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Reaction Rate (reaction + rate)
Kinds of Reaction Rate Terms modified by Reaction Rate Selected AbstractsMesityltriphenylbismuthonium Tetrafluoroborate as an Efficient Bismuth(V) Oxidant: Remarkable Steric Effects on Reaction Rates and Chemoselectivities in Alcohol Oxidation.CHEMINFORM, Issue 31 2007Yoshihiro Matano Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF. [source] Elucidation of the mechanism and end products of glutaraldehyde crosslinking reaction by X-ray structure analysisBIOTECHNOLOGY & BIOENGINEERING, Issue 3 2007Yariv Wine Abstract Glutaraldehyde has been used for several decades as an effective crosslinking agent for many applications including sample fixation for microscopy, enzyme and cell immobilization, and stabilization of protein crystals. Despite of its common use as a crosslinking agent, the mechanism and chemistry involved in glutaraldehyde crosslinking reaction is not yet fully understood. Here we describe feasibility study and results obtained from a new approach to investigate the process of protein crystals stabilization by glutaraldehyde crosslinking. It involves exposure of a model protein crystal (Lysozyme) to glutaraldehyde in alkaline or acidic pH for different incubation periods and reaction arrest by medium exchange with crystallization medium to remove unbound glutaraldehyde. The crystals were subsequently incubated in diluted buffer affecting dissolution of un-crosslinked crystals. Samples from the resulting solution were subjected to protein composition analysis by gel electrophoresis and mass spectroscopy while crosslinked, dissolution resistant crystals were subjected to high resolution X-ray structural analysis. Data from gel electrophoresis indicated that the crosslinking process starts at specific preferable crosslinking site by lysozyme dimer formation, for both acidic and alkaline pH values. These dimer formations were followed by trimer and tetramer formations leading eventually to dissolution resistant crystals. The crosslinking initiation site and the end products obtained from glutaraldehyde crosslinking in both pH ranges resulted from reactions between lysine residues of neighboring protein molecules and the polymeric form of glutaraldehyde. Reaction rate was much faster at alkaline pH. Different reaction end products, indicating different reaction mechanisms, were identified for crosslinking taking place under alkaline or acidic conditions. Biotechnol. Bioeng. 2007;98:711,718. © 2007 Wiley Periodicals, Inc. [source] Nitrate Removal Rates in a 15-Year-Old Permeable Reactive Barrier Treating Septic System NitrateGROUND WATER MONITORING & REMEDIATION, Issue 3 2008W.D. Robertson Permeable reactive barriers (PRBs) have gained popularity in recent years as a low-cost method for ground water remediation. However, their cost advantage usually requires that these barriers remain maintenance free for a number of years after installation. In this study, sediment cores were retrieved from a pilot-scale PRB consisting of a sand and wood particle (sawdust) mixture that has been in continuous operation for 15 years treating nitrate from a septic system plume in southern Ontario (Long Point site). Reaction rates for the 15-year-old media were measured in dynamic flow column tests and were compared to rates measured in year 1 using the same reactive mixture. Nitrate removal rates in the 15-year-old media varied, as expected, with temperature in the range of 0.22 to 1.1 mg N/L/d at 6 °C to 10 °C to 3.5 to 6.0 mg N/L/d at 20 °C to 22 °C. The latter rates remained within about 50% of the year 1 rates (10.2 ± 2.7 mg N/L/d at 22 °C). Near the end of the year 15 column test, media particles >0.5 mm in diameter, containing most of the wood particles, were removed from the reactive media by sieving. Nitrate removal subsequently declined by about 80%, indicating that the wood particles were the principal energy source for denitrification. This example shows that some denitrifying PRBs can remain maintenance free and be adequately reactive for decades. [source] Pyrazole formation: Examination of kinetics, substituent effects, and mechanistic pathways,INTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 7 2008Joseph C. Sloop Reaction kinetics for the condensation of 1,3-diketones 1a,o with selected arylhydrazines (aryl = Ph, 4-NO2Ph, 4-CH3OPh, and 2,4-diNO2Ph) was studied using 19F NMR spectroscopy. Product regioselectivity is modulated by reactant ratios, substituents, and acidity. Reaction rates were found to be influenced by substituents on the diketones and on phenylhydrazines as well as by acidity of the reaction medium with rates varying as much as 1000-fold. Hammett , values for these cyclizations were determined. The reaction was found to be first order in both the diketone and arylhydrazine. The rate-determining step for pyrazole formation shifts as a function of pH. Mechanistic details and reaction pathways supporting these findings are proposed. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 40: 370,383, 2008 [source] Kinetics of the thermal isomerization of 1,1,2,2-tetramethylcyclopropaneINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 8 2006David K. Lewis Reaction rates for the structural isomerization of 1,1,2,2-tetramethylcyclopropane to 2,4-dimethyl-2-pentene have been measured over a wide temperature range, 672,750 K in a static reactor and 1000,1120 K in a single-pulse shock tube. The combined data from the two temperature regions give Arrhenius parameters Ea=64.7 (±0.5) kcal/mol and log10(A, s,1) = 15.47 (±0.13). These values lie at the upper end of the ranges of Ea and log A values (62.2,64.7 kcal/mol and 14.82,15.55, respectively) obtained from three previous experimental studies, each of which covered a narrower temperature range. The previously noted trend toward lower Ea values for structural isomerization of methylcyclopropanes as methyl substitution increases extends only through the dimethylcyclopropanes (1,1- and 1,2-); Ea then appears to increase with further methyl substitution. In contrast, the pre-exponential factors for isomerization of cyclopropane and all of the methylcyclopropanes through tetramethylcyclopropane lie within ±0.3 of log10(A, s,1) = 15.2 and show no particular trend with increasing substitution. © 2006 Wiley Periodicals, Inc. Int J Chem Kinet 38: 483,488, 2006 [source] A paradigm for the mechanisms and products of spontaneous polymerizationsJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 24 2009H. K. Hall Jr. Abstract In spontaneous vinyl and ring-opening copolymerizations, polar and resonance effects on the intermediates from bond-forming initiation offer a continuous spectrum of reactivities and polymer structures. In bond-forming initiation, an electron-rich donor monomer forms a bond to an acceptor monomer. The donor monomer may be a vinyl monomer with O, N, or aryl substituent or it may be an aza- or oxacycle. The acceptor monomer may be a vinyl monomer carrying CN, COOR, or SO2R substituent or it may be a cyclic anhydride or maleimide. Beyond this, the donor may have a ,-like strained single bond, whereas the acceptor may be an electrophilic quinodimethane. Lewis acids may be used to enhance the electrophilicity of acceptor monomers. Reaction rates and polymer composition are determined by systematically varying the stability of the first intermediate, designated P (for polymethylene). The nature of the intermediate will vary from a highly reactive trans biradical, which initiates chain alternating copolymerization, to a cis/gauche zwitterion, which can initiate chain ionic homopolymerization, to an extremely stabilized zwitterion, which cannot add monomer, but builds up in concentration and terminates by combination, forming alternating copolymer. This model embraces the existing literature for a wide variety of monomers and possesses predictive power. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2009 [source] Starch- graft -(synthetic copolymer) latexes initiated with Ce4+ and stabilized by amylopectinJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 18 2007Hank De Bruyn Abstract A method is presented for synthesizing surfactant-free latexes comprising starch- graft -(vinyl polymer) starting with a suspension of amylopectin, either native or modified, then using cerium(IV) with either potassium persulfate or glucose to create grafting sites on the starch. Latex particles comprising polystyrene, poly(styrene- co -(n -butyl acrylate)) and poly(vinyl acetate) grafted onto high molecular weight amylopectin were developed, with up to 80% of the starch effectively grafted to the particles. These latexes were colloidally stable against electrolyte (several months in 4 M NaCl). Reaction rates of Ce4+ with simple sugars and polysaccharides were investigated, as well as the gelation mechanism of the latex. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 4185,4192, 2007 [source] Monitoring epoxy and unsaturated polyester reactions under pressure,Reaction rates and mechanical propertiesPOLYMER ENGINEERING & SCIENCE, Issue 11 2009Javier C. Cruz The effects of pressure on reaction rates and final mechanical properties were studied for an unsaturated polyester (UP) and epoxy resin. A pressure chamber where reactions can be monitored by use of Raman spectroscopy has been built for these purposes. The chamber allows for pressures up to 13.8 MPa at 200°C. An advanced temperature control system has been adapted to the vessel to precisely control and monitor sample temperature variations and overshoots. It is described how for an accelerated UP reaction increasing pressure will result in a competing effect on the reaction rate where the rate will initially lower, but with increased pressures it may accelerate due to acceleration of the reaction rate constants. The final mechanical properties exhibit a similar behavior slightly increasing with pressure but lowering as pressure is raised further. For epoxy, it was shown that the reaction kinetics were accelerated by pressure although no mechanical property differences could be noted for the pressure ranges tested. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers. [source] Relationship between sound pressure and reaction rate in sonochemical reactionTHE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, Issue 4 2009Yasuhiko Takuma Abstract Relationship between sound pressure in a sonochemical reactor and degradation rate of organic compounds, p -chlorophenol and p -nitrophenol, was investigated. Reaction rates were analyzed as first-order reaction. As a result, linear relationship was shown for each compound. Slope indicating the first reaction rate constant was remarkably affected by the position of the reaction flask. The sound pressure profile was measured and the relationship between the averaged pressure in a flask and the rate constant was shown. The relation was found to be remarkably different between the two. The threshold values were observed as 67kPa for p -chlorophenol and 22kPa for p -nitrophenol, respectively. From these results, it was shown that nature of compounds affects the dependency on sound pressure. On a étudié la relation entre les vitesses de dégradation des composés organiques, le p-chlorophénol et le p-nitrophénol, et la pression acoustique dans une réaction sonochimique. Les premières constantes de vitesse de réaction se sont avérées fortement influencée par la position de la cellule de réaction dans notre installation. Le profil de pression acoustique dans l'installation a été mesuré et la relation entre la pression moyennée dans la cellule et la constante de vitesse a été déterminée. On a trouvé que la relation était significativement différente entre les deux composés et que les valeurs seuils étaient de 100,kPa pour le p-chlorophénol. On a montré également que la nature des composés avait une influence sur leur dépendance à la pression acoustique. [source] Oxidation of ferrous iron by Thiobacillus ferrooxidans in a full-scale rotating biological contactorENVIRONMENTAL PROGRESS & SUSTAINABLE ENERGY, Issue 4 2001L. Nikolov The performance of a full-scale rotating biological contactor used for the oxidation of ferrous iron by Thiobacillus ferrooxidans in drainage waters was studied. It has been shown that high volumetric rates, up to 2.5 g/Lh can be obtained. We also examined the effects of input ferrous iron concentration and liquid retention time on reactor performance. It has been shown that when the input iron concentration increased, volumetric reaction rate increased while substrate conversion decreased. In general, the rotating biological contractor is a promising tool for the biological treatment of acid mine drainage containing ferrous iron. [source] Noble metal-based catalysts for total oxidation of chlorinated hydrocarbonsENVIRONMENTAL PROGRESS & SUSTAINABLE ENERGY, Issue 3 2001José M. Toledo Catalytic total oxidation of selected chlorinated hydrocarbons utilizing several noble metal (Pt, Pd, Ru)-based catalysts was studied. Chlorinated hydrocarbons used were trichloroethylene, dichloromethane, and chlorobenzene, alone or mixed with other hydrocarbons, such as toluene. Catalysts tested were both commercially available, manufactured by Degussa AG, Süd-Chemie AG, Kataleuna GmbH, Chimet, Johnson Matthey, Prototech Co., etc., and new ones developed by Universities of Leiden in The Netherlands, Budapest in Hungary, and Wroclaw in Poland. Both forms of catalyst, sphere (particulate) and monolith, were used. Selection of the best catalyst(s) was made based on its activity, selectivity, and life. Apparent energies of activation for the reactions on these catalysts, using an empirical firstorder reaction rate, are also given. [source] Oxidation kinetics of pentachlorophenol by manganese dioxideENVIRONMENTAL TOXICOLOGY & CHEMISTRY, Issue 11 2006Ling Zhao Abstract This study examined the abiotic transformation kinetics of pentachlorophenol (PCP) by manganese dioxide (MnO2) at different solution chemistry and initial concentrations of PCP and MnO2. The measured PCP transformation rates were found to be on the order of 1.07 with respect to [PCP] and 0.91 and 0.87 with respect to [MnO2] and [H+], respectively. Dissolved Mn2+ and Ca2+ as background electrolytes considerably decreased the reaction rate because of their adsorption and hence blocking of active sites on MnO2 surfaces. The dechlorination number, 0.59 chloride ions per transformed PCP after a 1-h reaction, suggests that a fraction of the transformed PCP was not dechlorinated and may be coupled directly to dimeric products. Gas chromatography/ mass spectrometry and liquid chromatography/mass spectrometry/mass spectrometry techniques were used to identify two isomeric nonachlorohydroxybiphenylethers as major products and 2,3,5,6-tetrachloro-1,4-hydroquinone and tetrachlorocatechol as minor products. Product identification suggested that the reaction may include two parallel reactions to form either dimers or 2,3,5,6-tetrachloro-1,4-hydroquinone and tetrachlorocatechol via simultaneous dehydrochlorination and hydroxylation. [source] Uncommon Solvent Effect in Oxidative Addition of MeI to a New Dinuclear Platinum Complex Containing a Platina(II)cyclopentane MoietyEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 32 2008S. Jafar Hoseini Abstract The reaction of the known complex cis,cis -[Me2Pta(,-dppm)(,-SMe2)Ptb,CH2(CH2)2CcH2(Ptb,Cc)] [dppm = bis(diphenylphosphanyl)methane] with phthalazine (NN) proceeded by replacement of the labile bridging SMe2 ligand with the bidentate N-donor ligand NN to give cis,cis -[Me2Pta(,-dppm)(,-NN)Ptb,CH2(CH2)2CcH2(Ptb,Cc)] (1) as a pale red solid in good yield. The complex was fully characterized by multinuclear (1H, 31P, 195Pt) NMR spectroscopy. The subsequent reaction of complex 1 with excess MeI gave the colorless diplatinum(IV) complex [Me3Pta(,-dppm)(,-I)2Ptb{CH2(CH2)2CcH2(Ptb,Cc)}Me], in which the bridging NN ligand is replaced by bridging iodido ligands. The reddish color of complex 1, which is due to a metal-to-ligand charge transfer (MLCT) band in the visible region, was used to monitor its reaction with MeI in the solvents acetone, CH2Cl2, and benzene. The kinetic data revealed that the reactions in nonpolar benzene or slightly polar CH2Cl2 proceeded in two steps, each following a common SN2 mechanism. In the first step, MeI attacked the platina(II)cyclopentane center rather than the dimethylplatinum(II) center, because the first center is more electron-rich than the second center. In the more polar acetone, the reaction proceeded similarly, with the exception that each step was accompanied by a solvolytic reaction, which was suggested to be responsible for the unusually slower reaction rate in acetone than in benzene or CH2Cl2. Consistently, the reaction rate in the highly polar solvent CH3CN was too slow for any meaningful measurement.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008) [source] Synthetic and Mechanistic Aspects of Acid-Catalyzed Disproportionation of Dialkyl Diarylmethyl Ethers: A Combined Experimental and Theoretical StudyEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 25 2009Margherita Barbero Abstract The disproportionation reactions of various dialkyl diarylmethyl ethers have been carried out in the presence of a catalytic amount (10 mol-%) of o -benzenedisulfonimide as a Brønsted acid catalyst; the reaction conditions were mild, and the yields of the diarylmethane target products were good. The catalyst was easily recovered and purified, ready to be used in further reactions. The theoretical study confirmed that the reaction proceeds in two steps: The formation of a carbocation from the protonated ether followed by hydride transfer. Although the hydride transfer is the rate-determining step, it is the stability of the carbocation that determines the reaction rate and therefore the yields. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009) [source] Hydroxyapatite/Bioactive Glass Films Produced by a Sol,Gel Method: In Vitro BehaviorADVANCED ENGINEERING MATERIALS, Issue 11 2009Nihat C. Köseo Abstract Hydroxyapatite (HA) and HA/bioactive glass (49S) films were deposited on Si(100) substrates by a sol,gel dip-coating method. The microstructure and in vitro bioactivity of the films were investigated by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS). Polycrystalline HA and amorphous bioactive glass films were obtained after annealing at 600 and 400,°C, respectively. The crystallization temperature of HA was determined to be around 568,°C. The surfaces of the HA films were covered with an apatite layer consists of spherulites formed by nanosized needle-like aggregates after the soaking in simulated body fluid (SBF) for 10 days, while amorphous HA/bioactive glass surface was covered with larger spherical crystallites. Both XPS and EDS results obtained from HA/bioactive glass film, after soaking in SBF, showed increasing P amounts on the surface at the expense of Si. The higher density of the newly formed layer on HA/bioactive glass surface than that of the HA surface after 10 days of soaking was evidence of increased reaction rate and apatite forming ability when bioactive glass layer is present on the HA films. [source] Identification of amino acids in antiplasmin involved in its noncovalent ,lysine-binding-site'-dependent interaction with plasminFEBS JOURNAL, Issue 9 2003Haiyao Wang The lysine-binding-site-mediated interaction between plasmin and antiplasmin is of great importance for the fast rate of this reaction. It also plays an important part in regulating the fibrinolytic enzyme system. To identify structures important for its noncovalent interaction with plasmin, we constructed seven single-site mutants of antiplasmin by modifying charged amino acids in the C-terminal part of the molecule. All the variants were expressed in the Drosophila S2 cell system, purified, and shown to form stable complexes with plasmin. A kinetic evaluation revealed that two mutants of the C-terminal lysine (K452E or K452T) did not differ significantly from wild-type antiplasmin in their reactions with plasmin, in either the presence or absence of 6-aminohexanoic acid, suggesting that this C-terminal lysine is not important for this reaction. On the other hand, modification of Lys436 to Glu decreased the reaction rate about fivefold compared with wild-type. In addition, in the presence of 6-aminohexanoic acid, only a small decrease in the reaction rate was observed, suggesting that Lys436 is important for the lysine-binding-site-mediated interaction between plasmin and antiplasmin. Results from computerized molecular modelling of the C-terminal 40 amino acids support our experimental data. [source] Chemical modification of pyroclastic rock by hot water: an experimental investigation of mass transport at the fluid,solid interfaceGEOFLUIDS (ELECTRONIC), Issue 1 2009J. HARA Abstract Hydrothermal water,(pyroclastic) rock interactions were examined using flow-through experiments to deduce the effect of mass transport phenomena on the reaction process. A series of experiments were conducted over the temperature range 75,250°C, with a constant temperature for each experiment, and at saturated vapour pressure, to estimate the apparent rate constants as a function of temperature. Based on the chemistry of analysed solutions, the water,rock interaction in the experiments was controlled by diffusion from the reaction surface and by the existence of a surface layer at the rock,fluid interface, which regulated the chemical reaction rate. The reaction progress depended to a high degree on flow velocity and temperature conditions, with element abundances in the fluid significantly affected by these factors. Mass transport coefficients for diffusion from the rock surface to the bulk solution have been estimated. Ca is selectively depleted under lower temperature conditions (T < 150°C), whereas Na is greatly depleted under higher temperature conditions (T > 150°C), and K reaction rates are increased when flow velocity increases. Using these conditions, specific alkali and alkali earth cations were selectively leached from mineral surfaces. The ,surface layer' comprised a 0.5,1.8 mm boundary film on the solution side (the thickness of this layer has no dependence on chemical character) and a reaction layer. The reaction layer was composed of a Si, Al-rich cation-leached layer, whose thickness was dependent on temperature, flow velocity and reaction length. The reaction layer varied in thickness from about 10,4 to 10,7 mm under high temperature/low fluid velocity and low temperature/high fluid velocity conditions, respectively. [source] Mineral precipitation associated with vertical fault zones: the interaction of solute advection, diffusion and chemical kineticsGEOFLUIDS (ELECTRONIC), Issue 1 2007CHONGBIN ZHAO Abstract This article is concerned with chemical reactions that occur between two interacting parallel fluid flows using mixing in vertical faults as an example. Mineral precipitation associated with fluid flow in permeable fault zones results in mineralization and chemical reaction (alteration) patterns, which in turn are strongly dependent on interactions between solute advection (controlled by fluid flow rates), solute diffusion/dispersion and chemical kinetics. These interactions can be understood by simultaneously considering two dimensionless numbers, the Damköhler number and the Z -number. The Damköhler number expresses the interaction between solute advection (flow rate) and chemical kinetics, while the Z -number expresses the interaction between solute diffusion/dispersion and chemical kinetics. Based on the Damköhler and Z -numbers, two chemical equilibrium length-scales are defined, dominated by either solute advection or by solute diffusion/dispersion. For a permeable vertical fault zone and for a given solute diffusion/dispersion coefficient, there exist three possible types of chemical reaction patterns, depending on both the flow rate and the chemical reaction rate. These three types are: (i) those dominated by solute diffusion and dispersion resulting in precipitation at the lower tip of a vertical fault and as a thin sliver within the fault, (ii) those dominated by solute advection resulting in precipitation at or above the upper tip of the fault, and (iii) those in which advection and diffusion/dispersion play similar roles resulting in wide mineralization within the fault. Theoretical analysis indicates that there exists both an optimal flow rate and an optimal chemical reaction rate, such that chemical equilibrium following focusing and mixing of two fluids may be attained within the fault zone (i.e. type 3). However, for rapid and parallel flows, such as those resulting from a lithostatic pressure gradient, it is difficult for a chemical reaction to reach equilibrium within the fault zone, if the two fluids are not well mixed before entering the fault zone. Numerical examples are given to illustrate the three possible types of chemical reaction patterns. [source] Effects of Mixing Granular Iron with Sand on the Kinetics of Trichloroethylene ReductionGROUND WATER MONITORING & REMEDIATION, Issue 2 2009Erping Bi A substantial cost of granular iron permeable reactive barriers is that of the granular iron itself. Cutting the iron with sand can reduce costs, but several performance issues arise. In particular, reaction rates are expected to decline as the percentage of iron in the blend is diminished. This might occur simply as a function of iron content, or mass transfer effects may play a role in a much less predictable fashion. Column experiments were conducted to investigate the performance consequences of mixing Connelly granular iron with sand using the reduction kinetics of trichloroethylene (TCE) to quantify the changes. Five mixing ratios (i.e., 100%, 85%, 75%, 50%, and 25% of iron by weight) were studied. The experimental data showed that there is a noticeable decrease in the reaction rate when the content of sand is 25% by weight (iron mass to pore volume ratio, Fe/Vp = 3548 g/L) or greater. An analysis of the reaction kinetics, using the Langmuir-Hinshelwood rate equation, indicated that mass transfer became an apparent cause of rate loss when the iron content fell below 50% by weight (Fe/Vp = 2223 g/L). Paradoxically, there were tentative indications that TCE removal rates were higher in a 15% sand + 85% iron mixture (Fe/Vp = 4416 g/L) than they were in 100% iron (Fe/Vp = 4577 g/L). This subtle improvement in performance might be due to an increase of iron surface available for contact with TCE, due to grain packing in the sand-iron mixture. [source] Quenching of Singlet Oxygen by Tertiary Aliphatic Amines.HELVETICA CHIMICA ACTA, Issue 10 2006Products, Structural Effects on Rates Abstract A kinetic and product study of the reaction of a series of , -methyl-substituted N -methylpiperidines with thermally generated 1O2 in MeCN was carried out. It was found that as the number of , -methyl groups (Me in , -position relative to the N-atom) increases, the rate of 1O2 quenching (physical plus chemical) slightly decreases. This finding shows that, with respect to the reaction rate, steric effects are much more important than electronic effects as the latter should have produced the opposite result. The opposite outcome was instead found for the chemical quenching that leads to the N -demethylation products and N -formyl derivatives. The same trend was observed for the ratio between N -demethylation and formation of the N -formyl derivatives (NH/NCHO ratio). All these results are consistent with the mechanism reported in Scheme,1 where an exciplex is first formed that by a H-atom transfer process produces an , -amino-substituted C-radical. The latter forms the product of N -demethylation by one electron oxidation, or affords the N -formyl derivative by radical coupling (Scheme,1). Similar results were obtained with N,N -dimethylcyclohexanamine. However, this ,acyclic' amine exhibited behaviors quite distinct from those of the N -methylpiperidines series, with respect to reaction rate, extent of chemical quenching, and NH/NCHO ratio. [source] Kinetics and Mechanism of the Demetallation of Macrocyclic Nickel(II) Complexes by CyanideHELVETICA CHIMICA ACTA, Issue 3 2005Liselotte Siegfried The kinetics and the mechanism of the cyanide-induced demetallation of a series of Ni2+ complexes with macrocyclic ligands of different ring size (12- to 14-membered; see 1,4) and steric constraints was studied. Although the rates differ by almost five orders of magnitude when compared to each other under fixed experimental conditions (pH,10.5, [CN,]=10,2,M), all reactions proceed through the relatively rapid formation of cyano adducts [Ni(CN)nL] (n=1, 2), which then react with additional CN, or HCN to give the final products. Of paramount importance for the reaction rate is the geometry and configuration of the cyano adducts [Ni(CN)nL] (n=1,2). cis -Dicyano derivatives with a folded macrocycle react faster than trans -compounds. In the case of (1,4,8,11-tetraazacyclotetradecane)nickel(2+) ([Ni (4)]2+), which gives a trans- dicyano adduct, the base-catalyzed N-inversion necessary to obtain the cis- dicyano derivative becomes rate determining at high CN, concentrations. [source] Denitrification in a hyporheic riparian zone controlled by river regulation in the Seine river basin (France)HYDROLOGICAL PROCESSES, Issue 5 2009F. Curie Abstract The purpose of this paper is to study denitrification and the conditions for its development in a hyporheic zone. The study site is the riparian zone of a former branch of the Seine River, where the river stage is kept almost constant during the year by hydraulic regulation. Hydrological and geochemical surveys were performed by monitoring four wells, ten shorter piezometers and the river over a 15-month period. The water fluxes originating from the chalky hillsides and the river converge in a zone parallel to the river that acts as a drainage flow path through the floodplain. The riparian zone between this flow path and the river shows an important depletion of nitrate during the summer and autumn period, which cannot be explained by a simple mixing of waters coming from the river and the chalky hillsides. It can be attributed to denitrification as it occurs when oxygen concentration is below 2 mg l,1, and goes along with a consumption of dissolved organic carbon and a decrease of redox potential. The river completely controls these hydro-geochemical conditions. It also keeps the wetness of the riparian zone almost constant, which allowed us to isolate the high temperatures in summer and autumn as an important triggering factor for denitrification through its influence on the reaction rate and oxygen deficits. We also found a small isotopic enrichment of nitrate, suggesting that denitrification occurs after diffusion of nitrate through the sediment and riparian zone matrix, which is consistent with the hyporheic functioning of the study site. Copyright © 2008 John Wiley & Sons, Ltd. [source] Rate constants for the gas-phase reactions of nitrate radicals with geraniol, citronellol, and dihydromyrcenol,INTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 11 2010Joel C. Harrison Terpenes and terpene alcohols are prevalent compounds found in a wide variety of consumer products including soaps, flavorings, perfumes, and air fresheners used in the indoor environment. Knowing the reaction rate of these chemicals with the nitrate radical is an important factor in determining their fate indoors. In this study, the bimolecular rate constants of k (16.6 ± 4.2) × 10,12, k (12.1 ± 3) × 10,12, and k (2.3 ± 0.6) × 10,14 cm3 molecule,1 s,1 were measured using the relative rate technique for the reaction of the nitrate radical (NO3,) with 2,6-dimethyl-2,6-octadien-8-ol (geraniol), 3,7-dimethyl-6-octen-1-ol (citronellol), and 2,6-dimethyl-7-octen-2-ol (dihydromyrcenol) at (297 ± 3) K and 1 atmosphere total pressure. Using the geraniol, citronellol, or dihydromyrcenol + NO3, rate constants reported here, pseudo-first-order rate lifetimes (k,) of 1.5, 1.1, and 0.002 h,1 were determined, respectively. © 2010 Wiley Periodicals, Inc. Int J Chem Kinet 42: 669,675, 2010 [source] Kinetic study of hydroperoxide epoxidation of 1-octene in the presence of molybdenum disilicide as a catalyst in a low concentration range of olefinINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 10 2009Yuriy B. Trach The kinetic regularities of the influence of low 1-octene concentrations on its epoxidation by tert -butyl hydroperoxide in the presence of molybdenum disilicide (MoSi2) as a catalyst were investigated. The minimum in the dependence of the initial rate of hydroperoxide consumption on 1-octene concentration was observed. The kinetic scheme of epoxidation, which includes the competition between hydroperoxide and olefin for the catalytic active centers, was proposed. The equation for the reaction rate was derived according to the kinetic scheme. The kinetic parameters of epoxidation were calculated. © 2009 Wiley Periodicals, Inc. Int J Chem Kinet 41: 623,628, 2009 [source] Mechanism of oxidation of alanine by chloroaurate(III) complexes in acid medium: Kinetics of the rate processesINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 7 2009Pratik K. Sen The kinetics of the oxidation of alanine by chloroaurate(III) complexes in acetate buffer medium has been investigated. The major oxidation product of alanine has been identified as acetaldehyde by 1H NMR spectroscopy. Under the experimental conditions, AuCl and AuCl3(OH), are the effective oxidizing species of gold(III). The reaction is first order with respect to Au(III) as well as alanine. The effects of H+ and Cl, on the second-order rate constant k2, have been analyzed, and accordingly the rate law has been deduced: k2, = (k1[H+][Cl,] + k3K4K5)/(K4K5 + [H+][Cl,]). Increasing dielectric constant of the medium has an accelerating effect on the reaction rate. Activation parameters associated with the overall reaction have been calculated. A mechanism involving the two effective oxidizing species of gold(III) and zwitterionic species of alanine, consistent with the rate law, has been proposed. © 2009 Wiley Periodicals, Inc. Int J Chem Kinet 41: 473,482, 2009 [source] Kinetic study of transesterification of methyl acetate with n -butanol catalyzed by NKC-9INTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 2 2009Baoyun Xu The transesterification of methyl acetate and n -butanol catalyzed by cation-exchange resin, NKC-9, was studied in this work to obtain the reaction kinetics. The experiments were carried out in a stirred batch reactor at different temperatures (328.15, 333.15, 338.15, 343.15, 345.15 K) under atmospheric pressure. The effects of temperature, molar ratio of reactants, and catalyst loading on the reaction rate were researched under the condition of eliminating the effect of diffusion. The experimental data were correlated with a kinetic model based on the pseudo-homogeneous catalysis. The kinetic equation describing the reaction catalyzed by NKC-9 was developed. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 41: 101,106, 2009 [source] Micellar and salt effects on the interaction of [Cu(II)-Gly-Gly]+ with ninhydrinINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 10 2007Mohd. Akram The effect of cationic micelles of cetyltrimethylammonium bromide (CTAB) on the kinetics of interaction of copper dipeptide complex [Cu(II)-Gly-Gly]+ with ninhydrin has been studied spectrophotometrically at 70°C and pH 5.0. The reaction follows first- and fractional-order kinetics, respectively, in complex and ninhydrin. The reaction is catalyzed by CTAB micelles, and the maximum rate enhancement is about twofold. The results obtained in the micellar medium are treated quantitatively in terms of the kinetic pseudophase and Piszkiewicz models. The rate constants (kobs or k,), micellar-binding constants (kS for [Cu(II)-Gly-Gly]+, kN for ninhydrin), and index of cooperativity (n) have been evaluated. A mechanism is proposed in accordance with the experimental results. The influence of different inorganic (NaCl, NaBr, Na2SO4) and organic (NaBenz, NaSal) salts on the reaction rate has also been seen, and it is found that tightly bound/incorporated counterions are the most effective. © 2007 Wiley Periodicals, Inc. Int J Chem Kinet 39: 556,564, 2007 [source] Effects of NO2, CO, O2, and SO2 on oxidation kinetics of NO over Pt-WO3/TiO2 catalyst for fast selective catalytic reduction processINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 10 2006Muhammad Faisal Irfan The selective catalytic reduction rate of NO with N-containing reducing agents can be enhanced considerably by converting a part of NO into NO2. The enhanced reaction rate is more pronounced at lower temperatures by using an equimolar mixture of NO and NO2. The kinetics of NO oxidation over Pt-WO3/TiO2 catalyst has been determined in a fixed-bed reactor with different concentrations of oxygen, nitric oxide, and nitrogen dioxide in the presence of 8% water. It has been found that the reaction is second order with respect to nitric oxide, first order for oxygen with a third-order rate constant. Also, it is found that there is no effect on the reaction order with an addition of NO2, CO, or SO2. It follows the same second order but the reaction rate is found to be changed. It is observed that in the case of NO2 and SO2, the reaction rate tends to decrease, but it increases with the addition of CO into the feed. © 2006 Wiley Periodicals, Inc. Int J Chem Kinet 38: 613,620, 2006 [source] Kinetics of Fe(II) reduction of cis -halogeno(dodecylamine) bis(ethylenediamine)- cobalt(III) complex ion in aqueous solutionsINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 2 2006K. Santhakumar Kinetics of reduction of the surfactant complex ions, cis -chloro/bromo (dodecylamine)bis(ethylenediamine)cobalt(III) by iron(II) in aqueous solution was studied at 303, 308, and 313 K by spectrophotometry method under pseudo-first-order conditions, using an excess of the reductant. The second-order rate constant remains constant below critical micelle concentration (cmc), but increases with cobalt(III) concentration above cmc, and the presence of aggregation of the complex itself alters the reaction rate. The rate of reaction was not affected by the added [H+]. Variation of ionic strength (,) influences the reaction rate. Activation and thermodynamic parameters have been computed. It is suggested that the reaction of Fe2+ (aq) with cobalt(III) complex proceeds by the inner-sphere mechanism. © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 38: 98,105, 2006 [source] Effect of surfactant micelles on the kinetics of oxidation of D -fructose by cerium(IV) in sulfuric acid mediumINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 1 2006Kabir-ud-Din Kinetics of the oxidation of D -fructose by cerium(IV) has been investigated both in the absence and presence of surfactants (cetyltrimethylammonium bromide, CTAB, and sodium dodecyl sulfate, SDS) in sulfuric acid medium. The reaction exhibits first-order kinetics each in [cerium(IV)] and [D -fructose] and inverse first order in [H2SO4]. The Arrhenius equation is found to be valid for the reaction between 30,50°C. A detailed mechanism with the associated reaction kinetics is presented and discussed. While SDS has no effect, CTAB increases the reaction rate with the same kinetic behavior in its presence. The catalytic role of CTAB micelles is discussed in terms of the pseudophase model proposed by Menger and Portnoy. The association constant Ks that equals to 286 mol,1 dm3 is found for the association of cerium(IV) with the positive head group of CTAB micelles. The effect of inorganic electrolytes (Na2SO4, NaNO3, NaCl) has also been studied and discussed. © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 38: 18,25, 2006 [source] | |||||||||||||||||||||||||||||||||||||||||||