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CH 18: Ethers and Epoxides

CH 18: Ethers and Epoxides. Renee Y. Becker Valencia Community College CHM 2211. Ethers and Their Relatives. An ether has two organic groups (alkyl, aryl, or vinyl) bonded to the same oxygen atom, R–O–R  Diethyl ether is used industrially as a solvent

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CH 18: Ethers and Epoxides

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  1. CH 18: Ethers and Epoxides Renee Y. Becker Valencia Community College CHM 2211

  2. Ethers and Their Relatives • An etherhas two organic groups (alkyl, aryl, or vinyl) bonded to the same oxygen atom, R–O–R • Diethyl ether is used industrially as a solvent • Tetrahydrofuran (THF) is a solvent that is a cyclic ether • Epoxides contain a C-O-C unit which make-up a three membered ring • Thiols (R–S–H) and sulfides (R–S–R) are sulfur (for oxygen) analogs of alcohols and ethers

  3. Naming Ethers • Ethers are named two ways: • As alkoxy derivatives of alkanes • Derived by listing the two alkyl groups in the general structure of ROR’ in alphabetical order as separate words and adding the word ether • When both alkyl groups are the same, the prefix di- precedes the name of the alkyl group • (Ethers can be described as symmetrical or unsymmetrical)

  4. Naming Ethers

  5. Naming Ethers • Epoxides (oxiranes) • “epoxy” always preceeds the name of the alkane

  6. Example 1: Name

  7. Example 2: Draw • Isopropyl methyl ether • 4-t-butoxy-1-cyclohexene • Phenyl propyl ether • O- nitro anisole

  8. Structure, Properties, and Sources of Ethers • R–O–R ~ tetrahedral bond angle (112° in dimethyl ether) • Oxygen is sp3-hybridized • Oxygen atom gives ethers a slight dipole moment (diethyl ether 1.2 D)

  9. Structure, Properties, and Sources of Ethers (comparative boiling points)

  10. Preparation of Ethers • Acid catalyzed synthesis of ethers: Limited to symmetrical ethers. WHY?

  11. Mechanism 1: Acid catalyzed synthesis of ethers

  12. Preparation of Ethers • Williamson ether synthesis • Metal alkoxides react with primary alkyl halides by an SN2 pathway to yield ethers. • Secondary and tertiary substrates react following an E2 mechanism

  13. Mechanism 2: Williamson Ether Synthesis

  14. Example 3: Williamson ether synthesis

  15. Example 4 How would you prepare the following compounds using a Williamson synthesis? • Methyl propyl ether • Anisole (methyl phenyl ether)

  16. Example 5: Predict the product:

  17. Alkoxymercuration of Alkenes • React alkene with an alcohol and mercuric acetate or trifluoroacetate • Demercuration with NaBH4 yields an ether • Overall Markovnikov addition of alcohol to alkene

  18. Reactions of Ethers: Acidic Cleavage • Ethers are generally unreactive • Strong acid will cleave an ether at elevated temperature • HI, HBr produce an alkyl halide from less hindered component by SN2 (tertiary ethers undergo SN1)

  19. Mechanism 3: Acidic Cleavage Note that the halide attacks the protonated ether at the less highly substituted site.

  20. Example 6

  21. Reactions of Ethers: Claisen Rearrangement • Specific to allyl aryl ethers, ArOCH2CH=CH2 • Heating to 200–250°C leads to an o-allylphenol • Result is alkylation of the phenol in an ortho position

  22. Mechanism 4: Claisen Rearrangement • Concerted pericyclic 6-electron, 6-membered ring transition state • Mechanism consistent with 14C labeling

  23. Example 7

  24. Cyclic Ethers: Epoxides • Cyclic ethers behave like acyclic ethers, except if ring is 3-membered • Dioxane and tetrahydrofuran are used as solvents

  25. Epoxides (Oxiranes) • Three membered ring ether is called an oxirane (root “ir” from “tri” for 3-membered; prefix “ox” for oxygen; “ane” for saturated) • Also called epoxides • Ethylene oxide (oxirane; 1,2-epoxyethane) is industrially important as an intermediate • Prepared by reaction of ethylene with oxygen at 300 °C and silver oxide catalyst

  26. Preparation of Epoxides Using a Peroxyacid • Treat an alkene with a peroxyacid

  27. Mechanism 5: Preparation of Epoxides Using a Peroxyacid

  28. Epoxides from Halohydrins • Addition of HO-X to an alkene gives a halohydrin • Treatment of a halohydrin with base gives an epoxide • Intramolecular Williamson ether synthesis

  29. Ring-Opening Reactions of Epoxides • Water adds to epoxides with dilute acid at room temperature • Product is a 1,2-diol (on adjacent C’s: vicinal) • Mechanism: acid protonates oxygen and water adds to opposite side (anti-addition)

  30. Mechanism 6: Acid catalyzed ring-openings

  31. Ethylene Glycol • 1,2-ethanediol from acid catalyzed hydration of ethylene • Widely used as automobile antifreeze (lowers freezing point of water solutions)

  32. Halohydrins from Epoxides • Anhydrous HF, HBr, HCl, or HI combines with an epoxide • Gives trans product

  33. Base-Catalyzed Epoxide Opening • Strain of the three-membered ring is relieved on ring-opening • Hydroxide cleaves epoxides at elevated temperatures to give trans 1,2-diols • Complete the reaction

  34. Mechanism 7: Base-Catalyzed Epoxide Opening

  35. Mechanism 7: Base-Catalyzed Epoxide Opening

  36. Addition of Grignards to Ethylene Oxide • Adds –CH2CH2OH to the Grignard reagent’s hydrocarbon chain • Acyclic and other larger ring ethers do not react

  37. Thiols • Thiols (RSH), are sulfur analogs of alcohols • Named with the suffix -thiol • SH group is called “mercapto group”

  38. Example 8: Name

  39. Example 9: Draw • Cyclopentanethiol • 3-methyl-4-heptanethiol • 4-ethyl-4-isopropyl-2-methyl-3-hexanethiol

  40. Sulfides • Sulfides (RSR), are sulfur analogs of ethers • Named by rules used for ethers, with sulfide in place of ether for simple compounds and alkylthio in place of alkoxy

  41. Example 10: Name or Draw

  42. Thiols: Formation and Reaction • From alkyl halides by displacement with a sulfur nucleophile such as SH • The alkylthiol product can undergo further reaction with the alkyl halide to give a symmetrical sulfide, giving a poorer yield of the thiol

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