Formation of <i>N</i>‑Nitrosodimethylamine during Chloramination of Secondary and Tertiary Amines: Role of Molecular Oxygen and Radical Intermediates

Abstract

<i>N</i>-Nitrosodimethylamine (NDMA) is a carcinogenic disinfection byproduct from water chloramination. Despite the identification of numerous NDMA precursors, essential parts of the reaction mechanism such as the incorporation of molecular O₂ are poorly understood. In laboratory model systems for the chloramination of secondary and tertiary amines, we investigated the kinetics of precursor disappearance and NDMA formation, quantified the stoichiometries of monochloramine (NH₂Cl) and aqueous O₂ consumption, derived ¹⁸O-kinetic isotope effects (¹⁸O-KIE) for the reactions of aqueous O₂, and studied the impact of radical scavengers on NDMA formation. Although the molar NDMA yields from five <i>N</i>,<i>N</i>-dimethylamine-containing precursors varied between 1.4% and 90%, we observed the stoichiometric removal of one O₂ per <i>N</i>,<i>N</i>-dimethylamine group of the precursor indicating that the oxygenation of N atoms did not determine the molar NDMA yield. Small ¹⁸O-KIEs between 1.0026 ± 0.0003 and 1.0092 ± 0.0009 found for all precursors as well as completely inhibited NDMA formation in the presence of radical scavengers (ABTS and trolox) imply that O₂ reacted with radical species. Our study suggests that aminyl radicals from the oxidation of organic amines by NH₂Cl and <i>N</i>-peroxyl radicals from the reaction of aminyl radicals with aqueous O₂ are part of the NDMA formation mechanism