Download Ethers, Epoxides and Sulfides: Synthesis, Properties and Reactions - Prof. John Edward Ant and more Study notes Chemistry in PDF only on Docsity! Ethers, epoxides and sulfides An ether is the logical (and nearly terminal) conclusion of a series of water alkylations: O H H "alkylate" O R H "alkylate" O R R "alkylate" O R R R Water Alcohol Ether Trialkyloxonium Salt (YECH!) Ethers come in a couple of flavors. There is the standard, dialkyl type: O O O O Ether, Diethyl Ether Ethyl ether (all 3 in common usage) Benzyl Methyl Ether Anisole (Phenyl Methyl Ether) Vinyl Methyl Ether And then there are the cyclic ethers – the names of these can be ugly, because they are often based on the molecule from which the ether was derived, rather than as an accurate description of the ether: O O O O O Furan Tetrahydrofuran (THF) Trimethylene Oxide (Oxetane) Ethylene Oxide Oxirane Epoxide Propylene Oxide Uses of ethers: Unlike alcohols, ethers in general are unreactive. They are more commonly used as reaction solvents than as reactants – ethers thus make great protecting groups! The only real exceptions to this rule are the epoxides - the three-membered ring ethers. We will talk more about the reactivity of these compounds later. Preparation of Ethers: The most common preparation of simple ethers uses the Williamson Ether Synthesis. In short, this preparation utilizes a deprotonated alcohol and an alkyl halide: In General: R O - + R' BrNa + O R R' + NaBr An Example: HO OH 2 NaH NaO ONa (+ 2 H2) 2 CH3Br MeO OMe Notes and Restrictions: The alkoxide portion of the starting material is generally produced by the action of sodium hydride (NaH). This hydride is the reagent of choice because the only by-product is hydrogen gas (a very strong driving force!) Because the alkoxide is very basic, the alkyl halide should be primary - otherwise, deprotonation rather than nucleophilic attack is likely: O NaO ONa Br Br + OH + + SN2 @ 1° C E2 @ 3° C Destruction of Ethers: Now that you’ve gone through all the trouble of making an ether, we’re going to talk about how you tear them back apart. Ethers are generally cleaved by strong acids at high temperatures (i.e. at LEAST 2M acid at 100+°C). HBr and HI are the reagents of choice: O HBr (2M) H2O / 110°C HO Br O HBr (2M) H2O / 110°C OH + CH3Br "H+" O H Br Because they can form stable carbocations, tertiary, allylic and benzylic ethers can be cleaved under much milder conditions, and form an alkene in place of the alkyl halide. Epoxides: ( O ) Because of the strain induced by the presence of a three-membered ring, epoxides are significantly more reactive than other ethers. Thus, they become useful in organic synthesis. Preparation: Epoxides are easily prepared from alkenes, by oxidation with a peroxyacid (often called a peracid). This is really the only non-stereospecific epoxidation method currently in use:
