An experimental and theoretical study of the nitrosation of ammonia and thiourea

The nitrosation reactions of both thiourea and ammonia have been investigated, owing to their interesting kinetic behaviour and their industrial importance in the sensitisation of emulsion explosives. Kinetic experiments were conducted using a well-mixed aqueous reactor, from which samples were periodically removed, quenched, and then analysed for nitrite concentration using ion chromatography. We derived a number of plausible rate equations for both the ammonia and thiourea gassing processes, which we subsequently tested against experimental results to determine the operating kinetic mechanism, under a range of conditions. The reaction between ammonia and nitrite was seen to proceed by way of the nitrosating agent dinitrogen trioxide. In the presence of thiocyanate, however, dinitrogen trioxide was replaced by the thermodynamically more powerful nitrosating agent nitrosyl thiocyanate. Both reaction mechanisms exhibited relatively similar temperature dependencies, with respective activation energies of 56 and 68kJmol-1, determined between 25 and 45 °C, indicating reaction-controlled regimes. The effects of pH, temperature and ammonium nitrate concentration on the rate of thiourea nitrosation were studied. The rate of reaction was independent of ammonium nitrate concentration, while decreasing the pH of the reaction medium resulted in significant increases in the rate of reaction. The activation energy was evaluated as being 61kJmol-1 in the temperature range of 25 to 45 °C. Theoretical evidence at the G2MP2 level of theory supports a mechanism of thiourea nitrosation involving rapid deprotonation of SS-nitrosothiourea, followed by rate-limiting SS to NN migration of the nitroso group.