Synthesis and some properties of cyclic acetals of malonaldehyde

The Lewis acid-catalyzed addition of cyclic orthoformates to vinyl ethyl ether, which leads to the formation of malonaldehyde acetals, was studied. It is shown that of the linear-cyclic malonaldehyde acetals, 2-(2,2-diethoxyethyl)- and 5,5-dimethyl-2-(2,2-diethoxyethyl)-1,3-dioxanes are stable. The transacetalization of 1,1,3,3-tetraethoxypropane with 1,2- and 1,3-diols, which leads to the formation of cyclic malonaldehyde acetals, was studied. The physicochemical constants of the acetals were determined, and their ¹H and ¹³C NMR spectra are described.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 4, pp. 459–463, April, 1983.     

Solvent effects in the reaction of 2-thiophenesulfonyl chloride with aniline

The second order rate constants k₂ and the activation parameters for the reaction of 2-thiophenesulfonyl chloride with aniline together with solution enthalpies of the reactants have been measured in methanol, ethanol, 2-propanol, acetonitrile and acetone. The reaction rates are slower in dipolar aprotic solvents than in protic ones due to a remarkable activation negative entropy. The rate constants k₂ are correlated with empirical solvent polarity parameters. The data seem in accord with a S_AN reaction mechanism.     

Electrooxdation of Tricarbonyl(N,N-Diphenylcyclohexadienecarboxamide)Iron Complexes. Reversibility of Electron Transfer and Free Energy Change

Qualitative and quantitative criteria of irreversibility of electron transfer in cyclic voltammetry (CV) of tricarbonyl(N,N-diphenylcyclohexadienecarboxamide)iron complexes were investigated. The measurement of the heterogeneous rate constants for electron transfer according to CV data over an extended range of free energy change ( ΔG) for these complexes are described. Kinetic analyses of the electrochemical technique demonstrates that when the applied potential exceeds the standard potential E°, the experimental rate constant k_e represents an accurate measurement of the intrinsic rate constant k₁ for electron transfer. However, as the applied potential is less than E°, the reversibility of the electron-transfer process becomes increasingly more important, and the rate constants for reverse electron transfer k_-t and decomposition k₂ of the electrogenerated intermediate have to be taken into account.     

Reactivity Trends and Kinetic Inspection of Hydroxide Ion Attack and DNA Interaction on Some Pharmacologically Active Agents of Fe(II) Amino Acid Schiff Base Complexes at Different Temperatures

The reactivity of few novel high‐spin Fe(II) complexes of Schiff base ligands derived from 2‐hydroxynaphthaldehyde and some variety of amino acids with the OH^? ion has been examined in an aqueous mixture at the temperature range from 10 to 40°C. Based on the kinetic investigations, the rate law and a plausible mechanism were proposed and discussed. The general rate equation was suggested as follows: rate = k_obs[complex], where k_obs. = k₁ + k₂[OH^?]. Base‐catalyzed hydrolysis kinetic measurements imply pseudo–first‐order doubly stage rates due the presence of mer‐ and fac‐isomers. The observed rate constants k_obs are correlated with the effect of substituent R in the structure of the ligands. From the effect of temperature on the rate base hydrolysis reaction, various thermodynamic parameters were evaluated. The evaluated rate constants and activation parameters are in a good agreement with the stability constants of the investigated complexes. Moreover, the reactivity of the investigated complexes toward DNA was examined and found to be in a good agreement with the reported binding constants.     

Kinetics of polymerization by activated monomer mechanism

Polymerization of cyclic ethers by activated monomer mechanism involves consecutive additions of protonated monomer molecules to the growing macromolecules fitted with hydroxyl groups at their ends. For oxirane itself and symmetrically substituted oxiranes there is only one kind of hydroxyl groups and one, unique way of ring-opening. Unsymmetrically substituted oxiranes provide however two sites of attack and two different hydroxyls, resulting from these ring-openings. Kinetics of polymerization of epichlorohydrin (chloromethyloxirane) has been studied and all four rate constants determined, namely rate constants of the primary and secondary alcoholate chain ends with a protonated monomer, opening in result of the attack on substituted or unsubstituted carbon atom. These rate constants are (in mol⁻¹·1·s⁻¹ at 25°C, in CH₂Cl₂ solvent): k₁₁ = 0.055, k₁₂ = = 0.41, k₂₂ = 0.135, and k₂₁ = 0.0011 (e.g. k₁₂ is the rate of reaction of the primary alcohol producing the secondary alcohol). Thus, polymerization proceeds almost exclusively on the secondary alcoholate groups, reproducing themselves (k₂₂).     

Standard rate constants of charge transfer for Nb(V)/Nb(IV) redox couple in chloride-fluoride melts: Experimental and calculation methods

Standard rate constants k _s of charge transfer for Nb(V)/Nb(IV) redox couple in NaCl-KCl (the equimolar mixture) -K₂NbF₇, KCl-K₂NbF₇, and CsCl-K₂NbF₇ melts are determined by using cyclic voltammetry. An unconventional series of the standard rate constants is found: k _s (KCl) < k _s (CsCl) < k _s (NaCl-KCl). Ab initio calculations carried out by using a PC Gamess/Firefly quantum-chemical program showed that the charge transfer activation energy can change not monotonically in the Na-K-Cs series, in compliance with the reorganization energy relationship. This leads, in its turn, to nonmonotonic change in the charge transfer rate constants.     

Kinetic study of the oxidation of some catecholamines by digital simulation of cyclic voltammograms

Electrochemical oxidation of some catecholamines such as dopamine ( 1 ), L ‐dopa ( 2 ), and methyldopa ( 3 ) has been studied in various pH values, using cyclic voltammetry. The results indicate participation of catecholamines ( 1–3 ) in intramolecular cyclization reaction to form the corresponding o‐quinone derivatives ( 1d–3d ). In various pHs, based on ECE mechanism, the observed homogeneous rate constants (k_obs) of cyclization reaction were estimated by comparing the experimental cyclic voltammetric responses with the digital‐simulated results. Also, the cyclization rate constants (k_cyc) were calculated using microscopic acidic dissociation constant of ammonium groups. The significant differences in electrochemical behavior, k_obs and k_cyc, of L ‐dopa ( 2 ) and methyldopa ( 3 ) with dopamine ( 1 ) are due to the effects of the side chain carboxyl group. © 2004 Wiley Periodicals, Inc. Int J Chem Kinet 37: 17–24, 2005     

Polymerization of α-Methylstyrene at High Temperatures in Tetrahydrof uran with Potassium as Initiator. I. Thermodynamic Study and Gel-Permeation Chromatographic Analyses of the Polymers

The acetals of aryl aldehydes react with aryl amines to produce Schiff bases in quantitative yields. This reaction was also facile with diamines and diacetals. It involves a two-step elimination of alcohol; kinetic data indicate that k ₂ is equal to or greater than k ₁ and this complicates the isolation of any intermediate compound.

Because of overlap of absorption bands, the existence of intermediates was not confirmed by infrared spectral measurements. Adducts of the acetals and the aniline hydrochlorides were isolated as hydrochlorides; their molecular weights, as well as the products obtained by neutralization, indicated that the intermediate is not a monoalkoxy compound. The acetals react, also, with N-acyl aniline by the elimination k ₁ of the alcohol and k ₂ of the ester, in which k ₁ k ₂, to produce Schiff bases in less than quantitative yields. These reactions of acetals with amines and their N-acyl derivatives are of interest in the syntheses of polymers.     

Kinetik der Reaktion von Sulfhydrylverbindungen mit α,β-ungesättigten Aldehyden in wäßrigem System

2-alkenals react with SH-compounds to R?CH(SR)?CH₂?CHO; 4-hydroxy-2-alkenals react to the corresponding 4-hydroxy-alkanals which rearrange to the cyclic semiacetals \(\begin{gathered} R\_CH\_CH(SR)\_CH_2 \_CHOH \\ |\_\_\_\_\_\_\_O\_\_\_\_\_\_\_| \\ \\ \end{gathered} \) . The overall-reaction is reversible. The rate of product formation is given by:v=k (alkenal) (sulfhydryl). Methods for determination of the rate constantk are described. The apparent constantk is a function of concentration and kind of proton donors of the reaction system (H₂O, H₃O⁺, CH₃COOH, H₂PO₄ ^?1, HPO₄ ^?2 or other HX-compounds). An equation is developed which describes the interrelation between the overall-reaction rate constantk and the individual intrinsic rate constants and composition of the reaction system. By this formula from thek-values the individual intrinsic rate constants of some model reactions are calculated. The biological effects of hydroxyalkenals are discussed on the basis of the obtained data.     

A novel approach to measure all rate constants in the simplest enzyme kinetics model

Enzymes play vital roles in life processes. Almost all biochemical reactions are mediated by enzymes. The rate constants of enzyme kinetics are the most important parameters for the reactions catalyzed by enzymes. In 1902, Adrian Brown proposed a simple single-substrate-single-product model which contains only three rate constants k ₁, k ₋₁ and k ₂. So far, biologists can measure the Michaelis constant K _M and the catalytic constant k _cat , which actually is equal to k ₂, according to Michaelis–Menten equation. Using temperature jump method or transient state kinetics, k ₁, k ₋₁ and k ₂ can be determined. However, these methods are complicated. In this article, we design a novel simple method that could determine the rate constants k ₁ and k ₋₁ based on knowing k _cat and K _M . Our numerical experiments show that the three rate constants can be calculated rather precisely. Hence, we believe that biochemists could design experiments to measure the rate constants based on our method. This work was partially supported by the National Natural Science Foundation of China (NSFC) under Grant No. 10771206 and partially by 973 project (2004CB318000) of P. R. China.     

Effect of the solvent on reduction of azomethine dyes with ketyl radicals and recombination of ketyl radicals

The rate constants of transfer of a hydrogen atom from ketyl radicals to azomethine dyes (k_H) and the rate constants of recombination of ketyl radicals (2k_r) in different solvents were measured by the method of pulsed photolysis. The dependences of k_H and 2k_r on the polarity of the solvent are V-shaped: k_H and 2k_r are maximum in weakly solvating solvents and in water and are minimum in nucleophilic solvents of medium polarity. This is due to the fact that nucleophilic solvation decreases and electrophilic solvation increases the reactivity of the ketyl radicals.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 8, pp. 1745–1749, August, 1989.     

Über die SiN-bindung: XLIII. Kinetische untersuchungen zur hydrolyse von silylurethanen des typs (p-XC6H4)Me2SiN(Ph)COOEt

Hydrolysis reactions of silylurethanes (p-XC₆H₄)Me₂SiN(Ph)COOEt with X = MeO, Me, H or Cl in aqueous buffer solutions, with pH values from 1.73 to 10.00 and at temperatures of 20, 30, and 40°C were studied.The catalytic rate constants for the acid- and base-catalysed reactions and for the “non-catalysed” reaction k(H₃O⁺), k(CH₃COO^?), k(NH₃), k(OH^?), and k₀ and the Brönsted coefficients for the general base-catalysed reactions were evaluated from the pseudo first-order rate constants k_exp, determined by UV spectroscopy.The ? values of the reactions can be derived from the σ constants given by Jaffé. Activation parameters can be obtained by means of the Arrhenius or Eyring equation.From the catalytic constants, Brönsted coefficients, ? values and activation parameters we derive mechanisms for the acid- and general base-catalysed reactions.Conclusions about these mechanisms are compared with the results obtained with the silylurethanes Me₃Si(p-XC₆H₄)NCOOEt.     

Über die Si-N-bindung: XLI. Kinetische untersuchungen zur hydrolyse von trimethylsilylurethanen des typs Me3Si(p-XC6H4)NCOOEt

Hydrolysis reactions of silylurethanes Me₃Si(p-XC₆H₄)NCOOEt (I) with X = Cl, H or Me in aqueous buffer solutions, with pH values from 1.94 to 10.00 were studied.The catalytic rate constants for the acid and base catalysed reactions and for the “non-catalysed” reaction k(H₃O⁺), k(CH₃COO^?), k(H₂PO₄^?), k(HPO₄^2?), k(NH₃), k(OH^?) and k₀ were evaluated from the pseudo first-order rate constants k_exp determined by UV spectroscopy.The Brönsted coefficients for the base-catalysed reactions were obtained from the catalytic rate constants found and the known constants of dissociation K(HB⁺).The ρ values of the reactions could be derived from the σ constants given by Jaffé.The kientical results obtained are interpreted mechanistically and are believed to also have model character for other nucleophilic substitution reactions with silicon compounds.     

Standard rate constants of charge transfer of Nb(V)/Nb(IV) redox pair in equimolar NaCl-KCl melt

Standard rate constants of charge transfer (k _s) on platinum and glassy carbon electrodes for Nb(V)/Nb(IV) redox pair in the NaCl-KCl-K₂NbF₇ melt are determined using the method of cyclic voltammetry in the temperature range of 973 to 1123 K. It is found that k _s increases with increasing temperature and when we pass from glassy carbon to platinum electrode. The “apparent” activation energies of charge transfer are determined; it is shown that the charge transfer between the Nb(V) and Nb(IV) complexes is quasi-reversible and is controlled predominantly by the diffusion.     

Kinetics of substitution of cis-diaqua-bis[2-(m-tolylazo)pyridine]-ruthenium(II) ion with pyridine-2-aldoxime in EtOH-H2O mixtures

Summary Kinetic and mechanistic studies on the anation of cis-[Ru(tap)₂(H₂O)₂]₂₊ (where tap = 2-(m-tolylazo)pyridine) by pyridine-2-aldoxime (LH) have been made spectrophotometrically at different temperatures (35–50° C) in aqueous medium and various EtOH-H₂O mixtures. The following rate law has been established at pH 5.6: k _obs = k ₁ k ₂[LH]/(k ₋₁ + k ₂[LH]) where k ₁ is the H₂O dissociation rate constant of the substrate complex and k _-1 and k ₂ are the aquation and ligand capturing rate constants of the pentacoordinate intermediate, [Ru(tap)₂(H₂O)]²⁺. The rate constants are independent of ionic strength. The reaction rate increases with increasing pH. Activation parameters (ΔH‡ and ΔS‡) have been calculated for media of four different dielectric constants and compared with other substitution reactions in aqueous medium. A dissociative mechanism is proposed.     

Kinetics and quantification of the electrophilic reactivities of substituted thiophenes and structure‐reactivity relationships

Second‐order rate constants (k₁) have been measured spectrophotometrically for reactions of 2‐methoxy‐3‐X‐5‐nitrothiophene 1a‐c (X = NO₂, CN, and COCH₃) with secondary cyclic amines (pyrrolidine 2a , piperidine 2b , and morpholine 2 c ) in CH₃CN and 91:9 (v/v) CH₃OH/CH₃CN at 20°C. The experimental data show that the rate constants (k₁) values exhibit good correlation with the parameters of nucphilicity (N) of the amines 2a‐c and are consistent with the Mayr's relationship log k (20°C) = s(E + N). We have shown that the electrophilicity parameters E derived for 1a–c and those reported previously for the thiophenes 1d‐g (X = SO₂CH₃, CO₂CH₃, CONH₂, and H) are linearly related to the pK_a values for their gem‐dimethoxy complexes in methanol. Using this correlation, we successfully evaluated the electrophilicity E values of 12 structurally diverse electrophiles in methanol for the first time. In addition, a satisfactory linear correlation (r² = 0.9726) between the experimental (log k_exp) and the calculated (log k_calcd) values for the σ‐complexation reactions of these 12 electrophiles with methoxide ion in methanol has been observed and discussed.     

Kinetic and mechanistic study of the reduction of 2,6-dichlorophenol indophenol by dithionites

The reaction of 2,6-dichlorophenolindophenol (DCPI) and dithionites is studied kinetically by applying the stopped-flow technique. Reaction rate constants are given for the pH range 1.30–6.80. The reaction was found to follow first-order kinetics with respect to each of the reactants. For pH 3.97, 5.10 and 6.80, the second-order reaction rate constant was determined by applying four different technique. Mean values of k = 172±5, 200±2 and 276±4 l mol^?1s^?1 are given for pH 3.97, 5.10 and 6.80, respectively. A mechanism is proposed for the reaction, which suggests partial reactions of all possible species of DCPI and dithionites at any pH. An equation for the calculation of k at any pH is derived, which gives k as a function of [H⁺], the partial reaction rate constants and the dissociation constants of DCPI and H₂S₂O₄. Values of reaction rate constants of all possible partial reactions are also presented.     

Influence of the stereochemical configuration on the radical polymerization of methacrylic monomers: cis‐ and trans‐(2‐cyclohexyl‐1,3‐dioxan‐5‐yl) methacrylate

The synthesis of two new isomeric monomers, cis‐(2‐cyclohexyl‐1,3‐dioxan‐5‐yl) methacrylate (CCDM) and trans‐(2‐cyclohexyl‐1,3‐dioxan‐5‐yl) methacrylate (TCDM), starting from the reaction of glycerol and cyclohexanecarbaldehyde, is reported. The process involved the preparation of different alcohol acetals and esterification with methacryloyl chloride of the corresponding cis and trans 5‐hydroxy compounds of 2‐cyclohexyl‐1,3‐dioxane. The radical polymerization reactions of both monomers, under the same conditions of temperature, solvent, monomer, and initiator concentrations, were studied to investigate the influence of the monomer configuration on the values of the propagation and termination rate constants (k_p and k_t ).The values of the ratio k_p /k_t ^1/2 were determined by UV spectroscopy by the measurement of the changes of absorbance with time at several wavelengths in the range 275–285 nm, where an appropriate change in absorbance was observed. Reliable values of the kinetics constants were determined by UV spectroscopy, showing a very good reproducibility of the kinetic experiments. The values of k_p /k_t ^1/2, in the temperature interval 45–65 °C, lay in the range 0.40–0.50 L^1/2/mol^1/2s^1/2 and 0.20–0.30 L^1/2/mol^1/2s^1/2 for CCDM and TCDM, respectively. Measurements of both the radical concentrations and the absolute rate constants k_p and k_t were also carried out with electron paramagnetic resonance techniques. The values of k_p at 60 °C were nearly identical for both the trans and cis monomers, but the termination rate constant of the trans monomer was about three times that of the cis monomer at the same temperature. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 3883–3891, 2000     

Pulsed laser powered homogenous pyrolysis for reaction kinetics studies: Probe laser measurement of reaction time and temperature

Pulsed laser powered homogeneous pyrolysis (LPHP) is a technique which can be used to measure rate parameters for purely homogeneous unimolecular decomposition reactions at high temperatures (600–1500 K). The reaction temperature in pulsed LPHP may be obtained from the speed of sound in the reacting gas, which may be measured by observing the thermal lens effect of the gas on a probe laser beam. The reaction time may be obtained directly from the thermal lens measurements. In this work experiments were performed using ethyl acetate (EtAc) and isopropyl chloride (2-ClPr), two reactants whose unimolecular decomposition rate parameters are well established. This allowed us to assess the accuracy and precision attainable with this technique. Pulsed LPHP proved capable of providing rate parameters in good agreement with those in the literature. The results for which the measured activation energies were closest to the literature values gave the temperature dependence of the rate constants as log(k_EtAc) = (12.0 ± 0.9) − 47.7 ± 4.4(kcal/mol)/2.303RT and log(k_2-ClPr) = (13.3 ± 1.0) − 50.8 ± 4.8(kcal/mol)/2.303RT. These may be compared with the literature recommendations, log(k_EtAc) = 12.6 − 48.0(kcal/mol)/2.303RT and log(k_2-ClPr) = 13.6 − 51.1(kcal/mol)/2.303RT. In all cases the measured rate parameters agreed with the recommended values to within the error limits of the measured values. Potential sources of error in the temperature measurement and the kinetic parameters are explored. The expected accuracy of the experiments is assessed, and possible improvements in the experiment are suggested. © 1996 John Wiley & Sons, Inc.