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16.4 Crown Ethers

16.4 Crown Ethers. Dr. Wolf's CHM 201 & 202. 16-16. Crown Ethers. structure cyclic polyethers derived from repeating —OCH 2 CH 2 — units properties form stable complexes with metal ions applications synthetic reactions involving anions. Dr. Wolf's CHM 201 & 202. 16-17. O. O. O. O.

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16.4 Crown Ethers

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  1. 16.4Crown Ethers Dr. Wolf's CHM 201 & 202 16-16

  2. Crown Ethers structurecyclic polyethers derived from repeating —OCH2CH2— units propertiesform stable complexes with metal ions applicationssynthetic reactions involving anions Dr. Wolf's CHM 201 & 202 16-17

  3. O O O O O O 18-Crown-6 negative charge concentrated in cavity inside the molecule Dr. Wolf's CHM 201 & 202 16-18

  4. O O O O O O 18-Crown-6 negative charge concentrated in cavity inside the molecule Dr. Wolf's CHM 201 & 202 16-19

  5. O O O O O O 18-Crown-6 forms stable Lewis acid/Lewis base complex with K+ K+ Dr. Wolf's CHM 201 & 202 16-20

  6. O O O O O O 18-Crown-6 forms stable Lewis acid/Lewis base complex with K+ K+ Dr. Wolf's CHM 201 & 202 16-21

  7. Ion-Complexing and Solubility K+F– not soluble in benzene Dr. Wolf's CHM 201 & 202 16-22

  8. O O O O O O Ion-Complexing and Solubility K+F– benzene add 18-crown-6 Dr. Wolf's CHM 201 & 202 16-23

  9. O O O O O O O O O O O O Ion-Complexing and Solubility F– K+ benzene 18-crown-6 complex of K+ dissolves in benzene Dr. Wolf's CHM 201 & 202 16-24

  10. O O O O O O O O O O O O + F– Ion-Complexing and Solubility K+ benzene F– carried into benzene to preserve electroneutrality Dr. Wolf's CHM 201 & 202 16-25

  11. Application to organic synthesis Complexaton of K+ by 18-crown-6 "solubilizes" salt in benzene Anion of salt is in a relatively unsolvated state in benzene (sometimes referred to as a "naked anion") Unsolvated anion is very reactive Only catalytic quantities of 18-crown-6 are needed Dr. Wolf's CHM 201 & 202 16-26

  12. Example KF CH3(CH2)6CH2Br CH3(CH2)6CH2F 18-crown-6 (92%) benzene Dr. Wolf's CHM 201 & 202 16-27

  13. 16.5Preparation of Ethers Dr. Wolf's CHM 201 & 202 16-28

  14. H2SO4, 130°C Acid-Catalyzed Condensation of Alcohols* 2CH3CH2CH2CH2OH CH3CH2CH2CH2OCH2CH2CH2CH3 (60%) *Discussed earlier in Section 15.7 Dr. Wolf's CHM 201 & 202 16-29

  15. Addition of Alcohols to Alkenes H+ (CH3)3COCH3 (CH3)2C=CH2 + CH3OH tert-Butyl methyl ether Dr. Wolf's CHM 201 & 202 16-30

  16. 16.6The Williamson Ether Synthesis Think SN2! primary alkyl halide + alkoxide nucleophile Dr. Wolf's CHM 201 & 202 16-31

  17. Example CH3CH2CH2CH2ONa +CH3CH2I CH3CH2CH2CH2OCH2CH3 + NaI (71%) Dr. Wolf's CHM 201 & 202 16-32

  18. CH2Cl CH2OCHCH3 CH3 Another Example CH3CHCH3 + ONa (84%) Dr. Wolf's CHM 201 & 202 16-33

  19. CH2Cl CH2OCHCH3 CH3 Another Example Alkoxide ion can be derived from primary, secondary, or tertiary alcohol Alkyl halide must be primary CH3CHCH3 + ONa (84%) Dr. Wolf's CHM 201 & 202 16-34

  20. Origin of Reactants CH3CHCH3 CH2OH OH HCl Na CH3CHCH3 CH2Cl + ONa CH2OCHCH3 (84%) CH3 Dr. Wolf's CHM 201 & 202 16-35

  21. What happens if the alkyl halide is not primary? + CH3CHCH3 CH2ONa Br Dr. Wolf's CHM 201 & 202 16-36

  22. What happens if the alkyl halide is not primary? + CH3CHCH3 CH2ONa Br CHCH3 H2C + CH2OH Elimination by the E2 mechanism becomesthe major reaction pathway. Dr. Wolf's CHM 201 & 202 16-37

  23. 16.7Reactions of Ethers:A Review and a Preview Dr. Wolf's CHM 201 & 202 16-38

  24. Summary of reactions of ethers No reactions of ethers encountered to this point. Ethers are relatively unreactive. Their low level of reactivity is one reason why ethers are often used as solvents in chemical reactions. Ethers oxidize in air to form explosive hydroperoxides and peroxides. Dr. Wolf's CHM 201 & 202 16-39

  25. 16.8Acid-Catalyzed Cleavage of Ethers Dr. Wolf's CHM 201 & 202 16-40

  26. Example HBr CH3CHCH2CH3 CH3CHCH2CH3 + CH3Br heat OCH3 Br (81%) Dr. Wolf's CHM 201 & 202 16-41

  27. CH3CHCH2CH3 O •• •• •• Br H •• •• CH3CHCH2CH3 + O •• CH3 H Mechanism CH3 Dr. Wolf's CHM 201 & 202 16-42

  28. CH3CHCH2CH3 O •• •• •• Br H •• •• CH3CHCH2CH3 CH3CHCH2CH3 O •• + •• H – O •• •• CH3 •• H Br •• •• Br CH3 •• •• Mechanism CH3 •• Dr. Wolf's CHM 201 & 202 16-43

  29. CH3CHCH2CH3 O •• •• •• Br H •• •• CH3CHCH2CH3 CH3CHCH2CH3 O •• + •• H – O •• •• CH3 •• H Br •• •• Br CH3 •• •• Mechanism CH3CHCH2CH3 Br CH3 HBr •• Dr. Wolf's CHM 201 & 202 16-44

  30. O Cleavage of Cyclic Ethers HI ICH2CH2CH2CH2I 150°C (65%) Dr. Wolf's CHM 201 & 202 16-45

  31. O •• + O H Mechanism ICH2CH2CH2CH2I •• •• HI Dr. Wolf's CHM 201 & 202 16-46

  32. O – •• •• I •• •• •• •• I •• + O •• H Mechanism ICH2CH2CH2CH2I •• •• HI •• O •• H Dr. Wolf's CHM 201 & 202 16-47

  33. O – •• •• I •• •• •• •• I •• + O •• H Mechanism ICH2CH2CH2CH2I •• •• HI HI •• O •• H Dr. Wolf's CHM 201 & 202 16-48

  34. 16.9Preparation of Epoxides:A Review and a Preview Dr. Wolf's CHM 201 & 202 16-49

  35. Preparation of Epoxides Epoxides are prepared by two major methods.Both begin with alkenes. reaction of alkenes with peroxy acids(Section 6.18) conversion of alkenes to vicinalhalohydrins, followed by treatmentwith base (Section 16.10) Dr. Wolf's CHM 201 & 202 16-50

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