Transformation of cyclic amides and uracil-derived nitrogen heterocycles during chlorination

Nitrogen heterocycles are important structural components in biomolecules and anthropogenic chemicals, yet their transformation during chlorine disinfection remains poorly understood. This study investigated chlorination kinetics and product formation for six nitrogen heterocycles of increasing structural complexity, including cyclic amides (2-piperidone, glutarimide, 5,6-dihydrouracil) and uracil derivatives (uracil, uridine, and 1,3-dimethyluracil) to determine how structural variations influence reaction pathways. Apparent second-order rate constants varied widely from 9.2 × 10^-3 M^-1 s^-1 (2-piperidone) to >10³ M^-1 s^-1 (uracil, uridine), largely influenced by the nitrogen pK_a values. Chlorination proceeded through initial N-chlorination, forming organic chloramides. While most organic chloramides were transient, that derived from 2-piperidone persisted for days under excess chlorine conditions. For saturated heterocyclic imides (glutarimide, 5,6-dihydrouracil), hydrolysis of the organic chloramides between the imide nitrogen and an adjacent acyl group rapidly formed ring-opened organic acids. Among uracil derivatives, chlorine added across the double bond. For uracil, the resulting 5-chlorouracil rapidly fragmented between the C-4 and C-5 position to release trichloroacetaldehyde at ∼100 % yield. Substitution at heterocyclic nitrogens in uridine and 1,3-dimethyluracil limited such fragmentation, forming more stable C-chlorinated heterocyclic or ring-opened products. The reaction patterns observed for these six nitrogen heterocycles were further validated using phthalimide and thymine, demonstrating the broader applicability of the identified reaction trends. These findings enhance our understanding of nitrogen heterocycle chlorination mechanisms and their implications for drinking water disinfection, offering insights into minimizing the formation of potentially harmful DBPs during chlorination.