•Accurate free energy profile of several mechanisms.•Cu(I)/Cu(III) mechanism explains the experiments.•Three mechanisms forming Cu(II) radical are unfavorable. Addition of fluorine to organic molecules in the late stage of synthesis has been an increased interest in the past decade. Catalytic methods using fluoride salts as reagents could be especially useful, mainly with the use of organometallic catalysis with copper. Recent experimental advances have been reported for copper-mediated fluorination and the development of an efficient catalytic process needs a deep understanding of the reaction mechanism and the free energy profile. In this work, different mechanisms of copper-mediated fluorination of 2-(2-bromophenyl)pyridine using LCuF as reagent (L = N-heterocycle-carbene ligand) were investigated using very high level of theory, DLPNO-CCSD(T) method with up to quadruple-zeta basis set. The Cu(I)/Cu(III) mechanism is the predominant one, taking place via neutral (MS2) or cationic (MS2p) intermediates. Although the oxidative addition step has high barrier, the reductive elimination was found to be the rate-determining step. Three reaction pathways involving the Cu(II) radical mechanism was also investigated. The first one is the single-electron transfer mechanism and the second is the bromine atom transfer in the first step. Both pathways involve high free energy intermediates and are inviable. The third mechanism occurs via singlet-triplet spin crossover with bromine transfer from the oxidative addition intermediate MS2 to the LCuF initial reagent. However, the reductive elimination step from the MS2r doublet Cu(II) complex is much more difficult than the MS2 or MS2p singlets of Cu(III) complexes, leading to a very slow kinetics for this Cu(II) radical mechanism. The kinetics analysis of the theoretical free energy profile results in effective ΔG‡ of 30.6 kcal mol−1, in good agreement with an estimated experimental value and providing an important support for the Cu(I)/Cu(III) mechanism. Display omitted .