Theoretical modeling of the liquid-phase chlorination of 4-nitroaniline catalyzed by dimethyl sulfoxide.

The mechanism of interaction of molecular chlorine with 4-nitroaniline in the aqueous medium, leading to the production of 2-chloro-4-nitroaniline, was studied. The simulation was carried out using high-level quantum chemistry methods DFT/ωB97x-D4/aug-cc-pVTZ//DLPNO-CCSD(T)/CBS and the SMD solvent model, and correction of translational entropy was also applied during the transition to the liquid-phase process. It was found that the reaction proceeds through the formation of an intermediate σ-complex and its subsequent decomposition. The catalytic role of dimethyl sulfoxide (DMSO) was investigated. It has been shown that DMSO catalyzes not only the σ-complex formation, but also its decomposition, participating in the deprotonation reaction with the formation of the DMSO cation (C2H7OS+). The thermodynamic and kinetic parameters of the reaction were calculated, and it was determined that the catalyst reduces the free activation energy of the first stage from 45 to 33 kJ mol-1 (298 K), and makes the second stage practically barrier-free. The reaction rate constant increases by 107 times. The catalyst action mechanism is to reduce the electrostatic repulsion in the chain of C-Cl-Cl atoms in the transitional state by shifting the electron density to the 4-nitroaniline molecule. Pre-reaction π-complexes of chlorine with 4-nitroaniline are formed due to van der Waals forces, and the σ-complex can be stabilized by dimethyl sulfoxide due to the formation of intermolecular hydrogen bonds. The presented mechanism will allow a better understanding of the influence of protonable catalysts on the course of SEAr reactions.