S20-Lectures-Week3

S20-Lectures-Week3 - Chemistry S-20ab Chemistry E-2a:...

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Dehydration of Alcohols Predict the product and show a mechanism for each of the following reactions: OH conc. H 3 PO 4 OH conc. H 2 SO 4 Reading : Section 10.1 Chemistry S-20ab Week 3 124
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Making Alcohols Leave: Part 1–The Problem Explain why each of the following reactions is not effective: OH NaBr Br does NOT work OH NaOCH 3 O does NOT work CH 3 Reading : Section 10.2 Chemistry S-20ab Week 3 125
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Making Alcohols Leave: Part 2–Using Acid We can convert an alcohol into a good leaving group by protonating the alcohol in strong acid. Let’s see how this works: OH conc. HBr Br OH conc. HI I Reading : Section 10.2 Chemistry S-20ab Week 3 126
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Making Alcohols Leave: Part 2–Acid Is A Problem Strong acids are not generally useful for converting alcohols into good leaving groups. What is the problem with each of the following reactions? does NOT work OH conc. HBr Br OH HCN CN does NOT work OH HCl Cl does NOT work Reading : Section 10.2 Chemistry S-20ab Week 3 127
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Making Alcohols Leave: Part 3–Using Tosylates Show the mechanism for the following reaction, in which an alcohol is converted into a good leaving group known as a tosylate : OH TsCl = S Cl O O CH 3 = ClSO 2 R pyridine = N OTs What can we do with a tosylate? Reading : Section 10.3 Chemistry S-20ab Week 3 128
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Making Alcohols Leave: Part 3–Reactions of Tosylates For each of the following reactions, predict the product and show stereochemistry: OH TsCl pyr NaBr OH TsCl pyr NaCN OH TsCl pyr NaOEt Reading : Section 10.3 Chemistry S-20ab Week 3 129
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Making Alcohols Leave: Part 4–SOCl 2 and PBr 3 Provide complete curved-arrow mechanisms for the following reactions: Cl OH SOCl 2 , pyridine Br OH PBr 3 Reading : Section 10.3 Chemistry S-20ab Week 3 130
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Making Alcohols Leave: Part 4–Stereochemistry For each of the following reactions, predict the product and show stereochemistry: OH SOCl 2 pyr NaI acetone OH NaCN PBr 3 What reagents are needed to carry out the following transformations? OH 1. 2. Cl OH 1. 2. Br OH 1. 2. Reading : Section 10.3 Chemistry S-20ab Week 3 131
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Oxidation of Alcohols Alcohols can be oxidized with various oxidizing agents. The most common are various derivatives of CrO 3 . We can make the following general observations: For primary alcohols: OH CrO 3 ("dry") O H OH CrO 3 ("wet") O OH (Where else have you seen an “option” for forming aldehydes versus forming carboxylic acids?) For secondary alcohols: O OH CrO 3 What do you think will happen for tertiary alcohols? OH CrO 3 ??? Reading : Sections 10.5 and 10.6 Chemistry S-20ab Week 3 132
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Oxidation of Alcohols: Mechanisms Draw curved-arrow mechanisms for each of the following steps in the oxidation of isopropanol: OH + CrO 3 O CrO 3 H O CrO 3 H O + H 2 CrO 3 + H 3 O + H + H 3 O + Reading : Section 10.6 Chemistry S-20ab Week 3 133
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Oxidation of Primary Alcohols to Carboxylic Acids In water, the oxidation of primary alcohols proceeds beyond the aldehyde stage and ends up with a carboxylic acid. Provide a mechanism for these steps:
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S20-Lectures-Week3 - Chemistry S-20ab Chemistry E-2a:...

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