A continuous and selective method for synthesis of dinitro herbicides through one-step dinitration approach has been successfully developed in microreactor systems. Compared to a conventional two-step batch process, reaction can be conducted without the need to separate intermediates and much less solvent or solventfree condition has been employed.Dinitroaniline derivatives are an important class of compounds with wide applications in herbicides and pesticides. 1 For example, pendimethalin, N-(1-ethylpropyl)-3,4-dimethyl-2,6-dinitroaniline, is a widely-used selective herbicide in agricultural industry. 2 Dinitroaniline derivatives are generally produced by nitration of aniline derivatives. These nitration reactions are extremely hazardous due to highly exothermic thermodynamics as well as possible decomposition and explosion of nitro compounds. 3 The reaction heat of direct dinitration can be twice as much as that of mononitration. In the chemical industry, to carry out the reaction in a mild way, a two-step process is generally conducted, in which one mononitration step is followed by subsequent mononitration. 4 Moreover, large quantities of solvents or diluents are used in each nitration step since nitration reactions are usually fast for reactive species such as phenol and aniline derivatives. Thus, it requires long reaction time and large amounts of waste solvents. Also, selectivity is not satisfying due to easy oxidation or over-nitration of aniline derivatives when reagents and products are exposed to the reaction conditions over a long residence time, and protection of the amino group by acetylation may be required prior to nitration. 5 Still, the safety risk is not eliminated owing to mass and heat transfer limitations in conventional batch reactors.The broad applications of dinitroaniline herbicides provide enormous impetus for extensive study of nitration reactions. The well-established mechanisms for nitration of reactive species, such as aniline and phenol compounds, include both electrophilic and charge-transfer processes. 6 Recently, one solution complementary to the conventional batch or semibatch mode is to employ a continuous and selective nitration mode. The continuous process can offer much small reaction volume and relatively large surface-to-volume ratio; thus process safety is improved and both temperature and residence time can be precisely controlled, which may improve selectivity by suppression of side reactions. 7 Microreactors have been demonstrated as an effective tool in continuous processes for mononitration of some aromatic compounds, for instance benzene, toluene and chlorobenzene. 8 Beneficial for its characteristic submillimeter dimensions, microreactors have much short diffusion paths and large surface-to-volume ratios compared with batch reactors, which are beneficial to realize excellent heat and mass transfer. So far, most academic and industrial attention has been focused on mononitration processes, and the straightforward one-step dinitration in continuous-flow reactors...

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