Problem_Set_8_Answers

Problem_Set_8_Answers - Chem 358 Answers to Additional...

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Chem 358 Spring 2007-Problem Set #8 Answers to Additional Problems 1. (a) The barrier to rotation depends on the importance of the contributing resonance structures. R N R O H R N R O H .. R S R O R S R O .. .. .. overlap of 2p(C) and 2p (N) overlap of 2p (C) and 3p (S) Because of the poor overlap between 2p and 3p orbitals, the C—N overlap is more important than the C—S overlap. Hence the barrier to rotation is bigger in the amide than in the thioester. (b) R S R O .. .. OH R O OH S R R O H O .. .. SR R O O .. .. .. HSR Tetrahedral Intermediate addition elimination irreversible (c) A thioester is expected to hydrolyze more readily than an amide because the resonance in part (a) is more important to the amide. This in turn makes the amide more stable than the thioester. If addition is rate-limiting the amide hydrolysis will be slower because of the resonance stabilization. If elimination is rate-limiting the amide hydrolysis will still be slower because RS is a better leaving group than RNH .
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