Physico-chemical aspects of polyethylene processing in an open mixer. Part 32: Formal kinetics of γ-lactone formation from secondary products. Heterogeneous kinetics

Oxidation of aldehydes and γ-hydroxy-trans-vinylene groups can yield γ-lactones. These intermediates account for γ-lactone formation in the advanced stages of polyethylene processing in air. The acyl-peroxy radical formed on free radical induced oxidation of aldehydes can abstract intramolecularly a δ-hydrogen atom to yield a peracid. Reaction of the alkyl radical formed in this reaction with the hydroperoxide group of the peracid gives a γ-lactone with simultaneous release of a hydroxyl radical. The calculated rate of γ-lactone formation according to the mechanism envisaged decreases slightly with increasing temperature (activation energy of about −5 kcal/mol). It is in agreement with the experiments that do not show significant activation energy in the high temperature range for the advanced stages of polyethylene processing. The calculated rate of γ-lactone formation is found to increase by a factor of about 2.7 if the processing experiments are performed in pure oxygen instead of in air. This is close to the experimental factor of about 2.Peroxidation of γ-hydroxy-trans-vinylene groups can also yield γ-lactones. The first possibility involves addition of a peroxy radical to the double bond followed by oxygen addition to the alkyl radical. This reaction possibly yields an α-peroxy-hydroperoxide. Intramolecular decomposition involving the two reactive groups of the α-peroxy-hydroperoxide can give an ozonide that on thermal decomposition yields among others an acid group in 4-position to the alcohol. The activation energy calculated is strongly negative so that the rate should decrease strongly with increasing temperature. Hence, the mechanism cannot contribute significantly to γ-lactone formation in the whole temperature range of the experiments. This is so in spite of the fact that the rate is estimated to increase by a factor of about 1.7 on passing from air to pure oxygen, which is close to the experimental value of approximately 2. The second possibility of transformation of γ-hydroxy-trans-vinylene groups is based on stress-induced oxygen addition to the double bond. Acid catalyzed decomposition of the allylic hydroperoxide that is formed in the reaction yields a pair of aldehydes with one of the aldehyde groups in 4-position to the alcohol group. Peroxidation of the aldehyde pair can give an acid group in 4-position to the hydroxyl group so that a γ-lactone can be formed. The activation energy calculated for the process is very small and the effect of the oxygen concentration corresponds to an increase by a factor of approximately 4.5 on passing from air to pure oxygen. It is postulated that simultaneous contribution by different mechanisms might well account for the experimental value of about 2.The heterogeneous kinetics discussed in detail allows for complementary data interpretation. It is especially suited for the understanding of the advanced stages of polyethylene processing, after some induction time.     

Physico-chemical aspects of polyethylene processing in an open mixer. Part 20: Additional product yields on bimolecular hydroperoxide decomposition with an alcohol group

Most products formed on polyethylene oxidation result from hydroperoxide decomposition. The product yields can be calculated for various mechanisms of hydroperoxide decomposition. This work concerns the reaction of a hydroperoxide with an alcohol group thought to be dominant in the advanced stages of polyethylene processing in the high temperature range (170-200 °C). Besides hydrogen abstraction by caged alkoxy radicals already envisaged previously, the possibility of β-scission is taken into account. This additional reaction introduces significant complexity into the reaction schemes. This is especially so because additional caged radical pairs must be included into the schemes and the calculations. It becomes possible to calculate the yields of aldehyde and vinyl groups that do not result from hydroperoxide decomposition in the absence of β-scission. The yields of the main oxidation products such as alcohols and ketones are not much affected by taking into account β-scission. The yield of aldehydes is important in the whole temperature range and increases considerably if the temperature is raised from 170 to 200 °C. It becomes more important than the ketone yield. The vinyl groups are formed in amounts corresponding roughly to 10-15% of the trans-vinylene groups in the temperature range of 170-200 °C.     

Kinetics and mechanism of cyclohexane oxidation with molecular oxygen in the presence of propionic aldehyde

The kinetics and mechanism of the liquid-phase oxidation of cyclohexane with molecular oxygen in the presence of the additives of propionic aldehyde are studied at 303.0, 322.5, and 341.5 K by measuring the rates of oxygen and propionic aldehyde consumption and the yields of the main reaction products (cyclohexanol (COL), cyclohexanone (CON), cyclohexyl hydroperoxide, and propionic acid and peracid). A kinetic scheme is proposed and rate constants of elementary reactions are estimated based on the analysis of their rates and the yields of the main cyclohexane products. The key reactions of the main steps (including chain initiation, propagation, and termination) are determined. An increase in the rate of cyclohexane oxidation and the yield of the target products (cyclohexanol, cyclohexanone, and cyclohexyl hydroperoxide) in the presence of propionic aldehyde suggests that highly active acylperoxy radicals participate in chain propagation. The [CON]/[COL] ratio indicates that these products are mainly formed in chain propagation. The strong effect of the Baeyer-Villiger rearrangement on both the rate of oxygen consumption and the yield of the target products at the initial stages of the process and at high propionic aldehyde concentrations is explained.     

Physico-chemical aspects of polyethylene processing in an open mixer. Part 21: Improved product yields on true bimolecular hydroperoxide decomposition

The product yields from the reaction between two hydroperoxide groups have been re-calculated. This is a consequence of the fact that β-scission of secondary alkoxy radicals cannot be neglected in the high temperature range of the polyethylene processing experiments (170-200 °C). It must be taken into account in addition to disproportionation/combination and hydrogen abstraction by alkoxy radicals. The increased complexity caused by the additional reaction results mainly from the larger number of caged radical pairs involved in the reactions and also in the calculations. Among other products it becomes possible to calculate the yields of aldehyde and vinyl groups that would not result from hydroperoxide decomposition in the absence of β-scission. The yields of the main oxidation products such as alcohols, ketones and trans-vinylene groups are reduced to some extent in comparison with the values calculated if β-scission is neglected. The vinyl group yield corresponds to slightly more than 10% of the yield of trans-vinylene groups in the temperature range of the experiments. The aldehyde yield is significantly larger than the vinyl group yield and is important in the whole temperature range examined. Main-chain scissions are important at the temperatures of the experiments. They become more important than the sum of the different combination reactions from a temperature of 200 °C on.     

Physico-chemical aspects of polyethylene processing in an open mixer. Part 23: Mechanisms and kinetics of trans-vinylene group consumption

There are many potential reactions for trans-vinylene groups in oxidizing polyethylene melts. The main possibilities are reactions with peroxy radicals, molecular oxygen, hydroperoxides and peracids. These different reactions can all contribute to the removal of trans-vinylene groups to some extent. This is especially so, for the reactions with hydroperoxides that have been found to be the dominant reactions with vinylidene and vinyl groups in the low temperature range. The reaction with peroxy radicals is thought to be as important relatively as with vinylidene groups. Therefore, the importance of the reaction is decreasing with increasing temperature. However, the most characteristic reaction for trans-vinylene groups can be detected without any doubt only in the advanced stages of processing. It is mechanical stress induced oxygen addition to the double bond. The discussion shows that the reaction should be important from the beginning of processing. The reaction cannot operate with vinyl and vinylidene groups, which are not part of the polyethylene main chain. After oxygen addition to the trans-vinylene group, the “ene” reaction yields an allylic hydroperoxide so that the double bond is not immediately removed. It is acid catalyzed hydroperoxide decomposition that leads to chain scission with aldehyde formation at the new chain ends.     

Kinetics of the liquid-phase oxidation of cyclooctene in the presence of MnO2

The kinetics of the liquid-phase oxidation of cyclooctene by molecular oxygen in the presence of a MnC₂ catalyst is studied. It is found that MnC₂ is an initiator of this reaction and has no effect on the steps of chain propagation and termination. The oxidation occurs by a radical chain mechanism with the escape of radicals to the bulk of the reaction mixture. Radicals are formed by the interaction of the olefin with the solid catalyst surface. The kinetic parameters of the reaction are calculated.     

Kinetics and Mechanism of Oxidation of l‐Histidine by Permanganate Ions in Sulfuric Acid Medium

The reaction kinetics for the oxidation of l ‐histidine by permanganate ions have been investigated spectrophotometrically in sulfuric acid medium at constant ionic strength and temperature. The order with respect to permanganate ions was found to be unity and second in acid concentration, whereas a fractional order is observed with respect to histidine. The reaction was observed to proceed through formation of a 1:1 intermediate complex between oxidant and substrate. The effect of the acid concentration suggests that the reaction is acid catalyzed. Increasing the ionic strength has no significant effect on the rate. The influence of temperature on the rate of reaction was studied. The presence of metal ion catalysts was found to accelerate the oxidation rate, and the order of effectiveness of the ions was Cu²⁺ > Ni²⁺ > Zn²⁺. The final oxidation products were identified as aldehyde (2‐imidazole acetaldehyde), ammonium ion, manganese(II), and carbon dioxide. Based on the kinetic results, a plausible reaction mechanism is proposed. The activation parameters were determined and discussed with respect to a slow reaction step.     

Synthesis of fully substituted pyrazolo[3,4-b]pyridine-5-carboxamide derivatives via a one-pot four-component reaction

A one-pot four-component reaction of an aliphatic or aromatic amine, diketene, an aromatic aldehyde and 1,3-diphenyl-1H-pyrazol-5-amine in the presence of p-toluenesulfonic acid as a catalyst has been developed. In this reaction, a new class of fully substituted pyrazolo[3,4-b]pyridine-5-carboxamide derivatives is produced under mild reaction conditions and in good yields at ambient temperature.     

The photolysis of n-butyraldehyde in isooctane at 313 nm

The effects of aldehyde concentration, incident light intensity, and temperature on the quantum yields of reaction products were studied. Mechanisms for primary and secondary photochemical processes were suggested, and primary quantum yields as well as rate constant ratios were derived. Reversibility of intramolecular γ-hydrogen transfer and disproportionation of the radical pair formed in the reaction of an excited triplet and ground state molecule were shown to provide important pathways for radiationless decay of the triplet state.     

Oxidation of a mustard gas analogue using an aldehyde/O2 system catalyzed by V-doped mesoporous silica

Vanadium-doped mesoporous silica was shown to be an effective heterogeneous catalyst for the oxidation of a mustard gas analogue, 2-chloroethyl ethyl sulfide (CEES), in the presence of an aldehyde and molecular oxygen. The oxidation was shown to involve a radical mechanism, which was indicated by the appearance of an induction period when the reaction occurred in the presence of a free radical scavenger. The reaction was initially selective for the oxidation of CEES to the sulfoxide, CEESO, although oxidation of the sulfoxide to the sulfone occurred once all the CEES had been oxidized. Chemical analysis indicated that V species did not leach from the silica support when the reaction was performed in the fluorinated solvent HFE-7100.     

甲苯氧化制苯甲酸的新催化体系

以分子氧为氧化剂,探索了甲苯氧化制苯甲酸的新催化体系,第一次把N-羟基邻苯二甲酰亚胺(NHPI)与钴盐组成的催化剂用于甲苯氧化反应.实验揭示:以NHPI/Co(acac)2(乙酰丙酮酸钴)和NHPI/Co(OAc)2(醋酸钴)为催化剂时,苯甲酸的收率可分别为55.5%与41.2%,文中还对氧气压力、反应温度与时间等对反应的影响进行了讨论,同时提出了可能的反应机理.     

Ab initio study of the reaction of propionyl (C2H5CO) radical with oxygen (O2)

The reaction of propionyl radical with oxygen has been studied using the full coupled cluster theory with the complete basis set. This is the first time to gain a conclusive insight into the reaction mechanism and kinetics for this important reaction in detail. The reaction takes place via a chemical activation mechanism. The barrierless association of propionyl with oxygen produces the propionylperoxy radical, which decomposes to form the hydroxyl radical and the three-center alpha-lactone predominantly or the four-center beta-propiolactone. The oxidation of propionyl radical to carbon monoxide or carbon dioxide is not straightforward rather via the secondary decomposition of alpha-lactone and beta-propiolactone. Kinetically, the overall rate constant is almost pressure independent and it approaches the high-pressure limit around tens of torr of helium. At temperatures below 600 K, the rate constant shows negative temperature dependence. The experimental yields of the hydroxyl radical can be well reproduced, with the average energy transferred per collision -DeltaE=20-25 cm(-1) at 213 and 295 K (helium bath gas). At low pressures, together with the hydroxy radical, alpha-lactone is the major product, while beta-propiolactone only accounts for about one-fifth of alpha-lactone. At the high-pressure limit, the production of the propionylperoxy radical is dominant together with a fraction of the isomers. The infrared spectroscopy or the mass spectroscopy techniques are suggested to be employed in the future experimental study of the C2H5CO+O2 reaction.     

Partial oxidation of 4-tert-butyltoluene catalyzed by homogeneous cobalt and cerium acetate catalysts in the Br-/H2O2/acetic acid system: insights into selectivity and mechanism

The partial oxidation of 4-tert-butyltoluene to 4-tert-butylbenzaldehyde by hydrogen peroxide in glacial acetic acid, catalyzed by bromide ions in combination with cobalt(II) acetate or cerium(III) acetate, has been studied in detail. Based on the observed differences in reaction rates and product distributions for the different catalysts, a reaction mechanism involving two independent pathways is proposed. After the initial formation of a benzylic radical species, either oxidation of this intermediate by the metal catalyst or reaction with bromine generated in situ occurs, depending on which catalyst is used. The first pathway leads to the exclusive formation of 4-tert-butylbenzaldehyde, whereas reaction of the radical intermediate with bromine leads to formation of the observed side products 4-tert-butylbenzyl bromide and its hydrolysis and solvolysis products 4-tert-butylbenzyl alcohol and 4-tert-butylbenzyl acetate, respectively. The cobalt(II) catalysts Co(OAc)(2) and Co(acac)(2) are able to quickly oxidize the radical intermediate, thereby largely preventing the bromination reaction (i.e., side-product formation) from occurring, and yield the aldehyde product with 75-80 % selectivity. In contrast, the cerium catalyst studied here exhibits an aldehyde selectivity of around 50 % due to the competing bromination reaction. Addition of extra hydrogen peroxide leads to an increased product yield of 72 % (cerium(III) acetate) or 58 % (cobalt(II) acetate). Product inhibition and the presence of increasing amounts of water in the reaction mixture do not play a role in the observed low incremental yields.     

Mechanistic Aspects of the Effects of Stress on the Rates of Photochemical Degradation Reactions in Polymers

Abstract

This review examines the mechanistic origins of the effects of stress on the photochemical degradation rates of polymers. Recent studies have shown that tensile and shear stresses accelerate the rate of the photochemical degradation of polymers. Conversely, compressive stress generally retards the rate of photochemical degradation. After an initial discussion of the photochemical auto‐oxidation mechanism, the three primary hypotheses that purport to explain how stress affects photochemical degradation are examined. The first hypothesis is attributed to Plotnikov, who proposed that stress changes the quantum yields of the reactions that lead to bond photolysis. The second hypothesis, attributed to a number of researchers, says that stress affects the ability of the geminate radical pairs, formed in the photochemical bond cleavage reactions, to recombine. The third hypothesis proposes that stress changes the rates of radical reactions subsequent to radical formation. A further attempt to account for the effects of stress on degradation rates is a modification of the so‐called Zhurkov equation that has been used rather successfully to predict the effects of stress on degradation rates in thermal reactions. This empirical equation relates the quantum yield of degradation to a composite activation barrier for the overall photochemical reaction. Following the discussion of these hypotheses, experimental mechanistic studies of stress effects are summarized, and what little data there is is shown to be consistent with the hypothesis that proposes that stress primarily affects the ability of photochemically generated radical pairs to recombine. By decreasing the efficiency of radical–radical recombination, the effect is to increase the relative efficiencies of the radicals' other reactions and hence the rate of degradation. In addition to stress, other factors can affect the rates of polymer photodegradation. These factors include the absorbed light intensity, the polymer morphology, the rate of oxygen diffusion in the polymer, and the chromophore concentration. Each of these parameters must be carefully controlled in mechanistic studies that probe the effects of stress on degradation rates.     

Über die anodische oxidation von hydrazinderivaten an platin im sauren elektrolyten I. Phenylhydrazin

The investigation of the anodic oxidation of phenylhydrazine at smooth platinum in acid solutions by the triangular voltage scan method, by controlled potential electrolysis and by potential step measurements yields the following results:(1) The overall-reaction yields 4 electrons per PH-molecule.(2) Within the PH-oxidation an inhibiting intermediate is developed, whose further reaction is limited, and which can be reduced by hydrogen.(3) The kinetics of the oxidation reaction are determined by a 1-electron step. The anodic reaction orders of phenylhydrazine and H⁺-ions are z_PH = 0.4 and z_H⁺ = ?0.8, respectively.These results are interpreted by means of a reaction mechanism. The kinetics of the oxidation are explained with the aid of the Temkin adsorption isotherm.     

Mechanistic studies on the Mukaiyama epoxidation

A detailed mechanistic study on the Mukaiyama epoxidation of limonene with dioxygen as oxidant, bis(acetylacetonato)nickel(II) as catalyst, and an aldehyde as co-reagent is reported. All major products of the reaction have been quantitatively identified, both with isobutyraldehyde and 2-methylundecanal as co-reacting aldehydes. Limonene epoxide is formed in good yield. The main products evolving from the aldehyde are carboxylic acid, CO(2), CO, and lower molecular weight ketone and alcohol (K + A). A mechanism is proposed in which an acylperoxy radical formed by the autoxidation of the aldehyde is the epoxidizing species. The observation of carbon dioxide and (K + A) in a 1:1 molar ratio supports this mechanism. CO(2) and (K + A) are formed in molar amounts of 50-60% with respect to the amount of epoxide produced, indicating that epoxidation takes place not only via acylperoxy radicals but also via a peracid route. Cyclohexene epoxidation was also investigated with a number of different metal complexes as catalysts. Cyclohexene is very sensitive for allylic oxidation, which provides information about the action of the catalyst, e.g., metals that form strongly oxidizing stable high-valence complexes are more likely to induce allylic oxidation. Color changes in the reaction mixture indicate the presence of such high-valence species. In the case of nickel, it was found that low-valence compounds predominate during the reaction, which is in line with the fact that this metal displays the highest selectivity for epoxide. A mechanism that accounts for the observations is presented.     

Kinetics of polymeric radical oxidation with molecular oxygen in poly(methyl methacrylate) films

In poly(methyl methacrylate) films, the kinetics of the oxidation of polymeric radicals and azobenzenenitrenes with molecular oxygen dissolved in the polymer is studied. The free radicals are produced at 77 K by irradiating the polymer with UV light, fast electrons, or γ rays. The concentration of oxygen is varied from 4.5 × 10¹⁸ to 3.1 × 10¹⁹ cm^?3; the temperature of the reaction, from 90 to 130 K. The reaction is carried out in excess oxygen. The kinetics of radical oxidation is shown to be independent of the type of radiation that stimulates the formation of radicals and coincides with the kinetics of the oxidation of azobenzenenitrenes, which are uniformly dissolved in the polymer. It is concluded that the structure of the polymer in the vicinity of the radicals is virtually the same as the structure of the polymer bulk. The activation energy of the oxygen diffusion coefficient calculated according to the radical oxidation kinetics amounts to ～30 kJ/mol.     

Effect of structure in benzaldehyde oximes on the formation of aldehydes and nitriles under photoinduced electron-transfer conditions

The mechanistic aspects of the photosensitized reactions of a series of benzaldehyde oximes (1a-o) were studied by steady-state (product studies) and laser flash photolysis methods. Nanosecond laser flash photolysis studies have shown that the reaction of the oxime with triplet chloranil (3CA) proceeds via an electron-transfer mechanism provided the free energy for electron transfer (DeltaG(ET)) is favorable; typically, the oxidation potential of the oxime should be below 2.0 V. Substituted benzaldehyde oximes with oxidation potentials greater than 2.0 V quench 3CA at rates that are independent of the substituent and the oxidation potential. The most likely mechanism under these conditions is a hydrogen atom transfer mechanism as this reaction should be dependent on the O-H bond strength only, which is virtually the same for all oximes. Product studies have shown that aldoximes react to give both the corresponding aldehyde and the nitrile. The important intermediate in the aldehyde pathway is the iminoxyl radical, which is formed via an electron transfer-proton transfer (ET-PT) sequence (for oximes with low oxidation potentials) or via a hydrogen atom transfer (HAT) pathway (for oximes with larger oxidation potentials). The nitriles are proposed to result from intermediate iminoyl radicals, which can be formed via direct hydrogen atom abstraction or via an electron-transfer-proton-transfer sequence. The experimental data seems to support the direct hydrogen atom abstraction as evidenced by the break in linearity in the plot of the quenching rates against the oxidation potential, which suggests a change in mechanism. The nitrile product is favored when electron-accepting substituents are present on the benzene ring of the benzaldehyde oximes or when the hydroxyl hydrogen atom is unavailable for abstraction. The latter is the case in pyridine-2-carboxaldoxime (2), where a strong intramolecular hydrogen bond is formed. Other molecules that form weaker intramolecular hydrogen bonds such as 2-furaldehyde oxime (3) and thiophene-2-carboxaldoxime (4) tend to yield increasing amounts of aldehyde.     

Catalytic oxidative degradation of polyethylene crystals

Linear polyethylene crystals have been degraded with oxygen in the presence of a catalyst composed of cobaltous and bromide ions at temperatures between 60 and 130°. The number of carbonyl groups introduced into polyethylene increases as the oxidation temperature is raised. Infra-red spectroscopy and molecular weight evidence suggests that ketone groups are formed by a non-chain scission reaction, whereas aldehyde and carboxylic acid groups result from chain scission reactions. Molecular weight distributions of the degradation products show that reactions occur selectively in the amorphous regions at the crystal surfaces, when oxidations are performed below 110°. At higher temperatures, the chains are degraded randomly. The carbonyl groups introduced into polyethylene were oxidized to carboxylic acid groups with concentrated (～ 70 per cent) nitric acid. The results demonstrate that this two-stage process yields a polydisperse long-chain linear paraffin with carboxylic acid end-groups, when the catalytic oxidations are performed between 115 and 130°.     

The kinetics of tobacco pyrolysis

When tobacco is pyrolysed under non-isothermal flow conditions in an inert atmosphere, variation of the inert gas or its space velocity has only a minor effect on the profiles of formation rate versus temperature for seven product gases. Thus, mass transfer processes between the tobacco surface and the gas phase are very rapid, and the products are formed at an overall rate which is determined entirely by that of the chemical reactions.The effect of radical chain inhibitors (nitrogen oxides) on the pyrolysis is complex because of the resultant oxidation. Nevertheless, no evidence was found for the occurrence of radical chain reactions in the gas phase. A small proportion (less than 10%) of all the gases monitored are formed by homogeneous decomposition of volatile and semi-volatile intermediate products, in the furnace used.At temperatures above about 600°C the reduction of carbon dioxide to carbon monoxide by the carbonaceous tobacco residue becomes increasingly important. However, when tobacco is pyrolysed in an inert atmosphere, only a small amount of carbon dioxide is produced above 600°C and consequently its reduction to carbon monoxide contributes only a small proportion to the total carbon monoxide formed above that temperature. The rate of the tobacco/carbon dioxide reaction is controlled by chemical kinetic rather than mass transfer effects. Carbon monoxide reacts with tobacco to a small extent.When the tobacco is pyrolysed in an atmosphere containing oxygen (9–21% v/v), some oxidation occurs at 200°C. At 250°C the combustion rate is controlled jointly by both kinetic and mass transfer processes, but mass transfer of oxygen in the gas phase becomes increasingly important as the temperature is increased, and it is dominant above 400°C. About 8% of the total carbon monoxide formed by combustion is lost by its further oxidation.The results imply that inside the combustion coal of a burning cigarette the actual reactions occurring are of secondary importance, the rate of supply of oxygen being the dominant factor in determining the combustion rate and heat generation. In contrast, in the region immediately behind the coal, where a large proportion of the products which enter mainstream smoke are formed by thermal decomposition of tobacco constituents, the chemistry of the tobacco substrate is critical, since the decomposition kinetics are controlled by chemical rather than mass transfer effects. tobacco substrate is critical. In addition, the heat release or absorption due to the pyrolytic reactions occurring behind the coal will depend on the chemical composition of the substrate. Thus, together with the differing thermal properties of the tobacco, the temperature gradient behind the coal should depend on the nature of the tobacco.