Chapter6-6-Chem207

Chem 207 B. R. Kaafarani 1

+ A—BC C A C C B

The characteristic reaction of alkenes isaddition to the double bond.

Chapter 6Reactions of Alkenes:

Addition Reactions

+ H—H

C C H C C

Exothermic ∆H° = –136 kJ/mol.

Catalyzed by finely divided Pt, Pd, Rh, Ni.

The metals are insoluble in solvents used in this reaction:heteregeneous reactions. Reaction occurs at the interfaceof the two phases.

6.1. Hydrogenation of Alkenes Hydrogenation of Ethylene

H2, Pt

CH2H3C

Example

H2, Pt

Problem 6.1

What three alkenes yield 2-methylbutane on catalytic hydrogenation?

Mechanism of Catalytic Hydrogenation: Mechanism 6.1

Can be used to measure relative stabilityof isomeric alkenes.

Correlation with structure is same aswhen heats of combustion are measured.

6.2. Heats of Hydrogenation

CH3CH2CH2CH3

119115

Heats of Hydrogenation of Isomers

Ethylene 136

Monosubstituted 125-126

cis-Disubstituted 117-119

trans-Disubstituted 114-115

Terminally disubstituted 116-117

Trisubstituted 112

Tetrasubstituted 110

Heats of Hydrogenation (KJ/mol)

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

126 kJ/mol

118 kJ/mol

112 kJ/mol

highest heat ofhydrogenation;least stable isomer

lowest heat ofhydrogenation;most stable isomer

Problem 6.2

6.3. Stereochemistry of Alkene Hydrogenation

A reaction in which a single starting material can givetwo or more stereoisomeric products but yields one ofthem in greater amounts than the other (or even to theexclusion of the other) is said to be stereoselective.

Two spatial (stereochemical) aspects ofalkene hydrogenation:

(1) Syn addition of both H atoms to double bond.

(2) Hydrogenation is stereoselective, correspondingto addition to less crowded face of double bond.

syn addition anti addition

syn-Additon versus anti-Addition

CO2CH3

(100%)

H2, PtCO2CH3

CO2CH3H

Example of Syn Addition

H3C CH3

D2, cat

Both productscorrespond tosyn additionof H2.

CH3H3C

But only this one if formed!

Example of Stereoselective

Reaction

CH3H3C

H3C CH3

H2, catTop face of doublebond blocked bythis methyl group

H2 adds to bottom face of double bond.

Example of Stereoselective

Reaction

+ E—Y –

C C E YC C

General equation for electrophilic addition

6.4. Electrophilic Addition of Hydrogen Halides to Alkenes

+ H—Y –

C C H YC C

When E+ is a hydrogen halide:

CH3CH2 CH2CH3

CH3CH2CH2CHCH2CH3

CHCl3, -30°CC CHBr

Example

Electrophilic addition ofhydrogen halides to alkenesproceeds by rate-determiningformation of a carbocationintermediate.

Electrons flow from the system of the alkene (electronrich) toward the positivelypolarized proton of thehydrogen halide.

Mechanism

HC C....X:

..:X:–

Mechanism

HF << HCl < HBr < HI

least acidic most acidic

Reactivity of Hydrogen Halide

6.5. Regioselectivity of Hydrogen Halide Addition: Markovnikov's Rule

When an unsymmetrically substituted alkenereacts with a hydrogen halide, the hydrogenadds to the carbon that has the greater numberof hydrogen substituents, and the halogen addsto the carbon that has the fewer hydrogensubstituents.

acetic acidBr

CH3CH2CHCH3

Example 1

CH2CH3CH2CHHBr

Br(90%)

C Cacetic acid

HBrCH3

Example 2

Markovnikov's Rule

(100%)

Example 3

HClCH3

Cl0°C

Protonation of double bond occurs in directionthat gives more stable carbocation.

Markovnikov's Rule

CH3CH2CHCH3CH2CH3CH2CH

CH3CH2CH—CH3 + Br –+

CH3CH2CH2—CH2

primary carbocation is less stable: not formed

6.6. Mechanistic Basis for Markovnikov's Rule: Example 1

secondary carbocation is less stable: not formed

Mechanistic Basis for Markovnikov's Rule: Example 3

HCl, 0°C

CH3CHCH(CH3)2

H2C CHCH(CH3)2

+CH3CHC(CH3)2

CH3CHCH(CH3)2

Cl (40%)

CH3CH2C(CH3)2

Cl(60%)

6.7. Carbocation Rearrangements in Hydrogen Halide Addition to Alkenes

Rearrangements sometimes occur!

CH3CHCH3

OSO2OH

CH3CH CH2HOSO2OH

Isopropylhydrogen sulfate

6.8. Addition of Sulfuric Acid (H2SO4) to Alkenes

- Follows Markovnikov's rule- Yields an alkyl hydrogen sulfate

+CH3CH CH2 H..

SO2OH..

+CH3CH CH3 +

..SO2OH..:

fast CH3CHCH3

OSO2OH..:

Mechanism

H—OHCH3CHCH3

O SO2OH

HO—SO2OH+

CH3CHCH3

Alkyl hydrogen sulfates undergo hydrolysis in hot water

1. H2SO4

2. H2O, heat

Application: Conversion of alkenes to alcohols

Not all alkenes yield alkyl hydrogen sulfateson reaction with sulfuric acid

These do:

H2C=CH2, RCH=CH2, and RCH=CHR'

These don't:

R2C=CH2, R2C=CHR, and R2C=CR2

But...

H—OHC C + OHC CH

6.9. Acid-Catalyzed Hydration of Alkenes

Reaction is acid catalyzed; typical hydration medium is 50% H2SO4-50% H2O.

50% H2SO4

50% H2O

H3C CH3

C CH2CH3CH3

Follows Markovnikov's Rule

50% H2SO4

50% H2O

CH3CH2

Follows Markovnikov's Rule

+ H2OH+

C CH3CH3

Involves a carbocation intermediate. Is the reverse of acid-catalyzed dehydration ofalcohols to alkenes.

Mechanism

Step (1): Protonation of double bond

C CH2 O+

Mechanism

Step (2): Capture of carbocation by water

Mechanism

Step (3): Deprotonation of oxonium ion

Mechanism

Ethylene CH2=CH2 1.0

Propene CH3CH=CH2 1.6 x 106

2-Methylpropene (CH3)2C=CH2 2.5 x 1011

Acid-catalyzed hydration

The more stable the carbocation, the faster it isformed, and the faster the reaction rate.

Relative Rates

+ H2OH+

C CH3CH3

In an equilibrium process, the same intermediatesand transition states are encountered in the forwarddirection and the reverse, but in the opposite order.

Principle of Microscopic Reversibility

6.11Hydroboration-Oxidation of Alkenes

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

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

Synthesis

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

1. Hydroboration2. Oxidation

Synthesis

+ H—BH2C C H BH2C C

Hydroboration can be viewed as the addition ofborane (BH3) to the double bond. But BH3 is notthe reagent actually used.

Hydroboration Step

Hydroboration reagents:

Diborane (B2H6)normally used in an ether- like solventcalled "diglyme"

2-Methoxyethyl ether or diglymeO

Hydroboration Step

Hydroboration reagents:

Borane-tetrahydrofurancomplex (H3B-THF)+O

BH3–

••

Hydroboration Step

H2O2, HO–

H BH2C C H OHC C

Organoborane formed in the hydroborationstep is oxidized with hydrogen peroxide.

Oxidation Step

1. B2H6, diglyme2. H2O2, HO–

Example

C C1. H3B-THF

2. H2O2, HO–H

C CCH3

Example

Features of Hydroboration-Oxidation

Hydration of alkenes.

Regioselectivity opposite to Markovnikov's rule.

No rearrangement.

Stereospecific syn addition.

+ X2 X X

Electrophilic addition to double bond. Forms a vicinal dihalide.

C CC C

General features

6.14. Addition of Halogens to Alkenes

CH3CHCHCH(CH3)2

(100%)

CHCl30°C

CHCH(CH3)2CH3CHBr2

Example

- Limited to Cl2 and Br2.

- F2 addition proceeds with explosive violence.

- I2 addition is endothermic: vicinal diiodidesdissociate to an alkene and I2.

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

Example

6.15. Stereochemistry of Halogen Addition

anti additionanti addition

trans-1,2-Dichlorocyclooctane73% yield; only product

Example

6.16. Mechanism of Halogen Addition to Alkenes: Halonium Ions

Mechanism is electrophilic addition

Br2 is not polar, but it is polarizable.

Two steps:(1) Formation of bromonium ion.(2) Nucleophilic attack on bromonium

ion by bromide.

Ethylene H2C=CH2 1

Propene CH3CH=CH2 61

2-Methylpropene (CH3)2C=CH2 5400

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

Table 6.3. Relative Rates of Bromination

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

H2C CH2 BrCH2CH2Br+ Br2

..: :..Br

–C C

No obvious explanation for anti addition provided by this mechanism.

Mechanism?

H2C CH2 BrCH2CH2Br+ Br2

Br: :+

..: :..Br

Cyclic bromonium ion

Mechanism

Mutual polarization ofelectron distributions ofBr2 and alkene

Formation of Bromonium Ion

Electrons flow from alkene toward Br2

electrons ofalkene displaceBr– from Br.

Formation of Bromonium Ion

Attack of Br– from side oppositeC—Br bond of bromonium ion givesanti addition.

–Br: :..

Stereochemistry

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

Example

Cyclopentene +Br2

Bromonium ion

trans-Stereochemistry in vicinal dibromide

Bromide ion attacks thebromonium ion from sideopposite carbon-bromine bond.

+ X2 X XC CC C

+ H2O OH

+ H—X

+ X2 X C CC C

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

halohydrins.

6.17. Conversion of Alkenes to Vicinal Halohydrins

H2C CH2 BrCH2CH2OH+ Br2

anti addition: only product

Examples

Bromonium ion is intermediate.Water is nucleophile that attacks bromonium ion.

Mechanism

C CH2BrCH3

Regioselectivity

Markovnikov's rule applied to halohydrin formation:the halogen adds to the carbon having the greaternumber of hydrogens.

H3C CH2

SN2 StereochemistrySN1 Regiochemistry

Transition state for attack of water on bromonium ion hascarbocation character; more stable transition state (left) haspositive charge on more highly substituted carbon

Explanation

H2O/DMSO

N-Bromosuccinimide (NBS)

Few alkenes are soluble in water

NBS is a source of Br2 in low concentration:radical addition. Works for allylic susbstitution.

The "peroxide effect"

6.18. Free-radical Addition of HBr to Alkenes

acetic acidBr

CH3CH2CHCH3CH2CH3CH2CHHBr

Markovnikov's Rule

CH3CH2CH2CH2Br

only product inabsence of peroxides

only product when peroxides added to

reaction mixture

CH3CH2CHCH3

CH2CH3CH2CH

Addition of HBr to 1-Butene

CH3CH2CH2CH2Br

only product when peroxides added to reaction mixture

CH2CH3CH2CH

Addition opposite to Markovnikov's rule occurs with HBr (not HCl or HI)

Addition of HBr to 1-Butene

+ HBrh

Addition of HBr with a regiochemistry oppositeto Markovnikov's rule can also occur wheninitiated with light with or without added peroxides.

Photochemical Addition of HBr

..O R...

..O RR O....

O .R ....

+ BrO .R ....

H ....

: +OR ....

H . Br....

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

Initiation steps:

Mechanism

Propagation steps:

+CH3CH2CH CH2. Br..

.CH3CH2CH CH2 Br:

+.CH3CH2CH CH2 Br:

H Br....

CH3CH2CH2CH2Br:..

. Br....

Mechanism

H2C CH2

H2C CHCH3

Epoxides are examples of heterocyclic compoundsthree-membered rings that contain oxygen.

Ethylene oxide Propylene oxide

6.19. Epoxidation of Alkenes

H2C CHCH3

1,2-epoxypropane 2-methyl-2,3-epoxybutane

Substitutive nomenclature: named as epoxy-substituted alkanes.

“epoxy” precedes name of alkane.

Epoxide Nomenclature

cis-2-Methyl-7,8-epoxyoctadecane

Problem 6.22. Give the IUPAC name, including stereochemistry, for disparlure

peroxy acidC C +

Epoxidation of Alkenes

+ CH3COOH

+ CH3COH

Example

syn addition

Stereochemistry of Epoxidation

peroxy acid

Problem 6.23. Give the structure of the alkene,including stereochemistry, that you would choose asthe starting material in a preparation of syntheticdisparlure.

Ethylene H2C=CH2 1

Propene CH3CH=CH2 22

2-Methylpropene (CH3)2C=CH2 484

2-Methyl-2-butene (CH3)2C=CHCH3 6526

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

Relative Rates of Epoxidation

+ O3 C CO

Ozonolysis has both synthetic and analyticalapplications.

- Synthesis of aldehydes and ketones.

- Identification of substituents on the double bond ofan alkene.

6.20. Ozonolysis of Alkenes

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

+ O3 C CO

C O CO+

H2O, Zn

Second step is hydrolysis of the ozonide.Two aldehydes, two ketones, or an aldehydeand a ketone are formed.

Ozonolysis of Alkenes

As an alternative to hydrolysis, the ozonide canbe treated with dimethyl sulfide.

+ O3 C CO

C O CO+

(CH3)2S

Ozonolysis of Alkenes

1. O32. H2O, Zn

CH2CH3H

CH2CH3

(38%) (57%)

CH2CH3

Example

AcetoneHO2C CO2H

C=C bond cleavage

KMnO4: Potassium permanganateNaIO4: Sodium periodateOsO4: Osmium tetroxide

Cyclic periodate intermediate

Introduction to OrganicChemical Synthesis: RETROSYNTHESIS

Devise a synthetic plan:- Reason backward from the target molecule.- Always use reactions that you are sure of.

Prepare cyclohexane from cyclohexanol

Ask yourself the key question:

–"Starting with anything, how can I make cyclohexane in a single step by a reaction that will work?"

The only reaction covered so far for preparingalkanes is catalytic hydrogenation of alkenes.

This leads to a new question. "Starting withanything, how can I prepare cyclohexene in a singlestep by a reaction I am sure will work?"

Alkenes can be prepared by dehydration ofalcohols.

The synthesis is complete.

heatOH

(CH3)3COH (CH3)2CCH2Br

"Starting with anything, how can I make thedesired compound in a single step by a reaction thatwill work?"

The desired compound is a vicinal bromohydrin.How are vicinal bromohydrins prepared?

Prepare 1-bromo-2-methyl-2-propanol from tert-butyl alcohol

(CH3)2CCH2Br

Vicinal bromohydrins are prepared by treatment of alkenes with Br2 in water.

How is the necessary alkene prepared?

H2O(CH3)2C CH2

2-Methylpropene is prepared from tert-butyl alcohol by acid-catalyzed dehydration.

The synthesis is complete.

(CH3)2CCH2Br

H2O(CH3)2C CH2

(CH3)3COH

6.21. Reactions of Alkenes with Alkenes: Polymerization

Cationic polymerization

Free-radical polymerization

Coordination polymerization

monomer(C4H8)

Dimerization of 2-methylpropene

(CH3)2C CH2

two dimers(C8H16)

CH3CCH

C(CH3)2 CH3CCH2C

Dimerization

CH3CCH2C

Mechanism of Dimerization

CH3CCH

C(CH3)2CH3CCH2C

CH3CCH2C

Mechanism of Dimerization

Cationic Polymerization

Cl AlCl3

+AlCl4-

AlCl4-

200 °C2000 atm

O2 orperoxides

H2C CH2

polyethylene

CH2 CH2 CH2 CH2 CH2 CH2 CH2

Free-Radical Polymerization of Ethylene

H2C CH2

RO. H2C CH2

.H2C CH2

H2C CH2

.H2C CH2

H2C CH2

.H2C CH2

H2C CH2

H2C CH2.

H2C CH2

H2C CH2.H2C CH2

H2C CH2

.H2C CH2

Mechanism

H2C CHCH3

polypropylene

CH CH2 CHCH2CHCH CH2

CH3 CH3 CH3 CH3

Free-Radical Polymerization of Propene

H2C CHCH3

RO. H2C CHCH3

. H2C CHCH3

.H2C CHCH3

H2C CHCH3

.H2C CHCH3

H2C CHCH3

.H2C CHCH3

H2C CHCH3

H2C CHCH3.

H2C CHCH3

H2C CHCH3.H2C CHCH3

H2C CHCH3

.H2C CHCH3

Mechanism

H2C=CHCl polyvinyl chlorideH2C=CHC6H5 polystyreneF2C=CF2 Teflon

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