Second-order Rate Constants For Reactions Research Articles

Click Functionalization of a Dibenzocyclooctyne-Containing Conjugated Polyimine.

A conjugated poly(phenyl-co-dibenzocyclooctyne) Schiff-base polymer, prepared through polycondensation of dibenzocyclooctyne bisamine (DIBO-(NH2)2) with bis(hexadecyloxy)phenyldialdehyde, is reported. The resulting polymer, which has a high molecular weight (M(n)>30 kDa, M(w)>60 kDa), undergoes efficient strain-promoted alkyne-azide cycloaddition reactions with a series of azides. This enables quantitative modification of each repeat unit within the polymer backbone and the rapid synthesis of a conjugated polymer library with widely different substituents but a consistent degree of polymerization (DP). Kinetic studies show a second-order reaction rate constant that is consistent with monomeric dibenzocyclooctynes. Grafting with azide-terminated polystyrene and polyethylene glycol monomethyl ether chains of varying molecular weight resulted in the efficient syntheses of a series of graft copolymers with a conjugated backbone and maximal graft density.

Kinetics and Mechanism of the Oxidation of Cyclic Methylsiloxanes by Hydroxyl Radical in the Gas Phase: An Experimental and Theoretical Study.

The ubiquitous presence of cyclic volatile methylsiloxanes (cVMS) in the global atmosphere has recently raised environmental concern. In order to assess the persistence and long-range transport potential of cVMS, their second-order rate constants (k) for reactions with hydroxyl radical ((•)OH) in the gas phase are needed. We experimentally and theoretically investigated the kinetics and mechanism of (•)OH oxidation of a series of cVMS, hexamethylcyclotrisiloxane (D3), octamethycyclotetrasiloxane (D4), and decamethycyclopentasiloxane (D5). Experimentally, we measured k values for D3, D4, and D5 with (•)OH in a gas-phase reaction chamber. The Arrhenius activation energies for these reactions in the temperature range from 313 to 353 K were small (-2.92 to 0.79 kcal·mol(-1)), indicating a weak temperature dependence. We also calculated the thermodynamic and kinetic behaviors for reactions at the M06-2X/6-311++G**//M06-2X/6-31+G** level of theory over a wider temperature range of 238-358 K that encompasses temperatures in the troposphere. The calculated Arrhenius activation energies range from -2.71 to -1.64 kcal·mol(-1), also exhibiting weak temperature dependence. The measured k values were approximately an order of magnitude higher than the theoretical values but have the same trend with increasing size of the siloxane ring. The calculated energy barriers for H-atom abstraction at different positions were similar, which provides theoretical support for extrapolating k for other cyclic siloxanes from the number of abstractable hydrogens.

Demonstration of Lignin-to-Peroxidase Direct Electron Transfer: A TRANSIENT-STATE KINETICS, DIRECTED MUTAGENESIS, EPR, AND NMR STUDY

Versatile peroxidase (VP) is a high redox-potential peroxidase of biotechnological interest that is able to oxidize phenolic and non-phenolic aromatics, Mn(2+), and different dyes. The ability of VP from Pleurotus eryngii to oxidize water-soluble lignins (softwood and hardwood lignosulfonates) is demonstrated here by a combination of directed mutagenesis and spectroscopic techniques, among others. In addition, direct electron transfer between the peroxidase and the lignin macromolecule was kinetically characterized using stopped-flow spectrophotometry. VP variants were used to show that this reaction strongly depends on the presence of a solvent-exposed tryptophan residue (Trp-164). Moreover, the tryptophanyl radical detected by EPR spectroscopy of H2O2-activated VP (being absent from the W164S variant) was identified as catalytically active because it was reduced during lignosulfonate oxidation, resulting in the appearance of a lignin radical. The decrease of lignin fluorescence (excitation at 355 nm/emission at 400 nm) during VP treatment under steady-state conditions was accompanied by a decrease of the lignin (aromatic nuclei and side chains) signals in one-dimensional and two-dimensional NMR spectra, confirming the ligninolytic capabilities of the enzyme. Simultaneously, size-exclusion chromatography showed an increase of the molecular mass of the modified residual lignin, especially for the (low molecular mass) hardwood lignosulfonate, revealing that the oxidation products tend to recondense during the VP treatment. Finally, mutagenesis of selected residues neighboring Trp-164 resulted in improved apparent second-order rate constants for lignosulfonate reactions, revealing that changes in its protein environment (modifying the net negative charge and/or substrate accessibility/binding) can modulate the reactivity of the catalytic tryptophan.

Kinetic Analysis of Haloacetonitrile Stability in Drinking Waters.

Haloacetonitriles (HANs) are an important class of drinking water disinfection byproducts (DBPs) that are reactive and can undergo considerable transformation on time scales relevant to system distribution (i.e., from a few hours to a week or more). The stability of seven mono-, di-, and trihaloacetonitriles was examined under a variety of conditions including different pH levels and disinfectant doses that are typical of drinking water distribution systems. Results indicated that hydroxide, hypochlorite, and their protonated forms could react with HANs via nucleophilic attack on the nitrile carbon, forming the corresponding haloacetamides (HAMs) and haloacetic acids (HAAs) as major reaction intermediates and end products. Other stable intermediate products, such as the N-chloro-haloacetamides (N-chloro-HAMs), may form during the course of HAN chlorination. A scheme of pathways for the HAN reactions was proposed, and the rate constants for individual reactions were estimated. Under slightly basic conditions, hydroxide and hypochlorite are primary reactants and their associated second-order reaction rate constants were estimated to be 6 to 9 orders of magnitude higher than those of their protonated conjugates (i.e., neutral water and hypochlorous acid), which are much weaker but more predominant nucleophiles at neutral and acidic pHs. Developed using the estimated reaction rate constants, the linear free energy relationships (LFERs) summarized the nucleophilic nature of HAN reactions and demonstrated an activating effect of the electron withdrawing halogens on nitrile reactivity, leading to decreasing HAN stability with increasing degree of halogenation of the substituents, while subsequent shift from chlorine to bromine atoms has a contrary stabilizing effect on HANs. The chemical kinetic model together with the reaction rate constants that were determined in this work can be used for quantitative predictions of HAN concentrations depending on pH and free chlorine contact times (CTs), which can be applied as an informative tool by drinking water treatment and system management engineers to better control these emerging nitrogenous DBPs, and can also be significant in making regulatory decisions.

Photolysis of three antiviral drugs acyclovir, zidovudine and lamivudine in surface freshwater and seawater

Photodegradation is an important elimination process for many pharmaceuticals in surface waters. In this study, photodegradation of three antiviral drugs, acyclovir, zidovudine, and lamivudine, was investigated in pure water, freshwater, and seawater under the irradiation of simulated sunlight. Results showed that zidovudine was easily transformed via direct photolysis, while acyclovir and lamivudine were mainly transformed via indirect photolysis. We found that in freshwater, nitrate enhanced the photodegradation of the three antiviral drugs, bicarbonate promoted the photodegradation of acyclovir, and dissolved organic matter (DOM) accelerated the photolysis of acyclovir and lamivudine. In seawater, the photolysis of acyclovir was not susceptible to Cl−, Br− and ionic strength; however, the photolysis of zidovudine was inhibited by Cl− and Br−, and the photolysis of lamivudine was enhanced by Cl−, Br− and ionic strength. Second-order reaction rate constants for the three antiviral drugs with 1O2 (k1O2) and OH (kOH) were also measured. These results are important for fate and ecological risk assessment of the antiviral drugs in natural waters.

Mass transfer and kinetics study on combined CO2 and SO2 absorption using aqueous ammonia

Combined capture of CO2 and SO2 using aqueous ammonia (NH3) is a good option to reduce capital and operational costs for post-combustion capture process. The mass transfer and kinetics of combined CO2 and SO2 absorption in aqueous NH3 solvent were determined at various SO2 concentrations, temperatures and NH3 concentrations using wetted wall column. A bi-liquid film assumption was proposed to analyze the mass transfer and kinetic characteristics by dividing the liquid film to SO2 instantaneous reaction region and CO2 pseudo-first-order reaction region. The high SO2 concentration significantly reduces the CO2 mass transfer coefficient in overall liquid phase and the SO2 instantaneous reaction region, but the high temperature and aqueous NH3 concentration benefit the CO2 mass transfer. In the CO2 pseudo-first-order reaction region, the apparent and second-order reaction rate constants of CO2 absorption were calculated and the second-order reaction rate constant can match very well with the previous work.

Improvement in iron activation ability of alachlor Fenton-like oxidation by ascorbic acid

The addition of a strong reductant, l-ascorbic acid (AA), to Fe3+/hydrogen peroxide (H2O2) improved alachlor degradation substantially. AA in the form of ascorbate monoanion undergoes a two-step oxidation to yield dehydroascorbic acid through the formation of intermediate semidehydroascorbate and ascorbyl radicals that are responsible for the autocatalytic transformation of Fe3+ to Fe2+. The ability of AA to reduce Fe3+ to Fe2+ and propagate the formation of hydroxyl radicals was confirmed by the indirect measuring of hydroxyl radicals. Hydroxyl radicals production and alachlor degradation by H2O2/AA resulted. Alachlor degradation rate by Fe3+/H2O2/AA was similar to that obtained by Fe2+/H2O2, although a much lower alachlor degradation and mineralization by Fe3+/H2O2 was achieved. The kinetics of alachlor removal by Fe2+/H2O2 and Fe3+/H2O2/AA were characterized by a rapid decay (0–1min) because of a higher formation of hydroxyl radicals and then by a slower decay (1–120min). The determination of a second-order reaction rate constant of hydroxyl radicals with alachlor relied on the competitive reactions of deoxyribose and alachlor with hydroxyl radicals. A rate constant of 6.9×109M−1s−1 (correlation coefficient of 0.997) implies that the reaction rate is close to that of diffusion-controlled reactions in water, and indicates that the hydroxyl radicals play a predominant role during oxidation. Phenylacetamide compounds that were primary by-products of alachlor degradation allowed for the prediction of the general reaction pathway. This study highlights the ability of AA to reduce Fe3+ making a presumption for non-regenerated ferric oxyhydroxide sludge reuse for activation of H2O2 oxidation in wastewater treatment at acidic pH.

Kinetics of the reaction of nitric oxide with polypyridylamine iron(II) complexes

The reactions of three polypyridylamine ferrous complexes, [Fe(TPEN)]2+, [Fe(TPPN)]2+, and [Fe(TPTN)]2+, with nitric oxide (NO) (where TPEN = N,N,N′,N′-tetrakis(2-pyridylmethyl)ethylenediamine, TPPN = N,N,N′,N′-tetrakis(2-pyridylmethyl)-1,2-propylenediamine, and TPTN = N,N,N′,N′-tetrakis(2-pyridylmethyl)trimethylenediamine) were investigated. The first two complexes, which are spin-crossover systems, presented second-order rate constants for complex formation reactions (kf) of 8.4 × 103 and 9.3 × 103 M−1 s−1, respectively (pH 5.0, 25 °C, I = 0.1 M). In contrast, the [Fe(TPTN)]2+ complex, which is in low-spin ground state, did not show any detectable reaction with NO. kf values are lower than those of high-spin Fe(II) complexes, such as [Fe(EDTA)]2− (EDTA = ethylenediaminetetraacetate) and [Fe(H2O)]2+, but higher than low-spin Fe(II) complexes, such as [Fe(CN)5(H2O)]3− and [Fe(bipyridine)3]2+. The release of NO from the [Fe(TPEN)NO]2+ and [Fe(TPPN)NO]2+ complexes were also studied, showing the values 15.6 and 17.7 s−1, respectively, comparable to the high-spin aminocarboxylate analogs. A mechanism is proposed based on the spin-crossover behavior and the geometry of these complexes and is discussed in the context of previous publications.

Kinetics of CO2 absorption in aqueous N,N-diethylethanolamine and its blend with N-(2-aminoethyl)ethanolamine using a stirred cell reactor

The absorption of CO2 into an aqueous solution of N,N-diethylethanolamine (DEEA) is studied at 293, 303, and 318K within the amine concentration range of 0.8–2kmolm−3 using a stirred cell with a flat gas–liquid interphase. Absorption rate experiments are carried out in the pseudo-first-order regime, which enabled determination of the kinetics from the fall-in pressure method. The second-order rate constants for the CO2 reaction with DEEA are obtained and then compared to the scarce literature data. At 303K, the obtained reaction rate constant equals 65.5m3kmol−1s−1. The activation energy is calculated to be 41.46kJ/mol.Additionally, the absorption of CO2 into aqueous solutions of DEEA activated with N-(2-aminoethyl) ethanolamine (AEEA) is studied at 303K. Three formulated solvents (2M DEEA+0.1M AEEA), (2M DEEA+0.2M AEEA) and (2M DEEA+0.3M AEEA) were investigated. The enhancement factor in the blended solution is significantly higher than for the pure DEEA solution. The overall reaction in amine blends can be regarded as a reaction between CO2 and DEEA in parallel with the reaction of CO2 with AEEA. It has been proven that the kinetic rate constants for the CO2 reaction with DEEA and AEEA, which were determined using the present experimental technique, can satisfactorily predict the measured kinetics for the aqueous amine blends DEEA/AEEA under pseudo-first-order regime.

Carcinogenesis of urethane: simulation versus experiment.

The carcinogenesis of urethane (ethyl carbamate), a byproduct of fermentation that is consistently found in various food products, was investigated with a combination of kinetic experiments and quantum chemical calculations. The main objective of the study was to find ΔG(⧧), the activation free energy for the rate-limiting step of the SN2 reaction among the ultimate carcinogen of urethane, vinyl carbamate epoxide (VCE), and different nucleobases of the DNA. In the experimental part, the second-order reaction rate constants for the formation of the main 7-(2-oxoethyl)guanine adduct in aqueous solutions of deoxyguanosine and in DNA were determined. A series of ab initio, density functional theory (DFT), and semiempirical molecular orbital (MO) calculations was then performed to determine the activation barriers for the reaction between VCE and nucleobases methylguanine, methyladenine, and methylcytosine. Effects of hydration were incorporated with the use of the solvent reaction field method of Tomasi and co-workers and the Langevine dipoles model of Florian and Warshel. The computational results for the main adduct were found to be in good agreement with the experiment, thus presenting strong evidence for the validity of the proposed SN2 mechanism. This allowed us to predict the activation barriers of reactions leading to side products for which kinetic experiments have not yet been performed. Our calculations have shown that the main 7-(2-oxoethyl)deoxyguanosine adduct indeed forms preferentially because the emergence of other adducts either proceeds across a significantly higher activation barrier or the geometry of the reaction requires the Watson-Crick pairs of the DNA to be broken. The computational study also considered the questions of stereoselectivity, the ease of the elimination of the leaving group, and the relative contributions of the two possible reaction paths for the formation of the 1,N(2)-ethenodeoxyguanosine adduct.

Experimental study of the kinetics of the homogenous reaction of CO2 into a novel aqueous 3-diethylamino-1,2-propanediol solution using the stopped-flow technique

In this work, the stopped-flow technique was used to determine the kinetic parameters in terms of pseudo first-order rate constants (k0) for homogenous reaction of CO2 into aqueous 3-diethylamino-1,2-propanediol (DEA-1,2-PD) solutions as temperature ranged from 293 to 313K and amine concentrations ranged from 0.20kmol/m3 to 1.00kmol/m3. It was found that k0 increased with increasing amine concentration and temperature. Both the base-catalyzed hydration and the termolecular models were applied to interpret the experimental data. The results showed that the predicted CO2 absorption rates exhibited good agreement with experimental data with an absolute average deviation (AAD) of 5.7% and 3.8% with respect to base-catalyzed hydration and the termolecular model, respectively. Furthermore, the pKa of DEA-1,2-PD was experimentally determined over the temperature range from 298 to 318K. The relationship between experimentally measured second-order reaction rate constants (k2) and pKa was correlated using the Brønsted relationship. The results suggest that the Brønsted relationship for DEA-1,2-PD developed in this work can predict the reaction rate constant very well with an AAD of 0.8%.

Critical evaluation and rate constants of chemoselective ligation reactions for stoichiometric conjugations in water.

Chemoselective ligation reactions have contributed immensely to the development of organic synthesis and chemical biology. However, the ligation of stoichiometric amounts of large molecules for applications such as protein-protein conjugates is still challenging. Conjugation reactions need to be fast enough to proceed under dilute conditions and chemoselective in the presence of unprotected functional groups; the starting materials and products must be stable under the reaction conditions. To compare known ligation reactions for their suitability under these conditions, we determined the second-order rate constants of ligation reactions using peptide substrates with unprotected functional groups. The reaction conditions, the chemoselectivity of the reactions, and the stability of the starting materials and products were carefully evaluated. In some cases, the stability could be improved by modifying the substrate structure. These data obtained under the ligation conditions provide a useful guide to choose an appropriate ligation reaction for synthesis of large molecules by covalent ligation reactions of unprotected substrates in water.

Reaction Kinetics of Carbon Dioxide (CO2) Absorption in Sodium Salts of Taurine and Proline Using a Stopped-Flow Technique

The pseudo–first-order reaction rate constants (k0, s−1) for the reaction of carbon dioxide in aqueous solutions of sodium taurate (NaTau) and sodium prolinate (NaPr) were measured using a stopped-flow technique at a temperature range of 298–313 K. The solutions concentration varied from 5 to 50 mol m−3 and from 4 to 12 mol m−3 for NaTau and NaPr, respectively. Comparing the k0 values, aqueous NaPr was found to react very fast with CO2 as compared with the industrial standard aqueous monoethanolamine (MEA) and aqueous sodium taurate (NaTau) was found to react slower than aqueous MEA at the concentration range considered in this work. For the studied amino acid salts, the order of the reactions was found to be unity with respect to the amino acid salt concentration. Proposed reaction mechanisms such as termolecular and zwitterion reaction mechanisms for the reaction of CO2 with aqueous solutions were used for calculating the second-order reaction rate constants (k2, m3 mol−1 s−1). The formation of zwitterion during the reaction with CO2 was found to be the rate-determining step, and the deprotonation of zwitterion was instantaneous compared to the reverse reaction of zwitterion to form an amino acid salt. The contribution of water was established to be significant for the deprotonation of zwitterion. Comparing the pseudo–first-order reaction rate constants (k0, s−1) of various amino acid salts with CO2, NaPr was found to be the faster reacting amino acid salt. The activation energy for NaTau was found to be 48.1 kJ mol−1 and that of the NaPr was found to be 12 kJ mol−1. The Arrhenius expressions for the reaction between CO2 and the studied amino acid salts are

Kinetics and polarity effects in the synthesis of the ionic liquid, 1-hexyl-3-methyl-imidazolium bromide, using compressed CO2

Compressed carbon dioxide, as a solvent medium, offers unique benefits for the synthesis of ionic liquids (ILs) especially for the ability to combine reaction and separation. The second-order reaction rate constants were determined for the synthesis of 1-hexyl-3-methylimidazolium bromide ([HMIm][Br]) at 313.15K and 333.15K and 30–150bar from the reactants, 1-methylimidazole and 1-bromohexane. Two different scenarios were encountered based upon the pressure: a one-phase supercritical mixture of the reactants becoming a vapor-liquid IL/CO2 system at complete conversion; or a two-phase CO2-expanded liquid (CXL) that yielded a three-phase vapor-liquid-liquid equilibrium (VLLE) at intermediate conversions. Despite multiphase phenomena, the vast majority of the reaction occurs in the reactant-rich liquid phase. The kinetic rate constants were comparable with traditional organic solvents. The reaction rates were higher in the CXL mode than in the initially supercritical mixtures. The changes in the kinetic rate constant with pressure (CO2 composition) could be explained and quantified using Kamlet–Taft (KT) polarity parameters that were measured in the mixtures using solvatochromic probes.

Kinetics and mechanisms of reactions for hydrated electron with chlorinated benzenes in aqueous solution

The reactions between chlorinated benzenes (CBzs) and hydrated electron (eaq−) were investigated by the electron beam (EB) and laser flash photolysis (LFP) experiments. Under the EB irradiation, the effects of irradiation dose, initial concentration and the number of Cl atoms on the removal efficiencies were further examined. At 10 kGy, the removal efficiencies of mono-CB, 1,3-diCB, 1,2-diCB and 1,4-diCB were 41.2%, 87.2%, 84.0%, and 84.1%, respectively. While irradiation dose was 50 kGy, the removal efficiencies increased to 47.4%, 95.8%, 95.0%, and 95.2%, respectively. Irradiation of CBzs solutions has shown that the higher the initial concentration, the lower the percentage of CBzs removal. In addition to this, the dechlorination efficiencies of 1,2-dichlorobenzene (1,2-diCB), 1,3-dichlorobenzene (1,3-diCB) and 1,4-dichlorobenzene (1,4-diCB) were much higher than that of chlorobenzene (mono-CB). The kinetics of the reactions was achieved with nanosecond LFP. The rate constants of second-order reaction between eaq− with mono-CB, 1,2-diCB, 1,3-diCB and 1,4-diCB were (5.3±0.4) × 108, (4.76±0.1) × 109, (1.01±0.1) × 1010 and (3.29±0.2) × 109 L·mol−1·s−1, respectively. Density functional theory (DFT) calculations were performed to determine the optical properties of unstable CBzs anion radicals, and the main absorption peaks lied in the range of 300–550 nm. The primary reaction pathway of CBzs with eaq− was gradual dechlorination, and the major products were Cl− and benzene (CBzs(-Cl−)). Furthermore, biphenyl (or chlorobiphenyl) was observed during the LFP, which was probably formed by recombination of benzene radicals.

Photochemical generation and kinetic studies of a putative porphyrin-ruthenium(V)-oxo species.

Photo-disproportionation of a bis-porphyrin-diruthenium(IV) μ-oxo dimer gave a porphyrin-ruthenium(III) species and a putative porphyrin-ruthenium(V)-oxo species that can be detected and studied in real time via laser flash photolysis methods. As determined by its spectral and kinetic behavior, the same oxo transient was also formed by photolysis of a porphyrin-ruthenium(III) N-oxide adduct. Second-order rate constants for reactions with several substrates at 22 °C were determined; representative values of rate constants were kox = 6.6 × 10(3) M(-1) s(-1) for diphenylmethanol, kox = 2.5 × 10(3) M(-1) s(-1) for styrene, and kox = 1.8 × 10(3) M(-1) s(-1) for cyclohexene. The putative porphyrin-ruthenium(V)-oxo transient reacted 5-6 orders of magnitude faster than the corresponding trans-dioxoruthenium(VI) porphyrins, and the rate constants obtained in this work were similar to those of the corrole-iron(V)-oxo derivative. The high reactivity for the photochemically generated ruthenium-oxo species in comparison to other porphyrin-metal-oxo intermediates suggests that it is a true ruthenium(V)-oxo species.

APEX (Aqueous Photochemistry of Environmentally occurring Xenobiotics): a free software tool to predict the kinetics of photochemical processes in surface waters

The APEX software predicts the photochemical transformation kinetics of xenobiotics in surface waters as a function of: photoreactivity parameters (direct photolysis quantum yield and second-order reaction rate constants with transient species, namely ˙OH, CO₃(-)˙, (1)O₂ and the triplet states of chromophoric dissolved organic matter, (3)CDOM*), water chemistry (nitrate, nitrite, bicarbonate, carbonate, bromide and dissolved organic carbon, DOC), and water depth (more specifically, the optical path length of sunlight in water). It applies to well-mixed surface water layers, including the epilimnion of stratified lakes, and the output data are average values over the considered water column. Based on intermediate formation yields from the parent compound via the different photochemical pathways, the software can also predict intermediate formation kinetics and overall yield. APEX is based on a photochemical model that has been validated against available field data of pollutant phototransformation, with good agreement between model predictions and field results. The APEX software makes allowance for different levels of knowledge of a photochemical system. For instance, the absorption spectrum of surface water can be used if known, or otherwise it can be modelled from the values of DOC. Also the direct photolysis quantum yield can be entered as a detailed wavelength trend, as a single value (constant or average), or it can be defined as a variable if unknown. APEX is based on the free software Octave. Additional applications are provided within APEX to assess the σ-level uncertainty of the results and the seasonal trend of photochemical processes.

The Kinetic Effect of Adding Piperazine Activator to Aqueous Tertiary and Sterically-hindered Amines Using Stopped-flow Technique

Abstract Aqueous amines as the solvents for CO 2 removal in low concentration streams have been exhaustively studied. This work focuses on the blending of piperazine (PZ) activator with aqueous tertiary amines (Methyldiethanolamine, MDEA) across concentration range of 200-900 mol/m3 for MDEA and 5-41 mol/m3 for PZ and sterically-hindered amines (2-amino-methyl-1-propanol, AMP) across concentration range of 6-24 mol/m3 for AMP and 7-48 mol/m3 for PZ at temperature range of 298-313 K using stopped-flow technique to determine the direct pseudo first-order reaction kinetics of the blends. The different proposed reaction mechanisms such as base-catalyzed hydration, zwitterion and termolecular reaction mechanisms for the reaction of CO 2 with aqueous solutions of amines were used to calculate the second-order reaction rate constants k 2 , (m3·mol-1s-1). Zwitterion mechanism correlated the stand-alone AMP perfectly while base catalysis mechanism correlated the stand-alone MDEA successfully. Hybrid of zwitterion mechanism correlated the blend of AMP/PZ while a hybrid of zwitterion and base catalysis mechanisms correlated the experimental data of MDEA/PZ system.

Development of a Flow-injection Spin-trapping ESR System for Estimation of Second-order Rate Constants for Reactions of a Superoxide Radical Anion and Selected Phenolic Compounds

Abstract A flow-injection ESR system was developed for the quantitative detection of a superoxide radical anion (O2•−), which was generated by the riboflavin-dependent photoreduction of oxygen. This system was applied for estimation of second-order rate constants (ks) of the reactions between the O2•− and selected phenolic compounds, such as caffeic acid, chlorogenic acid, rutin, trolox, vanillic acid, and 4-hydroxycinnamic acid.

Evaluation of the kinetic oxidation of aqueous volatile organic compounds by permanganate

The use of permanganate solutions for in-situ chemical oxidation (ISCO) is a well-established groundwater remediation technology, particularly for targeting chlorinated ethenes. The kinetics of oxidation reactions is an important ISCO remediation design aspect that affects the efficiency and oxidant persistence. The overall rate of the ISCO reaction between oxidant and contaminant is typically described using a second-order kinetic model while the second-order rate constant is determined experimentally by means of a pseudo first order approach. However, earlier studies of chlorinated hydrocarbons have yielded a wide range of values for the second-order rate constants. Also, there is limited insight in the kinetics of permanganate reactions with fuel-derived groundwater contaminants such as toluene and ethanol. In this study, batch experiments were carried out to investigate and compare the oxidation kinetics of aqueous trichloroethylene (TCE), ethanol, and toluene in an aqueous potassium permanganate solution. The overall second-order rate constants were determined directly by fitting a second-order model to the data, instead of typically using the pseudo-first-order approach. The second-order reaction rate constants (M−1s−1) for TCE, toluene, and ethanol were 8.0×10−1, 2.5×10−4, and 6.5×10−4, respectively. Results showed that the inappropriate use of the pseudo-first-order approach in several previous studies produced biased estimates of the second-order rate constants. In our study, this error was expressed as a function of the extent (P/N) in which the reactant concentrations deviated from the stoichiometric ratio of each oxidation reaction. The error associated with the inappropriate use of the pseudo-first-order approach is negatively correlated with the P/N ratio and reached up to 25% of the estimated second-order rate constant in some previous studies of TCE oxidation. Based on our results, a similar relation is valid for the other volatile organic compounds studied.