Answer: The aluminium trihalide assists in the heterolytic bond fission of the bromine molecule.
Step-by-step 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