Kinetics of the Decomposition of Ferrocenium Ion and Its Derivatives

The first-order kinetics of the decomposition of ferrocenium ion (Fc⁺) and its substitution derivatives have been studied in aqueous sulfuric acid and in the presence of excess Ce(IV) ion. The observed first-order rate constant (k_obs) is expressed as k_obs = k_d for the acyl-substituted ferrocenium ions and k_obs = k_d+ k_ox[Ce(IV)]_o for the unsubstituted and alkyl-substituted ferrocenium ions. Electron-donating alkyl substituents stabilize the ferrocenium ion whereas electron-withdrawing acyl substituents make it less stable. The order of relative stability toward decomposition is 1,1′-dimethyl Fc⁺ ≥ butyl Fc⁺ > 1,1-dimethylpropyl Fc⁺ > Fc⁺ > > formyl Fc⁺ > acetyl Fc⁺ > > benzoyl Fc⁺. A mechanism to interpret the kinetics is also given.     

Investigation of the kinetics of aquation of the 1:2 complex between Cr and nitrilotriacetic acid

Aquation of the 1:2 complex between Cr^III and nitrilotriacetic acid (NTA) was monitored using a combination of capillary electrophoresis (CE), ultraviolet–visible (UV–vis) spectrophotometry, and attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy. To our knowledge, this is the first published report of the use of either CE or ATR-FTIR to monitor the kinetics of ligand exchange reactions of Cr^III–aminocarboxylate complexes. The aquation products were identified as the 1:1 Cr^III complex with NTA and “free” NTA. The 1:1 complex dimerized to form a 2:2 complex in a slower subsequent reaction step. Rates of disappearance of the 1:2 complex were first-order under all experimental conditions. First-order rate constants for aquation, k_obs (h⁻¹), measured using all three techniques were similar at equivalent pH conditions, and with values reported previously in the literature. Measured k_obs values exhibited a complicated pH dependence with three distinct regions: (i) at pH < 6.5, k_obs values increased with decreasing pH, (ii) between pH 6.5 and 8.0, k_obs values were relatively constant, and (iii) at 8.0 < pH < 10.0, k_obs increased with increasing pH and then leveled off. A kinetic model incorporating five distinct aquation pathways was successfully employed to model the pH dependence of k_obs from 0.0 < pH < 10.0. These results show that CE and ATR-FTIR can be used as tools for better understanding ligand exchange processes occurring in aqueous solution.     

Catalytic effects of cationic nanoparticle (CTABr/NaX/H<Subscript>2</Subscript>O,X = Cl,Br) for the piperidinolysis of phenyl salicylate ions

The expression of pseudo-second-order rate constants (k _X) for cationic nanoparticle (CN) [CTABr/NaX/H₂O, X = Br, Cl, CTABr = cetyltrimethylammonium bromide] catalyzed piperidinolysis-ionized phenyl salicylate (PSa^–), at constant [CTABr]_T, 0.1 M piperidine (Pip), and 35°C, were calculated from the relationship: k _obs = (k ₀ + k _X[NaX])/(1 + K ^X/S[NaX]), in which k ₀, k _X, and K ^X/S are constant kinetic parameters and k _obs represents the pseudo-first-order rate constant for Pip reaction with phenyl salicylate ion in the presence of CN. The source of the large catalytic effect of CN catalyst was shown to be due to the transfer of PSa^– from pseudo-phase of the CNs to the bulk aqueous phase through X^–/PSa^– ion exchange at the surface of the CNs.     

The production of Cr(NH3)6 from the acid catalysed hydrolysis of Cr(NH3)5(NCO)

The rate of the reaction

has been investigated at 40–65°C with [HClO₄] varying from 0.04 to 0.6 M (μ = 0.6 M, NaClO₄). The observed rate law has the form: -d[Cr(NH₃)₅(NCO)²⁺]/dt = k_obs[Cr(NH₃)₅(NCO)²⁺] where k_obs = a[H+]²{1 + b[H⁺]²} and ^?1 at 55.0°C, a = 0.36 M^?1 s^?2 and b = 6.9 × 10^?3 M^?1 s^?1. The rate of loss of Cr(NH₃)₅(NCO)²⁺ increases with increasing acidity to a limiting value (at [H⁺] ～ 0.5 M) but the yield of Cr(NH₃)₆³⁺ decreases with increasing [H⁺] and increases with increasing temperature. In the kinetic studies the maximum yield of Cr(NH₃)₆³⁺ was 35% but a synthetic procedure has been developed to give a 60% yield.     

Kinetics of the alkaline hydrolysis of fenuron in aqueous and micellar media

The kinetics of the hydrolysis of fenuron in sodium hydroxide has been investigated spectrometrically in an aqueous medium and in cationic micelles of cetyltrimethylammonium bromide (CTAB) medium. The reaction follows first‐order kinetics with respect to [fenuron] in both the aqueous and micellar media. The rate of hydrolysis increases with the increase in [NaOH] in the lower concentration range but shows a leveling behavior at higher concentrations. The reaction followed the rate equation, 1/k_obs = 1/k + 1/(kK[OH^?]), where k_obs is the observed rate constant, k is rate constant in aqueous medium, and k is the equilibrium constant for the formation of hydroxide addition product. The cationic CTAB micelles enhanced the rate of hydrolytic reaction. In both aqueous and micellar pseudophases, the hydrolysis of fenuron presumably occurs via an addition–elimination mechanism in which an intermediate hydroxide addition complex is formed. The added salts decrease the rate of reaction. © 2007 Wiley Periodicals, Inc. Int J Chem Kinet 39: 638–644, 2007     

Reactivity trends in the base hydrolysis of 6-nitro-2H-chromen-2-one and 6-nitro-2H-chromen-2-one-3-carboxylic acid in binary mixtures of water with methanol and acetone at different temperatures

Kinetics of the base hydrolysis of 6-nitro-2H-chromen-2-one (NC) and 6-nitro-2H-chromen-2-one-3-carboxylic acid (NCC) in water-methanol and water-acetone mixtures was studied at temperature range from 283 to 313 K. The activation parameters of the reactions were evaluated and discussed. The change in the activation barrier of the investigated compounds from water to water-methanol and water-acetone mixtures were estimated from the kinetic data. The base hydrolysis of NC and NCC in the water-methanol and water-acetone mixtures follows a rate law with k _obs = k ₂[OH⁻] and k _obs = k ₁ + k ₂[OH⁻], respectively. The decrease in the rate constants of NC and NCC hydrolysis, as the proportion of methanol and acetone increases, is accounted for by the destabilization of the OH⁻ ion. The activation and thermodynamic parameters were determined.     

Kinetic evidence for the occurrence of trianionic tetrahedral intermediate in the hydroxide ion-catalyzed cleavage of 1, 2-benzisothiazolin-3-one-1, 1-dioxide (saccharin)

The kinetics of hydrolytic cleavage of saccharin has been studied at 60°C within the [ōH] range of 0.1 to 3.0 M. The observed pseudo first-order rate constants, k_obs, follow an empirical relationship: k_obs = B[ōH] + [C[ōH]]². The B and C terms are attributed to the formation of dianionic and trianionic tetrahedral intermediates on the reaction path. It is concluded that the ionized form of saccharin is the major reacting species under the present experimental conditions. The positive ionic strength effect and the negative effect of 1,4-dioxan on the rate of hydrolysis favor the proposed reaction mechanism. The analysis of the observed activation parameters indicates that the increase in the contribution of C term to k_obs causes the slight increase in both ΔH* and ΔS*. A significantly large negative value of ΔS* favors the proposed mechanism.     

Kinetic,solvation and reactivity studies of iron(II) complexes of monoxime and dioxime ligands

Complexes of Fe^II with monoxime and dioxime ligands have been isolated and characterised. Kinetic results and rate laws are reported for acid aquation and base hydrolysis of these complexes in H₂O and in MeOH–H₂O mixtures. Kinetics of acid catalysed aquation of Fe^II–monoxime complexes follow a rate law with k_obs = k₂[H⁺] + k₃[H⁺]², while kinetics of acid dissociation and base hydrolysis of the Fe^II–dioxime complex follow rate laws with k_obs = k₂[H⁺] and k_obs = k₂[OH⁻]. Acid aquation and base hydrolysis mechanisms are proposed. The solubilities of Fe^II–monoxime and –dioxime complex salts are reported and transfer chemical potentials of their complex cations are calculated. Solvent effects on reactivity trends have been analysed into initial and transition state components. These are determined from transfer chemical potentials of reactant and kinetic data. Rate constant trends from these complexes are compared and discussed in terms of ligand structure and solvation properties. Our kinetic results give information relevant to the application of these ligands as analytical reagents for trace Fe^II in acidic and neutral media, in water and in aqueous alcohols.     

Kinetics of nucleophilic attack on coordinated organic moieties. XVIII. Nucleophilicity of anions towards the tricarbonyl-(η-benzene) manganese (I) cation.

Kinetic results for the addition of OH^? to [Mn(CO)₃(η-C₆H₆)]⁺ (I) in water (eq. 1, X  OH) obey the expression k_obs  k_OH[OH^?], and give a k_OH value of 290 mol^?1 dm³ s^?1 at 20.0°C and ionic strength of 0.25 mol dm^?3. The analogous reaction of NaCN with I in water fits the two-term expression k_obs = k_OH[OH^?] + k_CN[CN^?], and leads to a k_CN value of 0.8 mol^?1 dm³ s^?1 at 20.0°C and ionic strength of 0.25 mol dm^?3. Interestingly, the related reaction (eq. 1, X  N₃) is too rapid to follow by stopped-flow spectrophotometry, indicating the overall rate trend N₃^? » OH^? » CN^?. This unusual nucleophilicity order, unexpected on the basis of both basicity and polarizability, is similar to that previously observed for anion addition to free carbonium ions.     

Kinetic Screening for the Acid‐Catalyzed Hydrolysis of Some Hydrophobic Fe(II) Schiff Base Amino Acid Chelates and Reactivity Trends in the Presence of Alkali Halide and Surfactant

Kinetics of acid‐catalyzed hydrolysis of some high‐spin Fe(II) Schiff base amino acid complexes were followed spectrophotometrically at 298 K under pseudo–first‐order conditions. The studied ligands were derived from the condensation of 5‐bromosalicylaldehyde with different four amino acids (phenylalanine, aspartic acid, histidine, and arginine). The acid hydrolysis reaction was studied in aqueous media and in the presence of different concentrations of the alkali halide (KBr) and cationic surfactant (cetyl‐trimethyl ammonium bromide, CTAB). The general rate equation was suggested to be rate = k_obs[complex], where k_obs = k₂[H⁺]. The increase in [KBr] enhances the reactivity of the reaction, and the addition of CTAB to the reaction mixture accelerates the reaction reactivity. The obtained kinetic data were used to determine the values of δ_mΔG^# (the change in the activation barrier) for the studied complexes when transferred from “water to water containing different [KBr]” and from “water to water containing altered [CTAB].”     

Reactive kinetics of carbon dioxide loaded aqueous blend of 2-amino-2-ethyl-1,3-propanediol and piperazine using a pressure drop method

In this study, a new pressure drop method has been used to investigate the kinetics of carbon dioxide reaction with aqueous blend of 2-amino-2-ethyl-1,3-propanediol (AEPD) with piperazine (PZ). The blending of a small amount of PZ with AEPD has a significant effect on the observed rate constant, k_obs. It was observed that k_obs values of the blend increased more than twice than the summation of k_obs values of individual alkanolamines. The reaction kinetics in this study were modeled by assuming a termolecular mechanism. The addition of 0.1 mol/L of PZ to 1 mol/L AEPD exhibited an observed rate constant, k_obs of 8824.1 s⁻¹, which is comparable to other alkanolamine mixtures. Hence, PZ/AEPD mixtures can be potentially used for rapid carbon dioxide capture.     

Kinetic investigation of aquation of some Fe(II) Schiff base amino acid complexes

Kinetics of aquation of some Fe(II) Schiff base amino acid complexes was followed spectrophotometrically. The Schiff base ligands were derived from salicylaldehyde and isoleucine, leucine, serine, methionine, tryptophan, or histidine. The reaction was studied in aqueous media, aqua–propanol mixtures, and in the presence of different concentrations of KBr. Moreover, the activation parameters were calculated and discussed for structures and other physical properties observed. The reaction was acid catalyzed and the general rate equation was suggested as follows: rate = k_obs [complex], where k_obs = k₂ [H⁺]. © 2010 Wiley Periodicals, Inc. Int J Chem Kinet 42: 372–379, 2010     

Kinetic evidence for the occurrence of general acid-base catalysis in N-methylhydroxylaminolysis of N-ethoxycarbonylphthalimide (NCPH)

Pseudo-first-order rate constants (k_{1 obs}) for the reaction of MeNHOH with NCPH obey the relationship: k_{1 obs}=k^′_b[MeNHOH]_T² where [MeNHOH]_T represents total concentration of N-methylhydroxylamine buffer. The rate constants, k_{1 obs} obtained at different total concentration of acetate buffer ([Buf]_T) in the presence of 0.004 mol dm⁻³ MeNHOH follow the relationship: k_{1 obs}=k_b[Buf]_T. The values of acetate buffer-catalyzed rate constant (k_b) at different pH reveal the occurrence of both general base- and general acid- or general base-specific acid-catalysis in the reaction of MeNHOH with NCPH. © 1997 John Wiley & Sons, Inc. Int J Chem Kinet 29: 647–654, 1997.     

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 and mechanism of the catalysed decomposition of sodium perborate by the ethylenediaminetetraacetatoiron(III) complex

The rate of reaction of [Fe^IIIY] (Y = EDTA anion) with NaBO₃ was studied in aqueous 0.1 M NaNO₃ at various temperatures. The observed rate constant,k _obs = kKKK ₁[FeEDTA(H₂O)^-]/{[H⁺] + KK ₁[FeEDTA(H₂O)^-]}applies over the pH range studied. The monohydroxy complex, [FeEDTA (OH)]^2– is the catalyst which reacts with the peroxy ion to produce a violet intermediate complex. The composition of perborate confirms to Michaelis–Menten kinetics, the rate-determining step involving breakdown of the intermediate complex. The activation enthalpy and activation entropy were calculated.     

Kinetics and Mechanism of Chromium(III)-Picolinato and Chromium(III)-Dipicolinato Complexes Aquation in HNO3 Solutions

The acid-catalyzed aquation of [Cr(pic)(H₂O)₄]₂ ²⁺ and [Cr(dpic)(H₂O)₃]⁺(pic = picolinic acid anion, dpic = dipicolinic acid dianion) in nitrate(V) media was studied. The reaction is reversible in the case of the pic-complex and practically irreversible in the case of the dpic-complex. It is assumed that the reactive form of the substrate undergoes fast chelate ring-opening followed by protolytic equilibria, followed by the rate of the Cr—O bond breaking of the monodentate bonded ligand which is the rate-determining step. The kinetics of pic/dpic ligand liberation were followed spectrophotometrically in the 0.4–2.0 M HNO₃ range at I= 2.0 M. The following dependences of the pseudo-first order rate constants on [H⁺] have been established:k _obs=a+b[H⁺](where b and a are apparent rate constants for the forward and the reverse reaction of the pic-complex) and k _obs=b[H⁺]+c[H⁺]²(where b and c are apparent rate constants for the dpic liberation). Fast protolytic pre-equilibria, leading to protonation of the carboxylic oxygen atom on the monodentate bonded ligand, preceeds ligand liberation.     

Kinetics and mechanism of ruthenium(III) catalyzed oxidation of tetrahydrofurfuryl alcohol by cerium(IV) in sulfuric acid media

The kinetics and mechanism of ruthenium(III) catalyzed oxidation of tetrahydrofurfuryl alcohol (THFA) by cerium(IV) in sulfuric acid media have been investigated spectrophotometrically in the temperature range 298–313 K. It is found that the reaction is first-order with respect to Ce^IV, and exhibits a positive fractional order with respect to THFA and Ru^III. The pseudo first-order ([THFA]≫[Ce^IV]≫[Ru^III]) rate constant k _obs decreases with the increase of [HSO ₄ ⁻ ]. Under the protection of nitrogen, the reaction system can initiate polymerization of acrylonitrile, indicating the generation of free radicals. On the basis of the experimental results, a reasonable mechanism has been proposed and the rate equations derived from the mechanism can explain all the experimental results. From the dependence of k _obs on the concentration of HSO ₄ ⁻ , has been found as the kinetically active species. Furthermore, the rate constants of the rate determining step together with the activation parameters were evaluated.     

The oxidation of thiosulphate ions by octacyanotungstate(V) in weak acidic medium

The kinetics of the oxidation of thiosulphate ions by octacyanotungstate(V) ions has been studied in the pH range 3.9–5.0. The reaction showed zero-order kinetics with respect to [W(CN)₈^3?] and is consistent with the rate law R = k[H⁺][S₂O₃^2?]². A reaction mechanism is proposed for the reaction with a third-order rate constant of 0.26 M^?2 s^?1 at 25°C.     

A kinetic study on the iron(III)-promoted aquation of [Cr(C2O4)2(picolinato)]2− and [Cr(C2O4)2(2-pyridineacetato)]2− complexes

Two [Cr(C₂O₄)₂(AB)]^2– type complexes, obtained from the reaction of cis-[Cr(C₂O₄)₂(H₂O)₂]^– with the AB ligand, [AB = picolinic (pyac) or 2-pyridine-ethanoic acid (pyeac) anions], were converted into [Cr(C₂O₄)(pyac)(H₂O)₂]⁰ and [Cr(C₂O₄)(pyeac)(H₂O)₂]⁰ compounds, respectively via Fe^III-induced substitution of the oxalato ligand. The aquation products were separated chromatographically and their spectral characteristics and acid dissociation constants determined. The kinetics of the oxalato ligand substitution were studied with a 10–40 fold excess of Fe^III over [Cr^III] at [H⁺] = 0.2 M and at constant ionic strength 1.0 M (Na⁺, H⁺, Fe³⁺, ClO^– ₄). The reaction rate law is of the form: r = k _obs[Cr^III], where k _obs = kQ[Fe^III]/(1 + Q[Fe^III]). The first-order rate constants (k), preequilibria quotients (Q) and activation parameters derived from the k values have been determined. The reaction mechanism is discussed in terms of a Lewis acid catalyzed (induced) ligand substitution.     

Study of equilibria and kinetics of the acid catalysed interaction of iron(III) with salicylaldehyde ando-hydroxyacetophenone

The equilibria and kinetics of the reaction of Fe^III with salicylaldehyde ando-hydroxyacetophenone, leading to 1∶1 chelate formation, have been studied at different temperatures (25–35°C) and ionic strength, I = 1.0 mol dm⁻³ (NaClO₄+HClO₄). A dual path mechanism involving both Fe _aq ³⁺ and Fe(OH) _aq ²⁺ species and undissociated free ligand (LH) is consistent with the experimental observations where [H⁺]≫[Fe]_T≫[L]_T (where [Fe]_T and [L]_T stand for total concentrations of iron and ligand respectively). The results conform to k_obs/B = k₁[H⁺]+k₂K_h where B = [Fe]_T/(K_h+[H⁺])+1/Q; K_h = hydrolysis constant of Fe _aq ³⁺ ; k₁, k₂ are the forward second order rate constants of Fe _aq ³⁺ and Fe(OH) _aq ²⁺ , respectively, and Q is the equilibrium constant of the reaction, Fe³⁺+LH⇋FeL²⁺+H⁺. Thermodynamic parameters for each of the steps have been determined. Fe(OH) _aq ²⁺ appears to react in a dissociative fashion (Eigen-Tamm mechanism), whilst Fe _aq ³⁺ appears to react through the associative inter-change (I_a) mechanism. The equilibrium constants (Q) obtained spectrophotometrically are compared with those obtained from kinetic studies. TMC 2638