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Chapter 14 ALKENES AND ALKYNES II.

Chapter 14 ALKENES AND ALKYNES II. Oxidation and Reduction Oxymercuration and Hydroboration Simple Ring Formation. Assigned Sections. Sections 14.1 through 14.4 (c. 352) Summary (Table 14-1) Skip section 14.3 Sections 14.5-14.13 will be in c. 353. Problem Assignment. Chemistry 352 and 353

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Chapter 14 ALKENES AND ALKYNES II.

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  1. Chapter 14ALKENES AND ALKYNES II. Oxidation and ReductionOxymercuration and HydroborationSimple Ring Formation

  2. Assigned Sections • Sections 14.1 through 14.4 (c. 352) • Summary (Table 14-1) • Skip section 14.3 • Sections 14.5-14.13 will be in c. 353

  3. Problem Assignment Chemistry 352 and 353 In-Text Problems: 1 – 3 4b 6 - 8 10-18 End-of-Chapter Problems: 25 - 26 28 – 31 33 – 35 38 42 (all but a) 45- 47 19, 20 22 - 24 39 – 41 44 48

  4. What is Oxidation? • Gain of oxygen • Loss of hydrogen • Skim section between p. 1 and 2 Oxidation

  5. OXIDATIONS OF ALKENES glycol formation epoxidation complete oxidation cleavage n + H2O

  6. What is Reduction? • Gain of hydrogen • Loss of oxygen reduced

  7. Sect. 14.1 EPOXIDES anti-glycol formation

  8. EPOXIDATION AND ANTI GLYCOL FORMATION First we will look at 1) the formation of epoxides, and 2) the opening of epoxides to form glycols generally “trans” anti epoxide glycol

  9. Carboxylic Acid Hydrogen peroxide Organic Peroxide A PERACID

  10. TWO COMMONLY-USED PERACIDS peracetic acid m-chloroperbenzoic acid (MCPBA)

  11. EPOXIDATION WITH A PERACID A CONCERTED REACTION both bonds formed at the same time epoxide STEREOSPECIFIC

  12. anti-Glycol Formation EPOXIDATION anti ring opening RING OPENING

  13. SN2 anti only OPENING EPOXIDES IN BASE See Chapter 12, Section 12.8 : .. .. .. : : : : : : : : : .. : : SN2 : hydroxyl attacks at leastsubstituted carbon, STEREOSPECIFIC and on the opposite side

  14. OPENING EPOXIDES IN ACID • See Chapter 12, Section 12.8 for a review. • My Chapter 12 webpage give some examples

  15. Sect. 14.2 syn-GLYCOL FORMATION

  16. syn -Glycol Formation syn glycols are made with OsO4 or KMnO4 OsO4 osmium tetroxide KMnO4 potassium permanganate

  17. H O O s O N a H S O 4 3 H O 2 O s O N a H S O 4 3 SYN ADDITION GIVES CISGLYCOLS C H 3 C H H 3 2 O H H O H cis C H 3 C H H 3 C H C syn conformation C H C H 3 3 C H O H C H O H meso cis -2-butene remember: addition is syn result is cis

  18. syn-Glycol Formation (I) osmium tetroxide syn hydroxylation

  19. Mechanism Notice the transfer of 2e- onto Os = REDUCTION OXIDIZED Both of the hydroxyl oxygens in the glycol come from OsO4 REDUCED

  20. syn-Glycol Formation (II) potassium permanganate syn hydroxylation

  21. Making epoxides via bromohydrins Bromohydrin (Chapter 8) ( peracid )

  22. Skip Sect. 14.3

  23. Sect. 14.4 OZONOLYSIS

  24. Ozone electric discharge or cosmic rays .. .. .. .. : .. .. .. - + + : : .. .. - EQUIVALENT RESONANCE STRUCTURES

  25. Ozonolysis FORMATION OF AN OZONIDE unstable HYDROLYSIS OF THE OZONIDE (WORKUP) aldehydes or ketones

  26. WORKUP PROCEDURES FOR OZONOLYSIS Two types of workup (decomposition of the ozonide) are possible : 1. OXIDATIVE Hydrogen peroxide is present Aldehydes are oxidized to carboxylic acids. Formaldehyde is oxidized to carbon dioxide, which is lost as a gas. 2. REDUCTIVE Add Zn and H2O or H3O+ METHOD A Reduce the ozonide with Pd / H2 , and then add acid ( H3O+ ). METHOD B Aldehydes survive intact and are not oxidized with reductive conditions.

  27. REDUCTIVE WORKUP OXIDATIVE WORKUP EXAMPLES O3 Zn / H2O OR O3 1) Pd/H2 • O3 2) H3O+ 2) H2O2 H3O+ + H2O

  28. “At one time” = before spectroscopy. AT ONE TIME OZONOLYSIS WAS WIDELY USED FOR STRUCTURE PROOF BY DEGRADATION Broken apart ( or degraded ) to simpler pieces that are easier to identify. Unknown compound The original structure can be deduced by reassembling the pieces.

  29. PROBLEM TO SOLVE 1) O3 / CH2Cl2 C7H12 2) H3O+ Pd / H2 C7H14 answer

  30. oxidative workup WHAT WAS THE ORIGINAL STRUCTURE ? H2O2

  31. ACETYLENES 1) O3, CH2Cl2 2) H3O+ KMnO4 or Oxidation of acetylenes, whether by KMnO4 or ozone, normally yields carboxylic acids.

  32. O3 NATURAL SOURCES OF SMOG temperature inversion traps bioemissions reacts with terpenes Terpenes Spruce, Cedar, Fir or Pine Forest

  33. Sect. 14.5 Hydrogenation of Alkenes catalyst + C C H H C C H H fine powder, very porous, suspended in solution a syn addition reaction The catalyst is Pt, PtO2, Pd, or Ni (in special cases Ru, Rh, Re)

  34. REVIEW OF HYDROGENATION CATALYTIC HYDROGENATION IS COVERED IN SEVERAL PLACES IN THE TEXT, PRINCIPALLY: Ch3 Section 3.18 Hydrogenation of Alkenes Ch4 Section 4.13 Relative Stabilities of Alkenes Ch6 Section 6.12 Heat of Hydrogenation Section 6.13 Resonance Energy of Benzene Ch8 Section 8.11 Hydrogenation of the Double Bond Pages 723-725 (Addition to Triple Bonds) Ch 14 Section 14.4 Hydrogenation of Alkynes

  35. Hydrogenation of Alkynes

  36. Lindlar Catalyst: syn addition syn addition Lindlar is a special catalyst that allows the hydrogenation of an alkyne to stop after one mole of hydrogen is added. Most amines, and compounds containing sulfur, reduce the activity of catalysts or “poison” them. quinoline

  37. Sect 14.6 Dissolving Metal Reduction anti This reaction proceeds with anti addition (trans compound). Catalytic reduction proceeds with syn addition, hence we have a choice of methods.

  38. MECHANISM OF Na-LIQUID-NH3 REDUCTIONS electron transfer 1 . - All intermediates prefer the trans geometry. ( from (NH3)n ) +2H+ +2e radical- anion anion anti addition radical :NH2- electron transfer 2

  39. TWO DIFFERENT REAGENTS ! SODIUM IN LIQUID AMMONIA = Reducing Agent Na / NH3(l) Strong Base = SODIUM AMIDE IN LIQUID AMMONIA NaNH2 / NH3(l)

  40. Sect. 14.7: Oxymercuration of an Alkene

  41. Mechanism of Oxymercuration Formation of Bridged Ion: step 1

  42. Step 2: Attack by water

  43. Step 3: Reduction

  44. Another example Continued next slide

  45. Sect. 14.8: Hydroboration of an Alkene

  46. Herbert Brown Nobel Prize, 1979 Source:Michigan State University, Department of Chemistry http://www.chemistry.msu.edu/Portraits/PortraitsHH_collection.shtml

  47. Preparation of Diborane Electron deficient

  48. concerted DIBORANE ADDS TO ALKENES ANTI-MARKOVNIKOFF substituent stabilizes transient carbocation center R R hydride transfer electrophile sp2 empty 2p orbital R Since there are 3 B-H bonds, addition can occur three times (3 moles of alkene) syn addition anti-Markovnikoff Boron adds to the carbon with the most hydrogens (!) which is the reverse of the Markovnikoff Rule.

  49. d+ R - d Concerted versus Carbocation Intermediate R R + - Carbocation intermediate would allow rotation; the reaction would not be stereospecific, giving both syn + anti addition. Concerted reaction, no intermediate; syn addition

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