Synthesis of 3,6-dichloro salicylic acid by Kolbe–Schmitt reaction. 1. The primary reaction mechanism through DFT analysis

Abstract

Density functional theory-based molecular modeling was used to study the mechanism of 2,5-dichloro phenoxide carboxylation with CO2 for the synthesis of 3,6-dichloro salicylic acid by Kolbe–Schmitt reaction, and the in situ Fourier transform infrared (FTIR) spectroscopy technology under reaction conditions was employed to confirm the validity of the calculated mechanism. The reaction route was optimized using B3LYP/6-311+G (d, p), showing a path including an electrophilic attack by CO2, and then followed by a proton transfer. The chlorine atoms substituted on the benzene ring as electron-attracting groups cause higher activation energy barriers for electrophilic attack than in the case of the corresponding unsubstituted phenol. The comparison between calculated vibrational spectra and experimental spectra showed good agreement. The results revealed the forming path of carboxyl, by showing a carboxyl stretching vibration absorption band at 1,739 cm−1, which then splits into two bands at 1,584 and 1,472 cm−1 assigned to the carbonate as the final product, giving hypostatic evidence for the reliability of the calculated 3-intermediates and 3-transition states mechanism for the Kolbe–Schmitt reaction.