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The bromination in acid solution makes use of the only

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Unformatted text preview: reactions: H O H2O O O O Br O 1. Mg Br2 2. PhCN 3. H+, H2O Br HOAc Purpose of the problem Revision of mechanisms plus exploration of reactions involving enols. Suggested solution The first reaction starts with acetal hydrolysis and the product is an enone that is not conjugated. It can become conjugated in acid solution via the enol. ■ The mechanism of acetal hydrolysis is on p. 226 of the textbook. acetal hydrolysis O H O O H HO O O H H The second sequence starts with the reaction of a nitrile with a Grignard reagent and the hydrolysis of the product to a ketone: this is mentioned on pp. 220 and 231 of the textbook. The bromination in acid solution makes use of the only enol possible. O H OH Br Br O Br Solutions for Chapter 20 – Formation and Reactions of Enols and Enolates 5 PROBLEM 5 Suggest mechanisms for these reactions and explain why these products are formed. O O O O O Br2 Br AcOH O O HClO4 Purpose of the problem Making sure that you understand why enols usually react through carbon but sometimes react through oxygen. Suggested solution The same ketone reacts in similar acidic conditions with different selectivity according to the electrophile. The bromination occurs at carbon as we should expect but the anhydride reacts at oxygen. So we can draw mechanisms to suit the product. O OH OH Br Br Br O O O –H Br O OH O –H O ■ This ‘double- headed curly O arrow’ short- hand is explained on p. 217 of the textbook. Acid anhydrides, being carbonyl electrophiles, respond to charge density (they are ‘hard’ ele...
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