Synthesis of 3,6-dichloro salicylic acid by Kolbe–Schmitt reaction. 2. Proton transfer mechanism for the side reaction
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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 Brønsted–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.
Keywords3,6-dichrolo salicylic acid Kolbe–Schmitt reaction di-metallic salts Density functional theory
Financial supports from Fundamental Research Funds for the Central Universities of China and the Science and Technology Committee of the Shanghai Municipal Government under Grant number 09DZ1100203 are gratefully acknowledged.
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