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Explain halogenation of benzene with example of chlorination. |
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Answer» Solution :(a) Chlorination and bromination of benzene gives chlorobenzene and bromobenzene respectively. Reactant of chlorination : Chlorobenzene Catalyst of chlorination: Anhydrous `FeCl_(3)` or `AlCl_(3)` Electrophiles : Cl + (Chloronium ion) Mechanism of reaction : electrophilic substitution reaction of aromatic ring.  (b) Mechanism of chlorination reaction of benzene : Reaction occurs in two steps. (i) Occurring of electrophilic reactant `Cl^(o+)` : Catalyst anhydrous `AlCl_(3)` or anhydrous `FeCl_(3)` react with `Cl_(3)` molecule and produce electrophilic chloronium ions `(Cl^(o+))`.  (ii) Carbocation OR Arenium ion OR Formation of `sigma`-COMPLEX OR Electrophilic substitution reaction : Step-1 : Electrophilic `Cl^(+)`, destroy `pi` electron CLOUD of benzene and attached one carbon. And form INTERMEDIATE sigma complex.  In ` sigma`-complex on carbon is `sp^(3)`. So `sigma`- complex is not aromatic. THOUGH intermediate resonance of `sigma`-complex is stable. It has (A), (B), (C) resonance structureand hybrid structure (D). The carbon of `sigma`-complex possess positive charge, it is carbocation.  Step-2 : In this step `pi`-bond is formed. `FeCl_(4)^(-)//AlCl_(4)^(-)` accept `H^(+)` from the `sigma`-complex. This step has bond formation so it is FAST step.  In step-I electrophilic group is added. So mechanism of this reaction is electrophilic in nature. And product obtained by substitution of H from the benzene by Cl group is electrophilic type. |
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