what happens if benzene reacts with Cl2 in presence of FeBr3?

Question

we know that halogenation of benzene adds a halogen to benzene in presence of a strong electrophile. but what if we use chlorine as a halogen but use an electrophile made out of a different halogen, like bromine?

Answer 1

Typically mixing a halogen electrophile with a catalyst using a different halide ion is not done in Friedel-Crafts halogenations. Here's why in the case described in the question.

• The electrophile could be changed through halogen displacement. Here, that would be chlorine reacting with $\ce{FeBr3}$, independently of the benzene, so the electrophile in subsequent reaction with benzene becomes bromine instead of chlorine. Since the chlorine could alternatively act as an electrophile first, you get a mixture of halobenzene products.

• When the proton is displaced, it will combine with a halide ion abstracted from the halide molecule/complex anion. But that could be bromide instead of chloride, giving a mixed hydrogen halide product and altering the catalyst (and perhaps its solubility and Lewis acidity).

So you risk mixing up the organic product, the displaced hydrogen halide and the catalyst. Don't go there. Match the halide ion in the catalyst with the halogen electrophile.