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Parabens and salicylates were examined as disinfection byproduct (DBP) precursors to explore the possible influence of ipso substitution (i.e., halogen exchange) on the yield and speciation of trihalomethanes (THMs) formed during water chlorination. Substoichiometric conversion of C–Br bonds into C–Cl bonds was confirmed for several parabens and salicylates. The co-occurrence of (mono)brominated and nonhalogenated precursors in the presence of free chlorine (but in the absence of added Br–) generated polybrominated THMs, implicating ipso substitution. The THM molar yield, bromine incorporation, and bromine recovery from brominated and nonhalogenated precursor mixtures were commensurate with those observed from equimolar additions of NaBr, indicating efficient displacement of aromatic bromine by free chlorine followed by reincorporation of liberated HOBr into DBP precursors. The THM molar yield from brominated precursors was enhanced by a factor of ≤20 relative to that from nonhalogenated precursors. Trends in THM molar yields and bromine incorporation differed between brominated parabens and brominated salicylates, suggesting that the influence of ipso substitution on THM formation varies with the structure of the organic precursor. Collectively, these results provide new evidence of the often-overlooked role ipso substitution can play in promoting halogen exchange and bromine enrichment among DBPs in chlorinated waters. 

Parabens are antimicrobial additives found in a wide array of consumer products. However, the halogenated compounds formed from parabens during wastewater disinfection are a potential environmental concern. In order to identify these transformation products and investigate their mechanism of formation, a synthetic route to ethyl parabens labeled with the stable isotope carbon‐13 at specific positions within the benzene ring was developed. This efficient two‐step procedure starts from commercially available¹³C‐labeled phenols and involves (1) initial acylation of the phenol via a Houben–Hoesch reaction with trichloroacetonitrile followed by (2) a modified haloform reaction of the resulting trichloromethyl ketone to afford the corresponding¹³C‐labeled ethyl parabens in 65%–80% overall yield. The scope of the modified haloform reaction was also investigated, allowing for the synthesis of other parabens derived from primary or secondary alcohols, including¹³C‐ and deuterium‐labeled esters. In addition, 4‐hydroxybenzoic acid can be formed directly from the common trichloromethyl ketone intermediate upon treatment with lithium hydroxide. This protocol complements existing methods for preparing¹³C‐labeled paraben derivatives and offers the specific advantages of exhibiting complete regioselectivity in the Houben–Hoesch reaction (to form thepara‐disubstituted product) and avoiding the need for protecting groups in the modified haloform reaction that forms the paraben esters.

 

Free chlorine and free bromine ( e.g. , HOCl and HOBr) are employed as disinfectants in a variety of aqueous systems, including drinking water, wastewater, ballast water, recreational waters, and cleaning products. Yet, the most widely used methods for quantifying free halogens, including those employing N , N -diethyl- p -phenylenediamine (DPD), cannot distinguish between HOCl and HOBr. Herein, we report methods for selectively quantifying free halogens in a variety of aqueous systems using 1,3,5-trimethoxybenzene (TMB). At near-neutral pH, TMB reacted on the order of seconds with HOCl, HOBr, and inorganic bromamines to yield halogenated products that were readily quantified by liquid chromatography or, following liquid–liquid extraction, gas chromatography-mass spectrometry (GC-MS). The chlorinated and brominated products of TMB were stable, and their molar concentrations were used to calculate the original concentrations of HOCl (method quantitation limit (MQL) by GC-MS = 15 nmol L −1 = 1.1 μg L −1 as Cl 2 ) and HOBr (MQL by GC-MS = 30 nmol L −1 = 2 μg L −1 as Cl 2 ), respectively. Moreover, TMB derivatization was efficacious for quantifying active halogenating agents in drinking water, pool water, chlorinated surface waters, and simulated spa waters treated with 1-bromo-3-chloro-5,5-dimethylhydantoin. TMB was also used to quantify bromide as a trace impurity in 20 nominally bromide-free reagents (following oxidation of bromide by HOCl to HOBr). Several possible interferents were tested, and iodide was identified as impeding accurate quantitation of HOCl and HOBr. Overall, compared to the DPD method, TMB can provide lower MQLs, larger linear ranges, and selectivity between HOCl and HOBr. 

Sodium sulfite, sodium thiosulfate, and ascorbic acid are commonly used to quench free chlorine and free bromine in studies of disinfection byproducts (DBPs) in drinking water, wastewater, and recreational water. The reducing capabilities of these quenchers, however, can lead to the degradation of some redox-labile analytes. Ammonium chloride, another common quencher, converts free chlorine into monochloramine and is therefore inappropriate for analytes susceptible to chloramination. Herein, we demonstrate the utility of 1,3,5-trimethoxybenzene (TMB) as a quencher of free chlorine and free bromine. The reactivity of TMB toward free chlorine was characterized previously. The reactivity of TMB toward free bromine was quantified herein ( k HOBr,TMB = 3.35 × 10 6 M −1 s −1 ) using competition kinetics. To explore the feasibility of TMB serving as a free halogen quencher for kinetic experiments, chlorination of 2,4-dichlorophenol, bromination of anisole, and chlorination and bromination of dimethenamid-P were examined. Although TMB does not react with free chlorine or free bromine as quickly as do some (but not all) traditional quenchers, there was generally no significant difference in the experimental rate constants with TMB (relative to thiosulfate) as the quencher. By monitoring the chlorination and bromination products of TMB, free halogen residuals in quenched samples were quantified. Furthermore, TMB did not affect the stabilities of DBPs ( e.g. , chloropicrin and bromoacetonitriles) that otherwise degraded in the presence of traditional quenchers. TMB could, therefore, be an appropriate quencher of free chlorine and free bromine in aqueous halogenation experiments involving redox-labile analytes and/or when selective quantification of residual free halogens is desired.