In this paper, hybrid quantum mechanical/molecular mechanical (QM/MM) calculations including 500 water molecules mold solvent effects on the molecular mechanisms of the alpha -chlorocyclobutanone and alpha -chlorocydohexanone transpositions to yield cyclopropane and cyclopentane carboxylic acids, respectively, as a model of the Favorskii rearrangement. The two accepted molecular mechanisms, the semibenzilic acid and the cyclopropanone transpositions, as well as the competition between both reaction pathways and the ring size effects are addressed in this investigation. Stationary points-reactants, products, transition structures, and intermediary species along both reaction pathways-have been located and characterized, involving a fully flexible active-site region, by means of GRACE and CHARMM software. The transition structures have been connected with their respective reactants and products by the intrinsic reaction coordinate procedure carried out in the presence of water media, thus obtaining for the first time a realistic reaction pathway for this chemical transposition. The analysis of the results obtained by QM/MM methods shows that the semibenzilic acid mechanism is favored over the cyclopropanone mechanism for the a-chlorocyclobutanone system. However, the study of the ring size effects reveals that the cyclopropanone mechanism is the energetically preferred reactive channel for the alpha -chlorocyclohexanone ring, probably due to the straining effects on bicycle cyclopropanone, an intermediate that appears on the semibenzilic acid pathway. This later mechanism is described as a two-step one, while the cyclopropanone or Loftfield mechanism is for the first time described as a four-step reaction. These results provide new information on an important chemical reaction and the key factors responsible for the behavior of reactant systems embedded in aqueous media. This methodology allows evaluation of specific solute-solvent interactions as well as weighing up of the different energy contribution terms.