Chapter 10 Lectures

Chapter 10 Lectures - Chapter 10 Substitution Reactions of...

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Chapter 10
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Substitution Reactions of Alcohols What happens if EtOH is reacted with NaCl? Does an S N 2 reaction occur? Et–OH + Et–Cl + NaCl NaOH strong base poor leaving group In general, R OH X=Cl, Br, I, F X So how do we convert a primary alcohol to an alkyl halide? R OH R X X=Cl, Br, I, or F H—X No Reaction R OH 2 + X R X + OH 2 weak base good leaving group
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Will the same transformation work for a tertiary alcohol? OH X=Cl, Br, I, F HX X H—X OH 2 + good leaving group + X X + OH 2 Key Features: 3° alcohols react at rt, 1° and 2° alcohols require heat no elimination products are observed 2° and 3° alcohols undergo S N 1, 1° undergo S N 2 ZnCl 2 can be used as a catalyst for the S N 2 case.
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HBr OH HCl OH HCl HBr OH OH Br Cl Cl Br + H + + +
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Other Methods for Converting Alcohols into Alkylating Agents OH P Br Br Br O PBr 2 H + N O PBr 2 Br OH MECHANISM pyridine OH pyridine OH pyridine PBr 3 PCl 3 SOCl 2 Br O PBr 2 + poor leaving group GOOD leaving group S N 2 reaction Br Cl Cl
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OH Common Sulfonates pyridine sulfonyl chloride reagent sulfonate ester replaced by alkoxy group tosylate ester (X = OR) mesylate ester (X = OR) tosyl chloride (X = Cl) mesyl chloride (X = Cl) triflate ester (X = OR) RSO 2 Cl Ms–X Tf–X Ts–X S O O Cl R RSO 2 Cl = S O O X H 3 C S O O X F 3 C S O O X O S O O R MeS OTs NaCN OMs SMe CN
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1. OH TsCl, pyridine 2. NaCN, acetone 1. OH PCl 3 , pyridine 2. NaCN, acetone Explanation of Stereochemistry CN CN INVERSION RETENTION Et Me O H Et Me O Ts Et Me CN S N 2 Ts Cl Et Me O PCl 2 Et Me Cl Et Me CN S N 2 S N 2 Cl– PCl 2 Cl CN CN double inversion = net retention
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Dehydration of Alcohols OH MECHANISM HO H 2 SO 4 H 2 SO 4 H 2 O + + H O SO 3 H E1 OH 2 + H O SO 3 H E2 Key Features: E1 mechanism (and possible rearrangements) for 2° and 3° alcohols E2 for 1° alcohols ease of dehydration 3°>2°>1° alcohols conditions are H 2 SO 4 or H 3 PO 4 and heat more stable (most substituted) alkene predominates
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OH H 2 SO 4 OH H 2 SO 4 + + + +
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Milder alternative OH pyridine 0 °C POCl 3 O P Cl O Cl H N E2 mechanism, no carbocation intermediate
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Chapter 10 Lectures - Chapter 10 Substitution Reactions of...

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