In this work, we use the ab initio cavity Quantum Electrodynamics approach to investigate a chemical reaction, the bromination of nitrobenzene, coupled to an optical cavity. Our approach is based on the previously developed parametrized QED (pQED) method, which uses the QED Pauli-Fierz Hamiltonian to describe light and matter interac-tions and uses adiabatic electronic states and all dipole matrix elements between them as inputs to compute the polariton eigenenergies. The bromination of nitrobenzene exhibits near 100% selectivity, favoring the meta-substituted isomer. Upon coupling to the cavity, we theoretically calculate the relative energies of the meta-, ortho-, and para-substituted cationic intermediates PhNO 2 -Br + , which are key intermediates that dictate the outcome of the reaction. Outside the cavity, the meta-substituted intermediate is 2 kcal/mol lower than the ortho-substituted intermediate and about 5 kcal/mol lower than the para-substituted intermediate, in agreement with reported experimental results. Upon coupling to the cavity and aligning the cavity polarization direction along the ê = (θ 1 , ϕ 1 ) (see Fig. ), the ortho-substituted intermediate is energetically more stable than the meta-substituted intermediate by up to 6 kcal/mol for a cavity frequency ω c and cavity volume V chosen to match state-of-the-art plasmonic cavity designs. When the cavity polarization direction is along the ê = (θ 2 , ϕ 2 ) (see Fig. ), the para-substituted intermediate is energetically more stable than the meta-substituted intermediate, with up to -2 kcal/mol. These changes in the selectivity of the various substituted intermediates are due to the quantum light-matter interactions between the molecules and cavity, which mixes the character of electronic excited states into the polariton ground state. These changes are characterized using the electronic density difference of the system inside and outside the cavity. We thus have theoretically shown that one can obtain ortho-or para-substituted bromonitrobenzene when coupling the reaction to an optical cavity, flipping the selectivity compared to outside the cavity.