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11-12-08 - gives trans-epoxide • Formation of...

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318M – Krische, Lecture 34: W - 11/12/08, Ethers, Sulfides and Epoxides Today, we continue chapter 11 on ethers, sulfides and epoxides. We commence by reviewing the reactions of ethers. Today, the primary focus will be on the reactions of epoxides. Structure of Epoxides : Epoxides are three-membered rings that contain a single oxygen atom. They are highly strained due to eclipsing interactions and deviation from ideal tetrahedral bond angles. The strain energy of an epoxide is ~27 Kcal/mol. This is a significant fraction of the typical C-O bond energy of 85- 91 Kcal/mol. Thus, it is not surprising that relief of strain causes epoxides to be much more reactive than normal ethers. Formation of Epoxides-Peracid Oxidation : Upon exposure of alkenes to peroxy acids such as meta- chloroperoxybenzoic acid (mCPBA), epoxides are obtained in a stereospecific fashion, i.e. trans-alkene
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Unformatted text preview: gives trans-epoxide. • Formation of Epoxides-Internal SN2 via Halohydrin : An alternative method for epoxide formation involves intramolecular SN2 reaction of a halohydrin. Like any SN2 reaction, the intramolecular SN2 reaction of a halohydrins is stereospecific. • Reactions of Epoxides-Acidic Cleavage : Upon exposure to aqueous acid, epoxides undergo stereospecific ring cleavage to give anti-1,2-diols. Markovnikov regiochemistry is observed. • Reactions of Epoxides-Basic Cleavage : Upon exposure to nucleophiles under basic conditions, strain relief drives opening of the epoxide with attack at the less substituted terminus. This regio- and stereospecific transformation works for epoxides only, i.e. not normal ethers....
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