Structure of zinc-5,15-di(o-methoxyphenyl)octaalkylporphyrines and their reaction with organic peroxides in the presence of pyridine

By combined spectral and calculation methods the structure of zinc 5,15-di(o-methoxyphenyl)-2,8,12,18 3,7,13,17-octaalkylporphyrinates (I, II) and their properties in the reaction with organic peroxides with addition of different amounts of pyridine were studied. The reaction of zinc porphyrinates with peroxides in the presence of pyridine leads to destruction of the complex chromophore. Kinetic parameters of the investigated reaction (effective k _ef and true k _V rate constants) are obtained. The presence of base in the reaction medium is found to lead to a change in the structure of the zinc porphyrinates and affects the rate of oxidation. By quantum-chemical method PM3 the geometry of the reagents was calculated and the deformation distortions of the reactants molecules and intermediates in the course of the oxidation reaction was demonstrated. The influence of electronic effects of substituents and the degree of deformation of the zinc porphyrinate macrocycle on their redox properties is revealed.     

Influence of a reaction medium on the oxidation of aromatic nitrogen-containing compounds by peroxyacids

The influence of different solvents on the oxidation reaction rate of pyridine (Py), quinoline (QN), acridine (AN), α-oxyquinoline (OQN) and α-picolinic acid (APA) by peroxydecanoic acid (PDA) was studied. It was found that the oxidation rate grows in the series Py < QN < AN, and the rate of the oxidation reaction of compounds containing a substituent in the α position from a reactive center is significantly lower than for unsubstituted analogues. The effective energies of activation of the oxidation reaction were found. It was shown that in the first stage, the reaction mechanism includes the rapid formation of an intermediate complex nitrogen-containing compound, peroxyacid, which forms products upon decomposing in the second stage. A kinetic equation that describes the studied process is offered. The constants of equilibrium of the intermediate complex formation (K _eq) and its decomposition constant (k ₂) in acetone and benzene were calculated. It was shown that the nature of the solvent influences the numerical values of both K _p and k ₂. It was established that introduction of acetic acid (which is able to form compounds with Py) into the reaction medium slows the rate of the oxidation process drastically. Correlation equations linking the polarity, polarizability, electrophilicity, and basicity of solvents with the constant of the PDA oxidation reaction rate for Py were found. It was concluded that the basicity and polarity of the solvent have a decisive influence on the oxidation reaction rate, while the polarizability and electrophilicity of the reaction medium do not influence the oxidation reaction rate.     

Cr(VI) oxidation using cetylpicolinium dichromate: Kinetics of oxidation of benzaldehydes with a green protocol

Oxidation kinetics of benzaldehyde and some of its ortho- and para-monosubstituted derivatives have been studied using cetylpicolinium dichromates, a class of novel phase transfer oxidants, in dichloromethane medium. The rate of reaction is first order with respect to oxidant and fractional order with respect to the substrates. The Michaelis–Menten type oxidation was observed with respect to the substrates. Benzaldehydes are found to be oxidized to their corresponding acids. The mechanism of oxidation reaction has been suggested based on the solvent isotope effect, Hammett plot, and thermodynamic study. The solvent isotope effect (k_CHCl3/k_CDCl3 = 1.57) indicates the involvement of hydrogen exchange with the medium during oxidation reactions. A strong influence of specific solute–solvent interactions on the rate of the reaction is observed. Both the electron-withdrawing and electron-releasing substituents on the substrates accelerate the rate of reaction.     

Oxidation of Methionine by Colloidal MnO2 in Aqueous and Micellar Media: A Kinetic Study

The oxidation of methionine by freshly prepared colloidal manganese dioxide in aqueous as well as micellar media was studied spectrophotometrically at 35°C. The reaction between methionine and MnO₂ in both media exhibits 1:1 stoichiometry (methionine:MnO₂). The oxidation reaction is first order with regard to the MnO₂ concentration, but is fractional-order in the methionine concentration and HClO₄ concentrations. A catalytic effect of nonionic surfactant TX-100 on the rate of oxidation was observed and reaction rate was found to be proportional to {k′ + k″ [TX-100]}, where k′ and k″ are the rate constants in absence and presence of surfactant, respectively. The use of surfactant micelles is highlighted as, in favorable cases; the micelles help the redox reactions by bringing the reactants in a close proximity through hydrogen bonding. The oxidation reaction in aqueous and micellar media is shown to proceed via methionine–MnO₂ and methionine–MnO₂–TX-100 complexes, respectively, which decomposes slowly in a rate determining step to give methionine sulfoxide as the product. A suitable mechanism is proposed for these observations.     

Oxidation of fluorenes by ammonium meta vanadate—a kinetic study

The oxidation of fluorene by vanadium(V) in aqueous acetic acid containing sulfuric acid (1.0M) at 50°C produces fluorenone and 2-hydroxy diphenyl 2′-carboxaldehyde. The order with respect to each reactant is found to be 1. The order dependence on sulfuric acid concentration is 4, indicating that V(OH)₂³⁺ could be the active species. An increase in the acetic acid percentage in the solvent medium increases the rate of the reaction. The effect of solvent variation has been discussed in the light of the acidity function and the polarity of the medium. The effect of substituents on the rate has been studied for seven substituted fluorenes, and a linear relationship exists between log k versus σ values with the slope ρ = -3.2. A small isotope effect is observed for the oxidation of the parent compound (k_H/k_D = 1.2). The effect of temperature on the rate of the reaction has been studied, and the activation parameters are discussed. A mechanism involving the rate-limiting formation of a cation-radical intermediate is proposed.     

Reaktivität von Koordinationsverbindungen XVII Die Autoxydation von CuI-Komplexen mit Ammoniak und Imidazol

The autoxidation of two cuprous complexes, Cu(NH₃)₂+ and Cu(imidazole)₂+, has been studied by following the oxygen consumption with a coated oxygen sensor, and the formation of Cu^II by means of a stopped flow technique. The reaction was found to be of third order being proportional to the concentrations of Cu^I, oxygen, and free ligand. pH variation was without effect in the range studied. The rate constants are k_IM = 5,5 ·10³ 1²· Mol^?2·s^?1 for imidazole, and k_NH3 = 1,6·10⁴ 1²· Mol^?2· s^?1 for NH₃ as ligand, resp. An apparent activation energy of less than 2 Kcal/mole has been found for the autoxidation of the cuprous imidazole complex. This can be explained by the assumption of a rapidly playing equilibrium proceeding the rate determining step.     

Biphasic oxidation of diaquadichloro(1,10-phenanthroline)chromium(III) by N-bromosuccinimide and the formation of chromium(IV) and chromium(V)

The kinetics of oxidation of diaquadichloro(1,10-phenanthroline)chromium(III) complex, [Cr^III(phen)(H₂O)₂Cl₂]⁺, by N-bromosuccinimide (NBS) is biphasic. The first faster step involves the oxidation of Cr(III) to Cr(IV). The second slower step is due to the oxidation of Cr(IV) to Cr(V). The reaction product is isolated and characterized by electron spin resonance (ESR), IR, and elemental analysis. The chromium(V) product is consistent with the formula [Cr^V(phen)Cl₂(O)]Br. The rate constants k_f and k_s, for the faster and the slower steps respectively, were obtained using an Origin 9.0 software program. Values of both k_f and k_s, varied linearly with [NBS] at constant reaction conditions. The effect of pH on the reaction rate is investigated over the pH (4.11–6.01) range at 25.0°C. The rate constants k_f and k_s increased with increasing pH. This is consistent with hydroxo forms of the chromium species being more reactive than the aqua forms. Chromium(III) complexes, more often than not, are inert. The oxidation of the Cr(III) complex to Cr(IV), most likely, proceeds by an outer sphere mechanism. Since chromium(IV) is labile the mechanism of its oxidation to chromium(V) is not certain.     

Kinetics of Zn-5,15-di(ortho-methyloxyphenyl)-2,8,12,18-tetramethyl-3,7,13,17-tetrabutylporphyrin oxidation by organic peroxides in o-xylene

A spectrophotometric and quantum chemical (PM3) study of the structure and properties of Zn-5,15-di(ortho-methyloxyphenyl)-2,8,12,18-tetramethyl-3,7,13,17- tetrabutylporphyrin (ZnP(I)) in its oxidation with organic peroxides in o-xylene at T = 295 K is presented. The kinetic parameters (k _app, k _v ) of this reaction are reported. The reactivities of strained zinc porphyrinates (ZnP(I), Zn-5,15-di(ortho-methyloxyphenyl)-2,3,7,8,12,13,17,18-octamethylporphyrin (ZnP(II)), and a zinc porphyrin with a 2,5-dimethoxyphenylene “cover” (ZnP(III))) are compared. The electronic and conformational properties of the macrocycle are significant factors in the interaction of the metalloporphyrins with the peroxides. The structures of the reactants and intermediates have been calculated by the PM3 method. The macrocycle of ZnP(I) has a distorted structure. The degree and type of distortion are estimated. When reacting with the peroxides, the metalloporphyrin changes its structure so that the macrocycle becomes still more strained, and this enhances its reactivity.     

Oxidation Reaction of Phenol with H2O2 Catalyzed by Metallomicelles Made of Co(II) and Cu(II) Complexes of Imidazole Groups and Micelle as Mimic Peroxidase

The imidazole derivatives (N,N‐bis(2‐ethyl‐5‐methyl‐imidazole‐4‐ylmethyl) amino‐propane (biap)) and its complexes containing cobalt or copper ion were synthesized in this study. The oxidation reaction of phenol with oxidant H₂O₂ catalyzed by the metallomicelle made of the complexes of imidazole groups and micelle (CTAB, Brij35, LSS) as the mimetic peroxidase was studied. The results show that the reaction rate for the catalytic oxidation of phenol increases by a factor of approximately 1×10⁵ in the metallomicelle over that in the simple micelles or the pure buffer solution at pH=6.9 and 25°C. The catalytic effects changed with H₂O₂, temperature, pH, and surfactant kind in the catalytic reactive process are discussed. A kinetic mathematic model of the phenol oxidation catalyzed by the metallomicelle is proposed.     

Kinetics of Ketone Oxidation by Ozone in Water Solutions

The spectrophotometric monitoring of ozone consumption in a liquid phase is used to study the kinetics of cyclopentanone and methyl butyl ketone oxidation. The rate of ozone reaction with ketones (RH) at 303–344 K in acidic (HClO₄) aqueous solutions is described by the equation w = k ₁[RH][O₃] + k _En[RH][HClO₄], where k ₁ is the rate constant for the reaction of ozone with RH and k _En is the rate constant for the enolization of RH. The kinetic parameters of the process are found.     

Kinetics and mechanism of the oxidation of aliphatic aldehydes by tetrabutylammonium tribromide

The oxidation of six aliphatic aldehydes by tetrabutylammonium tribromide (TBATB) in aqueous acetic acid resulted in the formation of corresponding carboxylic acids. The reaction is first order with respect to TBATB and the aldehydes. The oxidation of deuteriated acetaldehyde (MeCDO) showed the presence of substantial kinetic isotope effect (k_H/k_D = 5.92 at 298 K). Addition of tetrabutylammonium chloride has no effect on the reaction rate. Tribromide ion has been proposed as the reactive oxidizing species. The rate constants correlate well with Taft's σ* values, the reaction constant being negative. A mechanism involving a hydride‐ion transfer from the aldehyde hydrate to the oxidant in the rate‐determining step has been suggested. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33: 390–395, 2001     

Influence of the macrocycle nature on the redox properties of vanadium porphyrinates in their reaction with an organic peroxide

The redox properties of vanadium 5,15-bis(о-methoxyphenyl)-3,7,13,17-tetrabutyl-2,8,12,18-tetramethyl-5,10,15,20-tetraphenylporphyrinates in the reaction with an organic peroxide, specifically dicumyl peroxide, were studied. The kinetic parameters of the reaction were determined and its possible mechanism was suggested. The coordinating capacity of vanadium porphyrinates with respect to dicumyl peroxide and imidazole and the stability of the resulting molecular complexes were estimated. It was shown that the nature of the macrocyclic ligand and the surrounding of the coordination center affect the rate of redox transformations. The isolated structures of the reagents and oxidation intermediates were optimized and their geometric parameters were obtained by the quantum-chemical РМ3 method. The calculation results revealed a high degree of deformation of the intermediate molecule, leading to macrocycle destruction.     

Oxidation of valeraldehyde by chlorine dioxide

The product of the reaction of valeraldehyde with chlorine dioxide was determined, and the solvent effect on the reaction kinetics was studied. The major oxidation product is valeric acid. The reaction rate is described by the second-order equation w = k[RCHO]·[ClO₂]. The rate constants were measured in the 297–328 K interval, and the activation parameters of the reaction were determined.     

Kinetics and mechanism of the oxidation of primary alcohols by sodium N-chloroethylcarbamate

The oxidation of primary alcohols by sodium N-chloroethylcarbamate in acid solution, results in the formation of corresponding aldehydes. The reaction is first order with respect to the oxidant and alcohol. The rate increases with an increase in acidity. The oxidation of α,α-dideuterioethanol exhibited a primary kinetic isotope, k_H/k_D = 2.11 at 298 K. The value of solvent isotope effect k(H₂O)/k(D₂O) = 2.23 at 298 K. Addition of ethyl carbamate does not affect the rate. (EtOC(OH)NHCl)⁺ has been postulated as the reactive species. Plots of (log k₂ + Ho) against (Ho + log[H⁺]) are linear with the slope, ?, having values from 1.78–1.87. This suggested a proton abstraction by water in the rate-determining step. The rates of oxidation of alcohols bearing both electron-withdrawing and electron-donating groups are more than that of methanol. A concerted mechanism involving transfer of a hydride ion from the C? H bond of the alcohol tothe oxidant and removal of a proton from the O? H group by a water molecule has been proposed.     

Kinetics and mechanism of the oxidation of halotoluenes by acidic hexacyanoferrate(III)

The oxidation of halotoluenes by hexacyanoferrate(III) in aqueous acetic acid containing perchloric acid (0.5M) at 50°C gave the corresponding aldehyde as the major product, and a small amount of polymeric material. The order with respect to each of the reactants—substrate, oxidant, and acid—was found to be unity. Increasing proportions of acetic acid increased the rate of the reaction. The reaction was influenced by changes in temperature, and the activation parameters have been evaluated. The Hammett plot yielded a ρ⁺ value of ?1.8. A kinetic isotope effect k_H/k_D = 6.0 has been observed. The pathway for the conversion of the halotoluenes to the products has been mechanistically visualized as proceeding through the benzylic radical intermediate, formed in the rate-determining step of the reaction. The radical undergoes rapid conversion to the products.     

Kinetics and mechanism of oxidation of ferrocyanide by N-bromosuccinimide in aqueous acidic solution

The kinetics of oxidation of ferrocyanide by N-bromosuccinimide (NBS) has been studied spectrophotometrically in aqueous acidic medium over temperature range 20–35 °C, pH = 2.8–4.3, and ionic strength = 0.10–0.50 mol dm⁻³ over a range of [Fe²⁺] and [NBS]. The reaction exhibited first order dependence on both reactants and increased with increasing pH, [NBS], and [Fe²⁺]. The rate of oxidation obeys the rate law: d[Fe³⁺]/dt = [Fe(CN)₆]^4–[HNBS⁺]/(k ₂ + k ₃/[H⁺]). An outer-sphere mechanism has been proposed for the oxidation pathway of both protonated and deprotonated ferrocyanide species. Addition of both succinimide and mercuric acetate to the reaction mixture has no effect on the reaction rate under the experimental conditions. Mercuric acetate was added to the reaction mixture to act as scavenger for any bromide formed to ensure that the oxidation is entirely due to NBS oxidation.     

Kinetics of the Copolymerization of Methyl Vinyl Ketone and Acrylamide Initiated by the Adduct of Methyl Vinyl Ketone and Imidazole

N-(Butyl-3-one)imidazole acts as an initiating adduct which is formed in the anionic polymerization of methyl vinyl ketone (MVK) induced by imidazole (Im) and is directly formed from Im and the MVK monomer. The kinetics of the anionic homopolymerization of MVK and acrylamide (AAm) under argon in the presence of the adduct were investigated in tetrahydrofuran (THF). The rate of polymerization for the MVK system is expressed as R_p = k[Adduct] [MVK], where k = 3.1 × 10^?6 L/(mol·s)in THF at 30°C. The overall activation energy, E_a , was found to be 5.34 kcal/mol. The R_p for the AAm system is expressed as R_p = k[Adduct] [AAm], where k = 6.8 × 10^?6 L/(mol·s) in THF at 30°C, with E_a 7.78 kcal/mol. The mechanism of the polymerization induced by the initiator adduct is discussed on the basis of these results.     

Kinetics and mechanism of the oxidation of formic and oxalic acids by quinolinium fluorochromate

Kinetics and mechanism of oxidation of formic and oxalic acids by quinolinium fluorochromate (QFC) have been studied in dimethylsulphoxide. The main product of oxidation is carbon dioxide. The reaction is first-order with respect to QFC. Michaelis-Menten type of kinetics were observed with respect to the reductants. The reaction is acid-catalysed and the acid dependence has the form: k_obs =a +b[H⁺]. The oxidation of α-deuterioformic acid exhibits a substantial primary kinetic isotope effect (k_H/k_D = 6.01 at 303 K). The reaction has been studied in nineteen different organic solvents and the solvent effect has been analysed using Taft’s and Swain’s multiparametric equations. The temperature dependence of the kinetic isotope effect indicates the presence of a symmetrical cyclic transition state in the rate-determining step. Suitable mechanisms have been proposed.     

Kinetic Study of the Oxidation of N,N-Dimethylethanolamine by Bis(Hydrogenperiodato)Argentate(III) in Aqueous Solution

The oxidation of N,N-dimethylethanolamine (DMEA) by bis(hydrogenperiodato) argentate(III) ([Ag(HIO₆)₂]⁵⁻) was studied in aqueous alkaline medium. Formaldehyde and dimethylamine were identified as the major oxidation products after the oxidation of DMEA. The oxidation kinetics was followed spectrophotometrically in the temperature range of 25.0 °C–40.0 °C. It was found that the reaction was first order in [Ag(III)]; the oberved first-order rate constants k _obsd as functions of [DMEA], [OH⁻] and total concentration of periodate ([IO₄^-]_tot[\mathrm{IO}_{4}^{-}]_{\mathrm{tot}}) were analyzed and were revealed to follow a rate expression: k_obsd = (k₁ +k₂[OH^-])K₁K₂[DMEA]/{f([OH^-])[IO₄^-]_tot+ K₁ + K₁K₂[DMEA]}k_{\mathrm{obsd}} = (k_{1} +k_{2}[\mathrm{OH}^{-}])K_{1}K_{2}[\mathrm{DMEA}]/\{f([\mathrm{OH}^{-}])[\mathrm{IO}_{4}^{-}]_{\mathrm{tot}}+ K_{1} + K_{1}K_{2}[\mathrm{DMEA}]\}. Rate constants k ₁ and k ₂ and equilibrium constant K ₂ were derived; activation parameters corresponding to k ₁ and k ₂ were computed. In the proposed reaction mechanism, a peridato-Ag(III)-DMEA ternary complex is formed indirectly through a reactive intermediate species [Ag(HIO₆)(OH)(H₂O)]²⁻. In subsequent rate-determining steps as described by k ₁ and k ₂, the ternary complex decays to Ag(I) through two reaction pathways: one of which is spontaneous and the other is prompted by an OH⁻.     

Kinetics and mechanism of the oxidation of diols by pyridinium bromochromate

The kinetics of oxidation of four vicinal diols, four nonvicinal diols, and one of their monoethers by pyridinium bromochromate (PBC) have been studied in dimethyl sulfoxide. The main product of oxidation is the corresponding hydroxyaldehyde. The reaction is first-order with respect to each the diol and PBC. The reaction is acid-catalyzed and the acid dependence has the form: k_obs=a+b[H⁺]. The oxidation of [1,1,2,2-²H₄]ethanediol exhibited a primary kinetic isotope effect (k_H/k _D=6.70 at 298 K). The reaction has been studied in 19 organic solvents including dimethyl sulfoxide and the solvent effect has been analyzed using multiparametric equations. The temperature dependence of the kinetic isotope effect indicates the presence of a symmetrical transition state in the rate-determining step. A suitable mechanism has been proposed. © 1998 John Wiley & Sons, Inc. Int J Chem Kinet 30: 285–290, 1998.