1 / 36

Ethers and Epoxides

Ethers and Epoxides. 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 Tetrahydrofuran (THF) is a solvent that is a cyclic ether . Naming Ethers.

rasia
Download Presentation

Ethers and Epoxides

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Ethers and Epoxides

  2. Based on McMurry, Organic Chemistry, Chapter 18, 6th edition, (c) 2003

  3. 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 • Tetrahydrofuran (THF) is a solvent that is a cyclic ether Based on McMurry, Organic Chemistry, Chapter 18, 6th edition, (c) 2003

  4. Naming Ethers • Simple ethers are named by identifying the two organic substituents and adding the word ether • If other functional groups are present, the ether part is considered an alkoxy substituent Based on McMurry, Organic Chemistry, Chapter 18, 6th edition, (c) 2003

  5. Practice: Give the IUPAC of the following and draw the structure. A. B. C. para-diethoxy benzene D. cyclohexyl methyl ether

  6. Structure, Properties, and Sources of Ethers • Geometry: R–O–R ~ tetrahedral bond angle (112° in dimethyl ether) • Oxygen is sp3-hybridized • Oxygen atom gives ethers a slight dipole moment; slight boiling point elevation. • Some ethers react slowly with oxygen to give explosive peroxides. Based on McMurry, Organic Chemistry, Chapter 18, 6th edition, (c) 2003

  7. Diethyl ether prepared industrially by sulfuric acid–catalyzed dehydration of ethanol – also with other primary alcohols Based on McMurry, Organic Chemistry, Chapter 18, 6th edition, (c) 2003

  8. Synthesis of Ethers • Williamson Ether Synthesis • Alexander William Williamson • (1 May 1824 – 6 May 1904) was an English chemist of Scottish descent.

  9. The Williamson Ether Synthesis • Reaction of metal alkoxides and primary alkyl halides and tosylates • Best method for the preparation of ethers • Alkoxides prepared by reaction of an alcohol with a strong base such as sodium hydride, NaH Based on McMurry, Organic Chemistry, Chapter 18, 6th edition, (c) 2003

  10. The Williamson Ether Synthesis • Unsymmetrical ethers • Best prepared by reaction of the more hindered alkoxide partner with the less hindered alkyl halide partner, rather vice versa • Prepare: tert-butyl methyl ether

  11. Practice: How would you prepare the following ethers? 1. CH3-O-CH2CH2CH3 2. 3.

  12. Reactions of Ethers • Acidic cleavage • Claisen Rearrangement

  13. Acidic Ether Cleavage • Ethers are generally unreactive • Strong acid will cleave an ether at elevated temperature • HI and HBr produce an alkyl halide from less hindered component by SN2 (tertiary ethers undergo SN1) Based on McMurry, Organic Chemistry, Chapter 18, 6th edition, (c) 2003

  14. Acidic Ether Cleavage • Step-by –step mechanism Based on McMurry, Organic Chemistry, Chapter 18, 6th edition, (c) 2003

  15. Acidic Ether Cleavage • Ethers with tertiary alkyl group cleaves by an SN1 mechanism (to produce stable carbocation intermediate) • The ether oxygen atom stays with the less hindered alkyl group and the halide bonds to the tertiary group. • Cleavage is rapid, and happens at room temperature or below. • Example: cleavage of tert-butyl cyclohexyl ether using CF3CO2H at oC

  16. Predict the product of an ether cleavage. • Predict the product of the reaction of tert-butyl propyl ether with HBr.

  17. Predict the product of an ether cleavage. • What products would you expect from the reaction of the following ethers with HI?

  18. Predict the product of an ether cleavage. • Solution: • SN1 • SN2

  19. Reactions of Ethers: Claisen Rearrangement • Rainer Ludwig Claisen (January 14, 1851 – January 5, 1930) was a famous German chemist best known for his work with condensations of carbonyls and sigmatropic rearrangements.

  20. 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 Based on McMurry, Organic Chemistry, Chapter 18, 6th edition, (c) 2003

  21. Claisen Rearrangement Mechanism • Concerted pericyclic 6-electron, 6-membered ring transition state

  22. Cyclic Ethers: Epoxides • Cyclic ethers behave like acyclic ethers, except if ring is 3-membered • Dioxane and tetrahydrofuran are used as solvents Based on McMurry, Organic Chemistry, Chapter 18, 6th edition, (c) 2003

  23. 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 Based on McMurry, Organic Chemistry, Chapter 18, 6th edition, (c) 2003

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

  25. Formation of the 3-membered ring

  26. Epoxides from Halohydrins • Addition of HO-X (halo alcohol) to an alkene gives a halohydrin • Treatment of a halohydrin with base gives an epoxide • Intramolecular Williamson ether synthesis Based on McMurry, Organic Chemistry, Chapter 18, 6th edition, (c) 2003

  27. Based on McMurry, Organic Chemistry, Chapter 18, 6th edition, (c) 2003

  28. Reactions of Epoxides • Epoxide opening with aqueous acid • Epoxide opening with nucleophiles Based on McMurry, Organic Chemistry, Chapter 18, 6th edition, (c) 2003

  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 (trans addition) Based on McMurry, Organic Chemistry, Chapter 18, 6th edition, (c) 2003

  30. Ethylene Glycol • 1,2-ethanediol from acid catalyzed hydration of ethylene • Widely used as automobile antifreeze (lowers freezing point of water solutions) Based on McMurry, Organic Chemistry, Chapter 18, 6th edition, (c) 2003

  31. Halohydrins from Epoxides • Anhydrous HF, HBr, HCl, or HI combines with an epoxide • Gives trans product Based on McMurry, Organic Chemistry, Chapter 18, 6th edition, (c) 2003

  32. Regiochemistry of Acid-Catalyzed Opening of Epoxides • Nucleophile preferably adds to less hindered site if primary and secondary C’s • Also at tertiary because of carbocation character (See Figure 18.2) Based on McMurry, Organic Chemistry, Chapter 18, 6th edition, (c) 2003

  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 Based on McMurry, Organic Chemistry, Chapter 18, 6th edition, (c) 2003

  34. Addition of Grignards to Ethylene Oxide • Adds –CH2CH2OH to the Grignard reagent’s hydrocarbon chain • Acyclic and other larger ring ethers do not react Based on McMurry, Organic Chemistry, Chapter 18, 6th edition, (c) 2003

  35. 18.9 Crown Ethers • Large rings consisting repeating (-OCH2CH2-) or similar units • Named as x-crown-y • x is the total number of atoms in the ring • y is the number of oxygen atoms • 18-crown-6 ether: 18-membered ring containing 6 oxygens atoms • Central cavity is electronegative and attracts cations Based on McMurry, Organic Chemistry, Chapter 18, 6th edition, (c) 2003

  36. Uses of Crown Ethers • Complexes between crown ethers and ionic salts are soluble in nonpolar organic solvents • Creates reagents that are free of water that have useful properties • Inorganic salts dissolve in organic solvents leaving the anion unassociated, enhancing reactivity Based on McMurry, Organic Chemistry, Chapter 18, 6th edition, (c) 2003

More Related