Week_5_PurpleBook2007

Week_5_PurpleBook2007 - 116 Carbonyl Reactivity Week 5 1. A...

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+ OH + NR 2 O O Cl O OR O 2 O O Carbonyl Reactivity O H 1. A site on a molecule will be electrophilic if it has a partial positive charge and a low-lying LUMO. Explain, using resonance and molecular orbital theory, why the carbonyl carbon is electrophilic. O R 2. Using resonance and molecular orbital theory, rationalize the reactivity of the following carbonyl analogs towards nucleophilic attack: More reactive Less reactive More reactive Less reactive O 3. The electrophilicity of a carbonyl compound is affected by both inductive and resonance effects of neighboring groups. Rationalze the reactivity of the following carbonyl derivatives: electrophilic carbon Week 5 116
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H 3 C H O H 3 C H 3 C H O H 3 C OH Carbonyl Addition: Mechanisms 1. Provide curved-arrow mechanisms for the following transformations: H 3 C H O H + /H 2 O H + 2 O H 3 C H OH HO H 2 O, H + MgBr NaBH 4 Week 5 117
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R R' O R R' HO OH F 3 C CF 3 O F 3 C H O H H O H 3 C H O H 3 C CH 3 O 3 O K= [hydrate] [carbonyl] Carbonyl Compound Reactivity of Carbonyl Addition Reactions 1. Explain the observed ranking of equilibrium constants for carbonyl hydration in the following substrates: O 1x10 6 3x10 4 2x10 3 O 1 –3 9x10 –6 H 3 C O O O 3 3. In the reduction of ketones with NaBH 4 , cyclobutanone reacts 17 times faster than acetone. Explain this observation. NaBH 4 isopropanol, 0°C 17 times faster than: NaBH 4 isopropanol, 0°C 2. The following compound has 2 carbonyl groups. Which one will become hydrated under aqueous conditons? Show the structure of the hydrate. Explain why the hydration equilibrium ratio K is equal to 0.8, that is, significantly larger than the ratio for most unsubstituted ketones such as acetone (above). H 2 O K H 2 O Week 5 118
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H 3 C OH N Ph Ph O O N Ph Ph O OH Carbonyl Additions: Reactions and Synthesis 1. Provide a synthesis of the following compound using starting materials with 6 or fewer carbons . Me Me 2. Explain the selectivity observed in the following hydride reduction. (Ph = phenyl = C 6 H 5 ) NaBH 4 Week 5 119
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