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Nucleophilic Substitution Reactions of Dihalomethanes with Hydrogen Sulfide Species

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Nucleophilic Substitution Reactions of Dihalomethanes with Hydrogen Sulfide Species Environ. Sci. Technoi. 1992, 26, 2263-2274 Nucleophilic Substitution Reactions of Dihalomethanes with Hydrogen Sulfide Species A. Lynn Roberts, Philip N. Sanborn,t and Philip M. Gschwend" Ralph M. Parsons Laboratory, Department of Civll and Environmental Englneerlng, Massachusetts Institute of Technology, Cambridge, Massachusetts 02 139 transformations of primary alkyl halides. Such reactions Measurements of CH2Br2,CH2BrC1, and CH2Clzreac- may be of considerable si@icance because of the potential tion kinetics as a function of pH and temperature indicate toxicity of the organic sulfide reaction products. It is that HS--promoted reactions exceed hydrolysis rates at essential to obtain reliable estimates of reaction rates of HS- concentrations greater than 2-17 pM, well within ranges common in sulfate-reducing environments. Ex- halogenated organic contaminants with hydrogen sulfide periments suggest that polysulfide-mediatedreaction rates species under environmental conditions in order to in- may exceed HS--mediated rates in waters in equilibrium corporate these processes into fate and hazard assessment with orthorhombic S8 at pH values greater than 7.0. The models. One problem with previous studies is the absence Swain-Scott model of nucleophilic reactivity underesti- of data such as mass balances or detailed kinetic mea- mates rates at which these dihalomethanes react with HS-. surements to substantiate that observed transformations For CH2C12,the observed reaction rate is 5 times the are solely attributable to reactions with hydrogen sulfide predicted rate, greater discrepancies are observed with the species. Aqueous solutions of hydrogen sulfide are sen- ''softer" substrates CH2BrCland CH2Br2,for which ratios sitive to oxygenation, and several partidy oxidized sulfur of kohs/kpredare approximately 10-30. Examination of species may be more reactive than the bisulfide ion HS-. reaction products by FT-IR, solid-state 13C NMR,GC-MS, The products of dihalomethane reaction with these dif- and elemental analysis indicates the major product to be ferent species undoubtedly differ substantially in their poly(thiomethylene), although minor amounts of dithio- toxicity and mobility in the environment. A major purpose methane were also observed. Published data suggest such of the present study was to extend previous work to in- thiols possess toxic properties. Reactions of dihalo- vestigate the reactions of hydrogen sulfide species with methanes with HS- may thus yield products of greater gem-dihalides. In so doing, we have taken pains to verify toxicological significance than the parent compounds. that our observed rates are for reactions solely with HS-. We have also endeavored to characterize reaction products, Introduction both to confirm our proposed reaction mechanisms and Dihalomethanes are widely used as organic solvents, to provide information required in assessing the toxico- degreasing agents, and chemical intermediates. Owing to logical significance of these reactions. their popularity in industrial applications, they are ubi- A final purpose of the present study was to examine the quitous pollutants in aquatic systems. Existing informa- effect of alkyl halide structure on reactivity. The bisulfide tion suggests these compounds are only slowly transformed ion HS- is chemically analogous to the hydroxide ion OH-; by abiotic processes under aerobic conditions. Estimated reaction rates for the latter nucleophile are available from half-lives for reactions with water at pH 7,25 "C, are 704, the literature. By comparing the reactivity of these two 44, and 183 years for CH2C12,CH2BrC1, and CH2Br2,re- environmental reagents within a series of dihalomethanes, spectively (I),much greater than the corresponding values we can begin to assess the utility of models which have for CH3Cl and CH3Br. These half-lives are believed to been proposed to explain structurereactivity relationships. reflect bimolecular nucleophilic substitution (SN2)reac- tions, as dehydrohalogenation reactions do not occur Materials and Methods readily for dihalomethanes (2),and reactivity trends are Raagents. Reaction kinetics were measured under not consistent with an SN1 mechanism. pseudo-first-order conditions in solutions containing Under aerobic conditions, dihalomethanes are readily Na2S.9H20(Mallinckrodt); HC1 (Mallinckrodt) was used degraded by microorganisms or cell extracts thereof (3-14). to adjust the pH to the desired value and NaCl (Mal- In contrast, a number of studies have found dihalo- linckrodt) to adjust ionic strength to 0.05 equiv/L. When methanes to be relatively stable in cultures of anaerobic the reaction kinetics of CH2Br2and CH2BrCl were meas- microorganisms (15-18). If biological transformation is ured, ratios of alkyl halide to hydrogen sulfide species were slow under anoxic conditions, abiotic SN2pathways may such that supplemental pH buffering was unnecessary. be of considerable environmental significance, provided Additional buffering in the form of (NH4)2HP04(Mal- suitable nucleophiles exist. linckrodt) was used for experiments with CH2C12,owing One nucleophile of potential importance in anoxic sys- to the higher ratios of CH2C12to (H2S)~.All aqueous tems is the bisulfide ion HS-, produced by the microbially solutions were prepared using high-purity (>EMQ/cm) mediated dissimilatory reduction of sulfate. Total H2S deionized distilled water prepared with a Milli-Q water concentrations [(H2S)~Ias high as 470 pM to 12 mM have system (Millipore Corp., Bedford, MA). been encountered in landfill-contaminated groundwater Determinations of pH were performed at the appro- (19)and thermal springs and subsurface brines (201, and priate temperature with an Orion Model SA 720 pH/ISE concentrations on the order of tens of micromolar are quite meter and an Orion Ross combination pH electrode. common. Several researchers have studied the reactions Bisulfide ion concentrations were determined from (Hp9)T of hydrogen sulfide species with primary alkyl monohalides and pH from the expression for the pKa (at zero ionic and vic-dihalides (21-24). These investigations have strength) of H2Sas a function of temperature given in the confirmed that hydrogen sulfide species can promote the review by Miller0 (25),correcting for ionic strength ac- cording to the expression K,' = Kay~s/yHS-.Activity Current affiliation: Gradient Corp., 44 Brattle St., Cambridge, coefficients ym-for HS- were estimated hom the extended MA 02138. Debye-Huckel relationship, and activity coefficients yHg 0013-936X/92/0926-2263$03.00/0 0 1992 American Chemical Society Environ. Scl. Technoi., Vol. 26, No. 11, 1992 2283 for the neutral species H2S were assumed equal to 1. the end point determined via the Gran function. Stock solutions of CH2BrCl (99%, Aldrich), CH2Br2 Anoxic working solutions of HS- (1.5-22.5 mM) were (99+%, Aldrich), and CHZClz (99.99%; Omnisolv, EM prepared by adding weighed amounts of required salts Science) were prepared in methanol (99.9%; Omnisolv) for [NaCl and (NH4)2HP04] to the closed system before spiking experimental flasks. For most experiments with purging with argon. Aliquots of stock solutions of Na2S CHzBrCl and CH2Br2,C12C=CHCl (Mallinckrodt) was and HCl were transferred from reagent storage flasks using included in the spiking solution to establish the integrity glass syringes equipped with Teflon needles and nylon of our glass reaction vessels against losses of volatile com- stopcocks; all manipulations were performed via stopcocks pounds. Standards were prepared by diluting alkyl halides under a stream of argon gas. After being mixed, the directly into hexane (85% n-hexane; Baker Capillary-An- working solutions were transferred under argon pressure alyzed grade). All organic solvents were used as received to the reaction vessel, leaving no or very little headspace without further purification. Gas chromatographic (GC) to prevent volatilization (generally <1-mL headspace to analysis indicated the hexane contained traces of CH2C12 300-mL total volume). Aliquots of the excess working and CH2BrCl;corrections were made by subtracting peak solution were then removed under argon pressure for areas for these compounds in solvent blanks from peak measurement of pH and to check total hydrogen sulfide areas for standards or samples. concentrations via iodometric titration. Reagents used for measuring sulfite, thiosulfate, and Kinetic Experiments. Vessels used for studying rates polysulfides via reversed-phase high-pressure liquid of CH2BrC1and CH2Br2reaction were wrapped in several chromatography (HPLC) included sodium acetate (Fisher), layers of black plastic to minimize exposure to light and tetrabutylammonium hydrogen sulfate (>99%, Fluka), were placed in a water bath to reach thermal equilibrium 2,2'-dithiobis(5-nitropyridine) (98%, Aldrich), and aceto- at 25.01 f 0.02 or 35.00 f 0.02 "C. Flasks were then spiked nitrile (HPLC grade, Fisher). Standards were prepared (under a stream of argon gas) with 50-pL aliquota of a with Na2SzO3-5H20(Mallinckrodt) and anhydrous Na2S03 methanol solution containing CH2BrCl, CH2Br2, and (Mallinckrodt). Polysulfide solutions were prepared by C12C=CHC1to give initial concentrations of approximately adding excess Ss (sublimed, Mallinckrodt) to aqueous so- 17 pM CHzBrC1,5 pM CH2Br2,and 19 pM C12C=CHC1. lutions containing hydrogen sulfide species. After an initial mixing period and sampling, reaction Thiol products were methylated by adding aliquots of vessels were returned to the water baths for incubation. a solution containing
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