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 -