Oxidation of formic acid by bromine in aqueous,strongly acid media

Spectrophotometric methods were used to investigate the rate of the reaction of Br₂ with HCOOH in aqueous, acidic media. The reaction products are Br^? and CO₂. The kinetics of this reaction are complicated by both the formation of Br₃^? as Br^? is formed and the dissociation of HCOOH into HCOO^? and H⁺. Previous work on this reaction was carried out at acidities lower than the highest used here and led to the conclusion that only HCOO^? reacts with Br₂. It is agreed that this is by far the principal reaction. However, at the highest acidity experiments, an added small component of reaction was found, and it is suggested that it results from the direct reaction of Br₂ with HCOOH itself. On this assumption, values of the rate constants for both reactions are derived here. The rate constant for the reaction of HCOO^? with Br₂ agrees with values previously reported, within a factor of 2 on the low side. The reaction involving HCOOH is more than 2000 times slower than the reaction involving HCOO^?, but it does contribute to the overall rate as [H⁺] approaches 1M. These derived rate constants are able to simulate quantitatively the authors' absorbance-versus-time data, demonstrating the validity of their data treatment methods, if not mechanistic assignments. Finally, activation parameters were determined for both rate constants. The values obtained are: ΔE^?(HCOOH + Br₂) = 13.3 ± 1.1 kcal/mol, ΔS^? (HCOOH + Br₂) = ?28 ± 3 cal/deg mol, ΔE^? (HCOO^? + Br₂) = 13.1 ± 0.9 kcal/mol, and ΔS^?(HCOO^? + Br₂) = ?12 ± 1 cal/deg mol. That the activation energies of the two reactions turn out to be essentially identical does not support the authors' suggestion that both HCOOH and HCOO^? react with Br₂.     

Kinetics and mechanism of the oxidation of nitrous acid by bromine in aqueous sulfuric acid

The kinetics and mechanism of the reaction between nitrous acid and bromine are studied in dilute sulfuric acid medium, using both the stopped‐flow method and conventional spectrophotometry. The partial reaction order with respect to Br₂ moderately differs from 1, showing a saturation at a higher concentration of bromine. The second order of the reaction towards nitrous acid has been observed. Hydrogen and bromide ions significantly suppress the rate of reaction. Despite the apparent simplicity, the mechanism is rather complex, with two reaction pathways proposed. The first one is represented by the reaction of bromine with the intermediate dinitrogen trioxide. A direct nucleophilic attack of NO₂⁻ ion towards the bromine molecule is suggested as the second pathway. The proposed mechanism accounts for the observed behavior; in almost all cases a satisfactory quantitative agreement with the experiments is obtained. © 2000 John Wiley & Sons, Inc. Int J Chem Kinet 32: 279–285, 2000     

The kinetics of polyolefin oxidation in aqueous media

The thermal oxidation, at 75–92°C, of antioxidant-free low-density polyethylene (LDPE) is faster in most aqueous environments than in air. The accelerative environments include water, solutions of sodium salts, and a basic buffer. However, an acidic buffer retards oxidation. Transition metal salts are not examined here since their catalytic effect is wellknown. The acceleration is predominantly a surface phenomenon, and so is most obvious in thin polymer films. For antioxidant-containing polyolefins, complications arise because of the possibility of extraction of stabilizers, but the trend in oxidative lifetime is similar to that of the antioxidant-free specimens, and the antioxidants are observed to react chemically during the aging process. For example, a commercial crosslinked polyethylene (XLPE) shows a reduction in thermo-oxidative lifetime of at least a factor of two in aqueous environments compared to air. High-density polyethylene (HDPE) and polypropylene (PP) show similar behavior, but the magnitude of the effect of aqueous environments is less. This phenomenon is discussed in terms of nucleophilic attack by peroxy anions on hydroperoxides.     

Kinetics and mechanism of oxidation of formic acid by bismuth(V) in aqueous phosphoric acid medium

The solution of bismuth(V) was prepared by digesting sodium bismuthate in aqueous phosphoric acid (3.0 mol dm⁻³), the resulting pink colour solution absorbs in the visible region at 530 nm (640 dm³ mol⁻¹ cm⁻¹). The stoichiometry of the oxidation of formic acid by bismuth(V) corresponds to the reaction as represented by the Eq. ( 1 ). (1) The observed kinetic rate law is given by the Eq. ( 2 ); (2) where Bi^V and [HCO₂H] are the gross analytical concentrations of bismuth(V) and formic acid respectively. A plausible reaction mechanism corresponding to the rate law (2) has been proposed. Also the pattern of reactivity of bismuth(V) in HCIOHF mixture and H₃PO₄ respectively has been compared. © 2000 John Wiley & Sons, Inc. Int J Chem Kinet 32: 491–497, 2000     

Kinetics of the permanganate oxidation of formic acid in aqueous solution

The kinetics of the permanganate oxidation of formic acid in aqueous perchloric acid at 30°C were examined by the spectrophotometric method. The chemical reaction 2MnO + 3HCOOH + 2H⁺ → 2MnO₂ + 3CO₂ + 4H₂O, appears to proceed via several parallel reactions. The overall rate equation has been obtained by using statistical multilinear regression analysis of the 660 cases studied, and the presence in the rate equation of two new terms in relation to previous studies shows that both permanganate autocatalytic effects and acid media inhibition must be taken into account when the reaction proceeds at constant ionic strength.     

Kinetic and mechanism of oxidation of thiocarbohydrazide and its metal complex by acid bromate in aqueous and partial aqueous media

The kinetics of oxidation of thiocarbohydrazide in the free and zinc(Ⅱ)-bound states byacid bromate have been studied in aqueous and water-acetic acid(1:1,V/V)media under varyingconditions,both in the absence and presence of added bromide ion.The rates of oxidations show firstorder kinetics in[bromate]in all the cases,but exhibit different kinetic behaviour in[substrate]and[H~+].Oxidation of TCH in aqueous medium shows zero order in[TCH]and nearly second order in[H~+],while oxidation in aqueous acetic acid shows two ranges in[H~+].The rate shows first and fractionalorder kinetics in[TCH]in the first and second acid ranges.Kinetics observed in the presence of Br~-are similar to those observed for oxidation of TCH in second acid range.In addition,the reactionshows fractional order in[Br~-].Oxidation of TCH in Zn(Ⅱ)-bound state exhibits first order kinetics in[substrate]and second order in[H~+].Increase in ionic strength of the medium decreases the rate in allthe cases.Increase in acetic acid composition of the solvent increases the rate.Mechanisms consistentwith the observed results have been considered and the rate laws deduced.The rate limiting steps havebeen identified and the coefficients of these steps have been calculated at different temperatures.Therelated activation parameters have also been computed.The validity of the deduced rate laws has alsobeen tested by recalculating the rate constants from them as[TCH]and[H~+]are varied.     

Formation of aromatic radical-cations by oxidation with electronegatively substituted quinones in acid media; kinetics and mechanism

Electronegatively substituted quinones are shown to oxidize electronrich aromatic molecules to the corresponding radical-cations in acid medium via a reversible two step mechanism. The influence of acid strength on the rate of the reaction suggests that a protonated quinone molecule acts as the primary electron acceptor. The rate of formation of the radical cations depends on the one electron oxidation potential of the parent aromatic molecules in a way typical for endothermic outersphere electrontransfer.     

Suitable combination of promoter and micellar catalyst for chromic acid oxidation of formaldehyde to formic acid in aqueous acid media at room temperature

The chromic acid oxidation of formaldehyde in micellar solution of sodium dodecyl sulphate (SDS)/TX-100 at room temperature has been investigated under pseudo first-order condition. Heterocyclic bases such as picolinic acid (PA), 2,2′-bipyridine (bipy), 1,10-phenanthroline (phen) have been employed which acts as a promoter. The rate of this oxidation reaction increased 50 times by combination of bipy and SDS micelle compared to rate in pure aqueous media. Here, we can avoid hazardous organic solvents, high pressure and temperature which are required for conventional preparation of formic acid. The catalytic effect of SDS/TX-100 micelle has been explained.     

The kinetics of peroxydisulphate ion oxidation of glycolic acid in aqueous medium

The peroxydisulphate ion oxidation of glycolic acid has been studied kinetically under various conditions using different concentrations of reactants and of a neutral salt and at different temperatures. The reaction has been found to be first order and the thermodynamic constants have been calculated. The product, formic acid, has been identified and a probable mechamism for the reaction is proposed.     

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.     

The kinetics and mechanism of polymer-based NHC-Pd-pyridine catalyzed heterogeneous Suzuki reaction in aqueous media

One of the most important challenges of the Suzuki reaction is a green synthesis of reaction products. In terms of economy and ecology, the Suzuki reaction details must be characterized for the industrial-scale Suzuki reaction processes. In this paper, for the first time, a kinetic and mechanistic study on the Suzuki reaction catalyzed with hydrogel-supported PEPPSI (pyridine-enhanced precatalyst preparation stabilization (and) initiation) type NHC-Pd-pyridine composite has been investigated. To determine the rate-limiting step, the effects of reactants and experimental conditions on the heterogeneous Suzuki reaction have been experimentally defined. The experimental results demonstrated that it is possible to reach 100% yield under the optimum reaction conditions, which were found as 75 × 10⁻³ mol/L of phenylboronic acid (FBA), 50 × 10⁻³ mol/L of bromoacetophenone (Brac), 125 × 10⁻³ mol/L of K₂CO₃, 1 g/L of catalyst, 80°C of reaction temperature, 400 rpm of mixing rate, and 3 h of reaction time. The transmetalation step in the cycle was defined as the rate-limiting step. On the basis of kinetic results, a mathematical reaction rate expression was presented assuming the steady-state approach to steps of the catalytic cycle. The activation energy (E_a) of the reaction was estimated to be 34.88 kJ/mol.     

Kinetics and mechanism of hydrogen peroxide oxidation of chromone-3-carboxaldehydes in aqueous acid and micellar media

Oxidation of chromone-3-carboxaldehyde (CCA) and substituted analogues by H₂O₂ has been carried out in aqueous acid (HCl and H₂SO₄) and micellar media. Reaction kinetics indicated order in [CCA] as well as [H₂O₂] to be unity while it is a fraction (1 > n > O) in [acid]. Reaction rates were found to be faster in the solvents of low-dielectric constant (D). Added salt (KCl or (NH₄)₂SO₄) increased the rate of oxidation marginally. On the basis of observed linearity of Amis plot and marginal positive salt effect, protonated CCA (enol form of CCA, a cation) and H₂O₂ (neutral molecule) were considered as reactive species in the rate limiting step. Reaction rates were found to be enhanced significantly in anionic and nonionic micellar (sodium dodecylsulfate (SDS) and Triton X-100 (Tx), respectively) media. However, cationic micelles [cetyl trimethyl ammonium bromide (CTAB)] indicated marginal retardation effect. Effect of anionic and cationic micelles has been interpreted in terms of electrostatic interactions, while that of nonionic micelles in terms of hydrophobic interactions. Structure-reactivity correlations have been interpreted by Hammett's equation. Negative “ρ” (reaction constant) values indicated cationic transition state. © 1996 John Wiley & Sons, Inc.     

Studies on kinetics and mechanism of oxidation of D‐sorbitol and D‐mannitol by cerium(IV) in aqueous micellar sulfuric acid media

The kinetics and mechanism of cerium(IV) oxidation of hexitols, i.e. D ‐sorbitol and D ‐mannitol, in aqueous sulfuric acid media have been studied in the presence and absence of surfactants. Under the kinetic conditions, [S]_T ? [Ce(IV)]_T, where [S]_T is the total substrate (D ‐sorbitol or D ‐mannitol) concentration, the overall process shows a first‐order dependence on [Ce(IV)]_T and [S]_T. The process is acid catalyzed and inhibited by [HSO]. From the [HSO] dependence, it has been noted that the both Ce(SO₄)²⁺ and Ce(SO₄)₂ have been found kinetically active. The different rate constants in the presence and absence of surfactants have been estimated with the activation parameters. N‐cetylpyridinium chloride has been found to retard the oxidation process of hexitols, whereas sodium dodecyl sulfate has been found to accelerate the rate process. All these findings including the micellar effects have been interpreted in terms of the proposed reaction mechanism and partitioning behavior of the kinetically active different species of Ce(IV) between the aqueous and pseudomicellar phase. © 2008 Wiley Periodicals, Inc. 40: 445–453, 2008     

Kinetics and mechanism of the oxidation of L-ascorbic acid by 2,6-dichlorophenol-indophenol in aqueous solution

The kinetics of oxidation of L-ascorbic acid by 2,6-dichlorophenolindophenol in aqueous solution has been studied. The rate of the reaction decreases with increasing pH since the hydrogen ascorbate ion is less reactive than the unionized L-ascorbic acid. The rate constants for the oxidation of the two species have been evaluated and a plausible mechanism of the reaction is suggested.

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L- 2,6-- . pH, , L- . .

     

Kinetics and mechanism of the oxidation of p-aminobenzoic acid by periodate ion in aqueous medium

Kinetic results of the oxidation of p-aminobenzoic acid by periodate ion in aqueous medium are discussed. A suitable mechanism based on kinetic evidence and product analysis is proposed.

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Substrate selectivity and mechanism of oxidation of alkanes by peroxynitrous acid in aqueous solution

The relative rate constants (k_rel) were measured for the oxidation of alkanes (RH) by peroxynitrous acid (HOONO) in aqueous solutions at 25 °C. The krel values for the reactions of RH with HOONO and with OH radicals in the series of C₂-C₇ alkanes and the isotopic effects (c-C₆H₁₂/c-C₆D₁₂) agree. However, the k_rel value for methane was lower than for its reaction with OH. Possible reaction mechanisms are discussed. __________ Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 42, No. 5, pp. 295–299, September–October, 2006.     

Kinetics and mechanism of oxidation of sulfanilic acid by periodate ion in aqueous medium

Results of kinetic studies of the sodium metaperiodate oxidation of sulfanilic acid in aqueous medium are discussed. A mechanism for the formation of azobenzene-4,4-disulfonic acid, isolated and characterized as its S-benzylisothiuronium derivative, is proposed.

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. -4,4- , S- .

     

离子排斥色谱法测定甘油催化氧化产物中的H2CO3和HCOOH

建立了测定甘油催化氧化产物中H2CO3和HCOOH的离子排斥色谱分析方法。采用离子排斥柱分离,分别用纯水和4 mmol/L HCl作流动相进行H2CO3和HCOOH的分析。检测方式为非抑制电导检测。实验结果显示,H2CO3和HCOOH工作曲线的线性范围为2～100 mg/L和6.23～124.6 mg/L,检出限分别为0.45 mg/L和2.49 mg/L(S/N=3)。H2CO3的保留时间和峰面积的相对标准偏差分别为0.07%和4.0%,HCOOH的保留时间和峰面积的相对标准偏差分别为0.09%和2.2%。方法已用于甘油催化氧化产物中H2CO3和HCOOH的分析。