Influence of electron-donating groups on the aniline oxidative coupling reaction with promethazine: a comprehensive experimental and theoretical investigation

This study explores oxidative coupling of para-aniline derivatives, including (p-methylthio aniline, p-methyl aniline, p-methoxy aniline, p-hydroxy aniline and p-amino aniline). The study investigates the influence of substituents on the kinetics, thermodynamics and stability constants of the resulting compounds. A number of hybrid exchange–correlation functionals, namely, B3LYP, PBE0, BMK, CAM-B3LYP, M06-2X, HSE06 and BH&HLYP with a number of basis sets, namely, STO-3G, 3-21G, 6-31G, 6-31G*, DGDZVP, 6-311G, LanL2MB, LanL2DZ and SDD have been employed to calculate the electronic spectra of all the products. The results are compared with available experimental data. In most cases studied, the density functional theory results based on the B3LYP functionals with the DGDZVP basis set indicate a better agreement with the experimental absorption bands in the UV–Visible and IR spectra. The study reveals a first order reaction model with rate constants (0.06433–0.09053 min⁻¹) across temperatures for the product of aniline and p-amino aniline. Characterization of the compounds involves experimental and computational techniques such as FTIR and UV–Visible spectrophotometries. Activation energy (E_a) and pre-exponential factor (A) ranging (6.679–9.337 kJ mol⁻¹) and (1.2839–2.809 min⁻¹), with the highest values observed for the product of p-hydroxy aniline. Stability constants increase with temperature indicating an endothermic reaction. Thermodynamic analysis unveiled values for activation parameters: entropy (ΔS* = −0.2513 to −0.2447 kJ mol⁻¹ K⁻¹), enthalpy (ΔH* = 4.168–6.826 kJ mol⁻¹) for the product of p-hydroxy aniline and aniline; and Gibbs free energy (ΔG* = 80.1856–81.0466 kJ mol⁻¹) for the product of p-amino aniline and aniline. ΔG* values indicate a non-spontaneous and endothermic processes. Computational investigations using density functional theory (DFT) with the B3LYP/DGDZVP basis set, authorize reliability and accuracy of the experimental results.

Graphical Abstract