Kinetics and thermodynamics of the exchange reactions of peroxy radicals with hydroperoxides

The enthalpies and equilibrium constants of the exchange reactions of peroxy radicals with hydroperoxides of various structures are calculated. The experimental data on the reactions of hydrogen atom abstraction by the peroxy radicals from the hydroperoxides are analyzed, and the kinetic parameters characterizing these reactions are calculated using the intersecting parabolas method. The activation energies and rate constants for nine reactions of H atom abstraction by a peroxy radical from the OOH group of a peroxide are calculated using the above parameters. The geometric parameters of the transition states for the reactions are calculated. The low triplet repulsion plays an important role in the fast occurrence of the reactions. The polar interaction in the transition state is manifested in the reactions of the peroxy radicals with hydroperoxides containing a polar group.     

Solvation Accounts for the Counterintuitive Nucleophilicity Ordering of Peroxide Anions

The nucleophilic reactivities (N , s _N) of peroxide anions (generated from aromatic and aliphatic peroxy acids or alkyl hydroperoxides) were investigated by following the kinetics of their reactions with a series of benzhydrylium ions (Ar₂CH⁺) in alkaline aqueous solutions at 20 °C. The second‐order rate constants revealed that deprotonated peroxy acids (RCO₃⁻), although they are the considerably weaker Brønsted bases, react much faster than anions of aliphatic hydroperoxides (ROO⁻). Substitution of the rate constants of their reactions with benzhydrylium ions into the linear free energy relationship lg k =s _N(N +E ) furnished nucleophilicity parameters (N , s _N) of peroxide anions, which were successfully applied to predict the rates of Weitz–Scheffer epoxidations. DFT calculations with inclusion of solvent effects by means of the Integral Equation Formalism version of the Polarizable Continuum Model were performed to rationalize the observed reactivities.     

Solvent Effect on the Rate of Thermal Decomposition of Diacyl Diperoxides

The kinetics of thermal decomposition of didecanoyl diperoxyadipate in different organic solvents have been studied. The primary homolytic dissociation of the peroxide bond (O–O) is accompanied by secondary induced chain decomposition processes. The reaction medium affects both the rate of primary homolytic decomposition and secondary decomposition processes. Correlation equations have been proposed for the rate constants of the reactions under study and physicochemical parameters of the solvents.     

Solid-state photolysis of diacyl peroxides and radical decay

ESR studies are reported for the peroxides derived from butyric, caproic, caprylic, lauric, and stearic acids. In every case, a six-component soectrum is observed, which is transformed to the spectrum of the peroxy radical in the presence of oxygen, so the spectrum is assigned to alkyl radicals formed by rupture of the peroxide bond and decarboxylation of the alkoxy radical. The rate constants and activation energies are deduced for the decay of the radicals. The activity is inversely related to chain length. Buildup curves under UV are examined, since these are dependent on chain length and light intensity.     

Influence of Solvents on the Rate of Thermal Decomposition of Peroxydecanoic Acid

Thermal decomposition of peroxydecanoic acid has been investigated in various organic solvents. The reaction medium significantly affects the rate of peroxydecanoic acid thermal decomposition. It has been shown that primary homolytic decomposition of peroxy group is followed by induced radical chain decomposition reactions. The solvent nature influences both primary homolytic decomposition and secondary induced reactions. The correlation equations relating kinetic parameters of peroxydecanoic acid thermolysis and basic solvent properties have been derived.     

Interrelation between the electrochemical activity of alkyl-and thioalkylphenols and their antioxidant action

The oxidation potentials of alkyl-and thioalkylphenols of various structures and the rate constants for their reactions with styrene peroxide radicals were determined. The overall inhibiting activity of the substances in a model reaction of the thermal autooxidation of lard was studied. The oxidation potential of phenols was found to correlate with the O-H bond energy, the rate constants for phenol reactions with styrene peroxide radicals and phenoxyl radical reactions with Tetralin molecules, and the effectiveness of the overall inhibiting action of these compounds.     

Photometric and Gas-Chromatographic Determination of Hydrogen Peroxide and Peroxybutanoic Acid in Oxidized Butanoic Acid

Procedures were developed for determining hydrogen peroxide and peroxy acids mixed with peroxide compounds of other classes in the oxidation products of butanoic acid with atmospheric oxygen and hydrogen peroxide. Conditions were found for the selective decomposition of hydrogen peroxide with catalase in the presence of an excess of the carboxylic acid deactivating the enzyme. The errors introduced by the acylation of hydrogen peroxide with the carboxylic acid in the course of sample treatment with the enzyme were eliminated by adding diphenyl sulfide or dimethyl sulfoxide, which selectively reduced the peroxy acids. The concentrations of hydrogen peroxide and the peroxy acid were found from the difference between the total concentration of the peroxide compounds before and after treating a sample with catalase and a sulfur-containing reagent by the photometric method using a reagent containing Fe²⁺ ions and N, N-dimethyl-p-phenylenediamine. Peroxy acids were determined by GLC from the yields of the oxidation products of diphenyl sulfide with the peroxy acid (diphenyl sulfoxide and diphenyl sulfones).     

Evaluation of Initiators and Fillers in Diallyl Phthalate Resins by Differential Scanning Calorimetry

Seven organic peroxide initiators for the polymerization of diallyl-o-phthalate prepolymers were studied using differential scanning calorimetry. Included were t-butyl perbenzoate, dicumyl peroxide, α,α′-bis(t-butyl peroxy) diisopropyl-benzene, 2,5-dimethyl-2,5-di(t-butyl peroxy) hexane, 2,5-dimethyl-2,5-di(t-butyl peroxy)hexyne-3, di-t-butyl peroxide, and t-butyl hydroperoxide. Heats of reaction and reaction rate constants are presented for each initiator at 1, 2, 3, and 4 phr of diallyl-o-phthalate prepolymers. The differences in reaction temperature and rate are discussed. Effects of four types of commonly used fillers (asbestos floats, ground quartz, calcium silicate, and clay) on the heats of reaction of diallyl-o-phthalate prepolymers using t-butyl perbenzoate and dicumyl peroxide initiators show the large inhibiting effect of untreated kaolinite clays on this polymerization.     

Thermal hazard evaluation of lauroyl peroxide mixed with nitric acid

Many thermal runaway incidents have been caused by organic peroxides due to the peroxy group, -O-O-, which is essentially unstable and active. Lauroyl peroxide (LPO) is also sensitive to thermal sources and is incompatible with many materials, such as acids, bases, metals, and ions. From the thermal decomposition reaction of various concentrations of nitric acid (HNO3) (from lower to higher concentrations) with LPO, experimental data were obtained as to its exothermic onset temperature (T0), heat of decomposition (ΔHd), isothermal time to maximum rate (TMRiso), and other safety parameters exclusively for loss prevention of runaway reactions and thermal explosions. As a novel finding, LPO mixed with HNO3 can produce the detonation product of 1-nitrododecane. We used differential scanning calorimetry (DSC), thermal activity monitor III (TAM III), and gas chromatography/mass spectrometer (GC/MS) analyses of the reactivity for LPO and itself mixed with HNO3 to corroborate the decomposition reactions and reaction mechanisms in these investigations.     

Kinetics and mechanism of the liquid-phase oxidation of <Emphasis Type="Italic">n</Emphasis>-carboxylic acids

Short reaction chains participate in the oxidation of C₃-C₆, C₈, C₁₀, and C₁₂ n-carboxylic acids. The quadratic-law recombination of peroxy radicals occurs both without and with chain termination. The ratio of the rate constants of these reactions increases to k′/k _t = 4.5 on passing from propanoic acid to pentanoic acid, and then it decreases almost to zero for dodecanoic acid. The anomalous variation of the k′/k _t ratio is explained by the fact that the radicals resulting from carboxylic acid oxidation at the CH bonds nearest to the functional group make different contributions to the recombination process, depending on the carbon chain length. The cross recombination of secondary hydrocarbon peroxy radicals with the HO₂^· radicals resulting from the oxidation of carboxylic acids at the β-C-H bonds proceeds without chain termination.     

Aging and degradation of polyolefins. I. Peroxide-initiated oxidations of atactic polypropylene

Efficiencies of polymer radical production by thermal decomposition of di-tert-butylperoxy oxalate (DBPO) have been measured in bulk atactic polypropylene (PP) at 25–55°C; they range from 1 to 26%, depending on [DBPO], temperature, and presence of oxygen. Most of the polymer radicals thus produced disproportionate in the absence of oxygen but form peroxy radicals in its presence. Most of the pairs of peroxy radicals interact by a first-order reaction in the polymer cage. The fraction that escapes gives hydroperoxide in a reaction that is half order in rate of initiation. In interactions of polymer peroxy radicals, in or out of the cage, about one-third give dialkyl peroxides and immediate chain termination, two-thirds give alkoxy radicals. About one-third of the later cleave at 45°C; the rest abstract hydrogen to give hydroxy groups and new polymer and polymer peroxy radicals. The primary peroxy radicals from cleavage account for the rest of the chain termination. Cleavage of alkoxy radicals and crosslinking of PP through dialkyl peroxides nearly compensate. Up to 70% of the oxygen absorbed has been found in hydroperoxides. The formation of these can be completely inhibited, but cage reactions are unaffected by inhibitors. Concentrations of free polymer peroxy radicals have been measured by electron spin resonance and found to be very high, about 10^?3M at 58–63°C. Comparison with results on 2,4-dimethylpentane indicate that rate constants for both chain propagation and termination in the polymer are much smaller than those for the model hydrocarbon but that the ratio, k_p/(2k_t)^½, is about the same.     

Relationship between the electrochemical and antioxidant activities of alkyl-substituted phenols

Oxidation potentials and rate constants have been determined for reactions between alkyl-substituted phenols with various structures and the styryl peroxy radical. The overall inhibiting activity of the phenols in the thermal autooxidation of lard has been studied. The interrelation between the structure, physicochemical properties, and antioxidant activity of the phenols is discussed.     

Use of aqueous hydrogen peroxide solutions prepared by cathodic reduction of oxygen for indirect oxidation of chemical substances in situ: Achievements and prospects

The achievements and prospects in the use of aqueous hydrogen peroxide solutions prepared by cathodic reduction of oxygen in carbon black gas-diffusion and graphite electrodes for indirect oxidation of organic and inorganic substrates in situ are analyzed. Specific examples demonstrate the efficiency of using hydrogen peroxide solutions for in situ indirect electrocatalytic oxidation of organic and inorganic substrates to target products (indirect electrochemical synthesis), for decomposition (mineralization) of organic and inorganic pollutants in industrial waters and wastewaters, and for preparation of organic peroxy acids and inorganic peroxy solvates.     

On the mechanism of thermal oxidation of methane

Using the method of freezing radicals in conjunction with ESR spectroscopic measurements, the kinetics of the thermal oxidation of methane has been studied under atmospheric pressure depending on the temperature, composition of the mixture, and nature of the surface of the reaction vessel. It has been shown that in a reactor treated with boric acid, the intermediates methylhydroperoxide and hydrogen peroxide are responsible for chain branching. It has been established that the leading active centers of the reaction are the HO₂ radicals, while chain branching occurs as a result of the decomposition of peroxy compounds—methylhydroperoxide and hydrogen peroxide. In reactors treated with potassium bromide, the concentrations of radicals and peroxy compounds were found to be lower than the sensitivity of the method of measurement. Computations were performed for the scheme of methane oxidation at 738 K for a reactor treated with boric acid. Satisfactory agreement was found between the experimental and computed kinetic curves of accumulation of main intermediates CH₂O, H₂O₂, CH₃OOH. The influence of their addition on the kinetics of the reaction has been considered. It has been shown that the addition of formaldehyde does not lead to chain branching, however; it contributes to the formation of those peroxy compounds that bring about chain branching. Mathematical modeling confirmed conclusions made on the basis of experimental data concerning the nature of the leading active centers and the products that are responsible for the degenerate chain branching.     

Reactions of alkoxy and peroxy radicals formed upon the decomposition of hydroxyperoxide groups in the artemisinin derivatives

The enthalpies of intramolecular reactions of alkoxy and peroxy radicals formed from polyatomic artemisinin hydroperoxides and of their bimolecular reactions with C—H, S—H, and O—H bonds of biological substrates were calculated. The activation energies and rate constants of these reactions were calculated using the intersecting parabolas method. The decomposition of artemisinin hydroperoxides can initiate the cascade of intramolecular oxidation reactions involving radicals R·, RO·, HO·, HO₂·, and RO₂·. The main sequences of transformation of these radicals were established. The oxidative destruction of the artemisinin peroxy derivatives generates radicals RO₂·, HO·, and HO₂· in an amount of 4.5 radicals per peroxide derivative molecule on the average. The kinetic scheme of oxidative transformations of the hydroperoxide with four OOH groups and radicals formed from it was constructed using this radical as an example.     

Photooxidative degradation of cellulose: Reactions of the cellulosic free radicals with oxygen

Photooxidative degradation of cellulose resulted in decreases of degree of polymerization (DP) and α-cellulose content, concurrently producing chromophoric groups; namely, carbonyl, carboxyl, and hydroperoxide groups within the polymer. Electron spin resonance (ESR) studies revealed that cellulosic carbon free radicals readily reacted with oxygen molecules at 143–160 K to produce peroxy radicals, whereas cellulosic oxygen free radicals were inert toward oxygen molecules throughout the photooxygenation reactions. At 77 K it is feasible that only photoexcited oxygen molecules reacted with cellulosic carbon free radicals to produce peroxide radicals. These radicals were themselves stabilized at 273 K by abstraction of hydrogen atoms from cellulose to produce polymer hydroperoxides. Simultaneously, new radical sites, which exhibited three-line ESR spectra, were generated in cellulose.     

Intramolecular chain reaction of artemisinin oxidation

The intramolecular chain oxidation of artemisinin was analyzed using the parabolic model. The competition of the mono- and bimolecular peroxy radicals formed from artemisinin was considered. Artemisinin is predominantly oxidized via the intramolecular chain mechanism to form polyatomic hydroperoxides. This results in the situation when, under aerobic conditions, artemisinin is transformed from the monofunctional into polyfunctional initiator with several hydroperoxide groups. The enthalpy was calculated, and the activation energies and rate constants of the intramolecular reactions of the artemisinin peroxy radicals, as well as those of their bimolecular reactions with C-H, S-H, and O-H bonds of biological substrates and their analogs, were calculated in the framework of the parabolic model. A new kinetic scheme for artemisinin oxidation was proposed. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 267–275, February, 2008.     

过氧化脲在氧化反应中的应用研究进展

氧化反应是重要的有机合成单元反应,常采用H2O2或过氧酸等为氧化剂.近年来,过氧化脲作为一种清洁环保的固态H2O2,以其性质稳定、H2O2含量高(36.2%)且释放可控等优点,在众多氧化反应中得到了广泛应用.对过氧化脲在氧化反应中的应用进行了总结和概述,重点介绍了环氧化、Baeyer-Villiger氧化、N-氧化、硫醚氧化成砜和亚砜、氧化卤化等反应.     

The Theoretical and Experimental Studies on Oxidation of Straight Chain Amino Acids in Moderately Concentrated Sulfuric Acid Medium

The kinetics of the permanganic oxidation process of some straight chain amino acids in moderately concentrated sulfuric acid medium have been investigated using a spectrophotometric technique. Conclusive evidences have proven autocatalytic activity of Mn(II) for these reactions. It is determined that even and odd effects of the number carbon atom in a carbon chain are annihilated when it's the number of carbon atoms is increased more than of three in a noncatalytic oxidation pathway. Thus, rate constants belonging to glycine, l ‐α‐amino‐n‐butyric acid, l ‐norleucine, and l ‐α‐amino‐n‐heptanoic acid satisfy Taft's equation involving the induction factor in the noncatalytic pathway, whereas l ‐α‐amino‐n‐heptanoic acid has an odd number of carbon atom in its chain carbon. On the other hand, in the catalytic pathway, rate constants satisfy Taft' equation including inductive and steric factors, when rate constants belonging to amino acids with an even number of carbon atoms are separated from those with an odd number of carbon atoms. The oxidation process of amino acids in the noncatalytic pathway and those with the even number of carbon atoms in the carbon chain in the catalytic pathway speeds up by an increase in the length of chain that is accompanied with an increase in the carbon chain's electron‐donating characteristic. On the other hand, an increase in the length of the carbon chain is accompanied with more steric hindrance, which counteracts its electron‐donating character, thereby decreasing reaction rate in the catalytic pathway. Finally, amino acid–Mn(II) complexes were studied using a density functional theory method. Results obtained show that such a complex is less stable than reactants, namely it is formed in an endothermic reaction. The number and strength of hydrogen bonding belonging to amino acid is more than those of the amino acid–Mn(II) complex. Besides, it has been illustrated that natural bond orbital analysis and molecular orbital calculations satisfy the findings.     

Solvent effects on the rate of thermolysis of lauroyl peroxide

Thermolysis of lauroyl peroxide in various organic solvents was studied. It was shown that the primary homolytic dissociation of the peroxide group is accompanied by secondary reactions of chain-induced decomposition. The reaction medium affects the rate of both the primary homolytic dissociation and secondary induced decomposition processes. Correlation equations between the rate constants of the reactions in study and the physicochemical parameters of the solvents were proposed.