5_Nucleophilic_Aromatic_Substitution

5_Nucleophilic_Aroma - Chem 267 Basic Organic Chemistry II Prof Dr Xiangdong Fang University of Waterloo 1 Nucleophilic Aromatic Substitution

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Prof. Dr. Xiangdong Fang University of Waterloo Chem 267 Basic Organic Chemistry II 1
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2 Nucleophilic Aromatic Substitution Reactions 1) the S N Ar mechanism (addition-elimination) 2) benzyne mechanism (elimination-addition) 3) diazonium salts with “aryl-cation or radical” intermediate
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3 Nucleophilic Aromatic Substitution Reactions “for electron-deficient aromatic rings with strong deactivator substituent(s), e.g. –NO 2
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4 Stabilized Anionic Intermediate
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5 Different Mechanism Without Electron-Withdrawing Group “go via elimination-addition mechanism Cl NaOH 350 o C, 300 atm OH Cl NaOH 350 o C, 300 atm H OH Cl OH unstable benzyne +H 2 O + NaCl OH OH sp 2 carbanion H 2 O OH H + OH elimination nucleophilic addition
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6 side view front view benzyne sp 2 -sp 2 overlap not efficient Benzyne: a very strained cycloalkyne “extremely reactive and can not be isolated (the triple bond is very weak)”
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7 Supporting Evidence for Benzyne Intermediate 1) chlorobenzene is labelled with 13 C isotope at the carbon directly bonded to Cl group; 2) 13 C isotope is scrambled evenly between two adjacent carbon atoms;
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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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5_Nucleophilic_Aroma - Chem 267 Basic Organic Chemistry II Prof Dr Xiangdong Fang University of Waterloo 1 Nucleophilic Aromatic Substitution

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