Chapter 7. Alkenes: Reactions and Synthesis

*Diverse Reactions of AlkenesAlkenes 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)

*Reactions of Alkenes

*Preparation of Alkenes

*Part 1 - Synthesis of AlkenesThese reactions are used to produce alkenes.

*Synthesis of Alkenes: Synthesis 1#Alkenes are commonly made by elimination of HX from alkyl halide (dehydrohalogenation)Uses heat and KOH

*Synthesis of Alkenes: Synthesis 2#elimination of H-OH from an alcohol (dehydration) require strong acids (sulfuric acid, 50 C)

*Part 2 - Reaction of AlkenesThese reactions react alkenes to form a series of alkane products.

*Addition of Halogens to AlkenesBromine and chlorine add to alkenes to give 1,2-dihaldesF2 is too reactive and I2 does not add.

*Example: Mechanism of Bromine AdditionElectrophilic 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.

*Example: Addition of Br2 to CyclopenteneAddition is exclusively trans

*Halohydrin FormationThis 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)AlkeneHalohydrin

*An Alternative to BromineBromine is a difficult reagent to use for this reactionN-Bromosuccinimide (NBS) produces bromine in organic solvents and is a safer source.

*Addition of Water to Alkenes: OxymercurationHydration of an alkene is the addition of H-OH to to give an alcoholAcid catalysts are used in high temperature industrial processes: ethylene is converted to ethanol

*Addition of Water to Alkenes: OxymercurationHg(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.

*Addition of Water to Alkenes: HydroborationHerbert Brown (HB) invented hydroboration (HB)Borane (BH3) is electron deficient is a Lewis acid.Borane adds to an alkene to give an organoborane.

*BH3 Is a Lewis AcidSix electrons in outer shellCoordinates to oxygen electron pairs in ethers

* Addition of H-BH2 (from BH3-THF complex) to three alkenes gives a trialkylboraneOxidation with alkaline hydrogen peroxide in water produces the alcohol derived from the alkene Hydroboration-Oxidation Alcohol Formation from Alkenes

*Orientation in Hydration via HydroborationRegiochemistry is opposite to Markovnikov orientationOH is added to carbon with most HsH and OH add with syn stereochemistry, to the same face of the alkene (opposite of anti addition)

*Mechanism of HydroborationBorane is a Lewis acidAlkene is Lewis baseTransition state involves anionic development on BThe components of BH3 are across C=C

*Hydroboration, Electronic Effects Give Non-Markovnikov More stable carbocation is also consistent with steric preferences

*Hydroboration - Oxygen Insertion StepH2O2, OH- inserts OH in place of BRetains syn orientation

*Addition of Carbenes to AlkenesThe carbene functional group is half of an alkeneCarbenes are electrically neutral with six electrons in the outer shellThey symmetrically across double bonds to form cyclopropanes

*Formation of DichlorocarbeneBase removes proton from chloroformStabilized carbanion remainsUnimolecular Elimination of Cl- gives electron deficient species, dichlorocarbene

*Simmons-Smith ReactionEquivalent of addition of CH2: Reaction of diiodomethane with zinc-copper alloy produces a carbenoid speciesForms cyclopropanes by cycloaddition

*Reaction of DichlorocarbeneAddition of dichlorocarbene is stereospecific cis

*Reduction of Alkenes: HydrogenationAddition of H-H across C=CReduction in general is addition of H2 or its equivalent Requires Pt or Pd as powders on carbon and H2Hydrogen is first adsorbed on catalystReaction is heterogeneous (process is not in solution)

*Hydrogen Addition- SelectivitySelective for C=C. No reaction with C=O, C=NPolyunsaturated liquid oils become solidsIf one side is blocked, hydrogen adds to other

*Mechanism of Catalytic HydrogenationHeterogeneous reaction between phasesAddition of H-H is syn

*Oxidation of Alkenes: Hydroxylation and CleavageHydroxylation adds OH to each end of C=CCatalyzed by osmium tetroxideStereochemistry of addition is synProduct is a 1,2-dialcohol or diol (also called a glycol)

*Osmium Tetroxide Catalyzed Formation of DiolsHydroxylation - converts to syn-diolOsmium tetroxide, then sodium bisulfateVia cyclic osmate di-ester

*Section 3: Breakdown of AlkenesThese Reactions are used to breakdown alkenes into two products.

*Alkene Cleavage: OzoneOzone, O3, adds to alkenes to form molozonideReduce molozonide to obtain ketones and/or aldehydes

*Examples of Ozonolysis of AlkenesUsed in determination of structure of an unknown alkene

*Structure Elucidation With Ozone Cleavage products reveal an alkenes structure

*Permanganate Oxidation of AlkenesOxidizing reagents other than ozone also cleave alkenesPotassium permanganate (KMnO4) can produce carboxylic acids and carbon dioxide if Hs are present on C=C

*Cleavage of 1,2-diolsReaction of a 1,2-diol with periodic (per-iodic) acid, HIO4 , cleaves the diol into two carbonyl compindsSequence of diol formation with OsO4 followed by diol cleavage is a good alternative to ozonolysis

*Mechanism of Periodic Acid OxidationVia cyclic periodate intermediate

*Biological Alkene Addition ReactionsLiving organisms convert organic molecules using enzymes as catalystsMany reactions are similar to organic chemistry conversions, except they occur in neutral waterUsually much specific for reactant and stereochemistry

*Biological Hydration ExampleFumarate to malate catalyzed by fumaraseSpecific for trans isomerAddition of H, OH is anti

*Addition of Radicals to Alkenes: PolymersA polymer is a very large molecule consisting of repeating units of simpler molecules, formed by polymerizationAlkenes react with radical catalysts to undergo radical polymerizationEthylene is polymerized to poyethylene, for example

*Free Radical Polymerization of Alkenes Alkenes combine many times to give polymerReactivity induced by formation of free radicals

*Free Radical Polymerization: InitiationInitiation - a few radicals are generated by the reaction of a molecule that readily forms radicals from a non-radical moleculeA bond is broken homolytically

*Polymerization: PropagationRadical from intiation adds to alkene to generate alkene derived radicalThis radical adds to another alkene, and so on many times

*Polymerization: TerminationChain propagation ends when two radical chains combineNot controlled specifically but affected by reactivity and concentration

*Other PolymersOther alkenes give other common polymers

*Cationic PolymerizationVinyl monomers react with Brnsted or Lewis acid to produce a reactive carbocation that adds to alkenes and propagates via lengthening carbocations

*Take Home MessageLearn the REACTIONS (ALL OF THEM)

*Synthesis of Alkenes1) dehydrohalogenation2) dehydration

*Part 2 - Reaction of Alkenes1) Addition of Halogens to Alkenes2) Halohydrin Formation

*Part 2 - Reaction of Alkenes3) Addition of Water to Alkenes4) Hydroboration-Oxidation Alcohol Formation 5) Carbene Formation Cyclopropane synthesis

*Part 2 - Reaction of Alkenes6) Catalytic Hydrogenation7) Hydroxylation and Cleavage

*Part 3 - Breakdown of Alkenes1) Ozonolysis2) Permangante Oxidation 3) Periodic Acid Oxidation, Cleavage of 1,2-diols

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