Lecture 34: Formation, Reactions and Cleavage of Epoxides, Study notes of Organic Chemistry

Download Lecture 34: Formation, Reactions and Cleavage of Epoxides and more Study notes Organic Chemistry in PDF only on Docsity! 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 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.