ch 7: alkenes, reactions and synthesis
DESCRIPTION
Diverse Reactions of Alkenes Alkenes react with many electrophiles to give useful products by addition (often through special reagents)TRANSCRIPT
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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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The Reality of Bromonium Ions
• 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
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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
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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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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– 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
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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, 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 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