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1057-RG_7-09 - Chem1057 J Walcott LEARNING STRATEGIES...

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Chem1057 J. Walcott LEARNING STRATEGIES CENTER Fall 2009 Review Guide #7 Lectures: 10/5/09 – 10/16/09: Chapters 4, 6 and 7 I. Alkene Addition Reactions (continued from Review Guide #6) A. Syn-Anti Addition 1. Hydrohalogenation (adds H-X) 2. Hydration (adds H-OH) B. Anti Addition 1. Halogenation (adds Br-Br or Cl-Cl) 2 . Halohydrin formation (add Br 2 or Cl 2 and water or ether) 3. Oxymercuration - reduction (adds -OR and -H) C. Syn Addition Note: Syn addition usually involves a concerted mechanism ie. the reaction occurs in a single step without intermediates. As with anti addition, the number of possible products is fewer with syn addition than with syn-anti addition. In all of these reactions, steric factors must be considered. 1. Hydroboration-oxidation (adds -H and -OH) The addition of borane (BH 3 ) or dialkylborane (R 2 BH) to an alkene, followed by reaction with hydroxide ion and hydrogen peroxide will form alcohols. Direct substitution of the -BH 2 or - BR 2 from the first reaction by -OH from the second, explains the observed anti- Markovnikov regiochemistry. You do no need to know the mechanism for the oxidation step with peroxide and base. 2. Epoxidation with peracids (adds -O- makes an epoxide) a. Recall the mechanism of meta-chloroperoxybenzoic acid (m-CPBA) reacting with an alkene to produce an epoxide or oxirane. b. Epoxides with some stereospecificity can be produced by treating an alkene with Br 2 and water (hydrohalogenation), followed by treatment with a base. An intramolecular reaction of the alkoxide anion expelling a bromine anion produces the epoxide.
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