Chloroacetonitrile and N,2-dichloroacetamide formation from the reaction of chloroacetaldehyde and monochloramine in water

Combined chlorine is increasingly being used as an alternative disinfectant to free chlorine to maintain a residual in drinking water distribution systems mainly because it would reduce the formation of regulated disinfection byproducts (DBPs) trihalomethanes and haloacetic acids. However, the use of combined chlorine could promote the formation of currently unregulated nitrogenous DBPs (N-DBPs) such as haloacetonitriles and haloacetamides that are found to be more cyto- and genotoxic than regulated DBPs. Monochloramine quickly reacts with chloroacetaldehyde, a DBP formed during primary disinfection with free chlorine, forming and reaching pseudoequilibrium (equilibrium constant K₁ = 1.87 × 10³ M^-1) with the carbinolamine 2-chloro-1-(chloroamino)ethanol. 2-Chloro-1-(chloroamino)ethanol undergoes slow dehydration to form the imine 1-chloro-2-(chloroimino)ethane that decomposes at a faster rate to chloroacetonitrile. 2-Chloro-1-(chloroamino) ethanol is also oxidized by monochloramine to produce the previously unreported DBP N,2-dichloroacetamide. The carbinolamine dehydration step was found to be acid/base catalyzed (k₂⁰ = 3.30 × 10^-6 s^-1, k₂^H = 2.43 M^-1 s^-1, k₂^OH = 3.90 M^-1 s^-1). In contrast, N,2-dichloroacetamide formation was observed to be only base catalyzed (k ₃^OH = 3.03 × 10⁴ M^-2 s ^-1). N,2-dichloroacetamide cytotoxicity (LC₅₀ = 2.56 × 10^-4 M) was found to be slightly lower compared to that reported for chloroacetamide but higher than those of di- and trichloroacetamide.