The Reactions of Alkenes

The Stereochemistry of Addition Reactions

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

Diverse Reactions of Alkenes

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• Alkenes are commonly made by

– elimination of HX from alkyl halide (dehydrohalogenation)

• Uses heat and KOH

– elimination of H-OH from an alcohol (dehydration)

• requires strong acids (sulfuric acid, 50 ºC)

Preparation of Alkenes: A Preview of Elimination Reactions

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A Regioselective Reaction

A reaction in which one structural isomer is favored over another, leading to its predominance in the mixture of products.

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A Stereoselective Reaction

A reaction in which one stereoisomer in a mixture isproduced more rapidly than another, resulting in predominance of the favored stereoisomer in the mixture of products.

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A Stereospecific Reaction

A reaction in which a particular stereoisomeric formof reactant gives one specific stereoisomer of product,while a different stereoisomeric form of reactant leadsto a different single pure streoisomer of product.

Stereospecific reaction is also stereoselective; however,stereoselective reaction is not stereospecific.

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An Electrophilic Addition Reaction

where HX = HF, HCl, HBr, and HI

Reactivity of HF << HCl < HBr < HI since

HF is less acidic and HI is most acidic.The rate of addition of HI is too fast to measure.

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The Mechanism of the Reaction

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Relative Stabilities of Carbocations

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Hyperconjugation Stabilizes a Carbocation

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The Difference in Carbocation Stability Determines the Products

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The Product Does Not Have Stereoisomers

Not Regioselective or Regiospecific

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Formation of a Racemic Mixture

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The Major Product is a Surprise

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The Major Product is a Surprise

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The Carbocation Does Not Rearrange(No Improvement in Carbocation Stability)

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Anti-Markovnikov Regiochemistry; Radical Intermediate

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Mechanism of the Reaction

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Radical Stabilities

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• 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

Addition of Halogens to Alkenes

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• Addition is exclusively trans

Addition of Br2 to Cyclopentene

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• 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

Bromonium Ion Mechanism

The Mechanism for the Addition of a Halogen

The intermediate is a cyclic bromonium ion.

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• Bromonium ions were postulated more than 60 years ago to explain the stereochemical course of the addition (to give the trans-dibromide from a cyclic alkene)

• Olah showed that bromonium ions are stable in liquid SO2 with SbF5 and can be studied directly

The Reality of Bromonium Ions

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No Carbocation Rearrangements

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Alkenes Do Not Add I2

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• 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)

Halohydrins from Alkenes: Addition of HOX

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Formation of Halohydrins

Markovnikov regiochemistry (OH group) and Anti-addition

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Mechanism for Halohydrin Formation

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Why Does it Follow the Same Rule?

The electrophile adds to the sp2 carbon bonded to the most hydrogens.

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• Bromine is a difficult reagent to use for this reaction

• N-Bromosuccinimide (NBS) produces bromine in organic solvents and is a safer source

An Alternative to Bromine

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More Reactions

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Alcohols from Alkenes

1) Acid Catalyzed Hydration of AlkeneMarkovnikov regiochemistry and Carbocation formation

2) Oxymercuration –demurcuration (reduction)Markovnikov regiochemistry

3) Hydroboration-oxidationanti-Markovnikov regiochemistry andsyn-addition

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

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

Hydration of Alkenes: Addition of H2O

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Acid-Catalyzed Addition of Water

Mechanism for the Acid-Catalyzed Addition of Water

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Acid-Catalyzed Addition of an Alcohol

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• For laboratory-scale hydration of an alkene• Use mercuric acetate in THF followed by sodium

borohydride• Markovnikov orientation

– via mercurinium ion

Oxymercuration Intermediates

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Mechanism for Oxymerucation

The final step of this mechanism is not well understood.

In another words, just know how it works.

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Hydration of Alkenes: Addition of H2O by Hydroboration

Borane (BH3) is electron deficient Borane adds to an alkene to give an

organoborane

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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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• Regiochemistry is opposite to Markovnikov orientation (anti-Markovnikov regiochemistry)

– 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)

Orientation in Hydration via Hydroboration

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• 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

Mechanism of Hydroboration

R2BH Allows Only Monoalkylation

Because of its bulky R groups, it has a stronger preference for the less substituted sp2 carbon.

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The Mechanism is the Same

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OH Replaces BR2

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Mechanism for the Oxidation Reaction

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No Carbocation Rearrangements

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The Electrophile Adds to the sp2 Carbon Bonded to the Most Hydrogens

The reagents are numbered because the second set of reagents is not added until the first reaction is over.

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• 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)

Reduction of Alkenes: Hydrogenation

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Addition of Hydrogen

catalytic hydrogenationa reduction reaction

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• 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

Hydrogen Addition - Selectivity

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Mechanism for Hydrogen Addition

catalytic hydrogenation

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Relative Stabilities of Alkenes

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The Most Stable Alkene Has the Smallest Heat of Hydrogenation

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Relative Stabilities of Alkenes

relative stabilities of dialkyl-substituted alkenes

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Formation of an Epoxide

the mechanism is similar to that for the addition of Br2

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• Epoxidation results in a cyclic ether with an oxygen atom

• Stereochemistry of addition is syn

Oxidation of Alkenes: Epoxidationand Hydroxylation

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Nomenclature of Epoxides

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• Hydroxylation - converts to syn-diol

• Osmium tetroxide, then sodium bisulfite

• Via cyclic osmate di-ester

Osmium Tetroxide Catalyzed Formation of Diols

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Hydroxylation Reaction-Syn Diol

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Ozonolysis

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Mechanism for Ozonide Formation

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The Ozonide is Converted to Ketones and/or Aldehydes

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• Ozone, O3, adds to alkenes to form molozonide

• Molozonideis converted to ozonide that may be reduced to obtain ketones and/or aldehydes

Oxidation of Alkenes: Cleavage to Carbonyl Compounds

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• Used in determination of structure of an unknown alkene

Examples of Ozonolysis of Alkenes

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Ozonolysis

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• 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

Permangate Oxidation of Alkenes

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• 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

Cleavage of 1,2-diols

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• The carbene functional group is “half of an alkene”

• Carbenes are electronically neutral with six electrons in the outer shell

• They add symmetrically across double bonds to form cyclopropanes

Addition of Carbenes to Alkenes: CyclopropaneSynthesis

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• Addition of dichlorocarbene is stereospecific cis

Reaction of Dichlorocarbene

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• Base removes proton from chloroform

• Stabilized carbanionremains

• Unimolecularelimination of Cl- gives electron deficient species, dichlorocarbene

Formation of Dichlorocarbene

• Equivalent of addition of CH2:

• Reaction of diiodomethane with zinc-copper alloy produces a carbenoid species

• Forms cyclopropanes by cycloaddition

Simmons-Smith Reaction

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Electrophilic Addition

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Four Stereoisomers are Obtained if the Reaction Forms a Carbocation Intermediate

syn and anti addition74

Syn Addition to a Cis Isomer Forms Only the Erythro Stereoisomers

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Syn Addition to a Trans Isomer Forms Only the Threo Stereoisomers

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• 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 polyethylene, for example

Radical Additions to Alkenes: Chain-Growth Polymers

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• 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

Free Radical Polymerization: Initiation

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

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

Polymerization: Propagation

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• Chain propagation ends when two radical chains combine

• Not controlled specifically but affected by reactivity and concentration

Polymerization: Termination

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• Other alkenes give other common polymers

Other Polymers

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• 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

Biological Additions of Radicals to Alkenes

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