Ionic strength effect on the kinetics and mechanism of N-vinyl compound formation in the presence of heterocyclic biological base: empirical and theoretical approaches

Abstract

In the current work, kinetics and mechanism of the reaction between dimethyl acetylenedicarboxylate (DMAD), thiazolidine-2,4-dione (heterocyclic biological base, TZD) and triphenylarsine (TPA) has been studied for the synthesis of N-vinyl compound in the presence of various salts and solvents by the UV–visible technique. Through an alternation in the reaction environment from lone methanol (kobs = 0.3121 M−1Min−1) or acetone alone (kobs = 0.008 M−1Min−1) to a mixture of methanol and salt (NaBr) (kobs = 21.35 M−1Min−1), the reaction rate extremely accelerates 70 or 2600 times more, respectively. These remarkable results indicated that the mentioned reaction has greatly proceeded and extremely faster in the mixture of a polar solvent and salt in comparison with the solvent alone. A mechanism involving four steps was proposed. Unfortunately, experimental data from the UV–visible technique were not adequate to determine the rate-determining step of the reaction mechanism. To clear this, Molecular Electron Density Theory computations have been consulted at the B3LYP level of theory using the standard 6-311 + G (d, p) and extra basis set of 6-311 + G (2df, 2pd) for the arsenic atom. The analysis of the relative schematic representation of the Gibbs free energy profiles revealed that 1,2-hydrogen shift proton is the rate-determining step of the reaction mechanism. The comparison of the rate constant of the reaction between DMAD, TZD and TPA in the presence of salt such as NaBr with this reaction in the absence of salt.