Kinetics and mechanism of oxidation of aniline and substituted anilines by isoquinolinium bromochromate in aqueous acetic acid

Oxidation of anilines by isoquinolinium bromochromate (IQBC) in aqueous acetic acid leads to the formation of corresponding azobenzenes. The reaction is first order with respect to both aniline and IQBC and is catalyzed by hydrogen ion. The rate of oxidation decreases with increasing dielectric constant of solvent, indicating the presence of an ion-dipole interaction. The rate of oxidation decreases with increase in concentration of KCl, possibly due to the formation of less reactive species by interaction of Cl^- and protonated IQBC. The specific rate of oxidizing species anilines reaction correlates with substituents constant affording a negative reaction constant. Hammett plot is found to be valid and the correlation between enthalpies and free energies of activation is reasonably linear with an isokinetic temperature of 401 K.     

Coulometric generation of acetylions by oxidation of mercury in acetic anhydride and in acetic acid/acetic anhydride mixture

Coulometric generation of acetyl (CH₃CO⁺) ions by oxidation of mercury in acetic anhydride and in acetic acid/acetic anhydride (5:95, v/v) is described. Current/potential curves for solvents, titrated bases, indicator and mercury showed that in both these solvents mercury is oxidized at potentials which are much more negative than those for the titrated bases and other components present in the solution. Quinoline, triethanolamine, triethylamine, pyridine and quinolin-8-ol in acetic anhydride, as well as triethylamine, 2,2′-bipyridine, 2,4,6-collidine, pyridine and sodium acetate in acetic acid/acetic anhydride were titrated with acetyl ions generated by the oxidation of mercury. In this way, it was established that the oxidation of mercury to mercury (I) ions proceeds quantitatively with 100% current efficiency.     

乳酸直接聚合的热力学、动力学研究

以乳酸单体为原料,在无催化剂存在和常压下直接缩聚合成聚乳酸,并研究了乳酸直接聚合的动力学,结合其热力学的分析,指出提高真空度脱除水分、控制合适的温度、选择合适的催化剂是获得高分子量聚乳酸的关键,为乳酸地接聚合工艺提供了理论参考。     

Kinetics and mechanism of oxidation of anisole and substituted anisoles by acid bromate in acetic acid-water mixture

Oxidation of anisoles by acid bromate has been studied in acetic acid-water system in the presence of sulphuric acid. The reaction is first order each in [anisole] and [Br(V)]. The rate of reaction increased with increase in [H⁺] and percentage of acetic acid. The products of oxidation have been identified as ortho and para hydroxyanisoles. From the effect of [H⁺] and [acetic acid] on rate, H ₂ ⁺ BrO₃ has been established as the reactive species. Anisoles having electron-donating substituents in the benzene ring accelerate the rates and vice versa with a Hammett ρ value of −0.6. A mechanism involving the attack of H ₂ ⁺ BrO₃ on ortho/para position of the anisole in the rate-determining step has been proposed.     

Physico-chemical aspects of polyethylene processing in an open mixer. Part 26: Formal kinetics of aldehyde and carboxylic acid formation at a constant rate

Formation of carboxylic acids at a constant rate can be easily explained. It seems to result from the formation and decomposition of α,γ-keto-hydroperoxides. Formal kinetics based on formation and decomposition of these structural units is in agreement with the experimental findings. The activation energy deduced from the calculations is negligible, in agreement with the experimental data showing the constant rate to be practically temperature independent. Comparison of the acids with the hydroperoxides and ketones formed initially shows that the rate of oxygen addition to alkyl radicals is significantly smaller than in low molecular mass liquids. The same conclusion is reached on comparing directly the acids formed on decomposition of α,γ-keto-hydroperoxides in polyethylene melt and in hexadecane. The rate of oxygen addition in polyethylene melt is closer to 2 × 10⁵ than to 6 × 10⁵ (s⁻¹) that is valid in hexadecane.It is possible to attribute the relatively small amount of aldehydes that might be formed at a constant rate to different reactions of alkoxy radicals that are not in a cage with other radicals. These alkoxy radicals result from the addition of peroxy radicals to unsaturated bonds. This addition is followed mainly by epoxide formation and simultaneous release of an alkoxy radical.     

Physico-chemical aspects of polyethylene processing in an open mixer: Part 27: Formal kinetics of aldehyde and carboxylic acid formation in the initial stages

There are many reactions susceptible to yield aldehydes and acids in polyethylene melts. It is β-scission of the alkoxy radicals formed on bimolecular hydroperoxide decomposition that is expected to be one of the main sources of the aldehydes that are formed at increasing rates in the early stages of polyethylene processing. Acid-catalyzed decomposition of allylic hydroperoxides is another source of substantial amounts of aldehydes. Formation and decomposition of α,γ- and α,β-di-hydroperoxides should yield acids. The activation energy estimated for these different processes is very large (about 57 kcal/mol) so that their contribution could be significant in the high temperature range only. This is different for the reaction of aldehydes with hydroperoxides to yield peroxy-hemiacetals. These intermediates can be expected mainly in the low temperature range where hydroperoxides are accumulating. Decomposition of the peroxy-hemiacetals gives acids as one of the main products. Free-radical induced oxidation of aldehydes is likely to yield peracids as far as oxygen addition is competitive with decarbonylation. The main problem is the transformation of the peracids into acids. The reaction with double bonds is expected to yield significantly more acids than thermal decomposition of peracids. If the last occurs, it will be followed mainly by decarboxylation. The overall activation energy for both processes of acid formation is negative (−18 to −20 kcal/mol). It is some combination of the various mechanisms examined that might account for the experimental activation energy for acid formation in the initial stages that is close to 18 kcal/mol.     

Physico-chemical aspects of polyethylene processing in an open mixer. Part 24: Experimental kinetics of aldehyde and carboxylic acid formation

The experimental kinetics for carboxylic acids shows more complexity than that for ketones. The fitting of the experimental results for the initial stages to the equation consisting of a linear and a quadratic term in processing time accounts well for the ketone data but not for the acid data. Instead of that, the data for the acids show fair fit to an equation containing a linear term and another term that is cubic in processing time. In the temperature range of the experiments the linear term is practically constant. The cubic term increases strongly with temperature. The combination of a linear and a quadratic term can account for the advanced stages of processing. The corresponding quadratic term shows strong increase if the processing temperature passes from 150 to 160 °C. However, for higher processing temperatures it remains constant within experimental error. The difference carbonyl absorbance measured after treatment of the polyethylene films with ammonia corresponds to the sum of the acids and aldehydes. It shows similarly complex kinetics. Some of the difficulties encountered with the experimental kinetics cannot be resolved with the data available. It is only the comparison with the formal kinetics based on potential mechanisms of product formation that allows for better understanding of the experimental results.     

Physico-chemical aspects of polyethylene processing in an open mixer. Part 28: Formal kinetics of aldehyde and carboxylic acid formation in the advanced stages

The rate of acid formation at high temperature is constantly increasing but temperature independent. Two main mechanisms can account for this behavior in the advanced stages of polyethylene processing. The first mechanism is based on free radical induced oxidation of aldehyde pairs that are formed on acid-catalyzed decomposition of allylic hydroperoxides. The last will be formed essentially on mechanical stress-induced oxygen addition to trans-vinylene groups. Peroxidation of one of the aldehydes might yield an acyl-peroxy radical that is likely to abstract the labile hydrogen atom from the second aldehyde. The acyl radical formed in the reaction will abstract a hydroxyl group from the peracid formed in the same reaction. This yields an acid and an acyl-oxy radical that will give a primary alkyl radical on decarboxylation. The second mechanism involves oxidation of ketones and alcohols that accumulate in the oxidizing melt. Acid-catalyzed decomposition of the α-keto-hydroperoxides yields simultaneously an acid and an aldehyde. Formal kinetics based on each mechanism shows that they do not involve significant activation energy, as it is required by the experimental data. The dependency on the oxygen concentration deduced from the formal kinetics for the oxidation of aldehyde pairs is in agreement with the experiments.     

Kinetics and mechanism of oxidation of cyclohexanol by 1-Chlorobenzotriazole (CBT) in acid medium

The kinetics of oxidation of cyclohexanol by 1-Chlorobenzotriazole (CBT) has been studied at 40°C in 50% aqueous acetic acid. The reaction is first order with respect to oxidant and first order with respect to substrate. The rate is found to increase with increase in acid concentration and percentage ofAcOH in the mixture. The kinetic parameters have been calculated. A suitable mechanism is proposed.

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Kinetik und Mechanismus der Oxidation von Cyclohexanol mit 1-Chlorbenzotriazol (CBT) in saurem Medium

Zusammenfassung Die Kinetik der Oxidation von Cyclohexanol mitCBT wurde bei 40°C in 50% wäßriger Essigsäure untersucht. Die Reaktion war sowohl bezüglich Oxidationsmittel als auch Substrat von erster Ordnung. Die Reaktionsgeschwindigkeit wird mit zunehmender Säurekonzentration (AcOH) erhöht. Die kinetischen Parameter wurden bestimmt und ein passender Mechanismus wird vorgeschlagen.

     

Kinetics and mechanism of oxidation of acetanilide by quinquevalent vanadium in acid medium

Summary The kinetics of the oxidation of acetanilide with vanadium(V) in sulphuric acid medium at constant ionic strength has been studied. The reaction is first order with oxidant. The order of reaction in acetanilide varies from one to zero. The reaction follows an acid catalyzed independent path, exhibiting square dependence in H⁺. A Bunnett plot indicates that the water acts as a nucleophile. The thermodynamic parameters have been computed. A probable reaction mechanism and rate law consistent with these data are given.

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Kinetik und Mechanismus der Oxydation von Acetanilid mit fünfwertigem Vanadium in saurem Medium

Zusammenfassung Es wurden kinetische Untersuchungen der Oxydation von Acetanilid mit Vanadium(V) in schwefelsaurem Medium bei konstanter Ionenstärke durchgeführt. Gegenüber dem Oxidans ist die Reaktion erster Ordnung, die Reaktionsordnung gegenüber Acetanilid variiert zwischen 1 und 0. Die Reaktion folgt einem von der Säurekatalyse unabhängigen Weg, wobei die Abhängigkeit von H⁺ quadratisch ist. Ein Bunnett-Plot zeigt, daß das Wasser als Nucleophil wirkt. Die thermodynamischen Parameter wurden berechnet. Ein möglicher Reaktionsmechanismus und ein Geschwindigkeitsnetz, das mit diesen Daten in Einklang ist, wird angegeben.

     

kinetics of the oxidation of b-picoline to nicotinic acid over vanadia-titania catalyst, 1. The network of the reaction and the effect of water

Over vanadia-titania catalysts, the oxidation of β-picoline into nicotinic acid proceeds under a parallel-consecutive network. Nicotinic acid is formed both directly from picoline and through pyridine-3-carbaldehyde as an intermediate. Products of total oxidation and nitrile are formed on a parallel path from picoline, and on a consecutive path via overoxidation of partial oxidation products. Introduction of water into the reaction mixture raises selectivity and activity due to an acceleration of the formation of carbaldehyde and nicotinic acid and not due to repressing the reactions of total oxidation. This revised version was published online in August 2006 with corrections to the Cover Date.     

Effect of bromide ion on the kinetics of bromination of o-hydroxy benzoic acid by bromine in aqueous solution

The kinetics of the bromination of o-hydroxy benzoic acid has been studied by rotating platinum electrode (RPE) technique. The bromide ion has been found to remarkably enhance the specific reaction rate. This is associated with a fall in energy of activation. The other added ions like nitrate, acetate and bicarbonate have shown that the base catalysis or salt effect is improbable. The mechanism suggested to explain the catalytic effect of bromide ions.     

Kinetics and activation energy of the oxidation of para‐tolyl radical by cobalt(III) in acetic acid: Competition kinetics

The title reaction gives rise to a benzylic cation that is rapidly transformed to its bromide in competition with the reaction of the radical with carbon tetrachloride. Experiments were carried out over 17–69°C in acetic acid containing cobalt(II) acetate, para‐xylene, hydrobromic acid, carbon tetrachloride, and meta‐chloroperoxybenzoic acid. The product ratio, ArCH₂Cl/ArCH₂Br, in combination with other pertinent rate constants, was used to determine the rate constant for the step of interest: log k (L mol^?1 s^?1 = 19.9 – (75 ± 11 kJ mol^?1/2.303 RT). The large pre‐exponential factor, which gives ΔS? = 128 J K^?1 mol^?1, signals an unusual transition state, because a negative value of ΔS? would be expected for a simple bimolecular reaction. The production of the ion pair ArCH‖OAc^? in HOAc, which has the same dielectric constant as benzene, may be responsible, at least in part. Furthermore, inner sphere reorganization of cobalt may also contribute. © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 37: 599–604, 2005     

Mechanistic studies on the oxidation of pyruvic acid by an oxo-bridged diiron(III,III) complex in aqueous acidic media

In aqueous solution [Fe₂(μ-O)(phen)₄(H₂O)₂]⁴⁺ (1, phen = 1,10-phenanthroline) equilibrates with its conjugate bases [Fe₂(μ-O)(phen)₄(H₂O)(OH)]³⁺ (2) and [Fe₂(μ-O)(phen)₄(OH)₂]²⁺ (3). In the presence of excess phen and in the pH range 2.5–5.5, the dimer quantitatively oxidizes pyruvic acid to acetic acid and carbon dioxide, the end iron species being ferroin, [Fe(phen)₃]²⁺. The observed reaction rate shows a bell-shaped curve as pH increases, but is independent of added phen. Kinetic analysis shows that (3) is non-reactive and (1) has much higher reactivity than (2) in oxidizing pyruvic acid. The basicity of the bridging oxygen increases with deprotonation of the aqua ligands. The reaction rate decreases significantly in media enriched with D₂O in comparison to that in H₂O, with a greater retardation at higher pH, suggesting the occurrence of proton coupled electron transfer (PCET; 1e, 1H⁺), which possibly drags the energetically unfavorable reaction to completion in presence of excess phen.     

Removal of nickel ions by adsorption on nano-bentonite: Equilibrium,kinetics, and thermodynamics

Nano-bentonite was used as an adsorbent to remove nickel ions from aqueous solutions. Experimental investigation was carried out to identify the effect of pH, contact time, initial concentration, and adsorbent dose of Ni(II). Equilibrium data were described by and fitted to Langmuir, Freundlich, and Dubinin–Radushkevich models. Results showed that the optimum conditions for the removal of the Ni(II) are initial concentration 100 mg/L, adsorbent dose 0.5 g, and pH 6. Surface morphology and functionality of nano-bentonite were characterized by SEM and FTIR. The kinetics data were more accurately described by pseudo-second-order model. The intra-particle diffusion model gave multi-linear curves, so more than one step controlled the adsorption process. Nano-bentonite removed nickel with maximum adsorption capacity of 39.06 mg/g (30°C, pH) and thermodynamic data indicated that adsorption reaction is spontaneous and of an endothermic nature.     

Kinetics and mechanism of oxidation of hypophosphite by Waugh-type enneamolybdomanganate(IV) in perchloric acid

The reaction between hypophosphite and enneamolybdomanganate(IV) in perchloric acid was carried out under pseudo-first-order conditions keeping large excess of hypophosphite. The order in oxidant was found to be unity and that of hypophosphite was found to be less than unity. The reaction proceeds with prior formation of complex between the reactants followed by its rate determining decomposition. The accelerating effect of hydrogen ions on the reaction is due to the formation of active hexaprotonated oxidant species. The formation of the complex is supported by kinetic results and also by spectrophotometric study. The product of the reaction was found to be phosphitomolybdate, [H₁₀(HP)Mo₆O₂₆]²⁻, which was confirmed by FTIR study and AAS analysis. The reaction involves direct two-electron transfer step without any free radical intervention. The effect of ionic strength, solvent polarity and the activation parameters were also in support of the mechanism proposed.