Ch_322b_15.17

Ch_322b_15.17 - Chapter 15.17 lecture note

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Allylic and Benzylic Halides in Nucleophilic Substitution Reactions Allylic and benzylic halides undergo nucleophilic substitutions by either the S N 1 or S N 2 mechanisms. -C=C-C-X Ar-C-X allylic halide benzylic halide The S N 1 Mechanism: Carbocation Intermediates Since allylic and benzylic halides yield relatively stable carbocations (resonance stabilized), they react well by the S N 1 mechanism. -C=C-C-X slow RDS bond heterolysis -C=C-C + -C-C=C + + X - C C C + allylic cation
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C-X slow RDS bond heterolysis C + = C + etc. + X - C + benzyl cation Bond Heterolysis of Benzylic Halides Substitutent Effects on the Reactivity of Benzylic Halides Substituents in the aromatic ring predictably influence the reactivity of benzylic halides in S N 1 reactions. Because the bond heterolysis step forming the high energy carbocation intermediate is the RDS, electronic influences of the substituent on the stability of the carbocation (and the transition state) are expected to influence the rate of the reaction.
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Ch_322b_15.17 - Chapter 15.17 lecture note

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