Elementary reaction step model of the N‐nitrosation of ammonia

This contribution develops a comprehensive kinetic model of the N-nitrosation reaction mechanism, consisting almost entirely of elementary reaction steps and applies it to study the nitrosation of ammonia. The reaction mechanism features 26 species and 22 reactions, with 8 parallel reaction pathways for ammonia nitrosation and a side pathway for nitrous acid decomposition. We compiled forward and reverse rate constants for each of the reactions, either from the literature sources or by correlation with known rate and equilibrium constants. The concentration of each reaction species with respect to time can be obtained for any set of initial concentrations by invoking a simultaneous solution to the system of ordinary differential equations describing the reaction mechanism. The model successfully predicts previous experimental results for ammonia nitrosation, with and without the addition of the catalyst thiocyanate. The effect of pH on the rate and mechanism of ammonia nitrosation was studied with the model. For uncatalyzed nitrosation, the results indicate that between pH 6.0 and 1.5 the reaction proceeds predominantly via reaction with N₂O₃, whereas ON⁺ nitrosation becomes the preferred pathway below pH 1.5. ONSCN is the dominant nitrosating agent across the entire pH range studied when the nucleophile thiocyanate was added in appreciable quantities. However, with the weak nucleophile Cl^?, nitrosation by N₂O₃ and ON⁺ governed the reaction kinetics at high and low pH, respectively. We demonstrate that nitrosating agent formation is rapid and does not limit the rate of ammonia nitrosation; however, nitrosating agent formation could become the rate-limiting step for the nitrosation of highly reactive substrates. © 2007 Wiley Periodicals, Inc. Int J Chem Kinet 39: 645–656, 2007