Study of the mechanism of the Wacker oxidation is of great importance for many reasons. Thus far, from a theoretical point of view, particularly in connection with the rate-determining step, no mechanism has been presented which includes all experimental evidence, notably the isotopic effect, the energy barrier, and the rate constant of the reaction. In this work, we proposed a mechanism for nucleophilic addition which quantitatively and qualitatively will be in good agreement with the experimental evidence. The mechanism study of the Wacker process has been performed using a density functional method in the gas and solution (Onsager and PCM models) phases. The internal nucleophilic addition of the OH group on the coordinated alkene has been investigated as a direct attack or water chain attack. The reaction progress in both routes of syn and anti attack yields two different products for this step; therefore, modeling of attacks has been performed for both routes. The results obtained in the gas and solution phases showed that the direct attacks in both routes have a high energy barrier and applying water chains highly reduces the energy barrier. The anti attack of the three-water-chain model has the lowest energy barrier. Applying this model leads to results which have good consistency with the experimental evidence.

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