2. © 2011 Pearson Education, Inc. Spring 2012 Dr. Halligan CHM 236 Reactions of Alcohols, Ethers, Epoxides, Amines, and Sulfur-Containing Compounds Chapter 10

3. © 2011 Pearson Education, Inc. 2 Ch. 10 Overview Conversion of Alcohols to Alkyl Halides and Sulfonate Esters Substitution reactions of activated alcohols (ROH with better LG’s) Dehydration of alcohols via Elimination Reactions Oxidation of Alcohols Nucleophilic Substitution Reactions of Ethers Nucleophilic Substitution Reactions of Epoxides Amines Do Not Undergo Substitution or Elimination Reactions Quaternary Ammonium Hydroxiudes Undergo Elimination Reactions Phase-Transfer Catalysts Thiols, Sulfides, and Sulfonium Salts

4. © 2011 Pearson Education, Inc. 3 Substitution Reactions of Alcohols The OH group of an alcohol is a terrible LG. Why? We must convert the OH group to a better LG before doing a substitution reaction. In this chapter, we will use HCl, HBr, HI, ZnCl 2, PBr 2, SOCl 2, and R’SO 2 Cl/py to convert OH to a better LG.

5. © 2011 Pearson Education, Inc. 4 Convert the strongly basic leaving group (OH – ) into the good leaving group, H 2 O (a weaker base): Only weakly basic nucleophiles can be used Alcohols and ethers have to be activated before they can undergo a substitution or an elimination reaction:

6. © 2011 Pearson Education, Inc. 5 Primary, secondary, and tertiary alcohols all undergo nucleophilic substitution reactions with HI, HBr, and HCl:

7. © 2011 Pearson Education, Inc. 6 Secondary and tertiary alcohols undergo S N 1 reactions with hydrogen halides:

8. © 2011 Pearson Education, Inc. 7 Primary alcohols undergo S N 2 reactions with hydrogen halides: Reaction carried out in 48% aqueous HBr. Recall that Br – is an excellent nucleophile in water.

9. © 2011 Pearson Education, Inc. 8 ZnCl 2 can be used to catalyze certain S N 2 reactions: ZnCl 2 functions as a Lewis acid that complexes strongly with the lone-pair electrons on oxygen:

10. © 2011 Pearson Education, Inc. 9 The Lucas Reagent Distinguishes primary, secondary, and tertiary low- molecular-weight alcohols The Lucas reaction: Positive test: Solution becomes cloudy Test results and mechanisms at room temperature: Primary: No reaction Secondary: Reaction in ~5 minutes Tertiary, allylic, and benzylic: Reaction immediate SN2SN2SN1SN1 S N 2 and S N 1

11. © 2011 Pearson Education, Inc. 10 Look out for rearrangement product in the S N 1 reaction of the secondary or tertiary alcohol:

12. © 2011 Pearson Education, Inc. 11 Other Methods for Converting Alcohols into Alkyl Halides Utilization of phosphorus tribromide: Other phosphorus reagents can be used: PBr 3, phosphorus tribromide PCl 3, phosphorus trichloride PCl 5, phosphorus pentachloride POCl 3, phosphorus oxychloride

13. © 2011 Pearson Education, Inc. 12 Activation by SOCl 2 Pyridine is generally used as a solvent and also acts as a base:

14. © 2011 Pearson Education, Inc. 13 The Artificial Sweetener Splenda (Sucralose) Could Splenda be a cellular alkylating agent? No, it is too polar to enter a cell.

15. © 2011 Pearson Education, Inc. 14

16. © 2011 Pearson Education, Inc. 15 Summary: Converting of Alcohols to Alkyl Halides Recommended procedures:

17. © 2011 Pearson Education, Inc. 16 Converting Alcohols into Sulfonate Esters

18. © 2011 Pearson Education, Inc. 17 Several sulfonyl chlorides are available to activate OH groups:

19. © 2011 Pearson Education, Inc. 18 Dehydration of Secondary and Tertiary Alcohols by an E1 Pathway To prevent the rehydration of the alkene product, one needs to remove the product as it is formed

20. © 2011 Pearson Education, Inc. 19 The major product is the most stable alkene product: The most stable alkene product has the most stable transition state

21. © 2011 Pearson Education, Inc. 20 The rate of dehydration reflects the ease with which the carbocation is formed:

22. © 2011 Pearson Education, Inc. 21 Look out for carbocation rearrangement:

23. © 2011 Pearson Education, Inc. 22 Complex Alcohol Rearrangements in Strong Acid

24. © 2011 Pearson Education, Inc. 23 Pinicol Rearrangement Protonate alcohol:Eliminate water: Rearrange carbocation:Deprotonate: Resonance-stabilized oxocarbocation

25. © 2011 Pearson Education, Inc. 24 Ring Expansion and Contraction Show the mechanism for this reaction: Protonate the alcohol. Eliminate water. Rearrange carbocation to afford the more stable cyclohexane ring. Deprotonate.

26. © 2011 Pearson Education, Inc. 25 Primary Alcohols Undergo Dehydration by an E2 Pathway

27. © 2011 Pearson Education, Inc. 26 The Stereochemical Outcome of the E1 Dehydration Alcohols and ethers undergo S N 1/E1 reactions unless they would have to form a primary carbocation

28. © 2011 Pearson Education, Inc. 27 A Milder Way to Dehydrate an Alcohol

29. © 2011 Pearson Education, Inc. 28 Oxidation by Chromium (VI)

30. © 2011 Pearson Education, Inc. 29 Oxidation of Alcohols Oxidation by chromic acid: Secondary alcohols are oxidized to ketones

31. © 2011 Pearson Education, Inc. 30 Primary alcohols are oxidized to aldehydes and eventually carboxylic acids: Mechanism:

32. © 2011 Pearson Education, Inc. 31 The oxidation of aldehydes to acids requires the presence of water: In the absence of water, the oxidation stops at the aldehyde: PCC, a methylene chloride–soluble reagent: No water present

33. © 2011 Pearson Education, Inc. 32 A tertiary alcohol cannot be oxidized and is converted to a stable chromate ester instead: Di-tert-Butyl Chromate No hydrogen on this carbon

34. © 2011 Pearson Education, Inc. 33 Amines do not undergo substitution reactions because NH 2 – is a very strong base (a very poor leaving group): Protonation of the amine moiety does not solve the problem:

35. © 2011 Pearson Education, Inc. 34 Nucleophilic Substitution Reactions of Ethers Ethers, like alcohols, can be activated by protonation:

36. © 2011 Pearson Education, Inc. 35 Ether cleavage: an S N 1 reaction: Ether cleavage: an S N 2 reaction:

37. © 2011 Pearson Education, Inc. 36 Reagents such as SOCl 2 and PCl 3 can activate alcohols but not ethers Ethers are frequently used as solvents because only they react with hydrogen halides

38. © 2011 Pearson Education, Inc. 37 Nucleophilic Substitution Reactions of Epoxides Recall Section 4.9

39. © 2011 Pearson Education, Inc. 38 Epoxides are more reactive than ethers in nucleophilic substitution reactions because of ring strain:

40. © 2011 Pearson Education, Inc. 39 Acid-Catalyzed Epoxide Ring Opening HBr: Aqueous acid:

41. © 2011 Pearson Education, Inc. 40 Acidic methanol: Stereospecificity of epoxide ring opening: A trans diol Reason: Back-side attack of water on protonated epoxide:

42. © 2011 Pearson Education, Inc. 41 Reaction of an epoxide in the presence of methanol and acid Mechanism: Regioselectivity:

43. © 2011 Pearson Education, Inc. 42 When a nucleophile attacks an unprotonated epoxide, the reaction is a pure S N 2 reaction: Therefore:

44. © 2011 Pearson Education, Inc. 43 Epoxides Are Synthetically Useful Reagents Enantiomers

45. © 2011 Pearson Education, Inc. 44 Epoxy resins are the strongest adhesives known: Amine nucleophile mediated epoxide ring opening:

46. © 2011 Pearson Education, Inc. 45 Arene Oxides

47. © 2011 Pearson Education, Inc. 46 Arene oxide fate: Enantiomers

48. © 2011 Pearson Education, Inc. 47 Mechanism for arene oxide rearrangement:

49. © 2011 Pearson Education, Inc. 48 Arene oxides as a carcinogen: DNA backbone

50. © 2011 Pearson Education, Inc. 49 BENZO[a]PYRENE EPOXIDATION Not all smokers get cancer Benzopyrenes in chimneys were responsible for cancer of the scrotum in “chimney- boys.” The connection between cancer and chimney soot was made by Percivall Pott in 1775.

51. © 2011 Pearson Education, Inc. 50 Reactions of 4° Ammonium Hydroxides A fully alkylated amino group (quaternary), makes a good enough LG for the “E2” Hofmann Elimination Reaction. If the 4° amino group was protonated, the E2 base, would just deprotonate the 4° amino group and there would be no E2 reaction.

52. © 2011 Pearson Education, Inc. 51 Hofmann Elimination Favors anti-Zaitsev Products Do you remember the 3 exceptions for E2 reactions in terms of predicting the major products? Usually, the more substituted alkene is the major product except if there is a big bulky base, possibility of conjugation or a Fluorine LG. Here, the quaternary ammonium salt is a relatively slow LG just like Fluorine. As the E2 base comes in to abstract the proton, a negative charge starts to build up on the B-carbon. Remember that carbanion stability is the opposite of carbocation stability.

53. © 2011 Pearson Education, Inc. 52 Crown Ethers The ability of a host to bond only certain guests is an example of molecular recognition

54. © 2011 Pearson Education, Inc. 53 Thiols are sulfur analogs of alcohols: Thiols are not good at hydrogen bonding Thiols form strong complexes with heavy metal cations Thiols are stronger acids (pK a = 10) than alcohols

55. © 2011 Pearson Education, Inc. 54 In protic solvent, thiolate ions are better nucleophiles than alkoxide ions: Sulfur is an excellent nucleophile because its electron cloud is polarized