Chapter 7. Alkenes: Reactions and Synthesis

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Diverse Reactions of Alkenes

• Alkenes react with many electrophiles to give useful products by addition (often through special reagents)– alcohols (add H-OH)– alkanes (add H-H)– halohydrins (add HO-X)– dihalides (add X-X)– halides (add H-X)– diols (add HO-OH)– cyclopropane (add :CH2)

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Reactions of Alkenes

C C

OHH

Alcohol

HH

OHHO

1,2-Diol

O

CO

XH

XX

OHX

Alkane

Carbonyl Compound

CyclopropaneEpoxide

1,2-Dihalide

Halide

Halohydrin

alkene

4

Preparation of Alkenes

Elimination

Addition

C X YC + C C

X Y

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Part 1 - Synthesis of Alkenes

• These reactions are used to produce alkenes.

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Synthesis of Alkenes: Synthesis 1#

• Alkenes are commonly made by elimination of HX from alkyl halide (dehydrohalogenation)

• Uses heat and KOH

HH

BrH

KOH

CH3CH2OH

H

H

KBr H2O++

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Synthesis of Alkenes: Synthesis 2#

– elimination of H-OH from an alcohol (dehydration) • require strong acids (sulfuric acid, 50 ºC)

CH3

OHH2SO4, H2O

THF, 50 oC

CH3

H2O+

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Part 2 - Reaction of Alkenes

• These reactions react alkenes to form a series of alkane products.

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Addition of Halogens to Alkenes

• Bromine and chlorine add to alkenes to give 1,2-dihaldes

• F2 is too reactive and I2 does not add.

C CH

H

H

H

Cl Cl

C C

Cl Cl

HH

H H

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Example: Mechanism of Bromine Addition

• Electrophilic addition of bromine to give a cation is followed by cyclization to give a bromonium ion.

• This bromoniun ion is a reactive electrophile and bromide ion is a good nucleophile.– Gives trans addition.

C C

Br

Br

C CBr

Br- C

Br

C+

Br-

Br

C C

Br

+

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Example: Addition of Br2 to Cyclopentene

• Addition is exclusively trans

HH Br Br

Br

H

H

Br

Br

H

Br

H

found

not found

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Halohydrin Formation

• This is formally the addition of HO-X to an alkene (with +OH as the electrophile) to give a 1,2-halo alcohol, called a halohydrin.

• The actual reagent is the dihalogen (Br2 or Cl2 in water in an organic solvent)

C CX2

H2O C CX

HO

+ HX

Alkene Halohydrin

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An Alternative to Bromine

• Bromine is a difficult reagent to use for this reaction• N-Bromosuccinimide (NBS) produces bromine in organic

solvents and is a safer source.

N

O

O

Br

H2O, CH3SOCH3(DMSO)

(NBS)

OH

Br

2-Bromo-1-phenylethanol (76%)

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Addition of Water to Alkenes: Oxymercuration

• Hydration of an alkene is the addition of H-OH to to give an alcohol

• Acid catalysts are used in high temperature industrial processes: ethylene is converted to ethanol

C C

H

O H

H3PO4

250oC

H

H H

HHHO-

Intermediate

+ H

H H

HHHO

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Addition of Water to Alkenes: Oxymercuration

Hg(OAc)2CH3 CH3

HgOAcH H

H2O CH3

OH

HgOAc

NaBH4

H

CH3

OH

H

Hg(OAc)2 is used as an electrophillic sink. The double bond is then attacked by the water creating an alchohol. This is then REDUCED by NaBH4 that adds an H to the molecule.

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Addition of Water to Alkenes: Hydroboration

• Herbert Brown (HB) invented hydroboration (HB)• Borane (BH3) is electron deficient is a Lewis acid.• Borane adds to an alkene to give an organoborane.

C CB

H

H H

Borane

+BH2H

Organoborane

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BH3 Is a Lewis Acid

• Six electrons in outer shell• Coordinates to oxygen electron pairs in ethers

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• Addition of H-BH2 (from BH3-THF complex) to three alkenes gives a trialkylborane

• Oxidation with alkaline hydrogen peroxide in water produces the alcohol derived from the alkene

Hydroboration-Oxidation Alcohol Formation from Alkenes

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Orientation in Hydration via Hydroboration

• Regiochemistry is opposite to Markovnikov orientation– OH is added to carbon with most H’s

• H and OH add with syn stereochemistry, to the same face of the alkene (opposite of anti addition)

CH3

BH3

THF

H

CH3

H

B

HH -OH

H2O2

H

CH3

H

OH

1-methylcycopentene Alkylborane intermediate trans-2-methylcyclopentanol (85%)

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Mechanism of Hydroboration

• Borane is a Lewis acid• Alkene is Lewis base• Transition state involves

anionic development on B

• The components of BH3 are across C=C

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Hydroboration, Electronic Effects Give Non-Markovnikov

• More stable carbocation is also consistent with steric preferences

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Hydroboration - Oxygen Insertion Step

• H2O2, OH- inserts OH in place of B

• Retains syn orientation

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Addition of Carbenes to Alkenes

• The carbene functional group is “half of an alkene”• Carbenes are electrically neutral with six electrons in

the outer shell• They symmetrically across double bonds to form

cyclopropanes

CR1

R2

Carbene

+ C

R1 R2

CyclopropaneAlkene

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Formation of Dichlorocarbene

• Base removes proton from chloroform

• Stabilized carbanion remains

• Unimolecular Elimination of Cl- gives electron deficient species, dichlorocarbene

25

Simmons-Smith Reaction

• Equivalent of addition of CH2: • Reaction of diiodomethane with zinc-copper alloy

produces a carbenoid species• Forms cyclopropanes by cycloaddition

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Reaction of Dichlorocarbene

• Addition of dichlorocarbene is stereospecific cis

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Reduction of Alkenes: Hydrogenation

• Addition of H-H across C=C• Reduction in general is addition of H2 or its

equivalent • Requires Pt or Pd as powders on carbon and H2

• Hydrogen is first adsorbed on catalyst• Reaction is heterogeneous (process is not in

solution)

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Hydrogen Addition- Selectivity

• Selective for C=C. No reaction with C=O, C=N• Polyunsaturated liquid oils become solids• If one side is blocked, hydrogen adds to other

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Mechanism of Catalytic Hydrogenation

• Heterogeneous – reaction between phases

• Addition of H-H is syn

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Oxidation of Alkenes: Hydroxylation and Cleavage

• Hydroxylation adds OH to each end of C=C• Catalyzed by osmium tetroxide• Stereochemistry of addition is syn• Product is a 1,2-dialcohol or diol (also called a glycol)

Alkene

1. OsO4

2. NaHSO3

C CHO OH

A 1,2-diol

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Osmium Tetroxide Catalyzed Formation of Diols

• Hydroxylation - converts to syn-diol• Osmium tetroxide, then sodium bisulfate• Via cyclic osmate di-ester

CH3

CH3

1. OsO4 2. NaHSO3

A 1,2-diol (87%)NAME THIS MOLECULE

pyridine

CH3

CH3

O

OOs

O

O H2O

CH3

CH3

OH

OH

NAME THIS MOLECULE A cyclic osmate

Intermediate

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Section 3: Breakdown of Alkenes

• These Reactions are used to breakdown alkenes into two products.

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Alkene Cleavage: Ozone

• Ozone, O3, adds to alkenes to form molozonide• Reduce molozonide to obtain ketones and/or

aldehydes

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Examples of Ozonolysis of Alkenes

• Used in determination of structure of an unknown alkene

35

Structure Elucidation With Ozone

• Cleavage products reveal an alkene’s structure

36

Permanganate Oxidation of Alkenes

• Oxidizing reagents other than ozone also cleave alkenes

• Potassium permanganate (KMnO4) can produce carboxylic acids and carbon dioxide if H’s are present on C=C

+ KMnO4

O

O

H

O

OH

+

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Cleavage of 1,2-diols

• Reaction of a 1,2-diol with periodic (per-iodic) acid, HIO4 , cleaves the diol into two carbonyl compinds

• Sequence of diol formation with OsO4 followed by diol cleavage is a good alternative to ozonolysis

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Mechanism of Periodic Acid Oxidation

• Via cyclic periodate intermediate

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Biological Alkene Addition Reactions

• Living organisms convert organic molecules using enzymes as catalysts

• Many reactions are similar to organic chemistry conversions, except they occur in neutral water

• Usually much specific for reactant and stereochemistry

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Biological Hydration Example

• Fumarate to malate catalyzed by fumarase• Specific for trans isomer• Addition of H, OH is anti

OO

OO H OH

OO

OOH

OHFumarase

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Addition of Radicals to Alkenes: Polymers

• A polymer is a very large molecule consisting of repeating units of simpler molecules, formed by polymerization

• Alkenes react with radical catalysts to undergo radical polymerization

• Ethylene is polymerized to poyethylene, for example

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Free Radical Polymerization of Alkenes

• Alkenes combine many times to give polymer– Reactivity induced by formation of free radicals

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Free Radical Polymerization: Initiation

• Initiation - a few radicals are generated by the reaction of a molecule that readily forms radicals from a non-radical molecule

• A bond is broken homolytically

44

Polymerization: Propagation

• Radical from intiation adds to alkene to generate alkene derived radical

• This radical adds to another alkene, and so on many times

45

Polymerization: Termination

• Chain propagation ends when two radical chains combine• Not controlled specifically but affected by reactivity and

concentration

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Other Polymers

• Other alkenes give other common polymers

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Cationic Polymerization

• Vinyl monomers react with Brønsted or Lewis acid to produce a reactive carbocation that adds to alkenes and propagates via lengthening carbocations

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Take Home Message

• Learn the REACTIONS (ALL OF THEM)

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Synthesis of Alkenes

HH

BrH

KOH

CH3CH2OH

H

H

KBr H2O++

1) dehydrohalogenation

CH3

OHH2SO4, H2O

THF, 50 oC

CH3

H2O+

2) dehydration

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Part 2 - Reaction of Alkenes

C CH

H

H

H

Cl Cl C C

Cl Cl

HH

H H

+

C CX2

H2O C CX

HO

+ HX

1) Addition of Halogens to Alkenes

2) Halohydrin Formation

N

O

O

Br

H2O, CH3SOCH3(DMSO)

(NBS)

OH

Br

2-Bromo-1-phenylethanol (76%)

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Part 2 - Reaction of Alkenes

C CH

O HH3PO4

250oC

+H

H H

HHHO

CH3

BH3

THF

H

CH3

H

B

HH -OH

H2O2

H

CH3

H

OH

1-methylcycopentene Alkylborane intermediate trans-2-methylcyclopentanol (85%)

3) Addition of Water to Alkenes

4) Hydroboration-Oxidation Alcohol Formation

CR1

R2

Carbene

+ C

R1 R2

CyclopropaneAlkene

5) Carbene Formation – Cyclopropane synthesis

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Part 2 - Reaction of Alkenes6) Catalytic Hydrogenation

7) Hydroxylation and Cleavage

Alkene

1. OsO4

2. NaHSO3

C CHO OH

A 1,2-diol

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Part 3 - Breakdown of Alkenes1) Ozonolysis

2) Permangante Oxidation

+ KMnO4

O

O

H

O

OH

+

3) Periodic Acid Oxidation, Cleavage of 1,2-diols