Halogen Carriers

Benzene reacts with halogens only in the presence of a catalyst called a halogen carrier. The π-electrons in benzene are delocalised around the ring, so the electron density is too low to polarise a halogen molecule enough for it to attack. (An alkene's localised π-bond can do this, so alkenes react with bromine without a catalyst.)Common halogen carriers are AlCl₃, FeCl₃, AlBr₃ and FeBr₃. Iron filings can also be used, because iron reacts with bromine to form FeBr₃ in the reaction mixture. The reaction is carried out at room temperature under anhydrous conditions.Generating the electrophile
The halogen carrier accepts a lone pair from the halogen, producing a positive halogen ion:Br₂ + FeBr₃ → FeBr₄⁻ + Br⁺Br⁺ is attacked by the benzene ring in electrophilic substitution. The intermediate then loses H⁺, which reacts with FeBr₄⁻ to regenerate the catalyst:H⁺ + FeBr₄⁻ → FeBr₃ + HBrOverall: C₆H₆ + Br₂ → C₆H₅Br + HBr. The halogen carrier is used up and then remade, so it is a catalyst. Halogen carriers are also used in Friedel-Crafts reactions.Example: chlorination of benzene uses Cl₂ with AlCl₃. The electrophile forms by Cl₂ + AlCl₃ → AlCl₄⁻ + Cl⁺, and the products are chlorobenzene and HCl.