NONMETAL REDOX KINETICS - MONOCHLORAMINE, DICHLORAMINE, AND TRICHLORAMINE REACTIONS WITH CYANIDE ION

Trichloramine reacts with excess CN- in a stepwise series of Cl+ transfer reactions to give ClCN and to generate in sequence dichloramine, monochloramine, and ammonia: NCl3 + CN- + (HO)-O-2 --> NHCl2 + ClCN + OH-; NHCl2 + CN- + H2O --> NH2Cl + ClCN + OH-; and NH2Cl + CN- + H2O --> NH3 + ClCN + OH-. The reaction rates are first order in the corresponding concentrations of the chloramine species and in cyanide ion, with second-order rate constants (M(-1) s(-1), 25.0 degrees C, mu = 1.00 M) of 1.83 x 10(9) for NCl3, 576 for NHCl2, and 1.96 x 10(-2) for NH2Cl. The NH2CI reaction is general-acid assisted with third-order rate constants (M(-2) S-1) of 4.32 x 10(10) for H3O+ and 84 for HCO3-. The reactions of NHCl2 and of NCl3 with CN- are not acid assisted. Since HCN (pK(a) 8.95) is not reactive, the rates of the NHCl2 and NCl3 reactions decrease with decrease of pH (below pH 9). Off-setting effects of acid assistance and HCN formation cause the NH2Cl rate to increase from pH 13 to pH 9 and to level off below pH 9. At pH 7, the relative reactivities are NCl3 > HOCI much greater than NH2Cl much greater than NHCl2.