11-10-08 - pair followed by SN1 type ionization to form a...

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318M – Krische, Lecture 33: M - 11/10/08, Ethers, Sulfides and Epoxides Today, we continue our discussion on ethers, sulfides and epoxides. We commence by reviewing methods for the preparation of ethers. Today, the primary focus will be on the reactions of ethers. Acidic Cleavage of Ethers (SN2-Mechanism) : Upon exposure of primary ethers to HX (X = Br, I), one can obtain the corresponding alkyl halides. This reaction involves protonation of the ether’s oxygen lone pair followed by SN2 displacement using halide as nucleophile. Here, an alcohol serves as leaving group, which under the acidic conditions, is itself converted to an alkyl halide. Acidic Cleavage of Ethers (SN1-Mechanism) : Upon exposure of tertiary ethers to HX (X = Br, I), one also can obtain the corresponding alkyl halides. This reaction involves protonation of the ether’s oxygen lone
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Unformatted text preview: pair followed by SN1 type ionization to form a carbocation that is trapped via halide addition. Ethers as Protecting Groups : Because they are easily formed and cleaved, tert-butyl ethers can serve as protecting groups. For example, the attempted methylation of HC CCH 2 OH gives mostly the O-methylated product. Selective methylation at the acetylenic terminus requires that we first protect the hydroxyl group as the tert-butyl ether to give HC CCH 2 O t Bu. Now, with the acidic hydroxyl proton masked, we can methylate the acetylenic carbon under standard conditions (NaH, CH 3 I, DMSO). After this methylation is performed, one can unmask the hydroxyl moiety by cleaving the tert-butyl ether using aqueous acid to give H 3 CC CCH 2 OH....
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