Chapter 7. Alkenes: Reactions and Synthesis

1. Chapter 7. Alkenes: Reactions and Synthesis

2. 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)

3. Reactions of Alkenes

4. Preparation of Alkenes

5. Part 1 - Synthesis of Alkenes • These reactions are used to produce alkenes.

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

7. Synthesis of Alkenes: Synthesis 2# • elimination of H-OH from an alcohol (dehydration) • require strong acids (sulfuric acid, 50 ºC)

8. Part 2 - Reaction of Alkenes • These reactions react alkenes to form a series of alkane products.

9. Addition of Halogens to Alkenes • Bromine and chlorine add to alkenes to give 1,2-dihaldes • F2 is too reactive and I2 does not add.

10. Example: Mechanism of Bromine Addition • 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. • Gives trans addition.

11. Example: Addition of Br2 to Cyclopentene • Addition is exclusively trans

12. 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) Alkene Halohydrin

13. 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.

14. Addition of Water to Alkenes: Oxymercuration • Hydration of an alkeneis the addition of H-OH to to give an alcohol • Acid catalysts are used in high temperature industrial processes: ethylene is converted to ethanol

15. Addition of Water to Alkenes: Oxymercuration Hg(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.

16. 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.

17. BH3 Is a Lewis Acid • Six electrons in outer shell • Coordinates to oxygen electron pairs in ethers

18. Hydroboration-Oxidation Alcohol Formation from Alkenes • 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

19. 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)

20. 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

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

22. Hydroboration - Oxygen Insertion Step • H2O2, OH- inserts OH in place of B • Retains syn orientation

23. 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

24. Formation of Dichlorocarbene • Base removes proton from chloroform • Stabilized carbanion remains • Unimolecular Elimination of Cl- gives electron deficient species, dichlorocarbene

25. Simmons-Smith Reaction • Equivalent of addition of CH2: • Reaction of diiodomethane with zinc-copper alloy produces a carbenoid species • Forms cyclopropanes by cycloaddition

26. Reaction of Dichlorocarbene • Addition of dichlorocarbene is stereospecific cis

27. 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)

28. 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

29. Mechanism of Catalytic Hydrogenation • Heterogeneous – reaction between phases • Addition of H-H is syn

30. 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)

31. Osmium Tetroxide Catalyzed Formation of Diols • Hydroxylation - converts to syn-diol • Osmium tetroxide, then sodium bisulfate • Via cyclic osmate di-ester

32. Section 3: Breakdown of Alkenes • These Reactions are used to breakdown alkenes into two products.

33. Alkene Cleavage: Ozone • Ozone, O3, adds to alkenes to form molozonide • Reduce molozonide to obtain ketones and/or aldehydes

34. Examples of Ozonolysis of Alkenes • Used in determination of structure of an unknown alkene

35. Structure Elucidation With Ozone • Cleavage products reveal an alkene’s structure

36. Permanganate 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

37. 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

38. Mechanism of Periodic Acid Oxidation • Via cyclic periodate intermediate

39. 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

40. Biological Hydration Example • Fumarate to malate catalyzed by fumarase • Specific for trans isomer • Addition of H, OH is anti

41. 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

42. Free Radical Polymerization of Alkenes • Alkenes combine many times to give polymer • Reactivity induced by formation of free radicals

43. Free Radical Polymerization: Initiation • Initiation - a few radicals are generated by the reaction of a molecule that readily forms radicals from a non-radical molecule • A bond is broken homolytically

44. Polymerization: Propagation • Radical from intiation adds to alkene to generate alkene derived radical • This radical adds to another alkene, and so on many times

45. Polymerization: Termination • Chain propagation ends when two radical chains combine • Not controlled specifically but affected by reactivity and concentration

46. Other Polymers • Other alkenes give other common polymers

47. Cationic Polymerization • Vinyl monomers react with Brønsted or Lewis acid to produce a reactive carbocation that adds to alkenes and propagates via lengthening carbocations

48. Take Home Message • Learn the REACTIONS (ALL OF THEM)

49. Synthesis of Alkenes 1) dehydrohalogenation 2) dehydration

50. Part 2 - Reaction of Alkenes 1) Addition of Halogens to Alkenes 2) Halohydrin Formation