7. Alkenes: Reactions and Synthesis Based on McMurry’s Organic Chemistry, 6th edition ©2003 Ronald Kluger Department of Chemistry University of Toronto

Diverse Reactions of Alkenes Alkenes 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) McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

7.1 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) McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

7.2 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 McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

Addition of Br2 to Cyclopentene Addition is exclusively trans + McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

Mechanism of Bromine Addition Br+ adds to an alkene producing a cyclic ion Bromonium ion, bromine shares charge with carbon Gives trans addition McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

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 McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

The Reality of Bromonium Ions Bromonium were postulated more than 60 years ago to expain 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 McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003 7.3 Halohydrin Formation This 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) McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

Mechanism of Formation of a Bromohydrin Br2 forms bromonium ion, then water adds Orientation toward stable C+ species Aromatic rings do not react McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

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 McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

7.4 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 McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

Oxymercuration Intermediates For laboratory-scale hydration of an alkene Use mercuric acetate in THF followed by sodium borohydride Markovnikov orientation via mercurinium ion McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

7.5 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 McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003 BH3 Is a Lewis Acid Six electrons in outer shell Coordinates to oxygen electron pairs in ethers McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003 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 McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

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) McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

Mechanism of Hydroboration Borane is a Lewis acid Alkene is Lewis base Transition state involves anionic development on B The components of BH3 are across C=C McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

Hydroboration: Orientation in Addition Step Addition in least crowded orientation, syn Addition also is via most stable carbocation McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

Hydroboration, Electronic Effects Give Non-Markovnikov More stable carbocation is also consistent with steric preferences McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

Hydroboration - Oxygen Insertion Step H2O2, OH- inserts OH in place of B Retains syn orientation McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

7.6 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 symmetrically across double bonds to form cyclopropanes McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

Formation of Dichlorocarbene Base removes proton from chloroform Stabilized carbanion remains Unimolecular Elimination of Cl- gives electron deficient species, dichlorocarbene McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

Simmons-Smith Reaction Equivalent of addition of CH2: Reaction of diiodomethane with zinc-copper alloy produces a carbenoid species Forms cyclopropanes by cycloaddition McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

Reaction of Dichlorocarbene Addition of dichlorocarbene is stereospecific cis McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

7.7 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) McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

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 McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

Mechanism of Catalytic Hydrogenation Heterogeneous – reaction between phases Addition of H-H is syn McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

7.8 Oxidation of Alkenes: Hydroxylation and Cleavage Hydroxylation adds OH to each end of C=C Catalyzed by osmium tetroxide Stereochemistry of addition is syn Product is a 1,2-dialcohol or diol (also called a glycol) McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

Osmium Tetroxide Catalyzed Formation of Diols Hydroxylation - converts to syn-diol Osmium tetroxide, then sodium bisulfate Via cyclic osmate di-ester McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

Alkene Cleavage: Ozone Ozone, O3, adds to alkenes to form molozonide Reduce molozonide to obtain ketones and/or aldehydes McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

Examples of Ozonolysis of Alkenes Used in determination of structure of an unknown alkene McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

Structure Elucidation With Ozone Cleavage products reveal an alkene’s structure McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

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 McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003 Cleavage of 1,2-diols Reaction of a 1,2-diol with periodic (per-iodic) acid, HIO4 , cleaves the diol into two carbonyl compinds Sequence of diol formation with OsO4 followed by diol cleavage is a good alternative to ozonolysis McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

Mechanism of Periodic Acid Oxidation Via cyclic periodate intermediate McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

7.9 Biological Alkene Addition Reactions Living organisms convert organic molecules using enzymes as catalysts Many reactions are similar to organic chemistry conversions, except they occur in neutral water Usually much specific for reactant and stereochemistry McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

Biological Hydration Example Fumarate to malate catalyzed by fumarase Specific for trans isomer Addition of H, OH is anti McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

7.10 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 McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

Free Radical Polymerization of Alkenes Alkenes combine many times to give polymer Reactivity induced by formation of free radicals McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

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 McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

Polymerization: Propagation Radical from intiation adds to alkene to generate alkene derived radical This radical adds to another alkene, and so on many times McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

Polymerization: Termination Chain propagation ends when two radical chains combine Not controlled specifically but affected by reactivity and concentration McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003 Other Polymers Other alkenes give other common polymers McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

Unsymmetrical Monomers If alkene is unsymmetrical, reaction is via more highly substituted radical McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

Chain Branching During Polymerization During radical propagation chain can develop forks leading to branching One mechanism of branching is short chain branching in which an internal hydrogen is abstracted McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003 Long Chain Branching In long chains, a hydrogen from another chain is abstracted McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003

Cationic Polymerization Vinyl monomers react with Brønsted or Lewis acid to produce a reactive carbocation that adds to alkenes and propagates via lengthening carbocations McMurry Organic Chemistry 6th edition Chapter 7 (c) 2003