Chloramination of Nitromethane: Incomplete Chlorination and Unexpected Substitution Reaction

Nitromethaneis an ozone byproduct that can be efficientlychlorinated to chloropicrin. Chloramination of nitromethane to chloropicrinwas slower than free chlorination, and sometimes incomplete. Nitrateformed as an unexpected byproduct. Ozone is commonly used as a predisinfectant in potablewater reusetreatment trains. Nitromethane was recently found as a ubiquitousozone byproduct in wastewater, and the key intermediate toward chloropicrinduring subsequent secondary disinfection of ozonated wastewater effluentwith chlorine. However, many utilities have switched from free chlorineto chloramines as a secondary disinfectant. The reaction mechanismand kinetics of nitromethane transformation by chloramines, unlikethose for free chlorine, are unknown. In this work, the kinetics,mechanism, and products of nitromethane chloramination were studied.The expected principal product was chloropicrin, because chloraminesare commonly assumed to react similarly to, although more slowly than,free chlorine. Different molar yields of chloropicrin were observedunder acidic, neutral, and basic conditions, and surprisingly, transformationproducts other than chloropicrin were found. Monochloronitromethaneand dichloronitromethane were detected at basic pH, and the mass balancewas initially poor at neutral pH. Much of the missing mass was laterattributed to nitrate formation, from a newly identified pathway involvingmonochloramine reacting as a nucleophile rather than a halogenatingagent, through a presumed S(N)2 mechanism. The study indicatesthat nitromethane chloramination, unlike chlorination, is likely toproduce a range of products, whose speciation is a function of pHand reaction time.

Automated summary

Nitromethane, a byproduct of ozone, can be efficiently chlorinated to form chloropicrin. The chloramination reaction of nitromethane to chloropicrin is slower and sometimes incomplete compared to free chlorination, and unexpectedly forms nitrate. Ozone is commonly used as a pre-disinfectant in water treatment, but nitromethane has been found to be a common byproduct in wastewater and a key intermediate in the transformation to chloropicrin during secondary disinfection with chlorine. The reaction mechanism and kinetics of nitromethane transformation by chloramines are unknown. This study investigated the kinetics, mechanism, and products of nitromethane chloramination, revealing that it produces a range of products influenced by pH and reaction time.