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Ethers & Epoxides

Ethers & Epoxides. Reactions of Ethers. Ethers are relatively unreactive. Ethers are often used as solvents in organic reactions. Ethers oxidize in air to form explosive hydroperoxides and peroxides. “Crown” ethers are useful as enhancers in nucleophilic substitution and other reactions.

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Ethers & Epoxides

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  1. Ethers & Epoxides

  2. Reactions of Ethers Ethers are relatively unreactive. Ethers are often used as solvents in organic reactions. Ethers oxidize in air to form explosive hydroperoxides and peroxides. “Crown” ethers are useful as enhancers in nucleophilic substitution and other reactions

  3. Naming Ethers • Common names are used frequently: • Name each –R group. • Arrange them alphabetically. • End with the word “ether.”

  4. Naming Ethers • IUPAC systematic names are often used as well: • Make the larger of the –R groups the parent chain. • Name the smaller of the –R groups as an alkoxy substituent. • SEE: SKILLBUILDER 14.1.

  5. Crown Ethers

  6. Crown Ethers structurecyclic polyethers derived from repeating —OCH2CH2— units propertiesform stable complexes with metal ions applicationssynthetic reactions involving anions namingx = total # of atoms in ring: [x] Crown- # of oxygen atoms

  7. O O O O O O 18-Crown-6 negative charge concentrated in cavity inside the molecule

  8. O O O O O O 18-Crown-6 forms stable Lewis acid/Lewis base complex with K+ K+

  9. Ion-Complexing and Solubility K+F– not soluble in toluene

  10. O O O O O O Ion-Complexing and Solubility K+F– toluene add 18-crown-6

  11. O O O O O O O O O O O O Ion-Complexing and Solubility F– K+ toluene 18-crown-6 complex of K+ dissolves in benzene

  12. O O O O O O O O O O O O + F– Ion-Complexing and Solubility K+ toluene F– carried into toluene to preserve electroneutrality

  13. Application to organic synthesis Complexation of K+ by 18-crown-6 "solubilizes" potassium salts in toluene Anion of salt is in a relatively unsolvated state in toluene (sometimes referred to as a "naked anion") Unsolvated anions are very reactive Only catalytic quantities of 18-crown-6 are needed

  14. Example KF CH3(CH2)6CH2Br CH3(CH2)6CH2F 18-crown-6 (92%) toluene

  15. Question • Which reaction is the best candidate for catalysis by 18-crown-6? (Which reaction proceeds faster in the presence of the crown ether than in its absence?) • A) Bromobutane + KCN (in toluene) • B) Phenol + Br2 (in water) • C) Butanol + H2CrO4 (in water) • D) CH3CH2CH2CHO + H2 (in ethanol)

  16. Ion Size & Crown Ether Complexes K+ 18-Crown-6 Na+ 15-Crown-5 Li+ 12-Crown-4

  17. Question • What is the name of the crown ether show at the right? • A) 12-crown-4 • B) 10-crown-5 • C) 15-crown-5 • D) 18-crown-6

  18. Question • Which crown ether would provide the fastest rate for the following reaction? • A) 12-crown-4 • B) 10-crown-5 • C) 15-crown-5 • D) 18-crown-6

  19. The Williamson Ether Synthesis Just another SN2 reaction  primary alkyl halide (substrate) + alkoxide (nucleophile)

  20. Example CH3CH2CH2CH2ONa +CH3CH2I CH3CH2CH2CH2OCH2CH3+ NaI (71%)

  21. Alkoxide ion can be derived from primary, secondary, or tertiary alcohol Alkyl halide must be primary CH3CHCH3 CH2Cl + ONa CH2OCHCH3 (84%) CH3 Another Example

  22. CH3CHCH3 CH2OH OH HCl or SOCl2 Na (s) CH3CHCH3 CH2Cl + ONa CH2OCHCH3 (84%) CH3 1o Halides & Alkoxides

  23. Question What is the product of the following reaction?

  24. What is the correct order of reagents needed for the following transformation? Question • 1) Hg(OAc)2, THF:H2O 2) NaBH4, OH– • 1) BH3:THF 2) H2O2, OH– • 1) Hg(OAc)2, CH3CH2OH 2) NaBH4, OH– • 1) MCPBA 2) H+ 3) NaH 4) Ethyl iodide

  25. Mechanism

  26. Question • Which of the following best represents the rate-determining transition state for the reaction shown below? • A) B) • C) D)

  27. + CH3CHCH3 CH2ONa Br CHCH3 H2C + CH2OH Elimination producesthe major product. What if the alkyl halide is not primary?SN2 vs E2

  28. Question • The most effective pair of reagents for the preparation of tert-butyl ethyl ether is • A) potassium tert-butoxide and ethyl bromide. • B) potassium tert-butoxide and ethanol. • C) sodium ethoxide and tert-butyl bromide. • D) tert-butyl alcohol and ethyl bromide

  29. Limitation

  30. Preparation of Epoxides

  31. Preparation of Epoxides Two major methods: Reaction of alkenes with peroxy acids Conversion of alkenes to vicinalhalohydrins, followed by treatmentwith base.

  32. Preparation of Epoxidesw/ peroxyacids (MCPBA) .

  33. Conversion of Vicinal Halohydrinsto Epoxides

  34. H OH H Br •• – O via: •• •• H H Br •• •• •• Example H NaOH O H2O H (81%)

  35. NaOH O Epoxidation via Vicinal Halohydrins Corresponds to overall syn addition ofoxygen to the double bond. Br Br2 H2O OH antiaddition inversion

  36. H3C H H3C H H CH3 CH3 H Epoxidation via Vicinal Halohydrins Corresponds to overall syn addition ofoxygen to the double bond. Br H3C Br2 H NaOH H2O H O CH3 OH antiaddition inversion

  37. Question Which of the following will produce the epoxide below? a, b B.a, c C. b, c D. b, d E. c, d

  38. Stereochemistry / Optical Activity

  39. Epoxidation Stereochemistry • Epoxidationforms a racemic mixture because reaction occurs with equal probability on either face of the double bond.

  40. EnantioselectiveEpoxidation • In order to have an optically active product, one of the reactants, or reagents, or catalyst in a reaction must be chiral. • An example is aSharplesscatalyst, which forms such a chiral complex that favors the formation of one enantiomericepoxide versus the other. • Catalyst:

  41. EnantioselectiveEpoxidation • The desired epoxide can be formed in excess by choosing the appropriate catalyst. Note the position of the –OH group. • SEE: CONCEPTUAL CHECKPOINT 14.16.

  42. Question What is the product of the following reaction?

  43. Reactions of Epoxides

  44. Reactions of Epoxides All reactions involve nucleophilic attack at carbon and lead to opening of the ring. An example is the reaction of ethylene oxide with a Grignard reagent as a method for the synthesis of alcohols.

  45. R MgX R CH2 CH2 CH2 OMgX H2C O H3O+ RCH2CH2OH Reaction of Grignard Reagentswith Epoxides

  46. CH2MgCl CH2CH2CH2OH Example CH2 H2C + O 1. diethyl ether 2. H3O+ (71%)

  47. CH2 H2C O Nu—CH2CH2O—H In General... Reactions of epoxides involve attack by anucleophile and proceed with ring-opening.For ethylene oxide: + Nu—H

  48. Nucleophiles attack herewhen the reaction iscatalyzed by acids. Anionic nucleophilesattack here. (less hindered) R CH2 C H O In General... For epoxides where the two carbons of thering are differently substituted:

  49. Nucleophilic Ring-OpeningReactions of Epoxides

  50. QuestionTrue (A) / False (B) Refer to the reaction coordinate diagrams below. The epoxide reaction is exergonic and the Transition State resembles the reactants whereas the ether reaction is slower and the Transition State resembles the reactants

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