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CH 7: Alkenes, Reactions and Synthesis

Renee Y. BeckerCHM 2210

Valencia Community College

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Diverse Reactions of Alkenes

• Alkenes react with many electrophiles to give useful products by addition (often through special reagents)

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Why this chapter?

• To begin a systematic description of major functional groups

• Begin to focus on general principles and patterns of reactivity that tie organic chemistry

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Preparation of Alkenes: A Preview of Elimination Reactions

• Alkenes are commonly made by– elimination of HX from alkyl halide

(dehydrohalogenation)• Uses heat and KOH

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

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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

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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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Addition of Hypohalous Acids to Alkenes: Halohydrin Formation

• This is formally the addition of HO-X to an alkene 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

– Show correct mech!

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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• Markov., Br goes to C with more H’s

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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

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Oxymercuration Intermediates

• For laboratory-scale hydration of an alkene

• Use mercuric acetate in THF followed by sodium borohydride

• Markovnikov orientation (H to C with more H)– via mercurinium ion

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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

• Non Markov., (H to C with less H)

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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 Forms an Alcohol from an 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 (cis), to the same face of the alkene (opposite of anti addition (trans))

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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 added across C=C

• More stable carbocation is also consistent with steric preferences

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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 add 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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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: Epoxidation and Hydroxylation

• Epoxidation results in a cyclic ether with an oxygen atom

• Stereochemistry of addition is syn

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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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Oxidaton of Alkenes:Cleavage to Carbonyl Compounds

• 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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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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Cleavage of 1,2-diols

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

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

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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: 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 initiation 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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Biological Additions of Radicals to Alkenes

• Severe limitations to the usefulness of radical addition reactions in the lab

• In contrast to electrophilic additions, reactive intermediate is not quenched so it reacts again and again uncontrollably

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Biological Reactions

• Biological reactions different than in the laboratory

• One substrate molecule at a time is present in the active site of an enzyme

• • Biological reactions are more controlled, more specific

than other reactions

Mechanism Examples

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Group 1: Halogenation

Cl2

CH2Cl2

Regioselective? Why, why not?Cyclic, trans or cis? Why?

Br2

CH2Cl2+

+

Group 1: Halogenation

Cl2

CH2Cl2

Regioselective? Why, why not?Cyclic, trans or cis? Why?

Br2

CH2Cl2+

+

Mechanism Examples

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Group 2: dehydrohalogenation

Regioselective? Why, why not?

KOH

alcohol

major Minor KBr H2O

How many prod? Major and minor? KBr H2OKOH

alcohol

Br

Br

++

G r o u p 2 : d e h y d r o h a l o g e n a t i o n

R e g i o s e l e c t i v e ? W h y , w h y n o t ?

K O H

a l c o h o l

m a j o r M i n o r K B r H

2O

How many prod? Major and minor? KBr H2OKOH

alcohol

Br

Br

++

Mechanism Examples

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Group 3: dehydration

Regioselective? Why, why not?H2O

OHH3O+

THF 50C

Major Minor

OH

Regioselective? Why, why not?

H2OH3O+

THF 50C

Group 3: dehydration

Regioselective? Why, why not?H2O

OHH3O+

THF 50C

Major Minor

OH

Regioselective? Why, why not?

H2OH3O+

THF 50C

Mechanism Examples

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H2O

Group 4: Halohydrin formation

Br2

H2O

Regioselective? Why, why not?Cyclic, trans or cis? Why?

Br2

+

+

H2O

Group 4: Halohydrin formation

Br2

H2O

Regioselective? Why, why not?Cyclic, trans or cis? Why?

Br2

+

+

Mechanism Examples

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H2O

H2SO4

250 CH2O

Group 5: Hydration

250 C

H2SO4

Regioselective? Why, why not?

+

+

H

2O

H2SO4

250 CH2O

Group 5: Hydration

250 C

H2SO4

Regioselective? Why, why not?

+

+