Preparation of Alkenes

Alkenes: Reactions and Synthesis

2302261 Dr Anawat Ajavakom

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)

McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

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Preparation of Alkenes: A Preview of Elimination Reactions

Alkenes are commonly made by 1. elimination of HX from alkyl halide (dehydrohalogenation) Uses heat and KOH


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β-Elimination

E1 (Elimination unimolecular)


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E1 (Elimination unimolecular) Stability of carbocation

3 o

o

o

Reactivity of alkyl halide


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E1 (Elimination unimolecular)

กรณี ที่มี β-hydrogen มากกว่าหนึ่ง

Rearrangement


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E2 (Elimination bimolecular): anti-elimination Leaving group I- > Br- > Cl- > F-

Reactivity of alkyl halide

อัลคีนที่คาร์บอนของพันธะคู่มีหมู่อะตอมคาร์บอนเกาะอยูจ่ าํ นวนมากก็ยงิ่ มีความเสถียรมาก


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Preparations of Alkenes

2. elimination of H-OH from an alcohol (dehydration) require strong acids (sulfuric acid, 50 ºC) : E1


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2. Dehydration of alcohols +

1,2-Hydride shift +

1,2-alkyl shift +


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3. Dehalogenation of Dihalide: E2 mechanism


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4. Hydrogenation of alkynes and Reduction with Na/NH3

3

trans 2 2 2

3

cis


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Reduction with Na/NH3 Mechanism


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4. Hydrogenation and reduction of alkynes

5. Pyrolysis of alkane


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Reactions of alkenes: A. Addition reaction of alkenes 1. Addition of hydrogen halides to alkenes

General reaction mechanism: electrophilic addition Attack of electrophile (such as HBr) on π bond of alkene Produces carbocation and bromide ion Carbocation is an electrophile, reacting with nucleophilic bromide ion


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Example of Electrophilic Addition

Addition of hydrogen bromide to 2-Methylpropene H-Br transfers proton to C=C Forms carbocation intermediate More stable cation forms Bromide adds to carbocation


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Orientation of Electrophilic Addition: Markovnikov’s Rule

In an unsymmetrical alkene, HX reagents can add in two different ways, but one way may be preferred over the other

If one orientation predominates, the reaction is regiospecific

Markovnikov observed in the 19th century that in the addition of HX to alkene, the H attaches to the carbon with the most H’s and X attaches to the other end (to the one with the most alkyl substituents) This is Markovnikov’s rule McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

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Example of Markovnikov’s Rule

Addition of HCl to 2-methylpropene Regiospecific – one product forms where two are possible If both ends have similar substitution, then not regiospecific


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Mechanistic Source of Regiospecificity in Addition Reactions

If addition involves a carbocation intermediate and there are two possible ways to add the route producing the more alkyl substituted cationic center is lower in energy alkyl groups stabilize carbocation


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Mechanism of Electrophilic Addition: Rearrangements of Carbocations

Carbocations undergo structural rearrangements following set patterns 1,2-H and 1,2-alkyl shifts occur Goes to give more stable carbocation Can go through less stable ions as intermediates


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2. Addition of Water to Alkenes

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


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3. Addition of Sulfuric acid to Alkenes

3 3 3 .

2 +

4 2


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4. Addition of HX to alkenes with peroxides

Free-radical reaction Anti-Markovnikov addition


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5. 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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5. Hydrogenation of alkenes

Syn-addition

Examples


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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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6. Addition of Halogens to Alkenes

Bromine and chlorine add to alkenes to give 1,2-dihaldes, an industrially important process F2 is too reactive and I2 does not add Cl2 reacts as Cl+ Cl Br2 is similar


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Addition of Br2 to Cyclopentene Addition is exclusively trans (anti-addition)

+


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Mechanism of Bromine Addition

Br+ adds to an alkene producing a cyclic ion Bromonium ion, bromine shares charge with carbon Gives trans addition


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Bromonium Ion Mechanism

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


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7. 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)


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Mechanism of Formation of a Bromohydrin

Br2 forms bromonium ion, then water adds + Orientation toward stable C species Aromatic rings do not react


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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


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8. 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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8A. 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 nonradical molecule A bond is broken homolytically


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Polymerization: Propagation

Radical from intiation adds to alkene to generate alkene derived radical This radical adds to another alkene, and so on many times


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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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8B. 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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9. Hydroxylation using OsO4

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)


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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


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Hydroxylation using KMnO4

4

2

-

4 2

2

2

-


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10. Oxymercuration Intermediates

For laboratory-scale hydration of an alkene Use mercuric acetate in THF followed by sodium borohydride Markovnikov orientation via mercurinium ion


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11. Hydroboration-Oxidation

Herbert Brown (HB) invented hydroboration (HB) Borane (BH3) is electron deficient is a Lewis acid Borane adds to an alkene to give an 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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Hydroboration-Oxidation Forms an Alcohol from an Alkene

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


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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)


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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: Orientation in Addition Step

Addition in least crowded orientation, syn Addition also is via most stable carbocation


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Hydroboration, Electronic Effects Give NonMarkovnikov

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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12. 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


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Formation of Dichlorocarbene

Base removes proton from chloroform Stabilized carbanion remains Unimolecular Elimination of Cl- gives electron deficient species, dichlorocarbene


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Reaction of Dichlorocarbene

Addition of dichlorocarbene is stereospecific cis


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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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B. Cleavage reactions of alkenes 1. 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


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2. Permangante 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


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3. 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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4. Epoxidation Using a Peroxyacid

Treat an alkene with a peroxyacid.


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4. Epoxidation Using a Peroxyacid

The peroxyacid epoxidation is very selective. Double bonds that are more electron-rich react faster, and syn stereochemistry is always abserved.


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Dienes: Reactions and Synthesis

Types of Dienes


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Preparations of 1,3-Butadienes

1. Acid-catalyzed dehydration of diols +

3 3

+

3 3

2. Pyrolysis of alkenes


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Preparations of 1,3-Butadienes

3. Dimerization of acetylene

4. Others


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Preparations of 1,3- and 1,4-Butadienes

2

2

3


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Reactions of dienes

1. Addition reaction of H2 to dienes


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Reactions of dienes

2. Addition reaction of HX to dienes

Example


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Reactions of dienes

3. Addition reaction of X2 to dienes Cl2

Cl

CCl4 1,2-addition

1,4-addition

Cl

Cl

Cl

Cl Cl2

CCl4 Cl

Cl

Cl Cl

1,2,3,4-Tetrachlorobutene McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

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Reactions of dienes

4. Diels-Alder reaction


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