Kinetics and mechanism of nucleophilic halogen exchange in furan derivatives

Studies are made of the reaction of 5-bromofurfurylidenaniline with aromatic amines (reaction 1), of the disproportionation of 5-bromofurfurylidenaniline (reaction 2), and of the reaction of 5-bromofurfurylidenaniline and 5-bromofurfural with saturated secondary amines (reaction 3). It is shown that these reactions are of the autocatalytic type (the catalyst being a reaction product, the hydrobromide of the amine), that addition of acid accelerates them, while they are retarded by addition of alkali. The hypothesis is advanced that in these reactions the active form of the substrate is the salt containing the fragment. The latter arises by protonation of the Schiff's base (reactions 1 and 2), or by quaternization of aminoacetals (which are products formed by addition of a secondary amine across the (double) bonds in through the action of the hydrohalide salt of the amine (reaction 3).     

Kinetics and mechanism of hydrolysis of N-acyloxymethyl derivatives of azetidin-2-one

The pH-independent, acid-catalyzed and base-catalyzed hydrolyses of N-acyloxymethylazetidin-2-ones all occur at the ester function. The pH-independent hydrolysis involves rate-limiting alkyl C-O fission and formation of an exocyclic beta-lactam iminum ion. This iminium ion is then trapped by water at the exocyclic iminium carbon atom, rather than at the beta-lactam carbonyl carbon atom, to form the corresponding N-hydroxymethylazetidin-2-ones. Calculations carried out at the B3LYP/6-31+G(d) level of theory also support that nucleophilic attack by water takes place at the exocyclic carbon rather than at the beta-lactam carbonyl carbon of the iminium ion. The mechanism for the acid-catalyzed pathway involves a preequilibrium protonation, probably at the beta-lactam nitrogen, followed by rate-limiting alkyl C-O fission with formation of an exocyclic iminum ion. The base-catalyzed hydrolysis involves rate-limiting hydroxide attack at the ester carbonyl carbon. These results imply formation of a beta-lactam system containing a positively charged amide nitrogen atom that hydrolyzes via a pathway that preserves the beta-lactam structure in the product and provide further evidence that cleavage of the beta-lactam C-N bond is not as facile as is commonly imagined.     

Kinetics and mechanism of chelating reaction between chitosan derivatives with Ca(II)

Chitosan derivatives, such as chitosan alpha-ketoglutaric acid (KCTS) and hydroxamated chitosan alpha-ketoglutaric acid (HKCTS), are prepared and their coordination behavior toward Ca(II) was studied. The adsorption isotherms were correlated by dc/dt?=??kcⁿ at 20°C, 30°C, 40°C, 50°C, and 60°C. By linear correlation, the shapes of the isotherm curves were similar to the kinetic function of 1/c?=?kt and the rate equation was dc/dt?=??kc ²; the activation energies were 13.31 and 14.76?kJ?mol^?1 for KCTS and HKCTS, respectively. The overall rate of Ca(II) adsorption is likely to be controlled by the chemical process. The coordination mechanism of chitosan derivatives with Ca(II) was studied by infrared and X-ray photoelectron spectroscopy. The results indicated that –NH– of KCTS was coordinated. Nitrogen of amino, oxygens of hydroxamic acid, and carbonyl in HKCTS coordinated with Ca(II).     

Kinetics and mechanism of the oxidation of guanosine derivatives by Pt(IV) complexes

The kinetics of redox reactions of the PtIV complexes trans-Pt(d,l)(1,2-(NH2)2C6H10)Cl4 ([PtIVCl4(dach)]) and Pt(NH2CH2CH2NH2)Cl4 ([PtIVCl4(en)]) with 5'- and 3'-dGMP (G) have been studied. These redox reactions involve substitution followed by an inner-sphere electron transfer. The substitution is catalyzed by PtII and follows the classic Basolo-Pearson PtII-catalyzed PtIV-substitution mechanism. We found that the substitutution rates depend on the steric hindrance of PtII, G, and PtIV with the least sterically hindered PtII complex catalyzing at the highest rate. 3'-dGMP undergoes substitution faster than 5'-dGMP, and [PtIVCl4(en)] substitutes faster than [PtIVCl4(dach)]. The enthalpies of activation of the substitution, DeltaH double dagger s, of 3'-dGMP is only 70% greater than that of 5'-dGMP (50.4 vs 30.7 kJ mol(-1)), but the entropy of activation of the substitution, DeltaS double dagger s, of 3'-dGMP is much greater than that of 5'-dGMP (-59.4 vs -129.5 J K(-1) mol(-1)), indicating that steric hindrance plays a major role in the substitution. The enthalpy of activation of electron transfer, DeltaH double dagger e, of 3'-dGMP is smaller than that of 5'-dGMP (88.8 vs 137.8 kJ mol(-1)). The entropy of activation of electron transfer, DeltaS double dagger e, of 3'-dGMP is negative, but that of 5'-dGMP is positive (-27.8 vs +128.8 J K-1 mol-1). The results indicate that 5'-hydroxo has less rotational barrier than 5'-phosphate, but it is geometrically unfavorable for internal electron transfer. The electron-transfer rate also depends on the reduction potential of PtIV. Because of its higher reduction potential, [PtIVCl4(dach)] has a faster electron transfer than [PtIVCl4(en)].     

Kinetics and mechanism of the reactions of ozone with guaiacol,veratrol, and veratrol derivatives

The kinetics of the reactions of ozone with compounds modeling structural lignin fragments, viz., phenol, guaiacol, veratrol, veratric aldehyde, and veratric alcohol, was studied. The reaction rate constants were calculated using the mass transfer model for the chemical reaction based on the film theory. The rate constants of the reactions of ozone with compounds of the veratryl series were calculated by the Hammett equation. The major ozonation products of the studied compounds were determined by HPLC. The ozonation mechanism was proposed. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 276–281, February, 2008.     

Kinetics and mechanism of diels-alder additions of tetracyanoethylene to anthracene derivatives—I : Substituent effects

The rates of addition of tetracyanoethylene to a number of 9- and 9,10-substituted anthracenes have been measured spectrophotometrically in solvent CCl₄. Substituent effects correlated well using the extended form of the Hammett equation. The importance of steric effects on the reaction was assessed by a systematic variation of the components of the data used in the above correlation.Activation parameters (ΔG_exp^≠, etc.) and the corresponding overall thermodynamic parameters for adduct formation (ΔG_ad°, etc.) were evaluated. ΔG_exp^≠ was found to be linearly related to ΔG_c°, the free energy of formation of the intermediate complex which confirms the role of the latter as a true reaction intermediate. From correlations between ΔG_exp^≠ and ΔG_ad°, an “early” transition state is suggested. The above thermodynamic and activation data enable detailed reaction profiles to be drawn.     

Kinetics and mechanism of oxidation of ethylenediamine and related compounds by diperiodatoargentate (III) ion

The oxidation of ethylenediamine by diperiodatoargentate (III) ion has been studied by stopped‐flow spectrophotometry. Kinetics of this reaction involves two steps. The first step is the complexation of silver (III) with the substrate and is over in about 10 ms. This is followed by a redox reaction in the second step that occurs intramolecularly from the substrate to the silver (III) center. The rate of reduction of silver (III) species by ethylenediamine, ethanolamine, and 1,2‐ethanediol were observed to be 1.2 × 10⁴, 1.1 × 10², and 0.14 dm³ mol⁻¹ s⁻¹, respectively, at 20°C. The reaction rate shows an inverse dependence on [IO] and [OH⁻] in the low concentration range (≤1 × 10^‐3 mol dm⁻³). At higher [OH⁻] (>1 × 10⁻³ mol dm⁻³) the rate of reaction starts increasing and attains a limiting value at very high [OH⁻]. The rate of deamination of ethylenediamine is enhanced by its complexation with silver (III). The involvement of [Ag^III(H₂IO₆) (H₂O)₂] and [Ag^III(H₂IO₆) (OH)₂]²⁻ are suggested as the reactive silver (III) species kinetically in mild basic and basic conditions, respectively. © 2000 John Wiley & Sons, Inc. Int J Chem Kinet 32: 286–293, 2000     

Kinetics and mechanism of the reaction of 5-bromo-2- carbonyl derivatives of furan with amines

The kinetics of exchange of bromine in 5-bromo-2-carbonyl derivatives of furan by dimethylamino and morpholino groups was studied. The reaction is kinetically complicated for 5-bromofurfural (I) — aldehyde I initially reacts simultaneously at both the carbonyl group to give the aminocarbinol and at the bromine atom with exchange by an amino group. The accumulation of the ammonium salt in the latter reaction leads to the development of an autocatalytic reaction, the product of which is 5-N,N-dialkylaminofurfurylidene-N,N-dialkylammonium bromide. Amines react with the ketones without complications; the second-order rate constants are presented. The rate of exchange decreases in the order CHO > COC₆H₅ > COCH=CHC₆H₅ > COCH₃ > CH=CHCOC₆H₅.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 11, pp. 1465–1470, November, 1976.     

Kinetics and mechanism of peroxymonocarbonate formation

The kinetics and mechanism of peroxymonocarbonate (HCO(4)(-)) formation in the reaction of hydrogen peroxide with bicarbonate have been investigated for the pH 6-9 range. A double pH jump method was used in which (13)C-labeled bicarbonate solutions are first acidified to produce (13)CO(2) and then brought to higher pH values by addition of base in the presence of hydrogen peroxide. The time evolution of the (13)C NMR spectrum was used to establish the competitive formation and subsequent equilibration of bicarbonate and peroxymonocarbonate following the second pH jump. Kinetic simulations are consistent with a mechanism for the bicarbonate reaction with peroxide in which the initial formation of CO(2) via dehydration of bicarbonate is followed by reaction of CO(2) with H(2)O(2) (perhydration) and its conjugate base HOO(-) (base-catalyzed perhydration). The rate of peroxymonocarbonate formation from bicarbonate increases with decreasing pH because of the increased availability of CO(2) as an intermediate. The selectivity for formation of HCO(4)(-) relative to the hydration product HCO(3)(-) increases with increasing pH as a consequence of the HOO(-) pathway and the slower overall equilibration rate, and this pH dependence allows estimation of rate constants for the reaction of CO(2) with H(2)O(2) and HOO(-) at 25 °C (2 × 10(-2) M(-1) s(-1) and 280 M(-1) s(-1), respectively). The contributions of the HOO(-) and H(2)O(2) pathways are comparable at pH 8. In contrast to the perhydration of many other common inorganic and organic acids, the facile nature of the CO(2)/HCO(3)(-) equilibrium and relatively high equilibrium availability of the acid anhydride (CO(2)) at neutral pH allows for rapid formation of the peroxymonocarbonate ion without strong acid catalysis. Formation of peroxymonocarbonate by the reaction of HCO(3)(-) with H(2)O(2) is significantly accelerated by carbonic anhydrase and the model complex [Zn(II)L(H(2)O)](2+) (L = 1,4,7,10-tetraazacyclododecane).     

Kinetics and mechanism of coal flotation

 The flotation kinetics of coarse coal particles was studied in a modified version of the Hallimond tube at 25 °C using nitrogen as the carrier gas, in the pH range 2–12. The kinetics was followed by measuring the volume of the particles accumulated in the collector tube as a function of time. At each pH, the rate constants were determined at several buffer concentrations and were extrapolated to zero buffer concentration. The observed first-order rate constant was represented as the product of separable constants and functions such as f _D, f _V and f _pH, which depend only on the particle size, gas flow and the pH of the dispersion, respectively. The diameter, D, of the particles was in the range 505–127 μm. The observed rate constant decreased linearly with the diameter of the particles at constant flow and it was calculated that f _D=exp(−1.56D). The dependence of f _V on the flow is a consequence of the fact that the flotation occurs when a single particle is captured by two bubbles. f _V was shown to be independent of the particle diameter. The effect of the pH on the rate of flotation was considered as resulting from the adsorption of protons (or hydroxide ions) by the particles and bubbles through multiple equilibria, assuming that there is no interaction between the binding sites. The pH–rate profile showed that there were two species responsible for the flotation: one stable at pH below 5 and the other at high pH. Comparison of f _V, f _D and f _pH for the flotation of coal and pyrite allowed the prediction of the optimum conditions for the separation of mixtures of these particles by flotation. Received: 6 August 2001  Accepted: 19 September 2001     

Kinetics and mechanism of acetylene polymerization

The kinetics of acetylene polymerization initiated by Ti(OBu)₄/4AlEt₃ catalyst was studied by radioquenching with C^*O to count the number of active sites [C] and by CH₃OT^* to determine the total metal polymer bonds [MPB] and M?_n of the polymer. The amount of quenching agent and time of reaction required and the kinetic isotope effect for CH₃OT^* were determined. The effects of Al/Ti ratio, catalyst aging, catalyst concentration, temperature, and monomer pressure on the polymerization were investigated. Detailed kinetic data on the variation of rate of polymerization, R_p, [C] [MPB], and M?_n with time were obtained at 298 and 195°K. The results required the assumption that the catalytic species C, is initially active and within less than 30 min all are converted by bimolecular kinetics to a far less active species. Analysis of the data yielded rate constants of propagation and termination and their energies of activation. Estimates of chain transfer efficiency were obtained. The mechanisms for the propagation, termination, and transfer processes were discussed. By drawing on our earlier EPR results we propose probable structures for the catalytically active species.     

Kinetics and mechanism of the sensitized photodegradation of uracil--modeling the fate of related herbicides in aqueous environments

The dye-sensitized photodegradation of uracil (UR), the parent compound of several profusely employed herbicides, has been studied as a model of their environmental fate. In order to mimic conditions frequently found in nature, aqueous solutions of UR have been irradiated with visible light in the presence of the natural sensitizer riboflavin (Rf). The results indicate that UR is photostable in acid media, but is quickly degraded in pH 7 or pH 9 solutions, where singlet molecular oxygen [O2(1Delta(g))] and, to a lesser extent, superoxide radical anion (O2*-)-both species photogenerated from triplet excited Rf, 3Rf*-participate in the photodegradation. At pH 7, UR is slowly degraded through an O2*- -mediated mechanism, whereas Rf disappears through its reaction with O2(1Delta(g)) and, in the form of 3Rf*, with UR. On the contrary, at pH 9 Rf is photoprotected through two processes: its regeneration from the formed Rf radical species-a back electron transfer that also produces O2*- -and the elimination from the medium of O2(1Delta(g)) by its reaction with UR. The overall result of the preservation of ground state Rf is the continuity of the photosensitized process and, hence, of the UR degradation. Media with higher pH values could not be employed due to the fast photodegradation of Rf. With rose bengal (RB) as photosensitizer, the rate constants found for the overall interaction between UR and the photogenerated O2(1Delta(g)) were in the range 5 x 10(5) M(-1) s(-1) (at pH 7) to 1.3 x 10(8) M(-1) s(-1) (in 1 M NaOH aqueous solution, mainly physical quenching). The maximum O2(1Delta(g)0-mediated photooxidation efficiencies with RB were reached at pH 11, where only the O2(1Delta(g)0-reactive quenching with UR was observed.     

Kinetics and mechanism of the reaction between ascorbic acid derivatives and an arenediazonium salt: cationic micellar effects

The effects of tetradecyltrimethylammonium bromide, TTAB, and hexadecyl-trimethylammonium bromide, CTAB, micellar systems on the reaction of 3-methylbenzenediazonium, 3MBD, tetrafluoroborate with ascorbic acid, VC, and with the hydrophobic derivatives 6-O-dodecyl-L-ascorbic acid, VC12, and 6-O-palmitoyl-L-ascorbic acid, VC16, were investigated at different pH values by employing a combination of UV-vis spectroscopy and high-performance liquid chromatography, HPLC, techniques. Previous studies in the absence of surfactant showed that the reaction between 3MBD and VC derivatives takes place through a rate-limiting decomposition of a transient diazo ether, DE, formed from reaction between 3MBD and the monoanion form of ascorbic acid, VC-, in a rapid preequilibrium step. In the presence of a fixed [CTAB], the kinetics of the reaction of 3MBD with VC follows a saturation kinetics similar to that observed in its absence, but for the reaction with VC12 and VC16, only the first linear portions of the saturation profiles could be obtained because k(obs) values become too large. HPLC analyses of the reaction mixtures show that no unexpected products are detected, suggesting that cationic micelles do not modify the mechanism of the reaction. Analyses of the kinetic data allowed estimations of the rate constant for the decomposition of the diazo ether and of the equilibrium constant for the formation of DE in the presence of CTAB micelles, which is approximately 6 times higher than in its absence; this suggests that CTAB micelles promote diazo ether formation. At constant [antioxidant], the variations of k(obs) for the reactions with VC, VC12, or VC16 follow bell-shaped curves, with rate enhancements of up to 2-3-fold for VC with respect to the value in the absence of surfactant. The rate maximum for the reaction of 3MBD with VC is reached at [CTAB] = 0.02 M suggesting a CTAB-induced rate increase, i.e., micellar catalysis; meanwhile the rate maximum for the reaction with VC12 and VC16, which may behave as amphiphilic compounds, is reached at [CTAB] approximately 1 x 10(-4) M, a concentration about 10 times lower than its critical micelle concentration, cmc, in pure water, but only approximately 3 times lower than the cmc of VC16, suggesting the formation of reactive CTAB-VC12 and CTAB-VC16 premicellar aggregates. Kinetic and HPLC results are consistent with the predictions of the pseudophase model and are interpreted in terms of 3MBD ions sampling in the aqueous bulk phase and the micellar effects on the different equilibrium involved. The results should contribute to a better understanding of the role of compartmentalized systems on the efficiency with which hydrophilic and hydrophobic reductants such as ascorbic acid derivatives interact with potentially mutagenic and carcinogenic ArN2+ ions.     

Kinetics and mechanism of the decomposition of dibridged cobalt dioxygen complexes of amino acid derivatives in alkaline medium

The kinetics and thermodynamics of the decomposition of dibridged μ(O₂²⁻, OH⁻)cobalt complexes with the linear tetradentate ⁻OOCCH(R)NHCH₂CH₂NHCH(R)COO⁻ (L = EBAA*) ligand have been studied in alkaline medium. Under these conditions, the dioxygen complex Co₂L₂O₂OH⁻ undergoes irreversible metal-centered autoxidation to form a CO(III) complex with the unchanged ligand and H₂O₂. The reaction is base-catalyzed, and a mechanism has been proposed in which deprotonation of a coordinated nitrogen activates the hydrolysis of the CO^IIIO₂ bond. From the enthalpy—entropy relationship, the isokinetic temperature β=274 K has been found.     

5'-methylaristeromycin and related derivatives

The biological versatility of aristeromycin (carbocyclic adenosine) is limited by accompanying cytotoxicity caused ostensibly by the intracellular formation of its 5'-nucleotide derivatives. Aristeromycin derivatives that offered steric interference to this transformation at the C-5' center were sought. This paper describes the facile stereospecific synthesis, where necessary, of such C-5'-methylated aristeromycin derivatives.     

Kinetics and mechanism of azidomercuration of alkenes,cycloalkenes, and their derivatives with a mercuric acetate-sodium azide system

The kinetics of azidomercuration of alkenes, cycioalkenes, and their derivatives with a Hg(OAc)₂-NaN₃ system (1 1, 1 2, and 1 3) was studied. Based on the data on the product structure and kinetic results, it was concluded that HgOAcN₃ and Hg(N₃)₂ play the role of azidomercuration reagents. The reactions with alkenes having a strained double bond occur by a concerted scheme. With nonstrained alkenes, a multistep mechanism is realized, its first reversible step involving the formation of a mercurinium intermediate with an ion pair structure.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No.12, pp. 2479–2485, December, 1995.