A kinetic investigation of the oxidation of <Emphasis Type="SmallCaps">dl</Emphasis>-pipecolinate by bis(hydrogenperiodato)argentate(III) complex anion

Kinetics of oxidation of dl-pipecolinate by bis(hydrogenperiodato)argentate(III) complex anion, [Ag(HIO₆)₂]⁵⁻, has been studied in aqueous alkaline medium in the temperature range of 25–40 °C. The oxidation kinetics is first order in the silver(III) and pipecolinate concentrations. The observed second-order rate constant, decreasing with increasing [periodate] is virtually independent of [OH⁻]. α-Aminoadipate as the major oxidation product of pipecolinate has been identified by chromatographic analysis. A reaction mechanism is proposed that involves a pre-equilibrium between [Ag(HIO₆)₂]⁵⁻ and [Ag(HIO₆)(H₂O)(OH)]²⁻, a mono-periodate coordinated silver(III) complex. Both Ag(III) complexes are reduced in parallel by pipecolinate in rate-determining steps (described by k ₁ for the former Ag(III) species and k ₂ for the latter). The determined rate constants and their associated activation parameters are k ₁ (25 °C) = 0.40 ± 0.02 M⁻¹ s⁻¹, ∆H ₁^≠ = 53 ± 2 kJ mol⁻¹, ∆S ₁^≠ = −74 ± 5 J K⁻¹ mol⁻¹ and k ₂ (25 °C) = 0.64 ± 0.02 M⁻¹ s⁻¹, ∆H ₂^≠ = 41 ± 2 kJ mol⁻¹, ∆S ₂^≠ = −110 ± 5 J K⁻¹ mol⁻¹. The time-resolved spectra, a positive dependence of the rate constants on ionic strength of the reaction medium, and the consistency of pre-equilibrium constants derived from different reaction systems support the proposed reaction mechanism.     

Mechanistic aspects of ligand substitution on <Emphasis Type="Italic">cis</Emphasis>-diaqua(<Emphasis Type="Italic">cis</Emphasis>-1,2-diaminocyclohexane)platinum(II) by Glycine-<Emphasis Type="SmallCaps">l</Emphasis>-Leucine

The kinetics of the interaction of glycine-l-leucine (Glyleu) with cis-[Pt(cis-dach)(OH₂)₂]²⁺ (dach = 1,2-diaminocyclohexane) has been studied spectrophotometrically as a function of [cis-[Pt(cis-dach)(OH₂)₂]²⁺], [Glyleu] and temperature at pH 4.0, where the complex exists predominantly as the diaqua species and Glyleu as a zwitterion. The substitution reaction shows two consecutive steps: the first is the ligand-assisted anation and the second is the chelation step. The activation parameters for both the steps were evaluated using Eyring’s equation. The low ∆H₁^≠ (51.9 ± 2.8 kJmol⁻¹) and large negative value of ∆S₁^≠ (−152 ± 8 JK⁻¹mol⁻¹) as well as ∆H₂^≠ (54.4 ± 1.7 kJmol⁻¹) and ∆S₂^≠ (−162 ± 5 JK⁻¹mol⁻¹) indicate an associative mode of activation for both the aqua ligand substitution processes.     

Kinetics of hydrolysis of five-membered <Emphasis Type="Italic">C</Emphasis>-nitroheterocycles: pyrazole,imidazole, 1,2,4-triazole,and isoxazole derivatives

Alkaline hydrolysis of mono-and dinitro derivatives of five-membered heterocycles, viz., pyrazole, imidazole, 1,2,4-triazole, and isoxazole, is accompanied by the elimination of the nitro group in the form of a nitrite anion. The hydrolysis kinetics was studied by the polarographic and photometric methods. The experimentally determined hydrolysis rate constants depend on the nature of the heterocycle. A possible mechanism for hydrolytic transformations of the compounds under study was proposed on the basis of the calculated thermodynamic parameters of the reaction (Δ G ^≠, ΔH ^≠, ΔS ^≠). Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2719–2725, December, 2005.     

Kinetics of the Reaction of 2-Chloro-quinoxaline with Hydroxide Ion in ACN–H2O and DMSO–H2O Binary Solvent Mixtures

The kinetics of alkaline hydrolysis of 2-chloroquinoxaline (QCl) with hydroxide ion was investigated spectrophotometrically at different percentages of aqueous–organic solvent mixtures with acetonitrile (10–60% v/v) and with dimethylesulphoxide (10–80%) over the temperature range from 25 to 45 °C. The reaction was performed under pseudo first order conditions with respect to 2-chloroquinoxaline (QCl). An increase in the percentage of organic solvent (v/v) has different effects on the reaction rate constants, presumably due to hydrogen bond donor and acceptor differences of the media and other solvatochromic parameters. The data were discussed in terms of the Kamelt-Taft parameter and E _T(30). A nonlinear relation between the logarithm of the rate constant and reciprocal of the dielectric constant suggests the presence of selective solvation by the polar water molecules. Activation parameters ΔH ^#, ΔS ^# and ΔG ^# were determined and discussed.     

Solid state kinetics of Cu(II) complex of [2-(1,2,3,4-thiatriazole-5-yliminomethyl)-phenol] from thermo gravimetric analysis

The ligand [2-(1,2,3,4-thiatriazole-5-yliminomethyl)-phenol] (L) is a schiff base derived from condensation reaction of 1,2,3,4-thiatriazole-5-ylamine and Salicylaldehyde. Synthesis of the ligand (L) and the complex [Cu(II)(L)₂]·2H₂O have been studied in our previous work (Bharti et al., Asian J Chem 23(2):773–776, 2011). Thermal decomposition behavior of synthesized Cu(II) complex has been investigated by thermo gravimetric (TG) analysis at heating rate of 10 °C min⁻¹ under nitrogen atmosphere. The mechanism of decomposition of Cu(II) complex has been established from TG data. Kinetic parameters such as order of reaction (n), activation energy (E _a), frequency factor (Z) and entropy of activation (∆S ^≠) were calculated by using Freeman and Carroll (J Phys Chem 62:394–397, 1958) as well as Doyle’s methods as modified by Zsako (J Phys Chem 72(7):2406–2411, 1968).     

The thermal decomposition of N , N -dimethyl-3-oxaglutaramic acid and the kinetics of its second-stage thermal decomposition reaction

N,N-dimethyl-3-oxa-glutaramic acid was purified and characterized by ¹H-NMR, Fourier transform infrared spectroscopy (FT-IR) and elemental analysis. The thermal decomposition of the title compound was studied by means of thermogravimetry differential thermogravimetry (TG-DTG) and FT-IR. The kinetic parameters of its second-stage decomposition reaction were calculated and the decomposition mechanism was discussed. The kinetic model function in a differential form, apparent activation energy and pre-exponential constant of the reaction are 3/2 [(1−α)^1/3−1]⁻¹, 203.75 kJ·mol⁻¹ and 10^17.95s⁻¹, respectively. The values of ΔS ^≠, ΔH ^≠ and ΔG ^≠ of the reaction are 94.28 J·mol⁻¹·K⁻¹, 203.75 kJ·mol⁻¹ and 155.75 kJ·mol⁻¹, respectively. Supported by the National Natural Science Foundation of China (Grant No. 20106009)     

Synthesis,molecular structure,non-isothermal decomposition kinetics and adiabatic time to explosion of 3,3-dinitroazetidinium 3,5-dinitrosalicylate

The title compound 3,3-dinitroazetidinium (DNAZ) 3,5-dinitrosalicylate (3,5-DNSA) was prepared and the crystal structure has been determined by a four-circle X-ray diffractometer. The thermal behavior of the title compound was studied under a non-isothermal condition by DSC and TG/DTG techniques. The kinetic parameters were obtained from analysis of the TG curves by Kissinger method, Ozawa method, the differential method and the integral method. The kinetic model function in differential form and the value of E _a and A of the decomposition reaction of the title compound are f(α)=4α^3/4, 130.83 kJ mol⁻¹ and 10_13.80s⁻¹, respectively. The critical temperature of thermal explosion of the title compound is 147.55 °C. The values of ΔS ^≠, ΔH ^≠ and ΔG ^≠ of this reaction are −1.35 J mol⁻¹ K₋₁, 122.42 and 122.97 kJ mol⁻¹, respectively. The specific heat capacity of the title compound was determined with a continuous C _p mode of mircocalorimeter. Using the relationship between C _p and T and the thermal decomposition parameters, the time of the thermal decomposition from initiation to thermal explosion (adiabatic time-to-explosion) was obtained.     

Kinetic and thermodynamic studies on reactions of [PtCl(bpma)]<Superscript>+</Superscript> and [Pt(bpma)H<Subscript>2</Subscript>O]<Superscript>2+</Superscript> (bpma = <Emphasis Type="Italic">bis</Emphasis>-(2-pyridylmethyl)amine) with some azoles and diazines

Substitution reactions of the complexes [Pt(bpma)H₂O]²⁺ and [PtCl(bpma)]⁺, where bpma is bis-(2-pyridylmethyl)amine, with the nitrogen-donor ligands 1,2,4-triazole, pyrazole and pyridazine were studied in aqueous 0.1 M NaClO₄ using variable-temperature UV–VIS spectrophotometry. The reactions of the aqua complex were studied at pH 2.5, and those of the chloro complex were studied in the presence of 10 mM NaCl to prevent their hydrolysis. The values obtained for the second-order rate constants indicate that the complexes with bpma are more reactive than those with diethylenetriamine. In both cases, the aqua complexes are more reactive than the corresponding chloro complexes. The reactivity of the incoming ligands follows the order: 1,2,4-triazole > pyridazine > pyrazole. Activation parameters were determined for all the reactions, and the negative entropies of activation (∆S^≠) support an A or Ia mechanism.     

Oxidation of 1,3,7-trimethylxanthine by hypochlorite ion

The kinetics of the oxidative conversion of 1,3,7-trimethylxanthine upon treatment with hypochlorite ions (OCl⁻) in aqueous medium at 283–298 K and pH 8.2 was studied. The reaction order with respect to each component was determined and proved to be 1. It was established that the temperature dependence of the reaction rate follows the Arrhenius equation. The activation parameters of the reaction were measured: E _a = 33.58 kJ/mol, ΔH ^≠ = 31.12 kJ/mol, ΔS ^≠ = −170.02 J/(K mol), ΔG ^≠ = 81.45 kJ/mol. The stoichiometry of the reaction was studied, and the chemistry of the oxidative conversion of caffeine treated with OCl⁻ is discussed.     

Kinetics and mechanism of anation of cis -diaquo- bis -oxalatochromate(III) ion byDL -alanine in ethanol-water mixtures of varying dielectric constant

The kinetics of the anation reaction of cis-diaquo-bis-oxalatochromate(III) ion by DL-alanine has been studied spectrophotometrically in the pH range 3.8 to 7.3, where DL-alanine remains in zwitterionic form. A second-order rate law has been established. Reaction rates in three different ethanol-water mixtures were measured. In each solvent medium the anation rate is higher as compared to water exchange reaction at a particular temperature. The activation parameters (gDH^# and ΔS^#) in different ethanol-water mixtures were obtained from Eyring plots. ΔG^#(ΔH^# −TΔS ^#) values were calculated in each solvent medium and compared with that of the isotopic water exchange process. A reaction mechanism involving theS _N2 path has been suggested.     

Parameters of cation “hardness” and “softness” and application thereof for calculation of extraction constants

Analysis of published data was performed on the basis of solvation energies of a number of “transition” and “soft” cations. A dependence of the solvation energy of cations on the donor and acceptor numbers of the solvent was derived. Donor and acceptor ability parameters of Mn²⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Ag⁺, Cd²⁺, and Tl⁺ were calculated. The resultant parameters were used to calculate the extraction constants of the specified cations with di-2-ethylhexyl-phosphoric acid (DEHPA) solution in neutral solvents; the model represented has a good predictive power.     

Mechanistic investigations of oxidation of isatins by sodium N-chlorobenzenesulfonamide in alkaline medium: A kinetic study

Oxidation of isatins (isatin, 5-methylisatin, 5-bromoisatin and 5-nitroisatin) to their anthranilic acids was performed efficiently with sodium N-chlorobenzenesulfonamide or chloramine-B (CAB) in alkaline medium at 35±0.1°C. The reactions follow identical kinetics for all the isatins, being first-order dependence each in [CAB] _o and [Isatin] _o and inverse fractional-order on [NaOH]. Addition of halide ions and benzenesulfonamide, reduction product of CAB, do not significantly affect the rate. Variation of ionic strength of the medium had no effect on the rate, while the dielectric effect is negative. The solvent isotope effect was studied using D₂O. Activation parameters for the overall reaction have been computed. The rates satisfactorily correlate with the Hammett σ relationship and the reaction constant ρ is −0.31 signifies that electron releasing groups accelerate the reaction while the electron withdrawing groups retard the rate. Values of ΔH^≠ and ΔS^≠ are linearly related and an isokinetic relationship is observed with β=376 K, indicating the reaction is controlled by enthalpy. The stoichiometry of the title reaction is found to be 1∶1. Oxidation products of isatins were identified as their corresponding anthranilic acids and the yields were found to be around 90 %. The observed results have been explained by a plausible mechanism and the related rate law deduced. This method offers several advantages including high yield of the products, short reaction times, easier isolation of products, and stable, cost effective and relatively non-toxic reagents, which make the reaction process simple and smooth.     

Kinetic study of the promazine oxidation to promazine 5-oxide by trisoxalatocobaltate(III) in basic aqueous media

The kinetics of the oxidation of promazine by trisoxalatocobaltate(III) were studied in the presence of a large excess of the cobalt(III) in tris buffer solution using u.v.–vis spectroscopy ([Co^III] = (0.6 − 2) × 10⁻³ M, [ptz] = 6 × 10⁻⁵ M, pH = 6.6–7.8, I = 0.1 M (NaCl), T = 288−308 K, l = 1 cm). The reaction proceeds via two consecutive reversible steps. In the first step, the reaction leads to formation of cobalt(II) species and a stable cationic radical. In the second step, cobalt(III) is reduced to cobalt(II) ion and a promazine radical is oxidized to the promazine 5-oxide. Linear dependences of the pseudo-first-order rate constants (k ₁ and k ₂) on [Co^III] with a non-zero intercept were established for both redox processes. Rates of reactions decreased with increasing concentration of the H⁺ ion indicating that the promazine and its radical exist in equilibrium with their deprotonated forms, which are reactive reducing species. The activation parameters for reactions studied were as follows: ΔH^≠ = 44 ± 1 kJ mol⁻¹, ΔS^≠ = −100 ± 4 JK⁻¹ mol⁻¹ for the first step and ΔH^≠ = 25 ± 1 kJ mol⁻¹, ΔS^≠ = −169 ± 4 J K⁻¹ mol⁻¹ for the second step, respectively. Mechanistic consequences of all the results are discussed.     

Volume parameters of some Diels-Alder reactions involving C=C,C=S,and N=N bonds

The effect of a hydrostatic pressure of up to 1000 kg cm⁻² on the rate constants of the Diels-Alder reactions of maleic anhydride with 1,2,3,4-tetraphenylcyclopentadiene and with 6,13-dichloropentacene, of 4-phenyl-1,2,4-triazoline-3,5-dione with hexachlorocyclopentadiene, and of thiobenzophenone with isoprene was studied at 25 °C. The volume parameters and ratios of the activation to reaction volumes make it possible to exclude electrostriction of the solvent during transition state solvation in all the reactions studied, which corresponds to the nonpolar nature of the transition state. Dedicated to Academician A. L. Buchachenko on the occasion of his 70th birthday. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1973–1980, September, 2005.     

Influence of the Media on the Activation Parameters of the Chain Propagation Step in the Liquid Phase Oxidation of Methyl Ethyl Ketone

The effect of solvation on the activation parameters of the reaction RH + ROO^· R^· + ROOH, where RH is methyl ethyl ketone, has been studied. The increased enthalpy of activation of this reaction in water is not a result of specific solvation of the peroxy radical but is non-specific effect of the solvent as a dielectric continuum.     

Kinetic and mechanistic studies on the reaction of thiosemicarbazide with [(H<Subscript>2</Subscript>O)(tap)<Subscript>2</Subscript>RuORu(tap)<Subscript>2</Subscript>(H<Subscript>2</Subscript>O)]<Superscript>2+</Superscript> {tap = 2-(m-tolylazo)pyridine}

The interaction of thiosemicarbazide with the title complex has been studied spectrophotometrically in aqueous medium as a function of [complex], [thiosemicarbazide], pH and temperature at constant ionic strength. At pH 7.4, the reaction shows two distinct paths; both of which are [thiosemicarbazide] dependent. A parallel reaction scheme fits well with the experimental findings. An associative interchange mechanism is proposed for both the paths; the activation parameters calculated from Eyring plots are ΔH₁^≠ = 14.2 ± 0.8 kJ mol⁻¹, ΔS₁^≠ = −241 ± 2 JK⁻¹ mol⁻¹, ΔH₂^≠ = 30.8 ± 1.4 kJ mol⁻¹ and ΔS₂^≠ = −236 ± 4 JK⁻¹ mol⁻¹. From the temperature dependence of the outer sphere association complex equilibrium constants, the thermodynamic parameters calculated are ΔH₁^° = 34.25 ± 1.9 kJ mol⁻¹, ΔS₁^° = 146 ± 6 J K⁻¹ mol⁻¹ and ΔH₂^° = 9.4 ± 1.1 kJ mol⁻¹, ΔS₂^° = 71 ± 3 JK⁻¹ mol⁻¹, which gives a negative ΔG° at all temperatures studied, supporting the spontaneous formation of an outer sphere association complex.     

Isokinetic relationships in unimolecular heterolysis. Mechanism of ionization of covalent bond

Various types of isokinetic (isoparametric) relationships in heterolytic reactions were summarized and critically analyzed. It was presumed that the series of substrate reactivity is reversed after passing the isoparametric point, and the bimolecular reaction mechanism changes to unimolecular: S_N2-S_N1, S_N2-E1, S_E2-S_E1, S_E2-S_N1, and S_N2(SSIP)-S_N2(C⁺). Three particular cases of isoparametric relationships are discussed: (1) isoentropy (ΔS ^≠ = const) which reflects formation of contact ion pair; (2) isoenthalpy (ΔH ^≠ = const) which reflects formation of space-separated ion pair; and (3) isoenergy (ΔG ^≠ = const), when ΔH ^≠ = ΔG ^≠ = ΔE _r. The rate of heterolysis in cyclohexane does not depend on the substrate nature, and a universal minimal rate of heterolysis exists, k25 ≈ 10⁻¹⁰ s⁻¹, τ_1/2 = 220 years. There is no nucleophilic assistance by the solvent in unimolecular heterolysis.     

Kinetics and mechanism of aqua ligand substitution from cis-diaqua(cis-1,2-diaminocyclohexane)platinum(II)perchlorate by diethyldithiocarbamate anion in aqueous medium

The kinetics of the interaction of diethyldithiocarbamate (Et₂DTC) with [Pt(dach)(H₂O)₂]²⁺ (dach = cis-1,2-diaminocyclohexane) have been studied spectrophotometrically as a function of [Pt(dach)(H₂O)₂ ²⁺], [Et₂DTC] and temperature at a particular pH (4.0). The reaction proceeds via rapid outer sphere association complex formation followed by two slow consecutive steps. The first step involves the transformation of the outer sphere complex into an inner sphere complex containing a Pt–S bond and one aqua ligand, while the second step involves chelation when the second aqua ligand is replaced. The association equilibrium constant K _E and two rate constants k ₁ and k ₂ have been evaluated. Activation parameters for both the steps have been calculated (∆H ₁ ^# = 66.8 ± 3.7 kJ mol⁻¹, ∆S ₁^# = −81 ± 12 JK⁻¹ mol⁻¹ and ∆H ₂^# = 95.1 ± 2.8 kJ mol⁻¹, ∆S ₂^# = −34.4 ± 9.1 JK⁻¹ mol⁻¹). The low enthalpy of activation and negative entropy of activation indicate an associative mode of activation for both the steps.     

Multivariate statistical treatment of solvent effects in the photoreduction of CoIII(En)2(Br)(RC6H4NH2)2+ complexes in aqua-organic solvent media

Ultraviolet photolysis (λ = 254 nm) studies were carried out for a series of cobalt(III) complexes, Co^III(En)₂(Br)(RC₆H₄NH₂)²⁺, where R = m-OMe, p-F, H, m-Me, p-Me, p-OEt, and p-OMe, in various compositions of water-methanol/1,4-dioxane mixtures (0, 5, 10, 15, 20, 25, and 30 vol % organic cosolvent) at two different temperatures (287 and 300 K). The ligand-to-metal charge-transfer excited state produced in the excitation of the complex initially generated a solvent-caged {Co^II; ligand radical} pair, which eventually undergoes recombination/separation into products. The quantum yield sharply increased from a mixture containing a lower mole fraction of organic cosolvent (x _org) to higher one. In other words, when x _org in the mixture increases, a steady increase in the quantum yield is observed. The quantum yield of Co^III(En)₂(Br)(RC₆H₄NH₂)²⁺ in various solvent mixtures is found to exhibit a linear (logΦ_Co(II) − 1/ε_r) dependence. This is consistent with solvation or solvent cage effect, which may be nonspecific, specific, or both. In order to throw light on these effects, a phenomenological model of solvent effects was applied. Therefore, the quantum yield values have been correlated statistically with some success containing different solvent parameters. The solvent parameters considered in this work are Grunwald-Winstein’s Y, Krygowski-Fawcett’s E _T ^N and DN ^N along with Kamlet-Taft’s α, β, π* parameters. The regression model proposed is Y _S = Y ₀ + Σ _{i = 1} ⁿ a _i X _i , where Y_S is the solvent-dependent property (here logΦ_Co(II)) in a given solvent; Y ₀ is the statistical quantity corresponding to the value of property in the reference solvent; X ₁, X ₂, X ₃ … are explanatory variables, the solvent parameters, which can explain the various solvation effects on reactants/{Co^II; ligand radical} pair, and a ₁, a ₂, a ₃… etc., are the regression coefficients. The coefficient values can be quantified to measure the relative importance of solvent effects on the physicochemical quantity, that is, the photoreduction yields in the present investigation. The text was submitted by the author in English.     

Self-consistent reaction field calculation of solvent reorganization energy in electron transfer: a dipole-reaction field interaction model

Based on the continuum dielectric model, this work has established the relationship between the solvent reorganization energy of electron transfer (ET) and the equilibrium solvation free energy. The dipole-reaction field interaction model has been proposed to describe the electrostatic solute-solvent interaction. The self-consistent reaction field (SCRF) approach has been applied to the calculation of the solvent reorganization energy in self-exchange reactions. A series of redox couples, O₂/O⁻ ₂, NO/NO⁺, O₃/O⁻ ₃, N₃/N⁻ ₃, NO₂/NO⁺ ₂, CO₂/CO⁻ ₂, SO₂/SO⁻ ₂, and ClO₂/ClO⁻ ₂, as well as (CH₂)₂C-(-CH₂-) _n -C(CH₂)₂ (n=1 ∼ 3) model systems have been investigated using ab initio calculation. For these ET systems, solvent reorganization energies have been estimated. Comparisons between our single-sphere approximation and the Marcus two-sphere model have also been made. For the inner reorganization energies of inorganic redox couples, errors are found not larger than 15% when comparing our SCRF results with those obtained from the experimental estimation. While for the (CH₂)₂C–(–CH₂–) _n –C(CH₂)₂ (n=1 ∼ 3) systems, the results reveal that the solvent reorganization energy strongly depends on the bridge length due to the variation of the dipole moment of the ionic solute, and that solvent reorganization energies for different systems lead to slightly different two-sphere radii. Received: 19 April 2000 / Accepted: 6 July 2000 / Published online: 27 September 2000