Halogenation via electrophilic aromatic substitution (SEAr) initiates formation of disinfection byproducts (DBPs). The SEAr mechanism involves two steps: 1) electrophile (halogen) addition and 2) proton removal. Mechanistic DBP formation models typically assume that halogen addition is the rate-determining step (RDS). Proton removal has been shown to be rate-determining in iodination, although this has never been examined under drinking water treatment conditions. Contributing to the uncertainty, iodination was recently proposed to undergo a concerted mechanism, rather than stepwise. Iodinated DBPs tend to be more toxic than chlorinated and brominated analogues, which calls for better understanding of how iodination in disinfected water differs from bromination and chlorination. This study examines the RDS of halogenation in laboratory-simulated water disinfection using kinetic isotope effect (KIE) analysis. Dimethenamid (DM) was selected as an aromatic substrate because DM halogenation has been extensively characterized under water treatment conditions. Deuterated DM (DM-d) was synthesized with trifluoroacetic acid-d. DM and DM-d halogenation rates were measured in parallel reactors under identical conditions for each experiment. Chlorination and bromination did not exhibit KIEs (kH/kD ≈ 1), consistent with halogen addition as the RDS. In contrast, primary KIEs (&gt;1.4) were always observed in iodination, indicating involvement of proton removal in the RDS. Iodination rates increased as pH (5.2 – 8.9) and [I–] (10 – 100 µM) decreased, although the magnitudes of KIEs did not vary substantially (1.47 (±0.08) to 2.06 (±0.40)). The influences of pH and [I–] suggest that iodine addition is also involved in the RDS. Adding 1 mM [Cl–] enhanced iodination rates without affecting the KIE appreciably (1.71 (±0.08)), whereas [Cl–] addition above 25 mM increased the KIE (2.9 – 3.4) but not the rates of iodination further. These results suggest that kinetic control shifts from iodine addition to proton removal with increasing [Cl–], consistent with the putative stepwise SEAr mechanism, rather than a concerted mechanism. Under typical drinking water conditions ([Cl–] ≤ 7 mM), a kinetic model assuming iodine addition as the RDS performed well, even though the primary KIEs indicate that proton removal is part of the RDS. A model accounting for proton removal was required to capture the influence of [Cl–] ≥ 25 mM on iodination (e.g., brine, seawater).

Kinetic isotope effects in the halogenation of dimethenamid

Macromolecular Chemistry: The Second Century

[Author Benefits](https://scimeetings.acs.org/?utm_source=pubs_content_marketing&utm_medium=website&utm_campaign=0320_MCF_NPI_Launch_Spring_Homepage&ref=pubs_content_marketing)

[How to use SciMeetings](https://storage.googleapis.com/dl.morressier.com/acs/SciMeetings-ACSSpring2020-HowToGuide_M3020-Final.pdf)

[Frequently Asked Questions](https://storage.googleapis.com/dl.morressier.com/acs/SciMtng_ACSNMSp2020_FAQ_M3120_Final.pdf)

ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.