Question

Write the steps in the mechanism and predict the product for the following reaction:

Answer 1

FeBr₃ is a Lewis acid catalyst. It forms an adduct with the Lewis base Br₂. This enhances the electrophilic nature of Br₂ which then readily gets attacked by the pi electrons of the toluene. The resulting species is an arenium ion which is stabilized by multiple resonance structures. The sp³ proton on the ring is then picked up by the FeBr₄^- and the ring regains aromaticity to give the product - p-bromotoluene. FeBr₃ catalyst is regenerated at the end of the reaction.