Diverse Reactions of Alkenes

Transcription

1 Chapter 8- Alkenes: Reactions and Synthesis Ashley Piekarski, Ph.D. Diverse Reactions of Alkenes Alkenes react with many electrophiles to give useful products by addiaon (ocen through special reagents) 1

2 Why am I learning this, Dr. P? To begin a systemaac descripaon of major funcaonal groups Begin to focus on general principles and paierns of reacavity that Ae organic chemistry 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) 2

3 Addition of Halogens to Alkenes Bromine and chlorine add to alkenes to give 1,2- dihaldes, an industrially important process F 2 is too reactive and I 2 does not add Cl 2 reacts as Cl + Cl - Br 2 is similar Addition of Br 2 to Cyclopentene AddiAon is exclusively trans 3

4 Mechanism of Bromine Addition Br + adds to an alkene producing a cyclic ion Bromonium ion, bromine shares charge with carbon Gives trans addition Bromonium Ion Mechanism Electrophilic addiaon of bromine to give a caaon is followed by cyclizaaon to give a bromonium ion This bromoniun ion is a reacave electrophile and bromide ion is a good nucleophile 4

5 The Reality of Bromonium Ions Bromonium ions were postulated more than 60 years ago to explain the stereochemical course of the addiaon (to give the trans- dibromide from a cyclic alkene) Olah showed that bromonium ions are stable in liquid SO 2 with SbF 5 and can be studied directly Addition of Hypohalous Acids to Alkenes: Halohydrin Formation This is formally the addiaon of HO- X to an alkene to give a 1,2- halo alcohol, called a halohydrin The actual reagent is the dihalogen (Br 2 or Cl 2 in water in an organic solvent) 5

6 Mechanism of Formation of a Bromohydrin An Alternative to Bromine Bromine is a difficult reagent to use for this reacaon N- Bromosuccinimide (NBS) produces bromine in organic solvents and is a safer source 6

7 Addition of Water to Alkenes: Oxymercuration Hydra5on of an alkene is the addiaon of H- OH to to give an alcohol Acid catalysts are used in high temperature industrial processes: ethylene is converted to ethanol Hydration Mechanism 7

8 Oxymercuration Mechanism For laboratory- scale hydraaon of an alkene Use mercuric acetate in THF followed by sodium borohydride Markovnikov orientaaon via mercurinium ion Addition of Water to Alkenes: Hydroboration Herbert Brown (HB) invented hydroboraaon (HB) Borane (BH 3 ) is electron deficient is a Lewis acid Borane adds to an alkene to give an organoborane 8

9 HydroboraAon- OxidaAon Forms an Alcohol from an Alkene AddiAon of H- BH 2 (from BH 3 - THF complex) to three alkenes gives a trialkylborane OxidaAon with alkaline hydrogen peroxide in water produces the alcohol derived from the alkene Orientation in Hydration via Hydroboration Regiochemistry is opposite to Markovnikov orientaaon 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 ana addiaon) 9

10 Mechanism of Hydroboration Addition of Carbenes to Alkenes The carbene funcaonal 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 10

11 Formation of Dichlorocarbene Base removes proton from chloroform Stabilized carbanion remains Unimolecular eliminaaon of Cl - gives electron deficient species, dichlorocarbene Reaction of Dichlorocarbene AddiAon of dichlorocarbene is stereospecific cis in the example below: 11

12 Simmons-Smith Reaction Equivalent of addiaon of CH 2 : ReacAon of diiodomethane with zinc- copper alloy produces a carbenoid species Forms cyclopropanes by cycloaddi5on Reduction of Alkenes: Hydrogenation AddiAon of H- H across C=C ReducAon in general is addiaon of H 2 or its equivalent Requires Pt or Pd as powders on carbon and H 2 Hydrogen is first adsorbed on catalyst ReacAon is heterogeneous (process is not in soluaon) 12

13 Hydrogen Addition- Selectivity SelecAve for C=C. No reacaon with C=O, C=N Polyunsaturated liquid oils become solids If one side is blocked, hydrogen adds to other Mechanism of Catalytic Hydrogenation Heterogeneous reacaon between phases AddiAon of H- H is syn 13

14 Oxidation of Alkenes: Epoxidation and Hydroxylation EpoxidaAon results in a cyclic ether with an oxygen atom Stereochemistry of addiaon is syn Reaction of epoxide to make trans-1,2-diol 14

15 Osmium Tetroxide Catalyzed Formation of Diols Hydroxyla5on - converts to syn- diol Osmium tetroxide, then sodium bisulfate Via cyclic osmate di- ester Oxidation of Alkenes: Cleavage to Carbonyl Compounds Ozone, O 3, adds to alkenes to form molozonide Reduce molozonide to obtain ketones and/or aldehydes 15

16 Molozonide to ozonide mechanism Examples of Ozonolysis of Alkenes Used in determinaaon of structure of an unknown alkene 16

17 Permanganate Oxidation of Alkenes Oxidizing reagents other than ozone also cleave alkenes Potassium permanganate (KMnO 4 ) can produce carboxylic acids and carbon dioxide if H s are present on C=C Cleavage of 1,2-diols ReacAon of a 1,2- diol with periodic (per- iodic) acid, HIO 4, cleaves the diol into two carbonyl compounds Sequence of diol formaaon with OsO 4 followed by diol cleavage is a good alternaave to ozonolysis 17

18 Learning check Predict the products for the following reacaons: 1. Hg(OAc) 2, H 2 O 2. NaBH 4 1. BH 3, THF 2. H 2 O 2 CH 2 I 2, Zn/Cu N a HS OsO 4, HNaHSO 2 O 2 3 O 3 1. O 3 2. Zn, H 3 O + Predict the reactants that made the following products: Learning check O O Cl Cl + O OH Br OH O + O 18

19 Addition of Radicals to Alkenes: Polymers A polymer is a very large molecule consisang of repeaang units of simpler molecules, formed by polymeriza4on Alkenes react with radical catalysts to undergo radical polymerizaaon Ethylene is polymerized to polyethylene, for example Free Radical Polymerization: Initiation Ini5a5on - a few radicals are generated by the reacaon of a molecule that readily forms radicals from a nonradical molecule A bond is broken homolyacally 19

20 Polymerization: Propagation Radical from iniaaaon adds to alkene to generate alkene derived radical This radical adds to another alkene, and so on many Ames Polymerization: Termination Chain propagaaon ends when two radical chains combine Not controlled specifically but affected by reacavity and concentraaon 20

21 Other Polymers Other alkenes give other common polymers Learning check Styrene reacts with methyl methacrylate using free radical polymerizaaon. Draw a segment of the polymer produced. O + O styrene methyl methacrylate 21