Formation of cyanogen chloride from the reaction of monochloramine with formaldehyde

Methanediol dehydrates to give formaldehyde, which reacts rapidly and reversibly with monochloramine to form N-chloroaminomethanol. Under drinking water conditions, N-chloroaminomethanol undergoes a relatively slow decomposition that eventually leads to the formation of cyanogen chloride (ClCN) in apparently stoichiometric amounts. The following reaction sequence is proposed: CH₂(OH)₂ ⇆ CH₂O + H₂O; CH₂O + NH₂Cl ⇆ CH₂(OH)NHCl; CH₂(OH)NHCl → CH₂NCl + H₂O; CH₂NCl → HCl + HCN; CN^- + NH₂Cl + H⁺ → ClCN + NH₃. These reactions were studied at 25.0°C and an ionic strength of 0.10 M (NaClO₄). Stopped-flow photometry was used to monitor rapid, reversible reactions, and photometry was used to study relatively slow decomposition reactions. Equilibrium and rate constants for the addition of formaldehyde to monochloramine were (6.6 ± 1.5) x 10⁵ M^-1 and (2.8 ± 0.1) x 10⁴ M^-1 s^-1, respectively. The dehydration of N- chloroaminomethanol was catalyzed by both H⁺ and OH^-, with respective rate constants of 277 ± 7 and 26.9 ± 5.6 M^-1 s^-1. Under characteristic drinking water conditions, the decay of N-chloroaminomethanol is the rate- limiting step. N-Chloromethanimine, formed by the dehydration of N- chloroaminomethanol, had a decomposition rate constant of (6.65 ± 0.06) x 10^-4 s^-1. At the relatively high methanediol concentrations used in this study, the intermediary N-chlorodimethanolamine was formed by the rapid and reversible reaction of N-chloroaminomethanol with formaldehyde. N- Chlorodimethanolamine then decayed relatively slowly. The following reaction sequence is proposed: CH₂(OH)NHCl + CH₂O ⇆ {CH₂(OH)}₂NCl; {CH₂(OH)}₂NCl → CH₂NCl + CH₂O + H₂O. The equilibrium and rate constants for the addition of formaldehyde to N-chloroaminomethanol were (9.5 ± 2.5) x 10⁴ M^-1 and (3.6 ± 0.1) x 10³ M^-1 s^-1, respectively. The decomposition of N-chlorodimethanolamine was catalyzed by OH^-, with a rate constant of 19.2 ± 3.7 M^-1 s^-1. N-Chlorodimethanolamine would not be present under typical drinking water treatment conditions.