Synthesis of 3,6-dichloro salicylic acid by Kolbe-Schmitt reaction. 2. Proton transfer mechanism for the side reaction

Experimental and computational efforts were combined to clarify the primary reason for the low yield of 3,6-dichloro salicylic acid synthesized from 2,5-dichloro phenoxide and CO2 by the Kolbe-Schmitt reaction. Liquid chromatography-electrospray ionization-tandem mass spectrometry (LC-ESI-MS) analysis showed that di-potassium salt is the unique ionized existing form of 3,6-dichloro salicylate as the direct carboxylate product. In addition, a byproduct 2,5-DCP with equivalent 3,6-dichloro salicylate is also produced. Theoretical investigation by means of the density functional theory revealed that the formation of 2,5-DCP can easily occur through a Bronsted-Lowry proton transfer mechanism, which is characterized by the rotation of carboxyl with a favorable thermodynamic potential. The byproduct 2,5-DCP can reach 50 % in a maximum theoretical yield, which will seriously inhibit the positive reaction equilibrium, meanwhile it deteriorates the mass transfer due to its high viscosity. This side reaction is confirmed to be the controlling factor for the low yield of 3,6-DCSA.