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1、Chapter 12 Reactions of Arenes: Electrophilic Aromatic Substitution,12.1 Representative Electrophilic Aromatic Substitution Reactions of Benzene,Electrophilic aromatic substitutions include:,Nitration Sulfonation Halogenation Friedel-Crafts Alkylation Friedel-Crafts Acylation,H,Table 12.1: Nitration
2、 of Benzene,+,+,H2O,H2SO4,HONO2,NO2,Nitrobenzene (95%),H,Table 12.1: Sulfonation of Benzene,+,+,H2O,heat,HOSO2OH,SO2OH,Benzenesulfonic acid (100%),H,Table 12.1: Halogenation of Benzene,+,+,HBr,FeBr3,Br2,Br2,Bromobenzene (65-75%),H,Table 12.1: Friedel-Crafts Alkylation of Benzene,+,+,HCl,AlCl3,C(CH3)
3、3,tert-Butylbenzene (60%),(CH3)3CCl,H,Table 12.1: Friedel-Crafts Acylation of Benzene,+,+,HCl,AlCl3,1-Phenyl-1-propanone (88%),12.2 Mechanistic Principles of Electrophilic Aromatic Substitution,Step 1: attack of electrophile on p-electron system of aromatic ring,highly endothermic carbocation is all
4、ylic, but not aromatic,Step 2: loss of a proton from the carbocation intermediate,H,H,H,E,H,H,H+,H,H,H,H,H,H,E,+,highly exothermic this step restores aromaticity of ring,Based on this general mechanism:,what remains is to identify the electrophile in nitration, sulfonation, halogenation, Friedel-Cra
5、fts alkylation, and Friedel-Crafts acylation to establish the mechanism of specific electrophilic aromatic substitutions,12.3 Nitration of Benzene,H,Nitration of Benzene,+,+,H2O,H2SO4,HONO2,NO2,Electrophile is nitronium ion,Step 1: attack of nitronium cation on p-electron system of aromatic ring,Ste
6、p 2: loss of a proton from the carbocation intermediate,H,H,H,NO2,H,H,H+,H,H,H,H,H,H,NO2,+,Where does nitronium ion come from?,H2SO4,+,12.4 Sulfonation of Benzene,H,Sulfonation of Benzene,+,+,H2O,heat,HOSO2OH,SO2OH,Several electrophiles present: a major one is sulfur trioxide,Step 1: attack of sulfu
7、r trioxide on p-electron system of aromatic ring,Step 2: loss of a proton from the carbocation intermediate,H,H,H,SO3,H,H,H+,H,H,H,H,H,H,SO3,+,Step 3: protonation of benzenesulfonate ion,H2SO4,12.5 Halogenation of Benzene,H,Halogenation of Benzene,+,+,HBr,FeBr3,Br2,Br2,Electrophile is a Lewis acid-L
8、ewis base complex between FeBr3 and Br2.,The Br2-FeBr3 Complex,+,Lewis base,Lewis acid,FeBr3,Complex,The Br2-FeBr3 complex is more electrophilic than Br2 alone.,Step 1: attack of Br2-FeBr3 complex on p-electron system of aromatic ring,H,H,H,H,H,H,Br,Br,FeBr3,+,+ FeBr4,Step 2: loss of a proton from t
9、he carbocation intermediate,H,H,H,Br,H,H,H+,H,H,H,H,H,H,Br,+,12.6 Friedel-Crafts Alkylation of Benzene,H,Friedel-Crafts Alkylation of Benzene,+,+,HCl,AlCl3,C(CH3)3,Electrophile is tert-butyl cation,(CH3)3CCl,acts as a Lewis acid to promote ionization of the alkyl halide,Role of AlCl3,(CH3)3C,Cl,+,Al
10、Cl3,acts as a Lewis acid to promote ionization of the alkyl halide,Role of AlCl3,(CH3)3C,Cl,+,AlCl3,+,(CH3)3C,+,Step 1: attack of tert-butyl cation on p-electron system of aromatic ring,H,H,H,H,H,H,Step 2: loss of a proton from the carbocation intermediate,H,H,H,C(CH3)3,H,H,H+,H,H,H,H,H,H,C(CH3)3,+,
11、H,Rearrangements in Friedel-Crafts Alkylation,Carbocations are intermediates. Therefore, rearrangements can occur,(CH3)2CHCH2Cl,Isobutyl chloride,tert-Butylbenzene (66%),C(CH3)3,+,H,Rearrangements in Friedel-Crafts Alkylation,Isobutyl chloride is the alkyl halide. But tert-butyl cation is the electr
12、ophile.,(CH3)2CHCH2Cl,Isobutyl chloride,tert-Butylbenzene (66%),C(CH3)3,+,Rearrangements in Friedel-Crafts Alkylation,+,C,CH2,H3C,CH3,H,+,+,H,Reactions Related to Friedel-Crafts Alkylation,H2SO4,+,Cyclohexylbenzene (65-68%),Cyclohexene is protonated by sulfuric acid, giving cyclohexyl cation which a
13、ttacks the benzene ring,12.7 Friedel-Crafts Acylation of Benzene,H,Friedel-Crafts Acylation of Benzene,+,+,HCl,AlCl3,O,CH3CH2CCl,CCH2CH3,O,Electrophile is an acyl cation,Step 1: attack of the acyl cation on p-electron system of aromatic ring,H,H,H,H,H,H,H,H,H,H,H,H,+,Step 2: loss of a proton from th
14、e carbocation intermediate,H,H,H,H,H,H+,H,H,H,H,H,H,+,can be used instead of acyl chlorides,H,Acid Anhydrides,Acetophenone (76-83%),AlCl3,+,12.8 Acylation-Reduction,Reduction of aldehyde and ketone carbonyl groups using Zn(Hg) and HCl is called the Clemmensen reduction.,Acylation-Reduction,H,Zn(Hg),
15、 HCl,AlCl3,RCCl,CH2R,permits primary alkyl groups to be attached to an aromatic ring,Reduction of aldehyde and ketone carbonyl groups by heating with H2NNH2 and KOH is called the Wolff-Kishner reduction.,Acylation-Reduction,H,H2NNH2, KOH, triethylene glycol, heat,AlCl3,RCCl,CH2R,permits primary alky
16、l groups to be attached to an aromatic ring,Example: Prepare isobutylbenzene,No! Friedel-Crafts alkylation of benzene using isobutyl chloride fails because of rearrangement.,(CH3)2CHCH2Cl,AlCl3,CH2CH(CH3)3,Recall,(CH3)2CHCH2Cl,Isobutyl chloride,tert-Butylbenzene (66%),C(CH3)3,+,Use Acylation-Reduction Instead,+,AlCl3,Zn(Hg) HCl,CH2CH(CH3)3,
