OXIDATION AND REDUCTION OF ALKENES

1. ALKENES AND ALKYNES II OXIDATION AND REDUCTION OF ALKENES

2. With metals chemists usually define oxidation and reduction in terms of the loss or gain of electrons. Here are some new ways to look at oxidation and reduction from the viewpoint of organic compounds.

3. OXIDATION

4. CH4 + 2 O2 CO2 + 2 H2O What is Oxidation? gained 2 oxygens lost 4 hydrogens • Loss of electrons • Gain of oxygen atoms • Loss of hydrogen atoms + O - 2e + O + O - 2e - 2e - 2e - 2H+ INCREASING OXIDATION d- d+ Each oxygen, or an other electronegative element, takes electrons from carbon, oxidizing it. C H C O

5. What is Oxidation? more examples (oxidized) Co2+ - 1e- Co3+ C O - 2H+ - 2e- C=O (oxidized) (= H2) H H (oxidized) C H + O2 C=O + H2O H (= +O -H2) C + O2 CO2 (oxidized)

6. REDUCTION

7. What is Reduction? • Gain of electrons • Gain of hydrogen atoms • Loss of oxygen atoms REDUCED H2 can be viewed as 2H+ + 2e- , i.e., as adding two electrons lost (a bond to) oxygen gained two hydrogens

8. What is Reduction? more examples (reduced) Fe3+ + 1e- Fe2+ C=O + 2H+ + 2e- C O (reduced) (= H2) H H C=O + 4H+ + 4e- C H + H2O (= 2H2) H (reduced)

9. OVERVIEW OF ALKENE OXIDATIONS

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

11. OVERVIEW The first part of Chapter 14 deals with oxidations of alkenes. glycol cleavage syn over-oxidation ozonide anti complete oxidation epoxide glycol SOME OXIDATION REACTIONS OF ALKENES

12. 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 unless opened in acid and a good carbocation can form epoxide glycol

13. EPOXIDES

14. R R R R EPOXIDES (OXIRANES) cis trans O O planar ring 1,2-Disubstituted oxiranes can have cis - trans isomerism.

15. MAKING EPOXIDES TWO METHODS 1) via bromohydrin (Sections 8.7 and 12.8) cis cis bromohydrin alkene epoxide ( peracid ) 2) using peracid (Section 14.1)

16. METHOD ONE SYNTHESIS OF EPOXIDES BY CYCLIZATION OF BROMOHYDRINS

17. EPOXIDES FROM BROMOHYDRINS carbonic acid unstable, decomposes bicarbonate [ H2CO3 ] .. Na+ : .. CO2 + H2O : .. .. .. .. : : : + NaBr NaHCO3 : : .. internal SN2

18. METHOD TWO SYNTHESIS OF EPOXIDES USING PERACIDS

19. PERACIDS Carboxylic Acid Hydrogen peroxide Organic Peroxide A PERACID

20. TWO COMMONLY-USED PERACIDS peracetic acid m-chloroperbenzoic acid

21. PERACIDS ARE NOT REALLY ACIDS ! resonance -H+ Acid doesn’t happen Peracid -H+ no resonance resonance Peracids are subject to nucleophilic attack that breaks the O-O bond

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

23. . . . . . . . . . . . . . . . . . . STEPWISE ANALYSIS OF THE CONCERTED REACTION An open carbocation would allow cis - trans interconversion. _ : .. : : + : .. All these steps are merged .. .. .. + : : : _ .. .. REMEMBER: “Concerted” means everthing happens in one step.

24. THE REACTION IS STEREOSPECIFIC RCO3H RCO3H cis cis trans trans This stereospecificity argues against the reaction being a stepwise process. An “open” carbocation would have a lifetime and would be free to rotate, thereby interchanging the cis and trans arrangement of the R groups before the ring closes. .. .. .. .. + + cis trans

25. OPENING EPOXIDES

26. OPENING EPOXIDES IN AQUEOUS MEDIUM GIVES GLYCOLS (1,2-DIOLS) “anti” glycol “syn” glycol Ring opening can be either syn or anti depending on conditions. 1,2-diols “glycols” 1,2-diols are frequently called “glycols”

27. SN2 anti only OPENING EPOXIDES basic solution (Section 12.8) : .. .. .. : : : : : : : : : .. : : SN2 : hydroxyl attacks at leastsubstituted carbon, STEREOSPECIFIC and on the opposite side

28. ACID SOLUTION In acid solution epoxides can open by either SN1 or SN2 depending on the substitution pattern. Epoxides with only primary or secondary cabons react primarily via the SN2 mechanism. attack occurs at the least-substituted carbon 1o 2o Epoxides with tertiary carbons (or the possibility of resonance) will react via the SN1 mechanism. 3o attack occurs at the most-substituted carbon resonance

29. opens only if tertiary or conjugated asymmetric bridged ion anti only OPENING EPOXIDES water attacks at most substituted carbon acidic solution (Section 12.8) SN1 : : .. syn + anti .. : : : : : .. + : SN2 : : water attacks at least substituted carbon NOT STEREOSPECIFIC opening in acid frequently gives “anti” but more often gives a mixture: syn + anti

30. anti GLYCOL FORMATION SUMMARY

31. anti -Glycol Formation accomplished by opening of epoxides H2O (H+ or OH-) BASE : always anti (SN2) STEREOSPECIFIC ACID : usually anti , unless a stable carbocation can form then you get syn + anti(SN1) NOT RELIABLY STEREOSPECIFIC

32. SYN GLYCOLS AND CLEAVAGE glycol cleavage syn “syn” over- oxidation Next we will look at 1) the syn formation of glycols (aldehydes) 2) cleavage of syn glycols 3) oxidation of aldehydes

33. syn GLYCOL FORMATION trans - Glycols and anti -glycols are made by opening epoxides ….. is there a stereospecific method to make cis -glycols or syn -glycols ?

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

35. : : : : : : : : O O O O .. .. .. O s Mn O O O O : : : : .. .. .. .. - CYCLIC 5-RING INTERMEDIATES These reagents react via cyclic five-membered ring intermediates The ring controls the stereochemistry to be syn. Both oxygens attach to the same side of the C=C bond.

36. OSMIUM TETROXIDE

37. 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 What is the result with trans -2-butene?

38. MECHANISM Notice the transfer of 2e- onto Os = REDUCTION of Osmium NaHSO3 / Both of the hydroxyl oxygens in the glycol come from OsO4 OXIDIZED further reduced REDUCED Os + 4H2O NaHSO3

39. POTASSIUM PERMANGANTE

40. MECHANISM K+ KMnO4 oxidized purple reduced brown sludge

41. KMn O 4 KMn O 4 C H 3 C H H 3 O H dilute, neutral, 0oC H O H cis C H 3 C H H 3 C H C C H C H 3 3 C H O H C H dilute, neutral, 0oC O H meso What is the result with trans -2-butene? Same result as with OsO4.

42. TESTS FOR C=C DOUBLE BONDS

43. PERMANGANATE TEST FOR A C=C DOUBLE BOND POSITIVE TEST Baeyer Test KMnO4 reacts implies the presence of a double bond KMnO4 brown sludge MnO2 (+) purple NEGATIVE TEST (-) KMnO4 is not decolorized compound with a suspected double bond implies that a double bond is not present

44. BROMINE TEST FOR A C=C DOUBLE BOND POSITIVE TEST DO YOU REMEMBER ? Br2 reacts implies the presence of a double bond Br2 / CCl4 (+) purple colorless NEGATIVE TEST (-) Br2 is not decolorized (does not react) compound with a suspected double bond implies that a double bond is not present

45. KMnO4 MORE VIGOROUS CONDITIONS

46. MORE VIGOROUS OXIDATION WITH KMnO4 Given enough time, or if the temperature is increased, or if the solution is made strongly acidic or basic, KMnO4 will oxidize almost any organic compound to carbon dioxide and water. partial oxidation Notice how changing the conditions gives three different results. glycol formation the compound is destroyed Notice that the sidechain oxidizes more easily than the benzene ring.

47. Cleavage of syn Glycols glycol cleavage “syn” Cleavage of glycols over- oxidation (aldehydes)

48. CLEAVAGE OF GLYCOLS HIO4 periodic acid

49. CLEAVAGE OF syn - GLYCOLS HIO4 O O H H HO OH H H two carbonyl compounds Once again … 5-ring cyclic intermediates are involved.

50. PERIODIC ACID WILL CLEAVE syn - GLYCOLS H H H H HO OH HIO4 .. cyclic intermediate .. : : : : O O : : .. O O I I O O : : : : : : O O .. .. .. .. - - hydrolysis H O 2 O O H H H O O H .. HIO4 is a mild reagent aldehydes do not oxidize. I : : O O : .. .. -