Experimental Rate Law Research Articles

Iron−Manganese Redox Processes and Synergism in the Mechanism for Manganese-Catalyzed Autoxidation of Hydrogen Sulfite

The mechanism for manganese-catalyzed aqueous autoxidation of hydrogen sulfite at pH 2.4 has been revised on the basis of previous comprehensive kinetic studies and thermodynamic data for iron-manganese redox processes and manganese(II) and -(III) protolysis equilibria. The catalytically active manganese species is concluded to be an oxo- (or hydroxo-) bridged mixed-valence complex of composition (OH)Mn(III)OMn(II)(aq) with a formation constant beta' of (3 +/- 1) x 10(4) M(-)(1) from kinetics or ca. 7 x 10(4) M(-)(1) from thermodynamics. It is formed via rapid reaction between Mn(H(2)O)(6)(2+) and hydrolyzed manganese(III) aqua hydroxo complexes, and it initiates the chain reaction via formation of a precursor complex with HSO(3)(-), within which fast bridged electron transfer from S(IV) to Mn(III) takes place, resulting in formation of chain propagating sulfite radicals, SO(3)(*)(-). The very high acidity of Mn(3+)(aq), indicating a strong bond Mn(III)-OH(2) in hydrolyzed manganese(III), makes an attack by HSO(3)(-) on substitution labile Mn(II) in the bridged complex more favorable than one directly on manganese(III). The synergistic effect observed in systems containing iron as well as manganese and the chain initiation by trace concentrations of iron(III) of ca. 5 x 10(-)(8) M can also be rationalized in terms of formation of this bridged mixed-valence dimanganese(II,III) complex. The presence of iron(III) in a Mn(II)/HSO(3)(-) system results in rapid establishment of an iron-manganese redox equilibrium, increasing the concentration of manganese(III) and of the catalytically active bridged complex. The bridged complex oxidizes HSO(3)(-) several orders of magnitude faster than does iron(III) itself. Comparison with some previous studies shows that the different experimental rate laws reported do not necessarily indicate different reaction mechanisms. Instead, they can be rationalized in terms of different rate-determining steps within the same complex chain reaction mechanism, depending on the experimental conditions used.

Kinetic and mechanistic aspects for the tartaric acid oxidation by vanadium(V) in sulfuric acid medium

Tartaric acid oxidation by vanadium(V) in sulfuric acid medium was investigated spectrophotometrically at 760 nm and 30°C by appearance of the vanadium(IV), as vanadyl. The reaction rate was determined under pseudo-first-order conditions with an excess of hydroxyacid over the oxidant concentration. The oxidation showed a first-order dependence with respect to vanadium(V) concentration and fractional orders with respect to tartaric acid and sulfuric acid concentrations, with no control and with constant ionic strength. The reaction rate is enhanced by an increase of ionic strength, and slightly reduced by a decrease of the dielectric constant of the medium. The activation parameters were calculated based on the rate constants determined in the 293 to 313 K interval. The proposed oxidation mechanisms and the derived rate laws are consistent with the experimental rate laws. © 1998 John Wiley & Sons, Inc. Int J Chem Kinet: 30: 55–61, 1998.

Stoichiometric enantioselective alkene epoxidation with a chiral dioxoruthenium(VI) D4-porphyrinato complex

A dioxoruthenium(VI) complex containing a D4-porphyrinato ligand por* {H2por* = 5,10,15,20-tetrakis[(1S,4R,5R,8S)-1,2,3,4,5,6,7,8-octahydro-1,4∶5,8-dimethanoanthracen-9-yl]porphyrin} has been prepared by oxidation of its ruthenium(II) carbonyl precursor with m-chloroperoxybenzoic acid and characterised by spectroscopic methods. The [RuVI(por*)O2] complex undergoes enantioselective epoxidation of alkenes and the highest enantiomeric excess (ee) attainable is 77%. In the presence of pyrazole the complex transforms to [RuIV(por*)(pz)2] when reacting with alkenes. The kinetics of the epoxidation of para-substituted styrenes has been studied. The experimental rate law is –d[RuVI]/dt = k2[RuVI][alkene]. The second order rate constants k2 at 25 °C fall in a narrow range, 2.1 × 10–3–9.7 × 10–3 dm3 mol–1 s–1. Comparison of the Hammett plot (log krelvs. σ˙) with those for achiral analogues [RuVI(tpp)O2] (H2tpp = 5,10,15,20-tetraphenylporphyrin) and [RuVI(oep)O2] (H2oep = 2,3,7,8,12,13,17,18-octaethylporphyrin) suggests the formation of a radical intermediate for the alkene epoxidations. Both [RuII(por*)(CO)(EtOH)] and [RuVI(por*)O2] were examined for enantioselective catalysis. Enantioselectivities of the stoichiometric and catalytic reactions showed good correlation. There is no solvent dependence on enantioselectivity when changing the solvent from dichloromethane to benzene.

MECHANISTIC STUDIES OF OXIDATION OF DIPHENYLMETHANOLS BY SODIUMN-CHLOROBENZENESULPHONAMIDE CATALYSED BY RUTHENIUM(III)

The kinetics of oxidation of six para-substituted diphenyhmethanols (Y-DPM, where Y = H, CI, Br, NO2, CH3 and OCH3) by sodium N-chlorobenzenesulphonamide [chloramine-B (CAB)I In the presence of HCl and catalysed by RuCl3 In 30% (v/v) methanol medium was studied at 35 degrees C, The experimental rate law is rate=k'[CAB](0)[DPM](0)(x)[RuCl3](y)[H+](z), where x, y and z are fractions, Addition of reaction product, benzenesulphonamide (BSA), retards the reaction. An increase in the dielectic constant of the medium decreases the rate, Rate studies in D2O medium showed that the solvent isotope effect k'(H2O)/k(D2O)=0.53, Proton inventory studies were carried out using H2O-D2O mixtures, The rates correlate satisfactorily with the Hammett a relationship and the plot is biphasic, The reaction constant rho is - 2.8 for electron-releasing groups and -0.31 for electron-withdrawing groups at 35 degrees C, The activation parameters Delta H-double dagger, Delta S-double dagger, Delta G(double dagger) and logA were calculated, Delta H-double dagger and Delta S-double dagger are linearly related and an isokinetic relationship is observed with beta=343 Ei indicating enthalpy as a controlling factor, (C) 1997 by John Wiley & Sons, Ltd.

Oxidation of primary amines by bromamine-B catalyzed by Osmium(VIII) in alkaline medium: A kinetic and mechanistic study

The kinetics of oxidation of the aliphatic primary amines, n-propylamine, n-butylamine, and isoamylamine, by N-sodio-N-bromobenznesulfonamide or bromamine-B (BAB), in the presence of osmium(VIII), has been studied in alkaline medium at 35°C. In the presence of the catalyst, the experimental rate law for the oxidation of the amine substrate (S) takes the form, rate=k[BAB][OsO4][OH−]x, which in the absence of the catalyst changes to the form, rate=k[BAB][S][OH−]y, where x and y are less than unity. Additions of halide ions and the reduction product of BAB (benzenesulfonamide), and the variation of ionic strength of the solvent medium have no effect on the reaction rate. Activation parameters have been evaluated. The proposed mechanism assumes the formation of a complex intermediate between the active oxidant species, PhSO2NBr−, and the catalyst, OsO4, in the rate determining step. This complex then interacts with the substrate amine in fast steps to yield the end products. The average value for the deprotonation constant of monobromamine-B, forming PhSO2NBr−, is evaluated for the Os(VIII) catalyzed reactions of the three amines in alkaline medium as 9.80×103 at 35°C. The average value for the same constant for the uncatalyzed reactions is 1.02×104 at 35°C. © 1997 John Wiley & Sons, Inc.

Activation of Hydrogen Peroxide for Oxidation by Copper(II) Complexes

Copper(II) complexes of the general formula [LCu(H2O)4]n+ (where L is a bidentate ligand andn = 1 or 2) activate hydrogen peroxide for the oxidation of quinaldine blue, an oxidation indicator. The copper(II) complexes of tri- and tetradentate ligands are shown to be inactive, as are the bis-complexes of bidentate ligands. The proposed mechanism for peroxide activation involves the formation of a copper(II)-hydroperoxide complex, which then rapidly oxidized the substrate. Comparison of reaction rates with different ligand systems, and different ligand-to-metal ratios, lead to the conclusion that the most active complexes are those in which two equatorial coordination positions are occupied by easily displaced water to form the active catalyst. Rate studies are performed which give an experimental rate law which is first order in copper(II) complex, zero order in substrate, and variable order in peroxide. These kinetics are predicated by the rate law derived from our proposed mechanism. The variable order in peroxide can be explained in terms of Michaelis-Menten-type kinetics, as linear Lineweaver-Burk plots of (rate−1) vs ([O2H−]−1) are obtained from our experimental data. This is consistent with our proposed mechanism, as the derived rate law can be rearranged into the Michaelis-Menton equation.

Kinetics and mechanism of oxidation of (aqua-2-aminomethyl-pyridine) chromium(III) by N-bromosuccinimide

The kinetics of oxidation of (aqua-2-aminomethyl-pyridine) CrIII by N-bromosuccinimide (NBS) in aqueous solution to yield chromium(VI) has been studied spectrophotometrically over the 25–40 °C range. The reaction rate is first order with respect to both [NBS] and [CrIII], and increases with increasing pH between 7.6 and 8.6. The thermodynamic activation parameters were calculated. The experimental rate law is consistent with a mechanism in which the deprotonated [Cr(L)2(OH)]2+ was considered to be the most reactive form compared to its conjugate acid. It is assumed that electron transfer takes place via an inner-sphere mechanism.

On the simple Michaelis-Menten mechanism for chemical reactions

Several mathematical properties associated with the simple Michaelis-Menten mechanism for enzymatic reactions are proven. In particular it is shown that the usual interpretation of the slope of the experimental Michaelis-Menten rate law in terms of the reaction constants of the mechanism can be obtained, in the approximation in which the total concentration of the enzyme is small compared with the Michaelis-Menten constant, independently of the ratio between the total initial concentrations of the enzyme and substrate. Furthermore, the ratio of the total concentration of the enzyme to the Michaelis-Menten constant allows for the elimination of a fast variable in a singular perturbation method, yielding the Michaelis-Menten rate law as a first order approximation.

The adsorption of nickel dimethylglyoxime complex on mercury

The adsorption of the Ni(II) dimethylglyoxime complex, Ni(DMGH) 2, on mercury was studied as a function of time by voltammetric and differential capacitance methods in ammonia and borate buffers. The influence of the adsorption potential and the complex concentration was also studied. The adsorption equilibrium shows a reversible behaviour. The experimental data conform to the Frumkin adsorption isotherm. The standard electrochemical Gibbs energy of adsorption and the interaction parameter obtained from the fit indicate the presence of attractive forces between adsorbed molecules and a standard electrochemical Gibbs energy of adsorption comparable with those of pentanol and hexanol. The experimental rate of adsorption, at short times, is independent of time. It is also linearly dependent on the complex concentration and exponentially dependent on the adsorption potential. The experimental rate law can be explained with a model considering the adsorption kinetics as being controlled by the adsorption of the complex.

Kinetics and mechanisms of homogeneous catalytic reactions. Part 2. Hydrogenation of cyclohexane catalysed by [RuH(CO)-(NCMe)2(PPh3)2]BF4

Kinetic and mechanistic studies of the homogeneous hydrogenation of cyclohexane were carried out using the cationic complex [RuH(CO)(NCMe)2(PPh3)2]BF4 as the catalyst precursor, which was very efficient under mild reaction conditions in xylene solution. The experimental rate law was found to ber = {K1K2k3/([MeCN]+K1−(1+K2)[C6H10]}[Ru][C6H10][H2], which became {K2k3/(1 +K2)}[Ru][H2] at high cyclohexane concentrations. The activation parameters were calculated. The kinetic data are consistent with a mechanism involving the oxidative addition of hydrogen as the rate-determining step of the catalytic cycle.

Biphasic Synthesis of Poly(2,6-dimethyl-1,4-phenylene oxide) Using a Surface-Active Coupling Catalyst. 2. Process Improvements, Additional Kinetic Results, and Proposed Reaction Mechanism

Significant process improvements, additional kinetic findings, and a proposed mechanism are presented for a previously reported biphasic synthesis of poly(2,6-dimethyl-1,4-phenylene oxide) that avoids the need for a toxic organic solvent. A switch from magnetic stirring to the more vigorous mechanical stirring coupled with the use of an emulsifier increases the rate of formation and molecular weight of the polymer product each by more than 500%. New data indicate that the reaction has an activation energy of 14.7 kcal/mol and a first-, second-, and 0.15-order dependence on the monomer, catalyst, and oxygen concentrations, respectively. The optimal phase ratio for a fast reaction is around 1:1, and the optimal aqueous ammonia concentration is around 0.26 M. The reaction profile shows an unusual accelerating-rate feature that suggests a shifting balance between chain-growth and emulsion stabilization effects on one hand and a site competition effect at the catalyst on the other hand. A mechanism is proposed which gives a rate law in good agreement with the experimental rate law. A successful extension of the biphasic technique to several carbonylation and autoxidation reactions suggests a general utility of the technique for chemical synthesis

Kinetics and mechanism of electron-transfer in the aquaethylene-diaminetetraacetatochromium(iii)/n-bromosuccinimide reaction

The oxidation of aquaethylenediaminetetraacetatochromium(III) [Cr(HEDTA)(H2O)] with N-bromosuccinimide (NBS) to yield chromium(VI) has been studied spectrophotometrically over the 20–40° C range. The rate is first order with respect to both reactants and increases with decreasing [H+] between pH 6.0 and 6.8. The thermodynamic activation parameters were calculated. The experimental rate law is consistent with a mechanism in which the deprotonated [Cr(EDTA)(OH)]2- and protonated [Cr(EDTA)(H2O)]- are the reactive species. It is proposed that electron transfer proceeds via an inner sphere mechanism.

Thermal Stability of Zirconia as a Catalyst Support: Kinetics and Modelling

The textural changes of two zirconium dioxide powders, one containing only tetragonal crystallites and the other a mixture of monoclinic and tetragonal crystallites, were investigated. The crystallite size increase was followed by means of X-ray diffraction line broadening for powders annealed in a controlled gaseous atmosphere at 600 and 770°C, in order to establish the variations of its experimental rate versus partial pressures of water and oxygen. An accelerating influence of water vapour was observed for both powders. A model, based on a catalytic effect of water vapour, and involving six elementary steps, was proposed in order to calculate theoretical rates within the approximation of the rate-limiting step. The comparison between the experimental and theoretical rate laws attests to the validity of the model and defines the most probable rate-limiting step: tetragonal crystallite growth proceeds at a rate controlled either by adsorption (or desorption) of water molecules or diffusion of hydroxyl groups: monoclinic crystallite growth is controlled by the diffusion of zirconium ions.

Kinetics and mechanism of dimerization of arylmercurials

AbstractThe kinetics of dimerization of arylmercurials XC6H4HgCl (X  H, p‐CH3, m‐CH3, p‐Cl, m‐OCH3, m‐CO2C2H5, and o‐OCH3) in the presence of [ClRh(CO)2]2 was studied in hexamethylphosphoramide (HMPA). The experimental rate law obtained is −d[ArHgCl]/dt=k[ArHgCl]2. The kinetic parameters of these reactions have been reported, and the variation, therein, has been explained on the basis of steric effect of substituents. The apparent activation energy E is linearly proportional to pre‐exponential factor lnA. A most plausible mechanism has been proposed on the basis of experimental results. © 1993 John Wiley & Sons, Inc.

Kinetics and mechanisms of homogeneous catalytic reactions: Part 1. Hydrogenation of benzaldehyde catalyzed by [RuH(CO)(NCMe) 2(PPh 3) 2]BF 4

Kinetic and mechanistic studies of the homogeneous hydrogenation of benzaldehyde were carried out using the cationic complex [RuH(CO)(NCMe) 2(PPh 3) 2]BF 4 ( 1) as the catalyst precursor, which was very efficient under mild reaction conditions in 2-methoxyethanol as the solvent. The experimental rate law was found to be r=( K 1 k 2/1 + K 1[PhCHO])[Ru][PhCHO][H 2] ( K 1 = 1.9 M −1; k 2 = 12.5 M −1 s −1) which can be approximated to r= k cat[Ru][PhCHO][H 2] ( k cat = 20 M −2 s −1). The activation parameters and the kinetic isotope effect were also calculated. The kinetic data and the coordination chemistry related to the hydrogenation of benzaldehyde are consistent with a mechanism involving the addition of hydrogen as the rate-determining step of the catalytic cycle.

Kinetics of surface-catalyzed oxidation of sulphur(IV) by dioxygen in aqueous suspensions of cobalt(II) oxide

A comparative rate analysis of autoxidation of sulphur(IV) in CoO suspension, leachate solution and blank reaction suggests the reaction to be surface catalyzed. In acetate buffers, the experimental rate law is: R obs= R un + ( k 3 + k 4 [H +] )[CoO][S(IV)][O 2] where R un is the rate of uncatalyzed reaction. The suggested mechanism requires the adsorption of both S(IV) and O 2 on the particle surface. Subsequently, the adsorbed species are activated resulting in the oxidation of sulphur( IV), probably through intervention of multiple oxidation states of cobalt.

Kinetics of the Reaction of Molecular Bromine with Doped Polycrystalline Silicon

The reactions of molecular bromine with phosphorus‐doped polycrystalline silicon have been studied as a function of pressure and temperature. In the pressure range from 0.5 to 30 torr and at temperatures between 360 and 410°C, the etch rate was found to be half order in the pressure of , confirming earlier observations on the reaction of intrinsic silicon with molecular bromine. The experimental rate law is shown to be consistent with a mechanism for the reaction in which the gaseous is adsorbed on the surface and then dissociated, with both steps occurring reversibly. By comparing these results with the rate constants for the etching of intrinsic silicon, we find an enhancement factor of over 125 can easily be achieved with moderate phosphorus doping levels. Such enhancements with doping are found to be attributable principally to a decrease in the activation energy for the reaction.

Kinetics and mechanism of chloramine‐T oxidation of cinnamaldehyde in two acid media

AbstractKinetics of oxidation of cinnamaldehyde (Cinn) by chloramine‐T (CAT) has been studied in solutions containing HCl and H2SO4 at 313 K. The respective experimental rate laws obtained in HCl and H2SO4 media are as follows: equation image and equation image Here the value of x varies from 0.9 to zero while the values of y and z are 0.83 and 0.72, respectively. Effects, on the reaction rate, of ionic strength, reaction product (p‐toluenesulfonamide), dielectric constant of the solvent medium, and the anions, Cl− and SO42−, have been investigated. Activation parameters have been computed, based on the rate constants determined at different temperatures. Mechanisms proposed and the rate laws derived are consistent with the observed kinetic data represented by the experimental rate laws.

Kinetics and mechanism of oxidation of indigo carmine by hypohalites

AbstractKinetics of oxidation of indigo carmine (IC) by sodium hypohalites, NaOX (X  Cl or Br), in alkaline buffer of pH 9–11 has been studied spectrophotometrically at λ = 610 nm. The experimental rate law obtained is −d[IC]/dt = k[OX−][IC][OH−]x where x <1. Variation of ionic strength or dielectric constant of the medium had no effect on the reaction rate while the addition of halide ions slightly retarded the rate. A most plausible mechanism proposed on the basis of experimental results involves the formation of isatin sulphonate which undergoes further oxidation to anthranilate. Activation parameters have been evaluated.

The reductive dissolution of iron oxides by ascorbate: The role of carboxylate anions in accelerating reductive dissolution

There are four general pathways of dissolution of reducible metal oxides in acidic aqueous solution: proton-assisted (acid), ligand-promoted acid, reductive, and ligand-promoted reductive dissolution. The presence and reactivity toward the surface of protons, chelating ligands, and reductants dictate the mechanism(s) controlling the dissolution. For the massive reductive dissolution of magnetic by ascorbic acid, the experimental rate law R = k[ HA −] 1 2 [ H +] suggests the involvement of surface ≡Fe III A − complexes. Adsorption isotherms of ascorbic acid onto hematite at pH 3 and 25°C yield a Langmuir-type surface complexation constant K s = (9.57 × 10 8 M −1). Slow dissolution follows with an empirical rate law R = k obs(≡Fe III A). It is concluded that the formation and kinetic reactivity of surface complexes determine the rate of dissolution. Dehydroascorbic acid also dissolves magnetite, but at slower rates. Oxalate accelerates the reductive dissolution of hematite by ascorbate even though it competes with ascorbate for surface sites; enhanced detachment of ≡Fe II surface species by oxalate complexation may be involved. Autoacceleration of the reductive dissolution by dissolved Fe II-carboxylate complexes is observed in EDTA/ascorbic acid mixtures; the rate reaches a maximum at intermediate [EDTA] values, where synergistic effects between EDTA and Fe II-EDTA complexes are important. Autoacceleration may also operate in oxalate solutions.