Chap 15 Electrophilic aromatic substitution(芳香族 亲电取代) 15.1 Electrophilic aromatic substitution reactions These reactions are of the general type shown below: +E Ar-E H E+- Electrophilic reagent亲电试剂
Chap 15 Electrophilic aromatic substitution (芳香族 亲电取代） 15.1 Electrophilic aromatic substitution reactions These reactions are of the general type shown below; + E + E + H + ArH + E + Ar-E + H + E+ -------Electrophilic reagent 亲电试剂
Fig 15.1 Electrophilic aromatic substitution reactions (X=CL, Br) + hX Halogenation 卤化 HNO HSO H20 Nitration 硝化 So, SO3H H2SO4 Sulfonation 磺化 RCL, AICI3 R Friedel-Crafts Alkylation (R can rearrange) HC弗瑞德尔克拉伏茨 烷基化 RC-CL, AlCI R Friedel-Crafts Acylation 弗瑞德尔-克拉伏茨 酰基化
Fig 15.1 Electrophilic aromatic substitution X2 , Freactions. eX3 (X = Cl, Br) X + HX Halogenation 卤 化 NO2 HNO3 H2SO4 + H2O SO2 H2SO4 SO3H RCl, AlCl3 (R can rearrange) R + HCl RC-Cl, AlCl3 O R O Nitration 硝 化 Sulfonation 磺 化 Friedel-Crafts Alkylation 弗瑞德尔-克拉伏茨 烷基化 Friedel-Crafts Acylation 弗瑞德尔-克拉伏茨 酰基化
15.2 A general mechanism for electrophilic aromatic substitution(芳香族亲电取代反 应机理): Arenium ions(芳基离 子) a considerable body of experimental evidence indicates that electrophiles attack the Isystem of benzene to form a delocalized nonaromatic carbocation known as an arenium ion or sometimes as a complex)
15.2 A general mechanism for electrophilic aromatic substitution（芳香族亲电取代反 应机理）: Arenium ions（芳基离 子） A considerable body of experimental evidence indicates that electrophiles attack the πsystem of benzene to form a delocalized nonaromatic carbocation known as an arenium ion (or sometimes as a σcomplex) π
+E H Arenium ion(芳基正离子) σ- complex(o络合物) E E H H
E + H + H + E + E + E H + E H + E H + Arenium ion (芳 基 正 离 子 ） σ- complex (σ-络 合 物 ）
We wish to show the mechanism using the modern formula for benzene Rate-determining Step 1 Slow Arenium ion(芳基正离子) E σ- complex(-络合物) Fast E Step 2 H+
We wish to show the mechanism using the modern formula for benzene. + E + Arenium ion (芳 基 正 离 子 ） σ- complex (σ-络 合 物 ） Step 1 E + H Step 2 E + H E + H + Rate-determining Slow Fast
Fig 15.2 The potential energy diagram for an electrophilic aromatic substitution reaction 6+ Arenium ion(芳基正离子) σ- complex(G-络合物) H △G1 +e E Reaction coordinate(反应进程
Fig 15.2 The potential energy diagram for an electrophilic aromatic substitution reaction. + E + Arenium ion (芳 基 正 离 子 ） σ- complex (σ-络 合 物 ） E + H E + H E + H+ E δ + H δ G1 G2 PE Reaction coordinate (反 应 进 程 ）
5.3 Halogenation of benzene Benzene does not react with bromine or chlorine unless a Lewis acid is present in the mixture. When Lewis acids are present, however, benzene reacts readily with bromine or, chlorine, and the reactions give bromobenzene and chlorobenzenein good yields
15.3 Halogenation of benzene Benzene does not react with bromine or chlorine unless a Lewis acid is present in the mixture. When Lewis acids are present, however, benzene reacts readily with bromine or, chlorine, and the reactions give bromobenzene and chlorobenzenein good yields. + Cl2 FeCl3 25 oC Cl + HCl Chlorobenzene (90%) + Br2 FeBr3 25 oC Br + HBr Bromobenzene (75%)
Fluorine 2 reacts so rapidly with benzene that aromatic fluorination requires special conditions and special types of apparatus lodine (2), on the other hand, is so unreactive that a special technique has to be used to effect direct iodination: the reaction has to be carried out in the presence of an oxidizing agent such as nitric acid: HNO 86%
Fluorine (F2 ) reacts so rapidly with benzene that aromatic fluorination requires special conditions and special types of apparatus. Iodine (I2 ), on the other hand, is so unreactive that a special technique has to be used to effect direct iodination; the reaction has to be carried out in the presence of an oxidizing agent such as nitric acid: + I2 I HNO3 86%
The mechanism for aromatic bromination is as follows Step 1 BrBr Febr3 Br+ FeBr H H Step2 Br B B Arenium H Step3 BrBr—FeBr3 Br HBr feBr3 Bromobenzene
The mechanism for aromatic bromination is as follows; Step1 Br Br + FeBr3 Br + + FeBr4 - Step2 Br + + H Br + H Br + Arenium Br + HBr Bromobenzene Step3 H Br + Br FeBr3 + FeBr3
15.4 Nitration of benzene Benzene reacts slowly with hot concentrated nitric acid to yield nitrobenzene, The reaction is much faster if it is carried out by heating benzene with a mixture of concentrated nitric acid and concentrated sulfuric acid NO. H2SO4 2 + hNO H3o t hso 50-550C
15.4 Nitration of benzene Benzene reacts slowly with hot concentrated nitric acid to yield nitrobenzene. The reaction is much faster if it is carried out by heating benzene with a mixture of concentrated nitric acid and concentrated sulfuric acid + HNO3 H NO2 2 SO4 85% 50-55 o C + H3O + + HSO4 -