Mononitrotoluene and its products

The nitration of toluene leads to mono-, di- and trinitrotoluene derivatives, depending on the reaction conditions. 

Technical nitration of toluene is carried out in a manner similar to that of benzene nitration however, since the reactivity is higher because of the presence of the methyl group, lower temperatures suffice. 

The reaction temperature should not exceed 60 °C, since above this limit both the methyl group and the aromatic nucleus are attacked oxidatively by the nitration mixture this leads to an increased formation of by-products, including nitrophenols, nitrocresols, phenylnitromethane and tetranitromethane. 

The mononitration of toluene is carried out at temperatures between 30 and 45 °C and with relatively low NO2 concentrations a nitric acid/sulfuric acid mixture (20/60) produces 57 to 60% 2-nitrotoluene, 3 to 4% 3-nitrotoluene and 37 to 40% 4-nitrotoluene. The yield of nitrotoluenes is 97 to 98%, with approx. 0.1% di-nitrotoluene and small amounts of nitrocresols arising as by-products. (The nitration can be carried out batchwise or continuously.) 

The three nitrotoluene isomers are separated by sequential distillation and crystallization. 2-Nitrotoluene can be recovered as a highly concentrated overhead fraction because of its lower boiling point (221.7 °C). The higher-boiling 3- and 4-nitrotoluenes (232.6 °C and 238.4 °C respectively) are separated industrially by crystallization from the melt, which, with a difference in crystallizing points of 35.3 °C (16.1 °C and 51.4 °C respectively), leads to extremely pure isomers. 

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