Dinitrotoluenes and their derivatives

In terms of volume, the dinitration of toluene is of greater importance than mononitration. Reaction conditions for the dinitration are more severe than those for mononitration, with temperatures between 65 and 70 °C. A nitrating mixture with 65% sulfuric acid, 25% nitric acid and 10% water is used and nitration may be carried out as either a continuous or batch process. 

When working up the products from dinitration, the reaction mixture is settled and separated into a dinitrotoluene-rich stream and a spent-acid stream. The dini-trotoluene stream is refined by alkali washing and crystallization. The isomer composition is 20% 2,6-dinitrotoluene, 76% 2,4-dinitrotoluene, 0.6% 3,5-dinitrotoluene and small quantities of 2,5-dinitrotoluene (80/20 DNT). The yield from dinitration is around 96 to 98% the commercial product has a crystallizing point of 55 to 58 °C. The decomposition of dinitrotoluenes at higher temperatures is highly exothermic, and hence they should not be stored at a temperature exceeding 75 °C. 

Dinitrotoluenes are used predominantly for the manufacture of toluene diisocyanate (TDI), which is produced from the corresponding diaminotoluenes. 

Reduction of dinitrotoluenes is carried out industrially with Raney-nickel or palla-dium/carbon catalysts, under a hydrogen pressure of 70 bar and at a temperature of up to 150 °C, in a cascade or loop reactor. The concentration of dinitrotoluene in the reaction mixture is kept very low to achieve yields of over 99%. The reaction mixture is divided into a recycle stream and a residue, from which the catalyst is separated in a filter or cyclone. Distillation of the reaction product then yields the components methanol, water, diamines and a residue. Distillation of the water-free amines is carried out under vacuum (approx. 50 mbar), with 2,3- and 3,4-tolu-enediamine being separated in a fronts column. A tar-like residue is removed as a bottom product in the main column, while a mixture containing 2,4- and 2,6-tolu-enediamine is collected as an overhead fraction. 

It is particularly important to remove 2,3- and 3,4-toluenediamine by distillation since, in the subsequent reaction with phosgene, these compounds would produce benzimidazolones which, having active amide hydrogen atoms, can react with the principal product, toluene diisocyanate, and reduce the yield. 

---