O

+

O

RCOH

syn addition

Problem 6.23 Give the structure of the alkene, including stereochemistry, that you would choose as the starting material in a preparation of synthetic disparlure (6.22, nomenclature).

peroxy acid

OH H

H H

disparlure

(cis-2-Methyl-7,8-epoxyoctadecane)

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• ethylene H2C=CH2 1

• propene CH3CH=CH2 22

• 2-methylpropene (CH3)2C=CH2 484

2 th l 2 b t (CH ) C CHCH 6526

Relative Rates of Epoxidation

• 2-methyl-2-butene (CH3)2C=CHCH3 6526

• More highly substituted double bonds react faster.Alkyl groups on the double bond make it more “electron rich.”

Mechanism of Epoxidation

• The peroxy acid reacts with the alkene to form a bicyclic ring intermediate.

• The ring dissociates to the epoxide. • Additon is syn; alkene stereochemistry is retained.

syn addition to trans-2-butene gives racemic

Epoxidation of 2-Butene

RCO3H RR S

S

50%

+

50%

syn addition to trans 2 butene gives racemic mixture; cis-2-butene gives the meso diastereomer

R

S R

S

RCO3H

meso

• Ozonolysis has both synthetic and analytical applications.

• Used for synthesis of aldehydes and ketones• Used for identification of substituents on the

double bond of an alkene• First step is the reaction of the alkene with ozone.

Ozonolysis of Alkenes

First step is the reaction of the alkene with ozone. • The product is an ozonide.

+ O3 CCO

O O

C C

• Second step is reduction of the ozonide. • Two aldehydes, two ketones, or an aldehyde

and a ketone are formed.

Ozonolysis of Alkenes

+ O3CC

O

C C

C O CO+ H2O, Zn

O O

• The ozonide can also be reduced with dimethyl sulfide.

1. O3

CH3

CH2CH3H

C C

CH2CH3

Example

1. O32. H2O, Zn

(38%) (57%)

CH2CH3

CO

CH2CH3

C O

CH3

H

+

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Polymerization of Alkenes

• Alkenes react with alkenes to form polymers by three mechanisms: cationic polymerization free-radical polymerization coordination polymerization

H2SO4

monomer(C4H8)

Dimerization of 2-methylpropene

(CH3)2C CH2

Cationic Polymerization

H2SO4

+

two dimers(C8H16)

CH3CCH

CH3

CH3

C(CH3)2 CH3CCH2C

CH3

CH3

CH2

CH3

Step (1) Protonation of double bond

H3C

H3C

C CH2+ O

+H

H

H

:

l

Mechanism of Dimerization

H3C Hslow

+H3C

H3C

C CH3

H

+ O

H

::

+

CH3

H2C C

CH3

CH3C

CH3

+

CH3


CH3CCH2C

CH3

CH3

CH3

CH3

+

The carbocation formed can lose a proton to form a dimer or react with another molecule of alkene to form a “polymer”


CH3CCH

CH3

CH3

C(CH3)2 CH3CCH2C

CH3

CH3

CH2

CH3

Two constitutional

CH3CCH2C

CH3

CH3

CH3

CH3

+

Two constitutional isomers are the products

200 ºC2000 atm

O2

peroxides

H2C CH2

CH CH CH CH CH CH CH

Free-Radical Polymerization of Ethylene

polyethylene

CH2 CH2 CH2 CH2 CH2 CH2 CH2

• Peroxides are used for free radical reactions as initiators;

• they readily form hydroxy or alkoxy radicals…

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..RO: •

..RO..

H2C CH2H2C CH2

•

CHH C

Mechanism

CH2H2C

H2C CH2

H2C CH2•

..RO:

The process continues…

H2C CHCH3

Free-Radical Polymerization of Propene

polypropylene

CH CH CHCHCHCH CH

CH3 CH3 CH3 CH3 CH3 CH3 CH3

•..

RO..

H2C CHCH3H2C CHCH3•

..RO:

CHCHH C

Mechanism

CHCH3H2C

H2C CHCH3

H2C CHCH3•

..RO:

As noted before, the process continues…

Likewise...

• H2C=CHCl polyvinyl chloride• H2C=CHC6H5 polystyrene• F2C=CF2 Teflon®