Dinitration of o-toluic acid in continuous-flow: process optimization and kinetic study

Two types of expeditious and highly efficient continuous-flow processes for the dinitration of o-toluic acid were described. The o-toluic acid was dinitrated with mixed acid (sulfuric acid and nitric acid) to give 3,5-dinitro-2-methylbenzoic acid with 96% isolated yield and achieved a productivity of 1586 g/h. Based on batch trials, an isothermal continuous-flow process was designed and intensified, and the intrinsic kinetics of the dinitration was studied to obtain the precise residence time. As a result, the amount of reagent and the reaction time were reduced significantly, purity and yield of target product were both greatly increased compared to batch process. Then, as a further trial, the dinitration was conducted in an adiabatic continuous-flow system. An online temperature detection was used to monitor reaction progress and obtain corresponding optimal residence time. These two continuous-flow processes have the similar characteristics of short residence time and high reaction yield, although the operation conditions are totally different.