The heterogeneous propagation of the chains in the reaction of aliphatic aldehydes oxidation

The low temperature gas-phase oxidation of CH₃CHO and C₂H₅CHO have been investigated as an example of heterogeneous-homogeneous reaction. It has been shown that with the increase of S/V (surface/volume) ratio of the reaction vessel the maximum rate of the process increases and the heterogeneous consumption of aldehyde prevails over the homogeneous one. Peroxy radicals have been discovered by EPR method in vessels with different S/V ratios. The concentration of peroxy radicals and the accumulation rate of peracid in the gas phase did not change with the increase of S/V ratio in the same interval. Under the conditions of these experiments excluding the homogeneous propagation of chains the aldehydes oxidized heterogeneously. The peroxy radicals initiate this heterogeneous process which indicate the existence of short chains on the surface. The reaction between the peroxy radicals and aldehyde on the solid surface (SiO₂, KCl/SiO₂) has been shown by EPR, IR-spectroscopy, and mass-spectrometry methods. A conclusion has been drawn that the solid surface takes part in the chain propagation stage of aldehydes oxidation. © 1994 John Wiley & Sons, Inc.     

Enthalpies of formation, bond dissociation energies, and molecular structures of the n-aldehydes (acetaldehyde, propanal, butanal, pentanal, hexanal, and heptanal) and their radicals

Aldehydes are important intermediates and products in a variety of combustion and gas-phase oxidation processes, such as in low-temperature combustion, in the atmosphere, and in interstellar media. Despite their importance, the enthalpies of formation and bond dissociation energies (BDEs) for the aldehydes are not accurately known. We have determined enthalpies of formation for acetaldehyde, propanal, and butanal from thermodynamic cycles, using experimentally measured reaction and formation enthalpies. All enthalpy values used for reference molecules and reactions were first verified to be accurate to within around 1 kcal mol-1 using high-level ab initio calculations. Enthalpies of formation were found to be -39.72 +/- 0.16 kcal mol-1 for acetaldehyde, -45.18 +/- 1.1 kcal mol-1 for propanal, and -49.27 +/- 0.16 kcal mol-1 for butanal. Enthalpies of formation for these three aldehydes, as well as for pentanal, hexanal, and heptanal, were calculated using the G3, G3B3, and CBS-APNO theoretical methods, in conjunction with bond-isodesmic work reactions. On the basis of the results of our thermodynamic cycles, theoretical calculations using isodesmic work reactions, and existing experimental measurements, we suggest that the best available formation enthalpies for the aldehydes acetaldehyde, propanal, butanal, pentanal, hexanal, and heptanal are -39.72, -45.18, -50.0, -54.61, -59.37, and -64.2 kcal mol-1, respectively. Our calculations also identify that the literature enthalpy of formation of crotonaldehyde is in error by as much as 1 kcal mol-1, and we suggest a value of -25.1 kcal mol-1, which we calculate using isodesmic work reactions. Bond energies for each of the bonds in the aldehydes up to pentanal were calculated at the CBS-APNO level. Analysis of the BDEs reveals the R-CH(2)CH=O to be the weakest bond in all aldehydes larger than acetaldehyde, due to formation of the resonantly stabilized vinoxy radical (vinyloxy radical/formyl methyl radical). It is proposed that the vinoxy radical as well as the more commonly considered formyl and acetyl radicals are important products of aldehyde combustion and oxidation, and the reaction pathways of the vinoxy, formyl, and acetyl radicals are discussed. Group additivity values for the carbon-oxygen-hydrogen groups common to the aldehydes are also determined. Internal rotor profiles and electrostatic potential surfaces are used to study the dipole induced dipole-dipole interaction in the synperiplanar conformation of propanal. It is proposed that the loss of this dipole-dipole interaction in RC(.-)HCH(2)CH=O radicals causes a ca. 1-2 kcal mol-1 decrease in the aldehyde C-H and C-C bond energies corresponding to RC(.-)HCH(2)CH=O radical formation.     

Oxidation mechanism of aromatic peroxy and bicyclic radicals from OH-toluene reactions

Theoretical calculations have been performed to investigate mechanistic features of OH-initiated oxidation reactions of toluene. Aromatic peroxy radicals arising from initial OH and subsequent O(2) additions to the toluene ring are shown to cyclize to form bicyclic radicals rather than undergoing reaction with NO under atmospheric conditions. Isomerization of bicyclic radicals to more stable epoxide radicals possesses significantly higher barriers and, hence, has slower rates than O(2) addition to form bicyclic peroxy radicals. At each OH attachment site, only one isomeric pathway via the bicyclic peroxy radical is accessible to lead to ring cleavage. The study provides thermochemical and kinetic data for quantitative assessment of the photochemical production potential of ozone and formation of toxic products and secondary organic aerosol from toluene oxidation.     

Initiated oxidation of dibutyl ether in the presence of inhibitors

It was found that unlike ionol, amines effectively inhibit the oxidation of dibutyl ether. The stoichiometric inhibition coefficient for amines is close to two. During the initiated oxidation of dibutyl ether in the presence of inhibitors, an exchange takes place of cyanoisopropyl peroxy radicals for the peroxy radicals of dibutyl ether. The mechanism of the reaction with p-phenylenediamine (p-PDA) probably consists in the formation of a fairly stable complex consisting of an amine molecule and two peroxy radicals. This reaction does not result in the formation of a hydroperoxide.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 6, pp. 1268–1273, June, 1990.     

Identification of 1‐palmitoyl‐2‐linoleoyl‐phosphatidylethanolamine modifications under oxidative stress conditions by LC‐MS/MS

Phosphatidylethanolamines are a major class of phospholipids found in cellular membranes. Identification of the alterations in these phospholipids, induced by free radicals, could provide new tools for in vivo diagnosis of oxidative stress. In this study, 1‐palmitoyl‐2‐linoleoyl‐phosphatidylethanolamine oxidation products, induced by the hydroxyl radical, were studied using LC‐MS and LC‐MS/MS. Data obtained allowed the identification and separation of isomeric oxidative products with modifications in the sn‐2 acyl chain, attributed to long‐ and short‐chain products. Among long‐chain products keto, keto‐hydroxy, hydroxy, poly‐hydroxy, peroxy and hydroxy–peroxy derivatives were identified. Product ions formed by loss of two H₂O molecules vs loss of HOOH, allowed the identification of, respectively, di‐ (or poli‐) hydroxy vs peroxy derivatives. Location of functional groups was determined by the product ions formed by cleavage of C–C bonds, in the vicinity of the oxidation positions, allowing the identification of C9, C12 and C13 as the predominant substituted positions. Short‐chain products identified comprised aldehydes, hydroxy‐aldehydes and carboxylic derivatives, with modified sn‐2 acyl lengths of C7–C9 and C11, C12. Among the short‐chain products identified, C9 products showed higher relative abundance. Copyright © 2009 John Wiley & Sons, Ltd.     

Peroxy radical reactivities in the cooxidation of ethylbenzene and substituted 3-phenylindans

The rate constants of peroxy radical recombination in the oxidation of 1-methyl-3-phenylindan, 1,3-dimethyl-3-phenylindan, and 1,1-dimethyl-3-phenylindan were measured using the nonstationary-state technique. A cooxidation method was applied to obtain the reactivities of these compounds with respect to α-phenylethyl peroxy radicals.     

Kinetics of radiolytic oxidation of ferrous ions in some aqueous aerated systems

The oxidation of ferrous ions by organic peroxy radicals at different doses is non-linear, however, the plots of inverse of ferric ion concentration against inverse of dose are linear. This behavior is explained by a set of three general reactions. Some of the organic free radicals produced in these systems either react with O₂ forming peroxy radicals or they get oxidized by ferric ions. Other organic free radicals do not involve in the above competition and react with O₂ only.     

NO spin trapping and EPR studies on the photochemistry of aliphatic aldehydes

Radicals, such as acyl, hydrated acyl, alkyl and ketyl radicals, from aliphatic aldehyde photochemistry were detected by NO spin trapping and EPR techniques. Deuterium effects on EPR spectra and the generation of radicals by 2-amido-2-propyl radical attack on substrate molecules in aqueous solution via hydrogen-atom abstraction were applied to identify radicals produced photochemically from aldehydes. Aliphatic aldehydes used in the present investigation were formaldehyde, acetaldehyde, acetaldehyde-d4, propionaldehyde, isobutyraldehyde, isopentanal and tert-pentanal. Possible reaction mechanisms are suggested.     

Profile of oxidation in irradiated polyethylene

Following gamma irradiation in air which causes bond scission and yields large concentrations of peroxy radicals, maximum oxidation and an increase in crystallinity occurs on the surface of ultrahigh molecular weight polyethylene. Here, bimolecular reactions of peroxy radicals generate carbonyls, mostly ketones. On the polymer surface, peroxy radicals continue to react over time periods of years to generate carbonyls and chain scission. Peroxy radicals in the interior of the polymer abstract hydrogens and form hydroperoxides, inducing chain reactions and a slow but continue increase of ketone. Within the polymer sample, to a decreasing depth with increasing dose, a reduced concentration of oxygen is available to react with radiolytic radicals, so that more efficient crosslinking and a low level of hydroperoxide chain reaction occur. After long periods of time a surface maximum in carbonyl concentration is produced. Heating polyethylene in high pressures of oxygen accelerates the oxidative process. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 329–339, 1998     

Hydroperoxide formation in irradiated polyethylene

Spectroscopic analysis for hydroperoxide in irradiated ultrahigh molecular weight polyethylene, on the basis of the formation of a nitrate derivative after exposure to dilute nitric oxide, is examined. Hydroperoxide is found to be an important intermediate in the oxidation of polyethylene and is believed to result from hydrogen abstraction reactions by peroxy radicals in a polyethylene matrix. During γ irradiation in air, the rates of bimolecular combination of peroxy radicals on the surface to form ketones or hydrogen abstraction to form hydroperoxides are similar. However, as a result of bimolecular combination, the concentration of peroxy radicals decreases. After irradiation and storage in ambient air, isolated peroxy radicals below the polymer surface induce a slow chain reaction leading to a long-term increase in hydroperoxides and carbonyls. Differences in hydroperoxide and oxygen content for samples irradiated in air or vacuum are primarily confined to or near the surface. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 3309–3316, 1999     

Role of surface-bound intermediates in the oxygen-assisted synthesis of amides by metallic silver and gold

A general mechanism for the oxygen-assisted synthesis of amides over metallic gold and silver surfaces has been derived from the study of acetaldehyde and dimethylamine in combination with previous work, allowing detailed comparison of the two surfaces' reactivities. Facile acetylation of dimethylamine by acetaldehyde occurs with high selectivity on oxygen-covered silver and gold (111) crystals via a common overall mechanism with different rate-limiting steps on the two metals. Adsorbed atomic oxygen activates the N-H bond of the amine leading to the formation of an adsorbed amide, which attacks the carbonyl carbon of the aldehyde, forming an adsorbed hemiaminal. Because aldehydes are known to form readily from partial oxidation of alcohols, our mechanism also provides insight into the related catalytic coupling of alcohols and amines. The hemiaminal β-H eliminates to form the coupled amide product. On silver, β-H elimination from the hemiaminal is rate-limiting, whereas on gold desorption of the amide is the slow step. Silver exhibits high selectivity for the coupling reaction for adsorbed oxygen concentrations between 0.01 and 0.1 monolayer, whereas gold exhibits selectivity more strongly dependent on oxygen coverage, approaching 100% at 0.03 monolayer. The selectivity trends and difference in rate-limiting steps are likely due to the influence of the relative stability of the adsorbed hydroxyl groups on the two surfaces. Low surface coverages of oxygen lead to the highest selectivity. This study provides a general framework for the oxygen-assisted coupling of alcohols and aldehydes with amines over gold- and silver-based catalysts in either the vapor or the liquid phase.     

Atmospheric oxidation mechanism of p-xylene: a density functional theory study

We report an investigation of the mechanistic features of OH-initiated oxidation reactions of p-xylene using density function theory (DFT). Reaction energies for the formation of the aromatic intermediate radicals have been obtained to determine their relative stability and reversibility, and their activation barriers have been analyzed to assess the energetically favorable pathways to propagate the p-xylene oxidation. OH addition is predicted to occur dominantly at the ortho position, with branching ratios of 0.8 and 0.2 for ortho and ipso additions, respectively, and the calculated overall rate constant is in agreement with available experimental studies. Under atmospheric conditions, the p-xylene peroxy radicals arising from initial OH and subsequent O(2) additions to the ring are shown to cyclize to form bicyclic radicals, rather than to react with NO to lead to ozone formation. With relatively low barriers, isomerization of the p-xylene bicyclic radicals to more stable epoxide radicals likely occurs, competing with O(2) addition to form bicyclic peroxy radicals. The study provides thermochemical and kinetic data for assessment of the photochemical production potential of ozone and formation of toxic products and secondary organic aerosol from p-xylene oxidation.     

Tertiary aminomethylphenols and methylene bisphenols with isobornyl substituents in the reaction with diphenylpicrylhydrazyl and peroxy radicals in ethylbenzene

The effect of the structure of aminomethylphenols and methylene bisphenols with isobornyl substituents on their reactivity in interactions with peroxy radicals in ethylbenzene and with 1,1-diphenyl-2-picrylhydrazyl (DPPH) is studied. Isobornylphenols with o-aminomethyl substituents, as opposed to p-aminomethyl derivatives, were found to possess rather low activity in the initiated oxidation of ethylbenzene, due to the formation of intramolecular hydrogen bonds between the hydrogen atom of the OH group and the nitrogen atom of the aminomethyl substituent. An increase in activity of o-aminomethyl-substituted phenols with increasing polarity of the medium is observed in the reaction with DPPH. The reaction rate constants for the interaction between two isomeric 2,2′- and 4,4′-methylene-bisphenols having isobornyl moieties and ethylbenzene peroxy radicals are measured. The ratio between activities of the first and second OH groups in 2,2-methylene-bisphenol is shown to be close to 50.     

Chemiluminescence in thermo-oxidation of polypropylene

Chemiluminescence in thermo-oxidation of polypropylene was interpreted, noting its course with time at different temperatures and the corresponding spectra. The light emission was attributed to decomposition of α-ketone-hydroperoxides and to chain oxidation of polymer proceeding via secondary peroxy radicals.     

Oxidation of propene in the gas phase

A series of laboratory and modelling experiments on the oxidation of propene in the gas phase has been undertaken to determine conditions which give high yields of propene oxide. The conditions under which the experiments were conducted were 505–549 K and up to 4 bar pressure. It is proposed that propene oxide is formed from propene by reaction with several peroxy radicals including HO₂ and CH₃CO₃. However, one of the more important radicals is hydroxypropylperoxy. Its reaction with propene, under these conditions is more important than concerted decomposition to formaldehyde and acetaldehyde. © 1995 John Wiley & Sons, Inc.     

An ESR study of the decay reaction of isotactic polypropylene peroxy radicals in air

The decay of peroxy radicals trapped in irradiated isotactic polypropylene has been studied by ESR in air at various temperatures between 284 and 309 K. All the ESR spectra obtained at the various reaction stages are shown to be composed of two components arising from a mobile fraction and an immobile fraction. Only the mobile peroxy radicals decay; those belonging to the immobile fraction are stable. Various reaction mechanisms are examined in order to explain the experimental results; it is concluded that the decay reaction is controlled by diffusion of peroxy radicals and that the immobile peroxy radicals play no role in the decay reaction. Intermolecular hydrogen abstraction of the peroxy radicals, rather than intramolecular abstraction, is suggested as the rate-determining reaction.     

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.     

Characteristics of Chemiluminescence during Oxidation of Organic Substances Inhibited by Vegetable Phenols

An increased intensity in the induction period was observed when the inhibition of the oxidation of hydrocarbons by vegetable phenols was studied by the method of chemiluminescence. It was shown that the additional emitters of chemiluminescence are o- or p-quinones formed in the reaction between phenoxy and peroxy radicals.     

Density functional theory study on OH-initiated atmospheric oxidation of m-xylene

The OH-initiated oxidation reactions of m-xylene are investigated using density functional theory. The structures, energetics, and relative stability of the OH-m-xylene reaction intermediate radicals have been determined, and their activation barriers have been analyzed to assess the energetically favorable pathways to propagate the oxidation. OH addition occurs preferentially at the two ortho positions with the branching ratios of 0.97, 0.02, and 0.01 for ortho, meta, and ipso additions, respectively. The results reveal that the OH-m-xylene-O2 peroxy radicals preferentially cyclize to form bicyclic radicals under atmospheric conditions rather than reacting with NO to lead to ozone formation, and the decomposition to O2 and the hydroxyl m-xylene adduct is competitive with the cyclization process. The bicyclic radicals of m-xylene formed from the major OH-addition pathways (i.e., ortho positions) are more probable to form bicyclic peroxy radicals by reacting with O2. This study provides thermochemical and kinetic data of the OH-initiated reactions of m-xylene for assessment of the role of aromatic hydrocarbons in photochemical production of ozone, toxic products, and secondary organic aerosols.     

Mechanism of antioxidant interaction on polymer oxidation by thermal and radiation ageing

The mechanism of polymer oxidation by radiation and thermal ageing was investigated for the life evaluation of cables installed in radiation environments. The antioxidant as a stabilizer was very effective for thermal oxidation with a small content in polymers, but was not effective for radiation oxidation. The ionizing radiation induced the oxidation to result in chain scission even at low temperature, because the free radicals were produced and the antioxidant could not stop the oxidation of radicals with the chain scission. A new mechanism of antioxidant effect for polymer oxidation was proposed. The effect of antioxidant was not the termination of free radicals in polymer chains such as peroxy radicals, but was the depression of initial radical formation in polymer chains by thermal activation. The antioxidant molecule was assumed to delocalize the activated energy in polymer chains by the Boltzmann statics (distribution) to result in decrease in the probability of radical formation at a given temperature. The interaction distance (delocalization volume) by one antioxidant molecule was estimated to be 5–10 nm by the radius of sphere in polymer matrix, though the value would depend on the chemical structure of antioxidant.