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Ch_322b_21.06 - Chapter 21.6 lecture note

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Aryl Halides and Nucleophilic Aromatic Substitution Simple aryl halides do not easily undergo nucleophilic aromatic substitution . Cl + NaOH H 2 O heat no reaction Similarly, vinylic halides do not react under these conditions. CH 2 =CHCl + NaOH H 2 O heat no reaction This decreased reactivity relative to alkyl halides is in line with the greater bond strengths of aryl and vinylic halides. some bond dissociation energies (kcal/mol) halide X CH-X CH 3 CH 3 F 127 106 Cl 97 81 Br 82 69 I 66 54
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Resonance Stabilization The stronger bonds in aryl and vinylic halides are explained by resonance that provides some double bond character to the C-X bonds. : Cl : : : Cl : : = + - : Cl : : = + - etc. R-C=C-Cl : : : R R R-C-C=Cl : : + R R : -
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Two Mechanisms for Nucleophilic Aromatic Substitution The Addition-Elimination Mechanism This mechanisms occurs with "active" aryl halides (strong electron-withdrawing substituents) and is given the notation S N Ar . The Elimination-Addition Mechanism This mechanism operates with unactivated aryl halides and requires very strong bases. It also is called the "benzyne" mechanism.
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