Acetonitrile and N-Chloroacetamide Formation from the Reaction of Acetaldehyde and Monochloramine

Nitriles and amides are two classes of nitrogenous disinfection byproducts (DBPs) associated with chloramination that are more cytotoxic and genotoxic than regulated DBPs. Monochloramine reacts with acetaldehyde, a common ozone and free chlorine disinfection byproduct, to form 1-(chloroamino)ethanol. Equilibrium (K₁) and forward and reverse rate (k₁,k_-1) constants for the reaction between initial reactants and 1-(chloroamino)ethanol were determined between 2 and 30 °C. Activation energies for k₁ and k_-1 were 3.04 and 45.2 kJ·mol^-1, respectively, and enthalpy change for K₁ was -42.1 kJ·mol^-1. In parallel reactions, 1-(chloroamino)ethanol (1) slowly dehydrated (k₂) to (chloroimino)ethane that further decomposed to acetonitrile and (2) was oxidized (k₃) by monochloramine to produce N-chloroacetamide. Both reactions were acid/base catalyzed, and rate constants were characterized at 10, 18, and 25 °C. Modeling for drinking water distribution system conditions showed that N-chloroacetamide and acetonitrile concentrations were 5-9 times higher at pH 9.0 compared to 7.8. Furthermore, acetonitrile concentration was found to form 7-10 times higher than N-chloroacetamide under typical monochloramine and acetaldehyde concentrations. N-chloroacetamide cytotoxicity (LC₅₀ = 1.78 × 10^-3 M) was comparable to dichloroacetamide and trichloroacetamide, but less potent than N,2-dichloroacetamide and chloroacetamide. While N-chloroacetamide was not found to be genotoxic, N,2-dichloroacetamide genotoxic potency (5.19 × 10^-3 M) was on the same order of magnitude as chloroacetamide and trichloroacetamide.