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Trichloramine reacts with excess CN- in a stepwise series of Cl + transfer reactions to give CICN and to generate in sequence dichloramine, monochloramine, and ammonia : NCl 3 + CN- + H 2 O→NHCl 2 + ClCN + OH- ; NHCl 2 + CN- + H 2 O→NH 2 Cl + CICN + OH- ; and NH 2 Cl + CN- + H 2 O→NH 3 + CICN + 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°C, μ = 1.00 M) of 1.83 x 10 9 for NCl 3 , 576 for NHCl 2 , and 1.96 x 10 -2 for NH 2 Cl. The NH 2 Cl reaction is general-acid assisted with third-order rate constants (M -2 s -1 ) of 4.32 x 10 10 for H 3 O + and 84 for HCO 3 - . The reactions of NHCl 2 and of NCl 3 with CN- are not acid assisted. Since HCN (pK a 8.95) is not reactive, the rates of the NHCl 2 and NCl 3 reactions decrease with decrease of pH (below pH 9). Off-setting effects of acid assistance and HCN formation cause the NH 2 Cl rate to increase from pH 13 to pH 9 and to level off below pH 9. At pH 7, the relative reactivities are NCl 3 > HOCI >> NH 2 Cl >> NHCl 2 . |
