1057-RG_10-09 - Chem 1057 J Walcott LEARNING STRATEGIES...

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Chem 1057 J. Walcott LEARNING STRATEGIES CENTER Fall 2009 Review Guide #10 Lectures: 11/2/09 – 11/6/09: Chapters 10 and 11 I. Ethers Ethers are generally unreactive and thereby are often used as solvents. A. Preparation 1. The Williamson ether synthesis is a base catalyzed reaction whereby an alcohol is converted into an alkoxide anion via reaction with a base. The alkoxide then attacks a haloalkane in an S N 2 reaction. Inversion of configuration occurs. 2. Alkoxymercuration-reduction of alkenes will make ethers when an alcohol instead of H 2 O solvent is used in the first step. A mercurinium ion is formed with anti addition of the alcohol, and the same regiochemistry (Markovnikov addition) is observed. B. Reactions Ether cleavage can be achieved when an ether reacts with an acid halide making an alcohol and alkyl halide. The oxygen gets protonated first. Then, an S n 2 mechanism occurs if a 1 ° or 2 ° halide is formed - an S n 1 mechanism occurs if a 3 ° halide is formed. C. Epoxides 1. Synthesis of epoxides can be achieved by the reaction of an alkene with m -CPBA ( m -chloroperoxybenzoic acid). The epoxide forms via a concerted mechanism, and as a consequence this reaction is a stereoselective syn addition. 2. When an alkene reacts with bromine in water, a halohydrin is produced via anti addition. The halohydrin can be treated with base to deprotonate the hydroxy group. The resulting anion can act as a nucleophile that attacks the bromine from the opposite side making an epoxide. 3. Epoxide ring-opening reactions. a. Under acidic conditions, the epoxide oxygen atom is protonated. A nucleophile can attack the more branched epoxide carbon atom because it has the larger partial positive charge. b. Under basic conditions, the nucleophile attacks the less branched epoxide carbon atom in an S N 2 reaction. The reaction of ethylene oxide with a Grignard reagent is quite useful in organic synthesis. The net transformation leads to formation of a primary alcohol that has
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