Solid acid catalysts for fluorotoluene nitration using nitric acid

Abstract

Fluorotoluenes have been nitrated regioselectively in the liquid phase under mild conditions using solid acid catalysts. Excellent yields were obtained with 70% nitric acid as nitrating agent. Nitration of 2-fluorotoluene at 90 °C showed 55% conversion with 90% selectivity for 2-fluoro-5-nitrotoluene whereas 3-fluorotoluene showed more than 79% conversion even at 60 °C with 67% selectivity for 3-fluoro-6-nitrotoluene and 30% selectivity for 3-fluoro-4-nitrotoluene. In the case of 4-fluorotoluene, side chain nitration was observed instead of ring nitration with 53% conversion and 59% selectivity for 4-fluoro-a-nitrotoluene. Among the different catalysts studied for fluorotoluene nitration, Fe/Mo/SiO₂, MoO₃/SiO₂ and H-beta showed higher conversion and selectivity in all three isomers due to higher acidity. A recycling study of catalyst H-beta for nitration of 3-fluorotoluene showed no significant loss in conversion as well as selectivity even after 5 recycles indicating the high stability of the catalyst. Compared to the conventional process, fluorotoluene nitration over solid acid catalysts is a highly regioselective, clean and environmentally friendly process with a simpler work up procedure.