Week_5.1_PurpleBook_Answers

Week_5.1_PurpleBook_Answers - 116 Carbonyl Reactivity Week...

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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 O Resonance shows partial positive charge on carbon. From MO point of view, we know that a π * antibonding orbital will already be a good LUMO, and the electronegative oxygen makesitevenlower,soitisan excellent LUMO. OH 2 O These resonance structures become increasingy worse from left to right, so there's less + charge on carbon. The energy of the π * LUMO is affected by the electronegativity of the neighboring group (which is why the Cl lowers the energy of the LUMO), but also by electron donation from neighboring lone pairs (which is why the ester, amide, and carboxylate raise the energy of the LUMO and make it a poorer acceptor.) electron withdrawing lone pairs are electron donating Week 5 116
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H 3 C H O H 3 C H 3 C H O H 3 C OH H 3 C H O H 3 C H OH HO MgBr H 3 C H H O H 3 C H O Carbonyl Addition: Mechanisms 1. Provide curved-arrow mechanisms for the following transformations: H 3 C CH 2 H + /H 2 O H + 2 O H 3 C H 2 "H: " H 2 O, H + H 3 C H –H + NaBH 4 H + H + H + :OH 2 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
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This note was uploaded on 05/01/2008 for the course CHEM 51 taught by Professor Gribble during the Spring '08 term at Dartmouth.

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Week_5.1_PurpleBook_Answers - 116 Carbonyl Reactivity Week...

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