10-20-08 - 318 M Krische, Lecture 23: M - 10/20/08,...

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318 M – Krische, Lecture 23: M - 10/20/08, Substitution/Elimination We start by considering the reaction of a chiral tertiary chloride with water. Even though this is a terrible substrate for S N 2, a product of substitution is obtained. The product is racemic, which tells us that the reaction does not proceed through an S N 2 mechanism. Additionally, the reaction exhibits first order kinetics, which again is inconsistent with S N 2. For the S N 2 reaction, concerted bond cleavage/formation demands that both the nucleophile and electrophile are present in the rate-determining step (RDS). The first order kinetics observed for the reaction of the tertiary chloride with water suggests that bond cleavage and bond formation are NOT concerted in this transformation. We explained racemization of the product and the first order kinetics by invoking a mechanism whereby the RDS involves ionization of the tertiary halide to give a carbocation. The nucleophile then traps the carbocation and, ultimately, gives the product of
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10-20-08 - 318 M Krische, Lecture 23: M - 10/20/08,...

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