Kinetics, thermodynamics and stability constants of aniline oxidative coupling reaction with promethazine: experimental and computational study

A spectrophotometric method was employed to investigate the kinetics and thermodynamics of the oxidative coupling reaction between aniline and promethazine at various temperatures. Experimental and computational techniques, including Fourier transform infrared spectroscopy, ultraviolet-visible spectrophotometry and elemental analysis were used to characterize the resulting product. The kinetic analysis revealed that the reaction followed a first-order model and rate constants were determined across different temperatures ranging from 0.05367 to 0.08947 min-1. Furthermore, the stability constant of the reaction was determined at different temperatures, demonstrating an increase with rising temperature indicating an endothermic nature of the reaction. The activation energy (Ea) and pre-exponential factor (A) were determined as 9.3369 kJ/mol and 2.809 min-1. Thermodynamic analysis unveiled values for activation parameters: entropy (Delta S* = - 0.2447 kJ/mol K), enthalpy (Delta H* = + 6.826 kJ/mol) and Gibbs free energy (Delta G* = + 79.7797 kJ/mol). The positive Delta G* and Delta H* values indicated the ability of aniline via oxidative coupling with promethazine to form a product with the process meaning toward being non-spontaneous and endothermic. Additionally, a computational investigation was carried out using density functional theory (DFT) with the B3LYP/DGDZVP basis set to theoretically determine and compare the results with experimental data. The computational results closely matched the experimental data, including thermodynamic parameters, UV-Vis spectrophotometry, IR spectroscopy and molecular orbital energy gap as elucidated by Gaussian software.