6_Chemistry_of_Benzene_Substituents

6_Chemistry_of_Benzene_Substituents - Chem 267 Basic...

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Prof. Dr. Xiangdong Fang University of Waterloo Chem 267 Basic Organic Chemistry II 1
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2 H 2 C H + Li H 2 C Li + p K a ~41 p K a ~50 stronger acid weaker acid benzyllithium Adjacent Phenyl Group Can Stabilize Radical, Anion, or Cation C H H C H H C H H C H H radical anion cation
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Benzylic Resonance Stabilization Br 2 No Reaction CH 3 Br 2 CH 2 Br +H B r CH 3 Cl 2 h CH 2 Cl Cl 2 h -HCl -HCl CHCl 2 Cl 2 h -HCl CCl 3 or h “phenylmethyl(benzyl) group is more reactive than related isolated functional group” 1) benzylic C-H bond can easily be radically halogenated (benzylic radical is more stable); 3
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4 ethylbenzene Cl 2 heat + + major minor Cl Cl Cl Cl more stabilized less stabilized benzylic resonance Result of Low-Lying LUMO of Benzene Molecular Orbital
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5 “bromine radical is more selective (stops at mono-bromonated product)” N O O Br N -bromosuccinimide (NBS) NBS uv light Br “using radical initiator, HBr addition to alkene often generates benzylic radical” HO OH heat OH 2 HB r Br -H 2 O CH 3 Br CH 3 Br benzylic radical r CH 3 Br Br +
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This note was uploaded on 07/07/2010 for the course CHEM 267 taught by Professor Fang during the Winter '09 term at Waterloo.

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6_Chemistry_of_Benzene_Substituents - Chem 267 Basic...

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