Kinetics of the reduction of nitrotoluenes by aqueous ammonium sulfide under liquid–liquid phase transfer catalysis

The reduction of nitrotoluenes (o-, m- and p-) using aqueous ammonium sulfide as the reducing agent was carried out in an organic solvent, toluene, under liquid–liquid mode with a phase transfer catalyst (PTC), tetrabutylammonium bromide (TBAB). The selectivity of toluidines was found to be 100%. The reaction rate of m-nitrotoluene was found to be highest among the three nitrotoluenes, followed by p- and o-nitrotoluene. The effects of different parameters, such as speed of agitation, temperature, ammonia concentration, elemental sulfur loading, catalyst concentration, sulfide concentration and concentration of nitrotoluene, on the conversion and reaction rate of nitrotoluene were studied to establish the mechanism of the reaction. The rate of reaction of nitrotoluene was found to be proportional to the concentration of catalyst, to the square of the concentration of sulfide and to the cube of the concentration of nitrotoluene. The apparent activation energy for this kinetically controlled reaction was estimated as 19.43, 21.45 and 25.54 kcal/mol for ONT, PNT and MNT, respectively. A generalized empirical kinetic model was developed to correlate the experimentally obtained conversion versus time data for the three nitrotoluenes.