A Novel Process for Synthesizing p-Nitrotoluene-o-sulfonic Acid

p-Nitrotoluene-o-sulfonic acid, a key intermediate for whitening agents, was prepared from p-nitrotoluene via sulfonation by using 50% oleum as sulfonation agent and waste sulfuric acid as diluted agent, as well as heat transfer medium. After the reactive mass was crystallized and filtrated, a certain amount of filtrate was reused in the sulfonation and the residue was hydrolysed to obtain the NTS dissolved. The yield of NTS could reach to 98.9% while the amount of 50% waste sulfuric acid aqueous solution could be reduced 75% approximately, with the comparison of the conventional procedure using 20% oleum as the sulfonation agent. The dependence of temperatures, time, ratio of raw materials and the amount of waste sulfuric acid added on the yield was investigated, and the proposed optimum conditions were to conduct the sulfonation 60min at 90℃, to maintain the sulfonation mixture 60min at 110℃, p-Nitrotoluene:SO_ 3 =1.0:1.1, and the amount of waste sulfuric acid reused:50% oleum=1:1.