Rate Of Dehalogenation Research Articles

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Bacteria enzyme enhances rate of dehalogenation. Component of turmeric may help fight melanoma and breast cancer.

Degradation of Drinking Water Disinfection Byproducts by Synthetic Goethite and Magnetite

Corrosion of iron pipes leads to the release of ferrous iron, Fe(II), and the formation of iron oxides, such as goethite and magnetite, on the pipe surface. Fe(II), a potent reductant when associated with iron oxide surfaces, can mediate the reduction of halogenated organic compounds. Batch experiments were performed to investigate the kinetics and pathways of the degradation of selected chlorinated disinfection byproducts (OBPs) by Fe(II) in the presence of synthetic goethite and magnetite. Trichloronitromethane was degraded via reduction, while trichloroacetonitrile, 1,1,1-trichloropropanone, and trichloroacetaldyde hydrate were transformed via both hydrolysis and reduction. Chloroform and trichloroacetic acid were unreactive. Observed pseudo-first-order reductive dehalogenation rates were influenced by DBP chemical structure and identity of the reductant. Fe(II) bound to iron minerals had greater reactivity than either aqueous Fe(II) or structural Fe(II) present in magnetite. For DBPs of structure Cl3C-R, reductive dehalogenation rate constants normalized by the surface density of Fe(II) on both goethite and magnetite correlated with the electronegativity of the -R group and with one electron reduction potential. In addition to chemical transformation, sorption onto the iron oxide minerals was also an important loss process for 1,1,1-trichloropropanone.

Simulated and experimental evaluation of factors affecting the rate and extent of reductive dehalogenation of chloroethenes with glucose

Carbohydrates such as molasses are being added to aquifers to serve as electron donors for reductive dehalogenation of chloroethenes. Glucose, as a model carbohydrate, was studied to better understand the processes involved and to evaluate the effectiveness for dehalogenation of different approaches for carbohydrate addition. A simulation model was developed and calibrated with experimental data for the reductive dehalogenation of tetrachloroethene to ethene via cis-1,2-dichloroethene. The model included fermentors that convert the primary donor (glucose) into butyrate, acetate and hydrogen, methanogens, and two separate dehalogenator groups. The dehalogenation groups use the hydrogen intermediate as an electron donor and the different haloethenes as electron acceptors through competitive inhibition. Model simulations suggest first that the initial relative population size of dehalogenators and H2-utilizing methanogens greatly affects the degree of dehalogenation achieved. Second, the growth and decay of biomass from soluble carbohydrate plays a significant role in reductive dehalogenation. Finally, the carbohydrate delivery strategies used (periodic versus batch addition and the time interval between periodic addition) greatly affect the degree of dehalogenation that can be obtained with a given amount of added carbohydrate.

Vinyl bromide as a surrogate for determining vinyl chloride reductive dechlorination potential.

Site evaluation for bioremediation of chlorinated ethenes may need treatability studies to assess the reductive dechlorination potential of vinyl chloride (VC). Dehalogenation of vinyl bromide (VB) was investigated as a surrogate measurement for the dechlorination potential of VC. VB dehalogenation rates and kinetics were studied and compared with those of VC by a methanogenic reductive dechlorinating enrichment culture that was dominated by Dehalococcoides species and by microcosms from two chloroethene-contaminated sites. The enrichment culture dehalogenated VB to ethene at higher rates than VC at similar concentrations. VB was dehalogenated with a higher enzyme affinity than was VC, as indicated by their half-velocity constants, 240 +/- 150 and 21 +/- 8 microM, for VC and VB, respectively. Cross-inhibition study exhibited some evidence for competitive inhibition between VC and VB, suggesting that their degradation might be catalyzed by the same enzyme in the culture. Laboratory microcosm studies using subsurface soil and groundwater from two contaminated sites demonstrated that the production of the reductive dehalogenation product (ethene) could be detected faster with VB as a substrate than with VC. As a result, a substantially shorter (up to 5-10 times) incubation time would be required to detect the same level of reductive dehalogenation activity using VB as a surrogate for VC in treatability assessments.

Carbon–Chlorine and Carbon–Bromine Bond Cleavage in the Catalytic Hydrodehalogenation of Halogenated Aromatics

The catalytic hydrodehalogenation of chlorobenzene (CB), bromobenzene (BB), CB/BB mixtures, and the three chlorobromobenzene (CBB) isomers was studied over the temperature range 473 K≤T≤603 K using Ni/SiO2 where the Ni loading was varied from 6.2 to 15.2% wt/wt. Each catalyst was 100% selective in terms of hydrodehalogenation and there was no evidence of any aromatic ring reduction. Steady-state conversion was readily achieved but there was a decided decline in activity with prolonged catalyst use. The catalyst samples were characterized, before and after use, by TEM, CO chemisorption/TPD, TPO, and potentiometric analysis. We recorded the levels of reversibly and irreversibly held Cl and/or Br on the spent catalysts. Appreciable Ni particle growth during catalysis was observed and this is attributed to a halide-induced metal agglomeration, while CO TPD revealed a significant disruption to the Ni particle electronic structure. Long-term deactivation is ascribed to a less effective activation of the haloarene and/or hydrogen reactant(s) allied to a halide-promoted carbonaceous deposition; the surface coke is characterized as predominantly amorphous. Reactivity increased in the order 2-CBB<3-CBB<4-CBB<BB<CB, which is taken to be diagnostic of an electrophilic mechanism where the presence of a second electron withdrawing a halogen substituent lowers the overall dehalogenation rate and steric hindrance governs reactivity in the case of the CBB isomers. In the hydrotreatment of CB/BB mixtures and CBB isomers, the fractional debromination increased due to a surface hydrochlorination and Cl exchange with the Br substituent on the aromatic ring. The reaction orders with respect to CB, BB, and H2 were determined, while the experimentally determined rate/pressure profiles were subjected to standard Langmuir–Hinshelwood kinetic modeling. The best overall (meaningful) fit to the experimental data was achieved with a model based on the associative adsorption of the haloarene and the dissociative adsorption of hydrogen on the same sites with no product inhibition.

Carbon–Chlorine and Carbon–Bromine Bond Cleavage in the Catalytic Hydrodehalogenation of Halogenated Aromatics

The catalytic hydrodehalogenation of chlorobenzene (CB), bromobenzene (BB), CB/BB mixtures, and the three chlorobromobenzene (CBB) isomers was studied over the temperature range 473 K≤ T≤603 K using Ni/SiO 2 where the Ni loading was varied from 6.2 to 15.2% wt/wt. Each catalyst was 100% selective in terms of hydrodehalogenation and there was no evidence of any aromatic ring reduction. Steady-state conversion was readily achieved but there was a decided decline in activity with prolonged catalyst use. The catalyst samples were characterized, before and after use, by TEM, CO chemisorption/TPD, TPO, and potentiometric analysis. We recorded the levels of reversibly and irreversibly held Cl and/or Br on the spent catalysts. Appreciable Ni particle growth during catalysis was observed and this is attributed to a halide-induced metal agglomeration, while CO TPD revealed a significant disruption to the Ni particle electronic structure. Long-term deactivation is ascribed to a less effective activation of the haloarene and/or hydrogen reactant(s) allied to a halide-promoted carbonaceous deposition; the surface coke is characterized as predominantly amorphous. Reactivity increased in the order 2-CBB<3-CBB<4-CBB<BB<CB, which is taken to be diagnostic of an electrophilic mechanism where the presence of a second electron withdrawing a halogen substituent lowers the overall dehalogenation rate and steric hindrance governs reactivity in the case of the CBB isomers. In the hydrotreatment of CB/BB mixtures and CBB isomers, the fractional debromination increased due to a surface hydrochlorination and Cl exchange with the Br substituent on the aromatic ring. The reaction orders with respect to CB, BB, and H 2 were determined, while the experimentally determined rate/pressure profiles were subjected to standard Langmuir–Hinshelwood kinetic modeling. The best overall (meaningful) fit to the experimental data was achieved with a model based on the associative adsorption of the haloarene and the dissociative adsorption of hydrogen on the same sites with no product inhibition.

Dechlorination of PCBs, CAHs, herbicides and pesticides neat and in soils at 25 °C using Na/NH 3

Na/NH 3 reductions have been used to dehalogenate polychlorinated biphenyls (PCBs), chlorinated aliphatic hydrocarbons (CAHs) and pesticides at diffusion controlled rates at room temperature in model compound studies in both dry NH 3 and when water was added. The rate ratio of dechlorination (aliphatic and aromatic compounds) versus reaction of the solvated electron with water is very large, allowing wet soils or sludges to be remediated without an unreasonable consumption of sodium. Several soils, purposely contaminated with 1,1,1-trichloroethane, 1-chlorooctane and tetrachloroethylene, were remediated by slurring the soils in NH 3 followed by addition of sodium. The consumption of sodium per mole of chlorine removed was examined as a function of both the hazardous substrate’s concentration in the soil and the amount of water present. The Na consumption per Cl removed increases as the amount of water increases and as the substrate concentration in soil decreases. However, remediation was still readily accomplished from 5000 to 3000 ppm to sub ppm levels of RCl in the presence of substantial amounts of water. PCB- and dioxin-contaminated oils were remediated with Na/NH 3 as were PCB-contaminated soils and sludges from contaminated sites. Ca/NH 3 treatments also successfully remediated PCB-contaminated clay, sandy and organic soils but laboratory studies demonstrated that Ca was less efficient than Na when substantial amounts of water were present. The advantages of solvated electron reductions using Na/NH 3 include: (1) very rapid dehalogenation rates at ambient temperature, (2) soils (even clay soils) break down into particles and slurry nicely in NH 3, (3) liquid ammonia handling technology is well known and (4) removal from soils, recovery and recycle of ammonia is easy due to its low boiling point. Finally, dechlorination is extremely fast even for the ‘corner’ chlorines in the substrate Mirex™ (structure in Eq. (5)).

Comparison of methods for the photochemical degradation of chlorophenols

Mono/polychlorophenols are currently used for a wide range of domestic, agricultural and industrial purposes. The improper disposal of mono/polychlorophenols in landfills and in low-temperature combustion contributes significantly to environmental contamination. Generally, chlorinated aromatic phenols are toxic to living organisms. For example, pentachlorophenol can affect sensitive organisms, even at low concentrations and lead to alterations in ecosystems. The objective of the present study is to investigate dehalogenation and destruction of halogenated phenols by using photochemical methods. The reactants and products were analyzed by gas chromatography. Dehalogenation was faster for monohalogenated phenols than poly-substituted phenols. Experimental data demonstrate that the reaction rate of dehalogenation in the presence of TiO2 is 4-chlorophenol>2,4,-dichlorophenol>2,4,6-trichlorophenol. To overcome the low solubility of higher chlorinated phenols in aqueous systems, the mixture of water/methanol (60:40) was used; however, the methanolic mixture affects photodegradation rates of the chlorophenols.

Quantitative structure–property relationship study on reductive dehalogenation of selected halogenated aliphatic hydrocarbons in sediment slurries

In this study, by the use of partial least squares (PLS) method and 26 quantum chemical descriptors computed by PM3 Hamiltonian, a quantitative structure–property relationship (QSPR) model was developed for reductive dehalogenation rate constants of 13 halogenated aliphatic compounds in sediment slurry under anaerobic conditions. The model can be used to explain the dehalogenation mechanism. Halogenated aliphatic compounds with great energy of the lowest unoccupied molecular orbital ( E lumo), total energy (TE), electronic energy (EE), the smallest bond order of the carbon–halogen bonds (BO) and the most positive net atomic charges on an atom of the molecule ( q +) values tend to be reductively dehalogenated slow, whereas halogenated aliphatic compounds with high values of molecular weight (Mw), average molecular polarizability ( α) and core–core repulsion energy (CCR) values tend to be reductively dehalogenated fastest.

Catalytic reductive dehalogenation of hexachloroethane by molecular variants of cytochrome P450cam (CYP101).

CYP101 (cytochrome P450cam) catalyses the oxidation of camphor but has also been shown to catalyse the reductive dehalogenation of hexachloroethane and pentachloroethane. This reaction has potential applications in the biodegradation of these environmental contaminants. The hexachloroethane dehalogenation activity of CYP101 has been investigated by mutagenesis. The effects of active-site polarity and volume were probed by combinations of active-site mutations. Increasing the active-site hydrophobicity by the Y96A and Y96F mutations strengthened hexachloroethane binding but decreased the rate of reaction. Increasing the polarity with the F87Y mutation drastically weakened hexachloroethane binding but did not affect the rate of reaction. The Y96H mutation had little effect at pH 7.4, but weakened hexachloroethane binding while increasing the rate of dehalogenation by up to 40% at pH 6.5, suggesting that the imidazole side-chain was partially protonated at pH 6.5 but not at pH 7.4. Substitutions by bulkier side-chains at F87, T101 and V247 weakened hexachloroethane binding but increased the dehalogenation rate. The effect of the individual mutations was additive in multiple mutants, and the most active mutant for hexachloroethane reductive dehalogenation at pH 7.4 was F87W-V247L (80 min-1 or 2.5 x the activity of the wild-type). The results suggested that the CYP101 active site shows good match with hexachloroethane, the Y96 side-chain plays an important role in both hexachloroethane binding and dehalogenation, and hexachloroethane binding and dehalogenation places conflicting demands on active-site polarity and compromises were necessary to achieve reasonable values for both.

Catalytic cysteine of thymidylate synthase is activated upon substrate binding.

The role of Ser 167 of Escherichia coli thymidylate synthase (TS) in catalysis has been characterized by kinetic and crystallographic studies. Position 167 variants including S167A, S167N, S167D, S167C, S167G, S167L, S167T, and S167V were generated by site-directed mutagenesis. Only S167A, S167G, S167T, and S167C complemented the growth of thymidine auxotrophs of E. coli in medium lacking thymidine. Steady-state kinetic analysis revealed that mutant enzymes exhibited k(cat) values 1.1-95-fold lower than that of the wild-type enzyme. Relative to wild-type TS, K(m) values of the mutant enzymes for 2'-deoxyuridylate (dUMP) were 5-90 times higher, while K(m) values for 5,10-methylenetetrahydrofolate (CH(2)H(4)folate) were 1.5-16-fold higher. The rate of dehalogenation of 5-bromo-2'-deoxyuridine 5'-monophosphate (BrdUMP), a reaction catalyzed by TS that does not require CH(2)H(4)folate as cosubstrate, by mutant TSs was analyzed and showed that only S167A and S167G catalyzed the dehalogenation reaction and values of k(cat)/K(m) for the mutant enzymes were decreased by 10- and 3000-fold, respectively. Analysis of pre-steady-state kinetics of ternary complex formation revealed that the productive binding of CH(2)H(4)folate is weaker to mutant TSs than to the wild-type enzyme. Chemical transformation constants (k(chem)) for the mutant enzymes were lower by 1.1-6.0-fold relative to the wild-type enzyme. S167A, S167T, and S167C crystallized in the I2(1)3 space group and scattered X-rays to either 1.7 A (S167A and S167T) or 2.6 A (S167C). The high-resolution data sets were refined to a R(crys) of 19.9%. In the crystals some cysteine residues were derivatized with 2-mercaptoethanol to form S,S-(2-hydroxyethyl)thiocysteine. The pattern of derivatization indicates that in the absence of bound substrate the catalytic cysteine is not more reactive than other cysteines. It is proposed that the catalytic cysteine is activated by substrate binding by a proton-transfer mechanism in which the phosphate group of the nucleotide neutralizes the charge of Arg 126', facilitating the transfer of a proton from the catalytic cysteine to a His 207-Asp 205 diad via a system of ordered water molecules.

Transformation of Chlorinated Hydrocarbons Using Aquocobalamin or Coenzyme F430 in Combination with Zero‐Valent Iron

AbstractMore effective methods are necessary for the remediation of soils, sediments, and ground waters contaminated with halogenated organic compounds. Our objective was to determine the feasibility and utility of using a tetrapyrrole‐Fe(0) mixture for reductive dehalogenation of synthetic organic contaminants. Aquocobaiamin or coenzyme F430 was combined with Fe(0) in aqueous systems containing either a single chlorinated compound or mixtures of chlorinated compounds, and substrate disappearance was monitored using gas chromatography‐mass spectrometry (GC‐MS). Zero‐valent iron effectively dehalogenated CCl4 at low to neutral pH values, while increases in CCl4 dehalogenation resulting from inclusion of tetrapyrrole catalysts along with Fe(0) occurred only at basic pH values. Rates of CCl4 disappearance increased with additional aquocobalamin, but reached a maximum and decreased at higher aquocobalamin concentrations. Overall dehalogenation rates may thus be a function of Fe(0)'s limited reactive surface area. There was a trend for both tetrapyrrole catalysts to promote the disappearance of halogenated compounds in a mixed substrate containing 20 compounds. Studies with five individual substrates likewise showed trends for increased substrate removal with F430 beyond that for Fe(0) alone. This increase is most important for compounds such as 1,2‐dichloroethane and 1,4‐dichlorobenzene that are not readily dehalogenated by Fe(0). Chloride concentrations in the reaction mixtures indicated that reductive dehalogenation was the dominant process responsible for substrate disappearance. Use of a combination of aquocobalamin or coenzyme F430 and Fe(0) may effectively promote dehalogenation, thus producing fewer products and more complete dehalogenation of the target substrates than can be achieved using only one of the abiotic reductants alone.

Anaerobic decomposition of halogenated aromatic compounds

Halogenated compounds constitute one of the largest groups of environmental pollutants, partly as a result of their widespread use as biocides, solvents and other industrial chemicals. A critical step in degradation of organohalides is the cleavage of the carbonhalogen bond. Reductive dehalogenation is generally the initial step in metabolism under methanogenic conditions, which requires a source of reducing equivalents, with the halogenated compound serving as an electron acceptor. Dehalogenation is greatly influenced by alternate electron acceptors; e.g. sulfate frequently inhibits reductive dehalogenation. On the other hand, a number of halogenated aromatic compounds can be degraded under different electron-accepting conditions and their complete oxidation to CO 2 can be coupled to processes such as denitrification, iron(III)-reduction, sulfate reduction and methanogenesis. Reductive dehalogenation was the initial step in degradation not only under methanogenic, but also under sulfate- and iron(III)-reducing conditions. Dehalogenation rates were in general slower under sulfidogenic and iron-reducing conditions, suggesting that dehalogenation was affected by the electron acceptor. The capacity for dehalogenation appears to be widely distributed in anoxic environments; however, the different substrate specificities and activities observed for the halogenated aromatic compounds suggest that distinct dehalogenating microbial populations are enriched under the different reducing conditions. Characterization of the microbial community structure using a combination of biomolecular techniques, such as cellular fatty acid profiling, and 16 S rRNA fingerprinting/sequence analysis, was used to discern the distinct populations enriched with each substrate and under each electron-accepting condition. These combined techniques will aid in identifying the organisms responsible for dehalogenation and degradation of halogenated aromatic compounds.

Radiolytic and Thermal Dehalogenation of CFC-113 Adsorbed on Molecular Sieve 13X

Abstract The reductive dehalogenation of 1,1,2-trichloro-1,2,2-trifluoroethane (CFC-113) has been radiolytically and thermally initiated on molecular sieve 13X. The molecular sieve adsorbing CFC-113 was irradiated with γ-rays or heated, and was then soaked in water. The Cl− and F− concentrations and the pH of the supernatant solutions were measured to follow the dehalogenation reaction. These values for samples irradiated at a dose rate of 7.8 kGy h−1 and heated at 150 °C were almost the same when the irradiation time was twenty-times longer than the heating time. The F− concentration once increased and then decreased with the reaction time, whereas the Cl− concentration increased continuously. The F−, once produced, may be consumed with the denatured molecular sieve. When the molecular sieve adsorbing 2-propanol together with CFC-113 was irradiated, the dechlorination proceeded by a chain reaction, but the defluorination was inhibited. On the other hand, when heated in the presence of 2-propanol, both the dechlorination and the defluorination were largely suppressed. The pH of the supernatant solutions decreased from an alkaline value to a neutral or acidic one along with dehalogenation. Radiolysis of CFC-113 on activated charcoal was also examined, but the dehalogenation rates were much slower than those on the molecular sieve.

Electrolytic Reduction of CCl4—Effects of Cathode Material and Potential on Kinetics, Selectivity, and Product Stoichiometry

Carbon tetrachloride (CCl4) was reductively transformed via electrolysis at metal cathodes consisting of pure Ag, Al, Au, Cu, Fe, Ni, Pd, and Zn. Primary products included CHCl3, CH2Cl2 and CH4. Traces of CH3Cl, C2H4, C2H6, C3H8, and other unidentified products were also observed in headspace samples. CHCl3 was reduced to CH2Cl2 and CH4 under the same experimental conditions, although at a slower rate. CH2Cl2 was essentially stable during the course of these experiments (ks < 0.05 L/m2 · min). CCl4 reduction was first order in the bulk liquid phase concentration. Pseudo-first-order rate constants were a function of cathode material, pH, and applied cathode potential (Ec) in the range −1.4 V ≤ Ec ≤ −0.4 V vs. SHE. At all electrodes, reaction rates conformed to Butler-Volmer kinetics, modified to account for mass transfer limitations. Under comparable conditions, the kinetics of CCl4 electrolysis at Ni and Cu electrodes were superior to those of transformation using any other metal tested. At Ec = −0.6 and −0.8 V, rate constants for CCl4 reduction at a Cu electrode decreased with increasing pH in the range 2 < pH < 6. At Ec ≤ −1.2 V, reduction was limited by mass transfer to the electrode surface and was independent of cathode material and pH. Electrode efficiency (ratio of dehalogenation rate to total electrolytic current) for CCl4 reduction was inversely related to the material-dependent exchange current density for hydrogen gas generation, suggesting that production of H2 (g) is the most significant competitive reaction under the conditions of the experiments. Based on efficiency considerations, the electrode materials were ordered Zn > Cu > Fe > Ni. Key words: Reductive dehalogenation; carbon tetrachloride; groundwater remediation; electrolysis

Chlorinated Ethene Half-Velocity Coefficients (KS) for Reductive Dehalogenation

Biological reduction of the chlorinated solvents tetrachloroethene (PCE) and trichloroethene (TCE) completely to ethene is of interest for engineered or intrinsic destruction of these prevalent groundwater contaminants. However, the transformation are frequently not complete, leading to the production of vinyl chloride (VC), a more hazardous compound. Factors affecting the relative rates of destruction of the solvents and their intermediate products are thus of interest. The maximum degradation rates (kX) and half-velocity coefficients (K{sub s}) for these chlorinated ethenes used as electron acceptors in reductive dehalogenation with hydrogen in excess were examined using an enrichment culture grown on benzoate, hydrogen, and PCE. Initial dehalogenation rates were measured at various chlorinated ethene concentrations in batch studies. With 38 mg/L volatile suspended solids of this culture, the kX and 95% confidence intervals for PCE, TCE, cis-dichloroethene (cDCE), and VC at 25 C were found to be 77 {+-} 5.59 {+-} 11, 14 {+-} 3, and 13 {+-} 3 {micro}M/day with K{sub s} values of 0.11 {+-} 0.04, 1.4 {+-} 0.9, 3.3 {+-} 2.2, and 2.6 {+-} 1.9 {micro}M, respectively. The lower maximum transformation rates and higher K{sub s} values for cDCE and VC partly explain why incomplete transformation of PCE and TCE oftenmore » occurs in the field.« less

Spectrum of the reductive dehalogenation activity of desulfitobacterium frappieri PCP-1

Desulfitobacterium frappieri PCP-1 was induced for ortho- and para-dechlorinating activities by different chlorophenols. Dehalogenation rates ranging from 25 to 1,158 nmol/min/mg of cell protein were observed according to the chlorophenol tested and the position of the chlorine removed. D. frappieri shows a broad substrate specificity; in addition to tetrachloroethylene and pentachloropyridine, strain PCP-1 can dehalogenate at ortho, meta, and para positions a large variety of aromatic molecules with substituted hydroxyl or amino groups. Reactions of O demethylation and reduction of nitro to amino substituents on aromatic molecules were also observed.

A new way to Si-Si, Ge-Ge bonds and diisopropyldihalotellurides

Reactions of diisopropyltelluride (Me 2CH) 2Te ( I) with various trimethylhalo-, dimethyldihalosilanes Me 4-nSiX n ( n= 1, 2; X = Cl, Br, I) and triethylbromogermane BrGeEt 3 have been investigated in air and in vacuum. Compound I reacts with trimethyliodosilane ISiMe 3 already at room temperature in vacuum to form hexamethyldisilane Me 3SiSiMe 3 and diisopropyldiiodotelluride (Me 2CH) 2TeI 2. In air this reactions proceeds at a higher rate than in vacuum, but leads to the formation of hexamethyldisiloxane (Me 3Si) 2O and diisopropyldihalogentellurides (Me 2CH) 2TeX 2 as main products. The reaction of diisopropyltelluride ( I) with dimethyldihalogensilanes X 2SiMe 2 (X = Cl, I) in vacuum leads to the corresponding diisopropyldihalogentellurides (Me 2CH) 2TeX 2 and a complex mixture of the products of dehalogenation of dimethyldihalogensilanes. The rate of dehalogenation considerably increases with the atomic number of the halogen: Cl < Br ⪡ I.

Mechanism-based Quantitative Structure–Biodegradability Relationships for Hydrolytic Dehalogenation of Chloro- and Bromo-Alkenes

Quantitative Structure–Biodegradability Relationships (QSBRs) have been developed for microbial hydrolytic dehalogenation of small chlorinated and brominated alkenes. Two different reaction mechanisms, S2 and S2′, were considered for the dehalogenation of small haloalkenes by hydrolytic dehalogenases. Multivariate QSBRs based on quantum-chemically calculated descriptors were used to distinguish between these dehalogenation mechanisms. A QSBR based on atom valence and partial atomic charge on electrophilic site, together with the energy of the lowest unoccupied molecular orbital, molecular weight and dipole moment were able to explain 92% (81% cross-validated) of the quantitative variance in the dehalogenation rates. This QSBR supported a S2 reaction mechanism. This reaction mechanism was also confirmed by GC-MS identification of the reaction products for the compounds which should give discriminating reaction products depending on mechanism. The substrate specificity of hydrolytic dehalogenases for unsaturated halogenated substrates is discussed in the light of chemical structure and properties of the substrate molecules.

Correlation Analysis of Rate Constants for Dechlorination by Zero-Valent Iron

Dehalogenation is among the most important processes involved in contaminant fate, but despite all the work that has been done on the kinetics of dehalogenation, there are few linear free energy relationships (LFERs) that can be used to explain or predict rates of dehalogenation by environmental reductants. Previously, we summarized kinetic data for dehalogenation of chlorinated alkanes and alkenes by zero-valent iron (Fe0) and showed that correlation analysis of these data with published two-electron reduction potentials did not give a simple relationship. In this study, we report successful LFERs based on estimated lowest unoccupied molecular orbital (LUMO) energies calculated from semiempirical (AM1 and PM3) and ab initio methods (6-31G*) and one-electron reduction potentials. Solvation effects can be modeled with COSMO and incorporated into semiempirical estimates of ELUMO, but this did not improve the correlation with k. The best LFER (log k = −5.7−1.5 ELUMO) explains 83% of the variability in surface area-normalized rate constants (k) with ab initio LUMO energies. The LFER is improved by correcting for statistical bias introduced by back transformation from log-linear regression models. New kinetic data for six compounds are compared with rate constants predicted using the unbiased LFER.