The effect of the mixing process on reaction kinetics

If the chemical rate constants of the initial reaction steps are large, any observed value of these constants may be affected by the mixing process. To investigate the effect of mixing efficiency on reaction kinetics we use two previously analyzed reaction systems, namely the glutamic aspartic aminotransferase system and the hemoglobin/O₂ system. A time constant k_a for the mixing process is introduced and four values for this parameter are considered, with the fastest essentially corresponding to complete mixing before any reaction can take place. The remaining three values are associated with mixing instruments, since they may be attainable in the real world. To evaluate the mixing efficiency (and conversely determine underlying k_a values) from observed kinetics, mixing was combined with chemical relaxation by temperature jumps initiated at different points in time during the mixing process. Computer simulations permit the development of specific criteria to determine (in reverse) appropriate values of k_a. In this respect, the aminotransferase system proved to be more revealing than the hemoglobin system. © 2003 Wiley Periodicals, Inc. Int J Chem Kinet 35: 484–496, 2003     

Trimerization products of trifluoroacetone: critical solvent effect on position and kinetics of anomeric equilibria

In the presence of bases, trifluoroacetone is known to trimerize leading to configurationally labile 6-methyl-2,4,6-tris(trifluoromethyl)tetrahydro-2H-pyran-2,4-diols 1a,b and 2a,b, structurally close to fluorinated carbohydrates. We report herein a complete study of their behavior in solution. The remarkable solvent effect on the two equilibria (1a <--> 1b; 2a <--> 2b) was rationalized using solvent basicity measures and polarity scales. Solvents of weak donor number were found to favor the diastereoisomers 1a and 2a, which were subsequently isolated. According to their X-ray analyses, they both possess a concave structure with 1,3-cis-diaxial hydroxyl groups. A complementary kinetic study illustrated that acidic conditions can drastically reduce the equilibration rate, allowing the use of a wide range of solvents. Finally, a reexamination of previously published trimerization conditions using sodium or magnesium amalgam revealed that, contrary to the suggestion by two independent reports, 1,3,5-tris(trifluoromethyl)cyclohexane-1,3,5-triol 3/4 was neither formed as the principal product in place of 1a,b and 2a,b nor could it be detected as a minor product.     

Mechanochemistry and co-crystal formation: effect of solvent on reaction kinetics

We demonstrate that significant improvements in kinetics of co-crystal formation by grinding can be achieved by the addition of minor amounts of appropriate solvent.     

The effect of electrolytes on the reaction rates and acid-base equilibria in ionic micelles

The influence of a number of electrolytes on the micellar effect of cetyltrimethylammoniurn bromide (CTAB) in the hydrolysis ofp-nitrophenyl acetate (1) and bis(p-nitrophenyl) methylphosphonate (2) and in the course of the acid-base dissociation of thep-nitroanilide of bis(chloromethyl)phosphinic acid (3) has been examined. The activity of the salts studied increases in the following order: MeCOOK2CO₃3<p-MeC₆H₄SO₃K. It has been found that in the presence of electrolytes the catalytic effect of CTAB micelles in the hydrolysis reactions of esters1 and2 decreases, and the pK _a value of anilide3 increases. The results obtained are interpreted in terms of the pseudophase model of micellar catalysis. The analysis of the experimental data carried out using logarithmic coordinates revealed a relationship between phase transitions in micellar catalysis and in micellization.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1396–1400, August, 1993.     

Studies on ion exchange equilibria and kinetics

The kinetics and equilibria of cation exchange HR+Na⁺=NaR+H⁺ with 001×7 strong acidic resin have been studied. The isotherm for this reaction has been determined, and the rational equilibrium constant, Ka, calculated. It is found experimentally that at low concentration, the reverse exchange RNa/H⁺ is faster than the forward exchange; on the contrary, at high concentration, the forward exchange is faster than the reverse exchange. The integral particle interdiffusion coefficients have been calculated and the film interdiffusion coefficients estimated.     

Studies on ion exchange equilibria and kinetics

¹⁶⁰Tb was used as radiotracer and the equilibria and kinetics of cation exchange with zeolite-Y were studied. The ion exchange isothems and the Kielland plots at 298 and 303 K were obtained. It was found that the ion exchange rate is controlled by particle diffusion. The integral interdiffusion coefficients for the direct and reverse exchanges at 298 K and 303 K were calculated. In addition, the isotopic ion exchange was studied too.     

FRET-based assay of the processing reaction kinetics of stimulus-responsive peptides: influence of amino acid sequence on reaction kinetics

In the field of chemical biology, methods for controlling peptidyl function by a stimulus are attracting increasing attention. Recently, we reported a stimulus-responsive peptide, which can be cleaved after exposure to a stimulus. In this study, we developed a FRET-based assay system to estimate the kinetics of the stimulus-induced processing (peptide bond cleavage) reaction. Based on the FRET system, it was clarified that introduction of a sterically less-hindered or polar residue at the position adjacent to the stimulus-responsive amino acid accelerates the processing reaction.     

The effect of platinum oxide films on reaction kinetics at platinum electrodes

It has been known for some time that the pre-treatment of platinum electrodes often effects subsequent electrochemical reactions. Part of the effect of pre-treatment is due to the fact that anodized or chemically oxidized platinum electrodes become coated to some degree with a film of platinum oxide. This work was concerned with quantitative measurement of kinetic parameters as a function of the extent of oxide film formation. Whenever possible, variations in reaction mechanisms are proposed.

Most of the experimental evidence has been taken from current-potential curves but the techniques of chronopotentiometry and microscopy were also used.

The reduction of vanadium^V chromium^VI, arsenic^V, iodate and oxygen were investigated as well as the oxidation of vanadium^IV, arsenic^III, oxalic acid, and formic acid. The presence of the platinum oxide film effected the reactions studied in a variety of ways but in every case some variation in the kinetic parameters of the reactions studied was recorded. For a number of cases, a modified oxygen bridge theory was found useful.     

A study of the kinetics of phenol oxidation on a fibrous catalyst

Kinetics of phenol oxidation was studied. The effect of the hydrogen peroxide concentration, temperature, acidity of the medium, and catalyst mass on the oxidation rate was examined. A mathematical relation for calculating the reaction rate of phenol oxidation was derived.     

Reactive compatibilization of SAN/EPR blends: effect of the kinetics of the compatibilization reaction on the interfacial adhesion

The very poor adhesion between films of styrene and acrylonitrile random copolymer (SAN) and maleic anhydride grafted polypropylene (PP‐g‐MA) can be dramatically improved by an intermediate thin layer of SAN bearing groups reactive toward maleic anhydride. The rate of the interfacial reaction, which is controlled by the reactive groups attached to SAN (amine vs. carbamate) and by the method used to build up the sandwich assembly, has a decisive effect on the capability of the SAN‐g‐PP graft copolymer formed at the interface to improve the fracture toughness in direct dependence on its molecular architecture.     

Trapping of a cycloheptatetraene in the reaction of atomic carbon with phenol

Reaction of atomic carbon with phenol generates tropone in a reaction postulated to proceed via the hydroxycycloheptatetraenes, which rearrange to tropone. When the hydroxyphenylcarbenes are generated by the C atom deoxygenation of the corresponding aldehydes, the meta and para isomers produce tropone; the ortho isomer does not.     

Wittig-Horner反应的动力学研究

Ｗｉｔｔｉｇ Ｈｏｒｎｅｒ反应是Ｗｉｔｔｉｇ反应的一种改进 ,二者可以互相补充 ,但Ｗｉｔｔｉｇ Ｈｏｒｎｅｒ反应还具有某些独特的优点[1] 。Ｗｉｔｔｉｇ Ｈｏｒｎｅｒ反应用于合成的研究有较多的报道 ,但对于该反应的理论研究却不多见[2 ] 。因此 ,我们合成了α 萘甲基磷酸二丁酯 ,并进行了它同一系列取代苯甲醛反应的动力学研究。在磷酸酯及醇钠恒定过量的条件下 ,测定了几种取代苯甲醛反应的假一级速率常数值。但实际上反应属于三级 ,对磷酸酯、乙醇钠和苯甲醛各为一级。因为当改变各反应组份的浓度时 ,速率发生改变。一般Ｗｉｔｔ…     

Positive equilibria of a class of power-law kinetics

This paper studies a class of power-law kinetics, PL-ILK, for whose subset, PL-TIK, analogues of the Deficiency Zero Theorem and the Deficiency One Theorem (DOT) for mass action systems are valid. The DOT also includes the necessary and sufficient condition of Boros for uniqueness in the non-weakly reversible case. To our knowledge, this is the first set of kinetics beyond mass action kinetics (MAK) for which the DOT has been shown to be valid. A further interesting property of PL-TIK is a certain “robustness” relative to dependence of linkage classes: existence of a positive equilibrium for each linkage class implies the existence of a positive equilibrium for the whole network. For MAK systems, the PL-ILK property is equivalent to the reactant deficiency of the linkage class containing the zero complex being one, and zero for all other linkage classes. As shown in the Supplementary Materials, an initial survey of MAK and BST systems already reveals numerous examples with PL-ILK kinetics.     

Spectrophotometric determination of phenol and resorcinol by reaction with p-aminophenol

Based on the reaction with p-aminophenol, a series of procedures have been developed for the spectrophotometric determination of phenol and resorcinol. Three basic approaches have been studied: (i) a batch procedure, using the dissolved oxygen as oxidant, (ii) a stopped-flow procedure by use of KIO(4) as oxidant and (iii) a flow injection (FI) procedure developed in the presence of KIO(4). Phenol can be accurately determined at 626 nm, in the stopped-flow mode, after a reaction time of 45 min in 0.005M NaOH, 0.004M KIO(4) and 500 mug/ml of PAP. The development of a manifold, which incorporates a sample parking, is a convenient approach which makes it possible to measure, in the stopped-flow mode, four solutions in one hour. The limit of detection of this method corresponds to 64 ng/ml of phenol Resorcinol can be determined by FI at 540 nm in 0.006M NaOH, 0.0002M KIO(4) and 50 mug/ml PAP with a limit of detection of 6.6 ng/ml and a sample throughput of 300 injections per hour. A combination between the FI procedure for the determination of resorcinol and the stopped-flow procedure for phenol determination provides accurate results in the analysis of spiked samples containing both phenol and resorcinol.     

Photodecomposition of phenol in a flow reactor: Adsorption and kinetics

Summary In this study, a new air lift loop photoreactor was used for both continuous and non-continuous water photodetoxification. Also, the kinetics of phenol photodegradation were measured under flow conditions. Both the kinetics of phenol adsorption on TiO₂/SiO₂ and of the total photo-mineralization of phenol were measured in order to gain more knowledge on the photocatalytic oxidation of phenol on illuminated titania.Adsorption of phenol on TiO₂/SiO₂ proceeds even from diluted solutions. However, based on our data, it is impossible to state precisely the influence of the adsorption on the overall rate of phenol photodecomposition. The photoreaction yields stable intermediates which subsequently undergo total mineralization to CO₂ and H₂O. It is postulated that the intermediates consist mainly of polymeric compounds, difficult to determine by conventional analytical methods like GC, HPLC, or UV/Vis spectroscopy.

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Photolyse von Phenol in einem Durchflußreaktor: Adsorption und Kinetik

Zusammenfassung Ein neuartiger Photoreaktor wurde auf seine Verwendbarkeit zur kontinuierlichen und nichtkontinuierlichen Wasseraufbereitung getestet. Die Kinetik des photolytischen Abbaus von Phenol wurde unter Durchflußbedingungen untersucht. Zum besseren Verständnis der photokatalysierten Oxidation von Phenol auf belichtetem Titanoxid wurde außerdem die Kinetik sowohl der Adsorption von Phenol an TiO₂/SiO₂ als auch seiner Gesamtphotomineralisierung erforscht.Adsorption von Phenol an TiO₂/SiO₂ erfolgt bereits aus verdünnten Lösungen. Nach unseren Ergebnissen ist es allerdings unmöglich, den Einfluß der Adsorption auf die Photolyse genau zu bestimmen. Die Photoreaktion ergibt stabile Zwischenprodukte, die sich im weiteren Verlauf zu CO₂ und H₂O umsetzen. Es wird postuliert, daß es sich bei den Zwischenprodukten hauptsächlich um Polymere handelt, die mit konventionellen analytischen Techniken wie GC, HPLC oder UV/Vis-Spektroskopie schwer nachzuweisen sind.

     

六氟化铀与卤化氢气体的反应动力学研究

研究了UF6+HX(HX=HCl, HBr和HI)反应动力学, 结果显示,UF6+HX反应速率随着HCl-HBr-HI次序增加, 在室温下它们的反应速率常数分别为2.32×10^-^6, 6.43×10^-^4, 5.89×10^-^3s^-^1.Pa^-^1。UF6+HCl和UF6+HBr反应的表观活化能分别为11.29和4.18kj/mol。以上反应速率依次增加, 表出活化能依次减小的趋向与HX的键能以HCl-HBr-HI次序减小相符合。