We apply a recently developed ab initio polariton chemistry approach, and theoretically demonstrate that coupling nitrobenzene to an optical cavity can fundamentally change the selectivity of the electrophilic bromination reaction of nitrobenzene, making possible the ortho-or parasubstituted products. We have further provided an analysis of how coupling to the cavity will change the charge distribution of the cationic intermediate, which therefore causes a modification to the preferred bromination site. As such, strong couplings between molecules inside the cavity offer a promising tool to fundamentally change the outcome of a known chemical reactions.