Dolbier HW Solutions 318

Dolbier HW Solutions 318 - 412 ETHERS, EPOXIDES, AND...

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Unformatted text preview: 412 ETHERS, EPOXIDES, AND SULFIDES 16.29 The proper approach to this problem is to first write the equations in full stereochemical detail. H CH3 (a) H HOCH2 O CH3 C OH (R)-1,2-Epoxypropane (R)-1,2-Propanediol It now becomes clear that the arrangement of groups around the stereogenic center remains unchanged in going from starting materials to products. Therefore, choose conditions such that the nucleophile attacks the CH2 group of the epoxide rather than the stereogenic center. Base-catalyzed hydrolysis is required; aqueous sodium hydroxide is appropriate. H HO H2C CH3 C H CH 3 H2 O HOCH2 NaOH C OH O The nucleophile (hydroxide ion) attacks the less hindered carbon of the epoxide ring. (b) H CH3 OH HOCH2 O C H CH3 (S )-1,2-Propanediol Inversion of configuration at the stereogenic center is required. The nucleophile must therefore attack the stereogenic center, and acid-catalyzed hydrolysis should be chosen. Dilute sulfuric acid would be satisfactory. OH2 H CH 3 H2C C OH H2 O H2SO4 HOCH2 O C H CH3 H The nucleophile (a water molecule) attacks that carbon atom of the ring that can better support a positive charge. Carbocation character develops at the transition state and is better supported by the carbon atom that is more highly substituted. 16.30 The key intermediate in the preparation of bis(2-chloroethyl) ether from ethylene is 2-chloroethanol, formed from ethylene by reaction with chlorine in water. Heating 2-chloroethanol in acid gives the desired ether. H2C CH2 Cl2, H2O Ethylene 16.31 Back Forward (a) ClCH2CH2OH H , heat 2-Chloroethanol ClCH2CH2OCH2CH2Cl Bis(2-chloroethyl) ether There is a temptation to try to do this transformation in a single step by using a reducing agent to convert the carbonyl to a methylene group. No reagent is available that reduces esters in this way! The Clemmensen and Wolff–Kishner reduction methods are suitable only for aldehydes and ketones. The best way to approach this problem is by reasoning backward. The desired Main Menu TOC Study Guide TOC Student OLC MHHE Website ...
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This note was uploaded on 09/28/2011 for the course CHM 2210 taught by Professor Reynolds during the Fall '01 term at University of Florida.

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