Contributions of BrCl, Br2, BrOCl, Br2O, and HOBr to Regiospecific

Mar 24, 2015 - When bromide-containing waters are chlorinated, conventional wisdom typically assumes HOBr is the only active brominating agent. Severa...
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Contributions of BrCl, Br2, BrOCl, Br2O, and HOBr to Regiospecific Bromination Rates of Anisole and Bromoanisoles in Aqueous Solution John D. Sivey,* Mark A. Bickley, and Daniel A. Victor Department of Chemistry and Urban Environmental Biogeochemistry Laboratory, Towson University, Towson, Maryland 21252, United States S Supporting Information *

ABSTRACT: When bromide-containing waters are chlorinated, conventional wisdom typically assumes HOBr is the only active brominating agent. Several additional and often-overlooked brominating agents (including BrCl, Br2, BrOCl, Br2O) can form in chlorinated waters, albeit at generally lower concentrations than HOBr. The extent to which these additional brominating agents influence bromination rates of disinfection byproduct precursors is, however, poorly understood. Herein, the influence of BrCl, Br2, BrOCl, Br2O, and HOBr toward rates of sequential bromination of anisole was quantified. Conditions affecting bromine speciation (e.g., pH, concentrations of chloride, bromide, and chlorine) were varied, and regiospecific second-order rate constants were calculated for reactions of each brominating agent with anisole, 2-bromoanisole, and 4-bromoanisole. The regioselectivity of anisole bromination changed with pH, consistent with the participation of more than one brominating agent. Under conditions representative of chlorinated drinking water, contributions to bromination rates decreased as BrCl > BrOCl > HOBr > Br2O (Br2 negligible). The second-order rate constant determined for net bromination of anisole by HOBr is up to 3000-times less than reported in previous studies (which assumed HOBr was the only active brominating agent). Accordingly, models that assume HOBr is the only kinetically relevant brominating agent in solutions of free bromine may be insufficient for reactions involving modestly nucleophilic organic compounds.

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INTRODUCTION Bromide is a ubiquitous constituent of natural waters, with concentrations of 50−200 μg/L typical for sources of drinking water.1 Significantly higher levels have, however, been reported for water sources impacted by seawater intrusion2 and by produced water from hydraulic fracturing operations.3−6 When bromide-containing waters are treated with free chlorine, bromide can be oxidized into free bromine: HOCl(aq) + Br − ⇌ HOBr(aq) + Cl−

The toxic effects of brominated DBPs generally exceed those of chlorinated DBPs possessing otherwise similar structures.19−22 With few exceptions,23−26 conventional wisdom holds that HOBr is “the” active brominating agent in solutions of free bromine. Indeed, most previous investigations of organic compound bromination, including studies of naturally occurring compounds (e.g., phenols,27,28 pyrene,29 and amino acid residues30) as well as xenobiotics (e.g., 17α-ethinylestradiol,31 amoxicillin,32 and chlorpyrifox33), assume HOBr is the only kinetically relevant brominating agent. Occasionally, bromination rate constants have also been reported for BrO−27,28,34 and Br2 (eq 3).23−25

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(log K1 = 5.18, 25 °C, ref 7; all equilibrium constants herein are at 0 M ionic strength). “Free bromine” ([Br(I)]tot) refers to all aqueous bromine species in the +I oxidation state (excluding bromamines). Hypobromous acid (HOBr) is typically the most abundant free bromine species. HOBr(aq) ⇌ BrO− + H+

Br2(aq) + H 2O ⇌ HOBr(aq) + Br − + H+

(log K3 = −8.40, 20 °C, ref 35). Several additional brominating agents can, however, form in solutions of free bromine, including BrCl (eq 4), Br2O (eq 5), and BrOCl (eq 6).24,36,37

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(pKa = 8.70, 20 °C, ref 8). Free bromine can also be generated via oxidation of bromide in waters treated with ozone9−11 or chloramines.12−14 Aqueous free bromine species are associated with the formation of brominated disinfection byproducts (DBPs).15−18 © 2015 American Chemical Society

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Received: Revised: Accepted: Published: 4937

January 13, 2015 March 22, 2015 March 24, 2015 March 24, 2015 DOI: 10.1021/acs.est.5b00205 Environ. Sci. Technol. 2015, 49, 4937−4945

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Environmental Science & Technology

Scheme 1. Reaction Pathways and Products for the Sequential Bromination of Anisole by Free Bromine Species (BrX, X = Cl, Br, OCl, OBr, or OH)

HOBr(aq) + Cl− + H+ ⇌ BrCl(aq) + H 2O

where “Br prod.” is the brominated product and kobs (s−1) is equal to the sum of second-order rate constants (M−1 s−1) multiplied by molar concentrations of each brominating agent (eq 8):

(4)

(log K4 = 4.09, 20 °C, ref 38) 2HOBr(aq) ⇌ Br2O(aq) + H 2O

(5)

kobs = kBrCl[BrCl] + kBr2[Br2] + kBrOCl[BrOCl]

(log K5 = 0.80, 25 °C, ref 25) HOCl(aq) + HOBr(aq) ⇌ BrOCl(aq) + H 2O

+ kBr2O[Br2O] + kHOBr[HOBr] (6)

Anisole and its brominated products possess several advantageous characteristics for examining bromination kinetics. Anisoles represent modestly activated aromatic compounds39 that are anticipated to exhibit selectivity toward more reactive brominating agents (e.g., BrCl, Br2, BrOCl, and Br2O). Anisoles were previously identified as nucleophilic components of natural organic matter in waters treated with free chlorine.40 In addition, anisoles lack ionizable functional groups, thereby simplifying their speciation and permitting parent compounds and brominated products to be simultaneously analyzed via gas chromatography.

(log K6 = −0.46, 25 °C, ref 25). The mixed halogen species BrCl and BrOCl are anticipated to function as brominating agents due to greater nucleofugality (leaving group ability) of Cl− and ClO− relative to Br− and BrO−, respectively. Despite their typically lower concentrations relative to HOBr (Supporting Information, SI, Figures S1 and S2), a recent study reported BrCl, Br2, BrOCl, and Br2O are 3 to 6 orders of magnitude more inherently reactive than HOBr toward the herbicide dimethenamid (a substituted thiophene).25 Several lines of evidence suggested brominating agents other than HOBr influenced bromination rates of dimethenamid.25 Specifically, bromination rates were: • First-order in chloride concentration (implicating BrCl, eq 4); • Approximately first-order in excess (unoxidized) bromide (implicating Br2, eq 3); • First-order in HOCl concentration (implicating BrOCl, eq 6); and • Nearly second-order in HOBr concentration (implicating Br2O, eq 5). These findings corroborate an earlier examination of p-xylene bromination, in which Br2 and BrCl were determined to be several orders of magnitude more inherently reactive than HOBr.24,36 To date, a set of second-order rate constants for BrCl, Br2, BrOCl, Br2O, and HOBr have only been reported for one compound (dimethenamid).25 The objective herein is to expand our understanding of these brominating agents by quantifying regioselective second-order rate constants for the sequential bromination of anisole in solutions of free bromine (Scheme 1). As with dimethenamid,25 we hypothesize that the high reactivities of BrCl, Br2, BrOCl, and Br2O can more than compensate for their low concentrations (relative to HOBr) and thereby influence overall bromination rates (eq 7) when free bromine is present in excess. d[Br prod.] = kobs[anisole] dt

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EXPERIMENTAL SECTION Reagents are described in the SI (Table S1). Kinetic Experiments. Regiospecific bromination rates of anisole, 2-bromoanisole (2-BA), and 4-bromonanisole (4-BA) were determined in batch reactors (total volume = 25 mL) incubated in a water bath at 20.00 ± 0.02 °C (to permit comparisons with previous bromination studies24,25). Reactions were conducted in 40 mL amber glass vials with Teflon-lined caps. Vials were rinsed with aqueous free chlorine (∼0.6 mM) and 18 MΩ·cm water before use. Solutions for kinetic experiments contained a pH buffer (sodium bicarbonate or sodium borate, typically 20 mM), NaNO3 (typically 90 mM) to fix ionic strength, NaCl (typically 10 mM), and NaBr (typically 120−550 μM). Experiments were also performed as a function of [NaCl] (ranging from no added to 30 mM NaCl); in such experiments, the concentration of NaNO3 was adjusted such that [NaNO3] + [NaCl] = 100 mM. Adjustments to solution pH were achieved by adding HNO3 or NaOH; pH was measured using an Accumet AB 150 pH meter (Fisher) with automatic temperature compensation, calibrated daily using certified buffers (pH 4.00, 7.00, and 10.00, Fisher). Working solutions of free chlorine were added to reactors to achieve targeted initial free chlorine concentrations (typically 570 μM). As rates of bromide oxidation by free chlorine vary with pH,41 incubation times of bromide + free chlorine were selected to permit ≥99% of bromide to be oxidized into free bromine. At pH < 9.0, an incubation time of 4 min was employed; at pH = 9.0−9.9, the incubation time was 30 min. Pseudo-first-order conditions were achieved by employing initial free bromine

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Figure 1. Pseudo-first-order rate constants corresponding to the formation of 4-bromoanisole (4-BA, filled circles) and 2-bromoanisole (2-BA, open circles) from the reaction of anisole with free bromine as a function of (A) chloride concentration and (B) total initial free chlorine concentration. Error estimates denote 95% confidence intervals (smaller than symbols if not shown). Frame A conditions: [NaBr]o = 114 μM, [NaCl] + [NaNO3] = 0.100 M, pH 7.05, [HOCl]tot,o = 510 μM, [anisole]o = 10 μM, and T = 20.0 °C. Frame B conditions: [NaBr]o = 230 μM, [NaNO3] = 93 mM, [NaCl] = 3.9 mM, pH 8.49, [anisole]o = 19 μM, and T = 20.0 °C.

and its monobrominated products could be simultaneously fit to eqs 9−11 via nonlinear least-squares regression analyses performed using Scientist 3.0 (MicroMath).

molar concentrations typically ≥10-times those of the parental anisole. Following the incubation period, anisole or one of its brominated products (Scheme 1) was added as a methanolic spike (final methanol concentration typically ≤0.6% v/v, always ≤1.3% v/v). Initial concentrations of the parental anisole typically ranged from 5−30 μM. Aliquots (0.900 mL) from reactors were periodically transferred to 4 mL amber vials preamended with Na2S2O3 (present at ∼1.4-fold molar excess relative to the initial concentration of free chlorine + free bromine to quench free halogens) and toluene (0.450 mL) containing 2-chlorobenzonitrile (4.2 μM) as an internal standard. Following vigorous manual mixing, toluene extracts were transferred to 2 mL glass autosampler vials (containing 200μL glass inserts) and sealed with screw-top caps containing PTFE-lined septa. The influence of ionic strength (30−120 mM), buffer concentration (10−40 mM for bicarbonate at pH 7.3 and borate at pH 9.0), and initial concentration of anisole (5−22 μM) on bromination rates were determined. Several additional chemical parameters (pH, concentration of bromide, and concentration of free chlorine) are capable of altering free bromine speciation and, hence, influencing observed bromination rates.24,25 Accordingly, experiments were performed such that one of the aforementioned parameters was systematically varied while all other parameters were held constant to isolate the effects of individual bromine species. Specific reagent concentrations for each experiment are compiled in the SI (Tables S2−S14). Analysis of Reactants and Products. Toluene extracts were analyzed via gas chromatography (GC, Agilent 7890A) with a mass selective (MS) detector (Agilent 5975C). Selected ion monitoring was used to quantify concentrations of parental anisoles and brominated products. Additional GC−MS method details are provided in the SI (Table S15). Example time courses are shown in SI Figures S3−S5. Calculation of Rate Constants. Regiospecific pseudo-firstorder rate constants were calculated by monitoring the formation of brominated products as a function of time. When anisole served as the parent compound, concentration data for anisole

d[anisole] = −(kanisole,obs)[anisole] dt

(9)

d[4−BA] = kI,obs[anisole] dt

(10)

d[2−BA] = kII,obs[anisole] dt

(11)

where kanisole,obs, kI,obs, and kII,obs are pseudo-first-order rate constants (s−1) corresponding to the loss of anisole, the formation of 4-BA, and the formation of 2-BA, respectively. Formation of higher brominated products was ≤1% of initial anisole concentrations and carbon mass balances were closed (e.g., SI Figure S3). These findings indicate that reactions other than bromination (e.g., chlorination) did not appreciably influence rates of anisole transformation and, therefore, kanisole = kI,obs + kII,obs. Accordingly, kI,obs and kII,obs can be calculated via eqs 12 and 13: kI,obs = (kanisole,obs)

[4−BA]f [2−BA]f + [4−BA]f

(12)

[2−BA]f [2−BA]f + [4−BA]f

(13)

kII,obs = (kanisole,obs)

where [4-BA]f and [2-BA]f denote the final measured concentration of 4-BA and 2-BA, respectively, with kanisole,obs determined from the slope of ln([anisole]) versus time plots. For reactions of anisole at pH > 8.7 and all reactions of 2-BA and 4-BA, less than 10% conversion of parent compound was observed and pseudo-first-order rate constants were calculated via the initial rate method. This approach involved plotting the concentration of product(s) versus time, performing a leastsquares linear regression, and dividing the resulting slope by the initial concentration of parental anisole to give a (regiospecific) pseudo-first-order rate constant. Bromination of 2,4-dibromo4939

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Figure 2. (A) Regiospecific rate constants and (B) ratios of rate constants for bromination of anisole to give 4-bromoanisole (4-BA, kI,obs) and 2bromoanisole (2-BA, kII,obs). Dashed line denotes predicted ratio (0.5) if ortho and para positions of anisole were equally reactive. Error bars denote 95% confidence intervals (smaller than symbol if not shown). Conditions: [anisole]o = 10 μM, [NaBr]o = 120 μM, [HOCl]tot,o = 570 μM, [NaNO3] = 90 mM, [NaCl] = 10 mM, [bicarbonate or borate buffer] = 20 mM, T = 20.0 °C.

[HOBr].25 The measured reaction order in [HOBr] is 1.22(±0.17) and 0.97(±0.33) for reactions of anisole to give 4BA and 2-BA, respectively (all error estimates herein denote 95% confidence intervals). In the case of 4-BA formation, the reaction order in [HOBr] greater than 1.00 suggests Br2O (whose concentration is proportional to [HOBr]2) influenced overall bromination rates under the examined conditions. Reactions in the Presence of Excess Bromide. For most reactors described herein, free bromine is prepared by combining NaBr with excess free chlorine. When free chlorine is the limiting reactant, however, unoxidized (i.e., “excess”) bromide is anticipated to exist. Experiments with excess bromide as the independent variable indicate that rates of anisole bromination increase as the concentration of excess bromide increases (SI Figure S10). This result suggests Br2 (whose concentration is proportional to the concentration of excess bromide) is an active brominating agent in these reactors. Influence of Solution pH. Rates of para and ortho bromination of anisole decrease with increasing pH (Figure 2A). At all examined pH values, para bromination of anisole is faster than ortho bromination (Figure 2A). The para/ortho rate constant ratio for bromination of anisole is approximately 7 from pH 5.4 to 6.4. As pH increases from 6.4 to 9.4, the para/ortho ratio increases to 16 (Figure 2B). As anisole is not ionizable under the conditions examined herein, the increase in selectivity (favoring para bromination) suggests the predominant brominating agent changes as a function of pH (under otherwise identical solution conditions). Determination of Second-Order Rate Constants. Modeling the rate data for conversion of anisole into 4-BA using HOBr as the sole brominating agent provides poor agreement with the experimental data (Figure 3). The “HOBr only” model predicts that values of log(kI,obs) will plateau at pH < 8.7 and decrease with a slope of −1 at pH > 8.7. Nevertheless, measured log(kI,obs) values continuously increase as pH decreases across all examined pH values (5.3−9.8); at pH > 8.7, log(kI,obs) decreases with a slope of −1.8. These results suggest brominating agents other than HOBr are influencing bromination rates. Of the additional probable brominating agents (BrCl, Br2, BrOCl, and Br2O), concentrations of both BrCl and Br2 are anticipated to increase at pH values less than the pKa of HOBr (8.70)8 and of HOCl (7.32).42 In the presence of

anisole and 2,6-dibromoanisole was sufficiently slow as to preclude quantitation of reaction rates. Second-order rate constants for individual brominating agents were calculated via nonlinear least-squares regression analyses of eq 8, using measured kobs and calculated concentrations of bromine species as input and second-order rate constants as fitting parameters. Additional details regarding the calculation of second-order rate constants are provided in the SI (Table S16).

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RESULTS AND DISCUSSION Effects of Buffer Concentration and Ionic Strength. The concentration of bicarbonate and borate did not appreciably influence bromination rates of anisole (SI Figure S6). Rates of anisole bromination also did not significantly differ as a function of ionic strength (SI Figure S7). Effects of Initial Concentration of Anisole. A plot of log(initial bromination rate) versus log(initial concentration of anisole) (SI Figure S8) shows a linear relationship for both para and ortho bromination with slopes not significantly different than 1.0. These results indicate reactions in our systems (in which bromide + free chlorine are added prior to addition of anisole) are first-order in [anisole] and, consequently, formation of reactive brominating agents is ostensibly not rate-limiting (see SI Table S17 for additional details). Influence of Chloride and of Excess Free Chlorine. In reactors containing free bromine, kobs corresponding to the formation of 4-BA and 2-BA increased linearly as a function of chloride concentration (Figure 1A). This result can be explained by invoking BrCl as an active brominating agent, noting that the concentration of BrCl is proportional to [Cl−]. In solutions containing free bromine and excess free chlorine (i.e., more free chlorine than required to stoichiometrically oxidize added bromide), kobs corresponding to the formation of 4-BA and 2-BA increased linearly as a function of free chlorine concentration (Figure 1B). These findings are consistent with the participation of BrOCl (whose concentration is proportional to [HOCl]) as an active brominating agent. Effects of Initial Concentration of Free Bromine. Regiospecific pseudo-first-order rate constants of anisole bromination are log−linearly related to the initial concentration of free bromine (SI Figure S9). Slopes of the resulting log(kobs) versus log([Br(I)]tot,o) plots represent the reaction order in 4940

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second-order rate constants as determined via nonlinear regression analyses of eq 8 are listed in Table 1. Relative Reactivity of Brominating Agents. Reactions of anisoles with aqueous brominating agents likely proceed via electrophilic aromatic substitution (Scheme 2), which generally includes two steps: (1) formation of a BrC σ bond (yielding a σ-complex, SI Figure S15), followed by (2) departure of an electrofuge (H+) from the aromatic ring.43,44 As the first step involves loss of aromaticity, this step is typically rate-limiting45 and can conceivably be influenced by the nucleofugality of the leaving group.25 For para bromination of anisole, the following inherent reactivity trend is observed: HOBr < Br2O < BrOCl < Br2 < BrCl. A similar trend was previously reported for bromination of dimethenamid, excepting that Br2 demonstrated approximately equal inherent reactivity toward dimethenamid as did BrOCl.25 These relative reactivities are consistent with the anticipated trend in leaving group nucleofugality among the brominating agents: OH− (from HOBr) < BrO− (from Br2O) < ClO− (from BrOCl) < Br− (from Br2) ≈ Cl− (from BrCl). This analysis assumes heterolytic bond cleavage during expulsion of the leaving group, consistent with previous examinations of aromatic bromination in the absence of both light and radical initiators.24,25,44 The greater reactivity of BrCl and Br2 (relative to HOBr, Br2O, and BrOCl) is also consistent with the lower bond dissociation energies of BrBr and BrCl relative to BrO (to the extent that heterolytic bond energies are proportional to homolytic bond energies, Table 2). That BrCl and Br2 possess lower LUMO energies (ELUMO) relative to the other brominating agents is also consistent with their greater rates of aromatic bromination.43 The greater reactivity of the mixed halogens BrCl and BrOCl compared to their brominated analogues (Br2 and Br2O) is, however, inconsistent with the reactivity trend predicted by bond dissociation and LUMO energies. The enhanced electrophilicity of the mixed halogens can, however, be explained by the greater partial positive charge on their bromine atom(s) (BrCl > Br2 and BrOCl > Br2O). For all reactions herein except para bromination of anisole, BrOCl is more inherently reactive than is Br2, and the overall reactivity trend is HOBr < Br2O < Br2 < BrOCl < BrCl. Accordingly, the magnitude of the positive charge on the bromine atom appears to be of even greater importance than nucleofugality as the nucleophilicity of the site of reaction on the parental anisole decreases. Indeed, the ratio kBrOCl/kBr2 increases

Figure 3. Pseudo-first-order rate constants (as log kI,obs) of anisole bromination to give 4-bromoanisole as a function of pH. Kinetic models are based on eq 8 (including only those terms specified in the legend). Error estimates denote 95% confidence intervals. Conditions: [anisole]o = 10 μM, [NaBr]o = 120 μM, [HOCl]tot,o = 570 μM, [NaNO3] = 90 mM, [NaCl] = 10 mM, [bicarbonate or borate buffer] = 20 mM, and T = 20.0 °C.

excess free chlorine (Figure 3), Br2 is unlikely to serve as a brominating agent. The formation of BrCl, however, is favored by the addition of 10 mM NaCl to these reactors. Accordingly, increases in kI,obs at pH < 7 likely result from the participation of BrCl as a brominating agent. Indeed, improved agreement is observed when HOBr and BrCl are considered concurrently as brominating agents. The “BrCl only” model provides an even better fit. The best agreement, however, is obtained when BrCl, BrOCl, Br2O, and HOBr are included as brominating agents (solid line, Figure 3). Inclusion of BrO− as a putative brominating agent did not improve model fits (see SI for additional discussion). A reactivity model which assumes BrCl, BrOCl, Br2O, and HOBr as reactive brominating agents also affords good agreement to measured rate constants corresponding to ortho bromination of anisole (kII,obs, SI Figure S11), bromination of 4BA (kIII,obs, SI Figure S12), and ortho and para bromination of 2BA (kIV,obs and kV,obs, SI Figures S13 and S14). The resulting

Table 1. Second-Order Rate Constants (M−1 s−1) for Sequential Bromination of Anisole at 20 °Ca

a Arrows denote site of bromination; see Scheme 1 for complete reaction pathway. Error estimates denote 95% confidence intervals. bInfluence of HOBr on overall bromination rates was too small to permit quantification of a second-order rate constant that was significantly different than 0 (at the 95% confidence level).

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Scheme 2. Proposed Electrophilic Aromatic Substitution Mechanism for para Bromination of Anisole (X = Cl, Br, OCl, OBr, or OH)

Table 2. Physical and Chemical Properties of Free Bromine Species (25 °C) brominating agent

homolytic bond dissociation energy (kJ/mol) (ref 46)

ELUMO (kJ/mol) (ref 47)

charge on Br (electrostatic model, ref 47)

normalized molecular volume (ref 47)

polarizability (×10−24 cm3) (ref 48)

Br2 BrCl Br2O BrOCl HOBr

193 219 235 (Br−O) 235 (Br−O) 235 (Br−O)

58.2 64.8 113 121 279

0.00 +0.091 +0.151 +0.176 +0.106

1.28 1.17 1.50 1.39 1.00

5.8 6.9 6.5 7.6 4.5

lesser steric hindrance,44 decreased electrostatic repulsion between the anionic nucleofuge and the methoxy group, and increased stability of the arenium ion intermediate49 relative to substitution at the ortho position.

as the nucleophilicity of the reactive site on the parental anisole (quantified as kBrCl) decreases (SI Figure S16), consistent with the aforementioned relationship between the positive charge on bromine and the nucleophilicity of the aromatic moiety. Polarizability of the brominating agents could also influence bromination rates (Table 2).25 At sterically hindered reaction sites (e.g., ortho to −OCH3), the molecular volume of the brominating agents may also be important. Regioselectivity Differences among Brominating Agents. Two of the parent compounds (anisole and 2-BA) can undergo bromine substitution at two nonequivalent sites. The regioselectivity of these reactions can be quantified as the ratio of second-order rate constants for each brominating agent (Table 3).

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ENVIRONMENTAL SIGNIFICANCE Relative Contributions of Brominating Agents in Chlorinated Drinking Water. The second-order rate constants in Table 1 can be used to calculate the relative contributions of individual brominating agents toward overall bromination rates in solutions with known compositions (and in the absence of additional species capable of competitively scavenging free bromine or free chlorine). Under typical drinking water chlorination conditions, BrCl is calculated to account for >53% of the bromination rate of anisole and its monobrominated products (Table 4). For all anisoles, the contribution of BrOCl exceeds 21%, but Br2O does not contribute more than 0.6%. Collectively, the nucleophilicity of the analytes in Table 4 span 6 orders of magnitude (as measured by kBrCl), and the anticipated contributions of brominating agents other than HOBr exceed 65%. These findings indicate HOBr may not be the only active brominating agent of modestly nucleophilic organic compounds under conditions representative of chlorinated drinking water. The contribution of HOBr increases as the nucleophilicity of the reactive site on the aromatic compound increases (Table 4), consistent with the reactivity-selectivity principle.39 Properties beyond aromatic compound nucleophilicity (e.g., steric constraints and polarizability) are, however, also likely to influence selectivity toward individual brominating agents. For additional discussions of reactivity versus selectivity, see SI (Figure S17). Under conditions typical of chlorinated drinking water, bromination rates of anisole are predicted to vary with chloride concentration (Figure 4A). For example, addition of 150 mg/L (0.92 mM) of ferric chloride as a coagulant can result in a 100 mg/L (2.8 mM) increase in [Cl−] and a more than 6-fold calculated increase in the rate of anisole bromination. As with chloride, HOCl can also influence rates of anisole bromination at free chlorine doses typical of disinfected drinking water (Figure 4B). Comparison to Previous Reports of Anisole Bromination. Echigo and Minear51 previously reported that kHOBr = 52 M−1 s−1 (20 °C) for the net bromination of anisole by assuming the following reactivity model:

Table 3. Regioselectivity of Anisole and 2-BA Brominationa brominating agent

anisole (kI/kII)

2-BA (kIV/kV)

BrCl Br2 BrOCl Br2O HOBr

6.6 ± 0.4 41 ± 5 11.8 ± 1.2 20 ± 3 24 ± 8

(1.47 ± 0.12) × 102 (3.2 ± 1.5) × 103 (1.7 ± 0.8) × 102 (3 ± 2) × 102 kV not quantified

a

Regioselectivity quantified as para versus ortho (with respect to the methoxy substituent) ratios of second-order rate constants for bromination. Error estimates denote propagated 95% confidence intervals. See Scheme 1 for reaction pathway.

Regioselectivity ratios of anisole as a function of pH (Figure 2B) ranged from 7 to 16 and are bracketed by the values shown in Table 3 (6.6−41), which represent regioselectivities that would be obtained in theoretical solutions containing only one brominating agent. Changes in regioselectivity can be explained by varying contributions of more than one active brominating agent. Specifically, the decreasing regioselectivity of anisole bromination with decreasing pH is consistent with the increasing influence of BrCl (whose regioselectivity ratio, kI/kII, is the lowest among the five brominating agents). Regioselectivity ratios for 2-BA as a function of pH (SI Figure S14) ranged from 100 to 400 and are also bracketed by the values shown in Table 3. If the unsubstituted carbon atoms para and ortho to the methoxy group were equally reactive toward bromine substitution, the para/ortho ratios for anisole and 2-BA would be 0.5 and 1.0, respectively. All measured regioselectivity ratios exceed these values. Preferential para substitution can be explained by 4942

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Table 4. Calculated Contributions of Brominating Agents to Overall Bromination Rate of Anisole, 4-BA, 2-BA, and Dimethenamid in Chlorinated Drinking Watera

a

Data for dimethenamid from ref 25. Relative nucleophilicity ranking based on normalized kBrCl values. Arrows denote site of bromination. Assumed conditions: pH 7.0, [HOCl]tot,o = 28 μM (2 mg/L as Cl2), [Br−]o = 1.25 μM (100 μg/L), [Cl−] = 0.30 mM (11 mg/L), and T = 20.0 °C. No appreciable contribution of Br2 is anticipated in the absence of excess bromide.

Figure 4. Calculated pseudo-first-order rate constants (kcalc) corresponding to the net bromination of anisole (kI + kII) as a function of the concentration of (A) chloride and (B) free chlorine. Values of kcalc are normalized to (A) 11 mg/L (0.3 mM) of chloride (the median concentration in drinking water50) and (B) 2 mg/L as Cl2 ([HOCl]tot = 28 μM). Broken lines denote reference concentrations of chloride and free chlorine used to calculate normalized kcalc values. Additional assumed conditions: pH 7.0, [Br−]o = 100 μg/L (1.25 μM), T = 20.0 °C. The dependence of kcalc on concentrations of chloride and free chlorine are associated with the influence of BrCl and BrOCl on overall bromination rates.

kHOBr =

kobs [HOBr]

reactive brominating agent. Accordingly, their kHOBr value likely overestimates the inherent reactivity of HOBr toward anisole. These examples illustrate the importance of considering brominating agents that are less abundant but potentially several orders of magnitude more inherently reactive than HOBr when calculating second-order rate constants for bromination reactions involving organic compounds of modest nucleophilicity. The potential for these often-overlooked brominating agents to influence bromination rates of more nucleophilic constituents of natural organic matter (e.g., phenolic moieties) and the formation rates of regulated disinfection byproducts merits future research.

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Compared to the net second-order rate constant for HOBr calculated from the data herein (kI,HOBr + kII,HOBr = 0.073 M−1 s−1), the previously reported kHOBr appears to overestimate the inherent reactivity of HOBr by more than a factor of 700. Echigo and Minear generated solutions of free bromine via oxidation of bromide by ozone;51 the reported presence of unoxidized bromide in their solutions of free bromine suggests Br2 (in addition to Br 2 O) could have influenced their overall bromination rates. Precise compositions of solutions (including total concentrations of bromide and chloride) were not reported in the previous study,51 thereby precluding a back-calculation of the contributions of brominating agents other than HOBr. Ximenes et al.52 have also reported a second-order rate constant for the net bromination of anisole; their kHOBr (230 M−1 s−1, 25 °C) exceeds the value reported herein (at 20 °C) by more than a factor of 3000. The 5 °C temperature difference between the two studies cannot sufficiently explain the difference in the kHOBr values. Ximenes et al.52 generated free bromine by combining free chlorine and NaBr in a 1:2 molar ratio. In such solutions, brominating agents such as Br2, BrCl, and Br2O are likely to exist in addition to HOBr. The kinetic model employed by Ximenes et al.52 assumed, however, that HOBr was the only

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ASSOCIATED CONTENT

S Supporting Information *

Additional speciation diagrams, reagent descriptions, reactor compositions, methodological details, and supplemental data. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: (410) 704-6087; E-mail: [email protected] (J.D.S.). 4943

DOI: 10.1021/acs.est.5b00205 Environ. Sci. Technol. 2015, 49, 4937−4945

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Environmental Science & Technology Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge three anonymous reviewers for providing insightful comments that significantly strengthened this manuscript. The authors also acknowledge funding from the American Chemical Society Petroleum Research Fund (Grant No. 54560-UNI4), Maryland Water Resources Research Center (Grant No. Z974005), Fisher College of Science and Mathematics and Department of Chemistry at Towson University.

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