4_Electrophilic_Aromatic_Substitution

4_Electrophilic_Aromatic_Substitution - Chem 267 Basic...

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Prof. Dr. Xiangdong Fang University of Waterloo Chem 267 Basic Organic Chemistry 2 1
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Electrophilic Aromatic Substitution Reaction 1) Benzene is more stabilized due to extensive aromatic delocalization of π -electrons, which makes it much less reactive; Br 2 KMnO 4 H 2 /Ni HBr 2) It is still possible to hydrogenate benzene with catalysts, e.g. RhCl(PPh 3 ) 3 , to give cyclohexane as the product. 2
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Reagent E + Product Name X 2 , FeX 3 X + Halogenation HNO 3 , H 2 SO 4 + NO 2 O=N=O Nitration H 2 SO 4 Fuming SO 3 Sulfonation R-X, AlCl 3 R + carbocation Friedel-Crafts Alkylation AlCl 3 acylium ion Friedel-Crafts Acylation NO 2 X SO 3 H R C O R + C O R Cl Electrophilic Aromatic Substitution Reaction 3
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Halogenation of Benzene 4 Fe Br Br Br Lewis acid electron-deficient + Br Br Lewis base electron-rich Fe Br Br Br Br Br Br Fe Br Br Br Br electrophile very electron-deficient weak base H Br H Br H Br H Br benzonium ion Fe Br Br Br Br Br FeBr 3 +H B r+ restore aromaticity Fe Br Br Br Br H Br H Br Br Br not observed lose
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5 Nitration of Benzene H 2 SO 4 + HNO 3 +H 2 O NO 2 S O O OH HO p K a -9 N O OH O nitric acid sulfuric acid -1 stronger acid weaker acid N O O O H H + + S O O OH O N O O +H 2 O nitronium ion very electron-deficient H NO 2 H NO 2 H NO 2 H NO 2 benzonium ion H 2 O NO 2 + H 2 OH Mechanism of Nitration
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6 Sulfonation of Benzene 1) Every step is under equilibrium 2) SO 3 is added to push the equilibrium to the product side S O O OH HO p K a -9 S O O OH HO + S O O O HO + S O O O O H H H S O O O S O O O +H 2 O H SO 3 - H SO 3 - H SO 3 - H SO 3 - benzonium ion H 2 O SO 3 - + H 2 OH SO 3 H H SO 3 - H S O O O Mechanism of Sulfonation or HSO 4 -
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7 Friedel-Crafts Alkylation RX + Cl Al Cl Cl empty porb ita l Lewis acid Lewis base RX Al Cl Cl Cl R + Al Cl Cl Cl X carbocation very electron-deficient weak base H R H R H R H R benzonium ion Al Cl Cl Cl X R + AlCl 3
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“carbocations are unstable and undergoes rearrangements” Friedel-Crafts alkylation only works for tertiary alkyl halides Cl + AlCl 3 25 o C + n -propylbenzene 30% isopropylbenzene 70% Cl + AlCl 3 25 o C + isobutylbenzene 0% tert -butylbenzene 100% AlCl 3 H H 1 o carbocation
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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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4_Electrophilic_Aromatic_Substitution - Chem 267 Basic...

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