Many electrophilic aromatic halogenations require the presence of aluminum trihalide (halide-chloride or bromide) as a catalyst. Why is it unnecessary to include an aluminum trihalide in the electrophillic aromatic bromination reaction of acetanilide with molecular bromide?

Answer: The aluminium trihalide assists in the heterolytic bond fission of the bromine molecule.

Explanation:

The electrophilic aromatic bromination of acetalinide with molecular bromine requires the formation of Br+. This specie is formed when an Aluminum trihalide reacts with bromine. The Br-Br bond breaks heterolytically such that charged species are produced. This Br + participates in the electrophilic aromatic substitution and AlX4 - is formed where X4 shows a tetrahalide complex of aluminium