Acetonitrile
and <i>N</i>‑Chloroacetamide
Formation from the Reaction of Acetaldehyde and Monochloramine
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Abstract
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 (<i>K</i><sub>1</sub>) and forward and reverse
rate (<i>k</i><sub>1</sub>,<i>k</i><sub>–1</sub>) constants for the reaction between initial reactants and 1-(chloroamino)ethanol
were determined between 2 and 30 °C. Activation energies for <i>k</i><sub>1</sub> and <i>k</i><sub>–1</sub> were 3.04 and 45.2 kJ·mol<sup>–1</sup>, respectively,
and enthalpy change for <i>K</i><sub>1</sub> was −42.1
kJ·mol<sup>–1</sup>. In parallel reactions, 1-(chloroamino)ethanol
(1) slowly dehydrated (<i>k</i><sub>2</sub>) to (chloroimino)ethane
that further decomposed to acetonitrile and (2) was oxidized (<i>k</i><sub>3</sub>) by monochloramine to produce <i>N</i>-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 <i>N</i>-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 <i>N</i>-chloroacetamide under typical monochloramine and acetaldehyde
concentrations. <i>N</i>-chloroacetamide cytotoxicity (LC<sub>50</sub> = 1.78 × 10<sup>–3</sup> M) was comparable to
dichloroacetamide and trichloroacetamide, but less potent than <i>N</i>,2-dichloroacetamide and chloroacetamide. While <i>N</i>-chloroacetamide was not found to be genotoxic, <i>N</i>,2-dichloroacetamide genotoxic potency (5.19 × 10<sup>–3</sup> M) was on the same order of magnitude as chloroacetamide
and trichloroacetamide