Chapter 15 with notes - Chapter 15 Nucleophilic...

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Chapter 15 – Nucleophilic substitution at saturated carbon Instructor: Dr. Daniel Seidel Chem 315 vs. addition/elimination mechanism S N 1 or S N 2 mechanism S N 1 = Substitution, Nucleophilic, 1st Order S N 2 = Substitution, Nucleophilic, 2nd Order
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Different rates of alcohol bromination What is the mechanism of the PBr 3 reaction?
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Mechanism: The S N 1 mechanism for nucleophilic substitution at saturated carbon
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A closer look at the S N 2 mechanism rate = k 2 [MeSNa][MeI] There is only one step in this reaction which is necessarily the rate determining step. Both reagents are involved in this step and are expected to impact the rate. This is indeed what is seen.
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Oxygen nucleophiles in the S N 2 reaction Oxygen nucleophile p K a of conjugate acid Rate in S N 2 reaction HO 15.7 (H 2 O) fast RCO 2 about 5 (RCO 2 H) reasonable H 2 O –1.7 (H 3 O + ) slow RSO 2 O 0 (RSO 2 OH) slow Different nucleophiles and leaving groups in the S N 2 reaction Halide leaving groups in the S N 2 reaction Halide X in MeX pKa of conjugate acid HX Rate of reaction with NaOH F +3 very slow Cl –7 moderate Br –9 fast I –10 very fast
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A closer look at the S N 1 mechanism rate = k 1 [t-BuBr] The first step is slow and rate determining.
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