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The Reactions of Alkenes

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The Reactions of Alkenes The Reactions of Alkenes The Stereochemistry of Addition Reactions 1 Diverse Reactions of Alkenes Alkenes react with many electrophiles to give useful products by addition (often through special reagents) 2 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) • requires strong acids (sulfuric acid, 50 ºC) 3 A Regioselective Reaction A reaction in which one structural isomer is favored over another, leading to its predominance in the mixture of products. 4 A Stereoselective Reaction A reaction in which one stereoisomer in a mixture is produced more rapidly than another, resulting in predominance of the favored stereoisomer in the mixture of products. 5 A Stereospecific Reaction A reaction in which a particular stereoisomeric form of reactant gives one specific stereoisomer of product, while a different stereoisomeric form of reactant leads to a different single pure streoisomer of product. Stereospecific reaction is also stereoselective; however, stereoselective reaction is not stereospecific. 6 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. 7 The Mechanism of the Reaction 8 Relative Stabilities of Carbocations 9 Hyperconjugation Stabilizes a Carbocation 10 The Difference in Carbocation Stability Determines the Products 11 The Product Does Not Have Stereoisomers Not Regioselective or Regiospecific 12 Formation of a Racemic Mixture 13 The Major Product is a Surprise 14 The Major Product is a Surprise 15 The Carbocation Does Not Rearrange (No Improvement in Carbocation Stability) 16 Anti-Markovnikov Regiochemistry; Radical Intermediate 17 Mechanism of the Reaction 18 Radical Stabilities 19 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 20 Addition of Br2 to Cyclopentene • Addition is exclusively trans 21 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 The Mechanism for the Addition of a Halogen The intermediate is a cyclic bromonium ion. 23 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) • Olah showed that bromonium ions are stable in liquid SO2 with SbF5 and can be studied directly 24 No Carbocation Rearrangements 25 Alkenes Do Not Add I2 26 Halohydrins from Alkenes: Addition of HOX • 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) 27 Formation of Halohydrins Markovnikov regiochemistry (OH group) and Anti-addition 28 Mechanism for Halohydrin Formation 29 Why Does it Follow the Same Rule? The electrophile adds to the sp2 carbon bonded to the most hydrogens. 30 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 31 More Reactions 32 Alcohols from Alkenes 1) Acid Catalyzed Hydration of Alkene Markovnikov regiochemistry and Carbocation formation 2) Oxymercuration –demurcuration (reduction) Markovnikov regiochemistry 3) Hydroboration-oxidation anti-Markovnikov regiochemistry and syn-addition 33 Hydration of Alkenes: Addition of H2O • 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 34 Acid-Catalyzed Addition of Water Mechanism for the Acid-Catalyzed Addition of Water 35 Acid-Catalyzed Addition of an Alcohol 36 Oxymercuration Intermediates • For laboratory-scale hydration of an alkene • Use mercuric acetate in THF followed by sodium borohydride • Markovnikov orientation – via mercurinium ion 37 Mechanism for Oxymerucation The final step of this mechanism is not well understood. In another words, just know how it works. 38 Hydration of Alkenes: Addition of H2O by Hydroboration . Borane (BH3) is electron deficient . Borane adds to an alkene to give an organoborane 39 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 40 Orientation in Hydration via Hydroboration • 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) 41 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 R2BH Allows Only Monoalkylation Because of its bulky R groups, it has a stronger preference for the less substituted sp2 carbon. 43 The Mechanism is the Same 44 OH Replaces BR2 45 Mechanism for the Oxidation Reaction 46 No Carbocation Rearrangements 47 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. 48 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) 49 Addition of Hydrogen catalytic hydrogenation a reduction reaction 50 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 51 Mechanism for Hydrogen Addition catalytic hydrogenation 52 Relative Stabilities of Alkenes 53 The Most Stable Alkene Has the Smallest Heat of Hydrogenation 54 Relative Stabilities of Alkenes relative stabilities of dialkyl-substituted alkenes 55 Formation of an Epoxide the mechanism is similar to that for the addition of Br2 56 Oxidation of Alkenes: Epoxidation and Hydroxylation • Epoxidation results in a cyclic ether with an oxygen atom • Stereochemistry of addition is syn 57 Nomenclature of Epoxides 58 Osmium Tetroxide Catalyzed Formation of Diols • Hydroxylation - converts to syn-diol • Osmium tetroxide, then sodium bisulfite • Via cyclic osmate di-ester 59 Hydroxylation Reaction-Syn Diol 60 Ozonolysis 61 Mechanism for Ozonide Formation 62 The Ozonide is Converted to Ketones and/or Aldehydes 63 Oxidation of Alkenes: Cleavage to Carbonyl Compounds • Ozone, O3, adds to alkenes to form molozonide • Molozonideis converted to ozonide that may be reduced to obtain ketones and/or aldehydes 64 Examples of Ozonolysis of Alkenes • Used in determination of structure of an unknown alkene 65 Ozonolysis 66 Permangate 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 67 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 68 Addition of Carbenes to Alkenes: Cyclopropane Synthesis • 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 69 Reaction of Dichlorocarbene • Addition of dichlorocarbene is stereospecific cis 70 Formation of Dichlorocarbene • Base removes proton from chloroform • Stabilized carbanion remains • Unimolecular elimination of Cl- gives electron deficient species, dichlorocarbene Simmons-Smith Reaction • Equivalent of addition of CH2: • Reaction of diiodomethane with zinc-copper alloy produces a carbenoid species • Forms cyclopropanes by cycloaddition 72 Electrophilic Addition 73 Four Stereoisomers are Obtained if the Reaction Forms a Carbocation Intermediate syn and anti addition 74 Syn Addition to a Cis Isomer Forms Only the Erythro Stereoisomers 75 Syn Addition to a Trans Isomer Forms Only the Threo Stereoisomers 76 Radical Additions to Alkenes: Chain-Growth 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 polyethylene, for example 78 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 79 Polymerization: Propagation • Radical from initiation adds to alkene to generate alkene derived radical • This radical adds to another alkene, and so on many times 80 Polymerization: Termination • Chain propagation ends when two radical chains combine • Not controlled specifically but affected by reactivity and concentration 81 Other Polymers • Other alkenes give other common polymers 82 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 83.
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