S-oxygenation of thiocarbamides. 3. Nonlinear kinetics in the oxidation of trimethylthiourea by acidic bromate

The oxidation of trimethylthiourea (TMTU) by acidic bromate has been studied. The reaction mimics the dynamics observed in the oxidation of unsubstituted thiourea by bromate with an induction period before formation of bromine. The stoichiometry of the reaction was determined to be 4:3, thus 4BrO(3)- + 3R(1)R(2)C=S+ 3H(2)O --> 4Br- + 3R(1)R(2)C=O + 3SO(4)(2-) + 6H+. This substituted thiourea is oxidized at a much faster rate than the unsubstituted thiourea. The oxidation mechanism of TMTU involves initial oxidations through sulfenic and sulfinic acids. At the sulfinic acid stage, the major oxidation pathway is through the cleavage of the C-S bond to form a reducing sulfur leaving group, which is easily oxidized to sulfate. The minor pathway through the sulfonic acid produces a very stable intermediate that is oxidized only very slowly to urea and sulfate. The direct reaction of aqueous bromine with TMTU was faster than reactions that form bromine, with a bimolecular rate constant of (1.50 +/- 0.04) x 10(2) M(-1) s(-1). This rapid reaction ensured that no oligooscillatory bromine formation was observed. The oxidation of TMTU was modeled by a simple reaction scheme containing 20 reactions.     

Kinetics and mechanism of oxidation of benzohydrazide by bromate catalyzed by vanadium(IV) in aqueous acidic medium

The reaction between benzohydrazide and potassium bromate catalyzed by vanadium(IV) was studied under pseudo‐first‐order condition keeping large excess of hydrazide concentration over that of the oxidant. The initiation of the reaction occurs through oxidation of the catalyst vanadium(IV), VO²⁺, to vanadium(V), VO, which then reacts with hydrazide to give N,N′‐diacylhydrazine and benzoic acid as the products. The order in [H⁺] is found to be two, and its effect is due to protonation and hydrolysis of oxidized form of the catalyst to form HVO₃. The oxidized form of the catalyst, VO, forms a complex with the protonated hydrazide as evidenced by the occurrence of absorption maxima at 390 nm. The rate of the reaction remains unaffected by the increase in the ionic strength. The activation parameters were determined, and data support the mechanism. The detailed mechanism and the rate equation are proposed for the reaction. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 40: 151–159, 2008     

S-oxygenation of thiocarbamides I: Oxidation of phenylthiourea by chlorite in acidic media

The oxidation of 1-phenyl-2-thiourea (PTU) by chlorite was studied in aqueous acidic media. The reaction is extremely complex with reaction dynamics strongly influenced by the pH of reaction medium. In excess chlorite concentrations the reaction stoichiometry involves the complete desulfurization of PTU to yield a urea residue and sulfate: 2ClO2- + PhN(H)CSNH2 + H2O --> SO4(2-) + PhN(H)CONH2 + 2Cl- + 2H+. In excess PTU, mixtures of sulfinic and sulfonic acids are formed. The reaction was followed spectrophotometrically by observing the formation of chlorine dioxide which is formed from the reaction of the reactive intermediate HOCl and chlorite: 2ClO2- + HOCl + H+ --> 2ClO2(aq) + Cl- + H2O. The complexity of the ClO2- - PTU reaction arises from the fact that the reaction of ClO2 with PTU is slow enough to allow the accumulation of ClO2 in the presence of PTU. Hence the formation of ClO2 was observed to be oligooscillatory with transient formation of ClO2 even in conditions of excess oxidant. The reaction showed complex acid dependence with acid catalysis in pH conditions higher than pKa of HClO2 and acid retardation in pH conditions of less than 2.0. The rate of oxidation of PTU was given by -d[PTU]/dt = k1[ClO2-][PTU] + k2[HClO2][PTU] with the rate law: -d[PTU]/dt = [Cl(III)](T)[PTU]0/K(a1) + [H+] [k1K(a1) + k2[H+]]; where [Cl(III)]T is the sum of chlorite and chlorous acid and K(a1) is the acid dissociation constant for chlorous acid. The following bimolecular rate constants were evaluated; k1 = 31.5+/-2.3 M(-1) s(-1) and k2 = 114+/-7 M(-1) s(-1). The direct reaction of ClO2 with PTU was autocatalytic in low acid concentrations with a stoichiometric ratio of 8:5; 8ClO2 + 5PhN(H)CSNH2 + 9H2O --> 5SO4(2-) + 5PhN(H)CONH2 + 8Cl- + 18H+. The proposed mechanism implicates HOCl as a major intermediate whose autocatalytic production determined the observed global dynamics of the reaction. A comprehensive 29-reaction scheme is evoked to describe the complex reaction dynamics.     

Kinetics and mechanism of oxidation of N, N'-dimethylaminoiminomethanesulfinic acid by acidic bromate

The major metabolites of the physiologically active compound dimethylthiourea (DMTU), dimethylaminoiminomethansesulfinic acid (DMAIMSA), and dimethylaminoiminomethanesulfonic acid (DMAIMSOA) were synthesized, and their kinetics and mechanisms of oxidation by acidic bromate and aqueous bromine was determined. The oxidation of DMAIMSA is much more facile and rapid as compared to a comparable oxidation by the same reagents of the parent compound, DMTU. The stoichiometry of the bromate-DMAIMSA reaction was determined to be 2BrO 3 (-) + 3NHCH 3(NCH 3)CSO 2H + 3H 2O --> 3SO 4 (2) (-) + 2Br (-) + 3CO(NHCH 3) 2 + 6H (+), with quantitative formation of sulfate. In excess bromate conditions, the stoichiometry was 4BrO 3 (-) + 5NHCH 3(NCH 3)CSO 2H + 3H 2O --> 5SO 4 (2) (-) + 2Br 2 + 5CO(NHCH 3) 2 + 6H (+). The direct bromine-DMAIMSA reaction gave an expected stoichiometric ratio of 2:1 with no further oxidation of product dimethylurea (DMU) by aqueous bromine. The bromine-DMAIMSA reaction was so fast that it was close to diffusion-controlled. Excess bromate conditions delivered a clock reaction behavior with the formation of bromine after an initial quiescent period. DMAIMSOA, on the other hand, was extremely inert to further oxidation in the acidic conditions used for this study. Rate of consumption of DMAIMSA showed a sigmoidal autocatalytic decay. The postulated mechanism involves an initial autocatalytic build-up of bromide that fuels the formation of the reactive oxidizing species HBrO 2 and HOBr through standard oxybromine reactions. The long and weak C-S bond in DMAIMSA ensures that its oxidation goes directly to DMU and sulfate, bypassing inert DMAIMSOA.     

S-oxygenation of thiocarbamides II: Oxidation of trimethylthiourea by chlorite and chlorine dioxide

The kinetics of the oxidation of a substituted thiourea, trimethylthiourea (TMTU), by chlorite have been studied in slightly acidic media. The reaction is much faster than the comparable oxidation of the unsubstituted thiourea by chlorite. The stoichiometry of the reaction was experimentally deduced to be 2ClO2- + Me2N(NHMe)C=S + H2O --> 2Cl- + Me2N(NHMe)C=O + SO4(2-) + 2H+. In excess chlorite conditions, chlorine dioxide is formed after a short induction period. The oxidation of TMTU occurs in two phases. It starts initially with S-oxygenation of the sulfur center to yield the sulfinic acid, which then reacts in the second phase predominantly through an initial hydrolysis to produce trimethylurea and the sulfoxylate anion. The sulfoxylate anion is a highly reducing species which is rapidly oxidized to sulfate. The sulfinic and sulfonic acids of TMTU exists in the form of zwitterionic species that are stable in acidic environments and rapidly decompose in basic environments. The rate of oxidation of the sulfonic acid is determined by its rate of hydrolysis, which is inhibited by acid. The direct reaction of chlorine dioxide and TMTU is autocatalytic and also inhibited by acid. It commences with the initial formation of an adduct of the radical chlorine dioxide species with the electron-rich sulfur center of the thiocarbamide followed by reaction of the adduct with another chlorine dioxide molecule and subsequent hydrolysis to yield chlorite and a sulfenic acid. The bimolecular rate constant for the reaction of chlorine dioxide and TMTU was experimentally determined as 16 +/- 3.0 M(-1) s(-1) at pH 1.00.     

Autocatalytic oxidation of hemin by acidic bromate

The kinetic behavior of the autocatalytic oxidation of hemin by acidic bromate was studied spectrophotometrically. The reaction was shown to differ significantly from both the acidic decomposition and the direct bromination of hemin. The dependence of its maximal rate on the reactant concentrations was established as v = k[hemin]^0.8[BrO^?₃][H⁺]^1.2. The value of the net rate constant was determined at two different ionic strengths. In the reaction path BrO^.₂ radicals rather than elementary bromine play a crucial role. A mechanism is presented which takes into account the results of bromate chemistry as described for the Belousov–Zhabotinsky (BZ) reaction completed with the appropriate steps of hemin. On the basis of the suggested mechanism, model calculations were carried out which allowed to give an estimation for the rate constant of the reaction between hemin and BrO^.₂ radicals. The results of computations account for the main experimental features of the reaction. © 2006 Wiley Periodicals, Inc. Int J Chem Kinet 38: 503–509, 2006     

Uncatalyzed chemical oscillations in the aqueous acidic bromate oxidation of veratraldehyde

Oscillatory behavior in the uncatalyzed aqueous acidic bromate oxidation of 3,4-dimethoxybenzaldehyde (veratraldehyde) is reported. The generally used H₂SO₄ can be substituted by HNO₃, HClO₄, CCl₃ COOH and H₃PO₄ in this system. The length of the induction period is found to be dependent on the initial concentrations of the reagents and this effect is more prominent in the case of acidity. The precipitate accumulated in the reaction during oscillations has been identified. A plausible mechanism is also suggested. Experiments with allyl alcohol, a bromine scavenger, suggest that elemental bromine formed in the reaction has a role in the mechanism together with or besides bromide inons.

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3,4- () . H₂SO₄ HNO₃, HClO₄, CCl₃ COOH H₃PO₄ . . , , . . , , , , , .

     

Kinetics of oxidation of ethanol by acid bromate

The kinetics of oxidation of ethanol by bromate ion in hydrochloric acid medium has been investigated. The reaction involves the formation of the intermediate bromate ester which is facilitated by the methyl group in ethanol but the electron attracting character (F>Cl>Br) of the halogens attached to the -carbon of the alcohol makes the esterification more difficult.

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. , , , (F>Cl>Br), - , .

     

Kinetics and mechanism of oxidation of ferrocyanide by N-bromosuccinimide in aqueous acidic solution

The kinetics of oxidation of ferrocyanide by N-bromosuccinimide (NBS) has been studied spectrophotometrically in aqueous acidic medium over temperature range 20–35 °C, pH = 2.8–4.3, and ionic strength = 0.10–0.50 mol dm⁻³ over a range of [Fe²⁺] and [NBS]. The reaction exhibited first order dependence on both reactants and increased with increasing pH, [NBS], and [Fe²⁺]. The rate of oxidation obeys the rate law: d[Fe³⁺]/dt = [Fe(CN)₆]^4–[HNBS⁺]/(k ₂ + k ₃/[H⁺]). An outer-sphere mechanism has been proposed for the oxidation pathway of both protonated and deprotonated ferrocyanide species. Addition of both succinimide and mercuric acetate to the reaction mixture has no effect on the reaction rate under the experimental conditions. Mercuric acetate was added to the reaction mixture to act as scavenger for any bromide formed to ensure that the oxidation is entirely due to NBS oxidation.     

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     

Kinetics of osmium(VIII) catalyzed oxidation of allyl alcohol by potassium bromate in aqueous acidic medium‐autocatalysis in catalysis

The kinetics of oxidation of allyl alcohol with potassium bromate in the presence of osmium(VIII) catalyst in aqueous acid medium has been studied under varying conditions. The active species of oxidant and catalyst in the reaction were understood to be Bro₃⁻ and H₂OsO₅, respectively. The autocatalysis exhibited by one of the products, that is, Br⁻, was attributed to complex formation between bromide and osmium(VIII). A composite scheme and rate law were possible. Some reaction constants involved in the mechanism have been evaluated. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 583–589, 1999     

Oscillatory reduction of bromate by aspirin in acidic aqueous solution

Summary The experimental behavior of the uncatalyzed Belousov - Zhabotinsky reaction between aspirin and bromate in acidic media in the batch reactor has been studied for the first time. Aspirin is an interesting substrate because it is one of the most used medicines. The medical aspirin behaves also in an oscillatory manner with bromate. The oscillating process was investigated under aerobic and anaerobic conditions. The complex dynamic behavior has been observed in the mixed aspirin - vitamin C - _BrO3 - H₂SO₄ system.     

Kinetics and mechanism of oxidation of methanol by acid bromate

The kinetics of oxidation of methanol by bromate ion in hydrochloric acid medium has been investigated. A mechanism consistent with the experimental observations is suggested.

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. , .

     

Kinetics of oxidation of sulphite by hexachloroplatinate(IV) in acidic solution

Summary The kinetics of the oxidation of sulphite by hexachloroplatinate(IV) has been studied over wide range of experimental conditions. The reaction is first-order in substrate and in platinum(IV). The rate decreases with the increase in acidity. The effect of salt and of changing dielectric constants on the reaction rate have been studied. Values of H and S have been calculated and are 26.3 kJ mol^–1 and –35.9 JK^–1 mol^–1, respectively. On the basis of experimental evidence, a two-electron reduction mechanism is proposed.     

Kinetics and mechanism of oxidation of secondary alcohols by potassium bromate

The oxidation by Br(V) of propan-2-ol follows the rate law (?d[Br(V)]/dt) = k₄ [alcohol][Br(V)][H⁺]². The initial reaction is complicated by the presence of the product bromide ion. The reaction is composed of two second order reactions—the first, a comparatively slow one and the second stage, a faster reaction which is mainly bromine oxidation. The pure bromate oxidation can be followed by the initial addition of mercuric acetate which prevents the accumulation of bromine in the system under these conditions. The reaction rate does not depend on the nature and structure of the alcohol. A mechanism involving a slow rate-determining formation of an alkyl-bromate ester followed by a fast decomposition to the products is in accord with the observed results.     

Kinetics of oxidation of ethyldigol by vanadium(V) in aqueous acidic medium

The kinetics of oxidation of ethyldigol by vanadium(V) in aqueous acidic medium has been carried out. The reaction is first order with respect to vanadium(V) and the substrate and is acid catalysed.Hammett acidity function (H ₀) andBunnett hypothesis have been applied. The formation of free radicals during the course of the reaction has been indicated. A probable reaction mechanism is proposed.

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Die Kinetik der Oxidation von Ethyldigol mit Vanadium(V) in wäßrigem saurem Medium

Zusammenfassung Es wurde die Kinetik der Oxidation von Ethyldigol mittels Vanadium(V) in wäßriger saurer Lösung untersucht. Die Reaktion ist erster Ordnung bezüglich Vanadium(V) und Substrat und ist säurekatalysiert. Es wurden dieHammett-Aciditätsfunktion (H ₀) und dieBunnett-Hypothese angewandt. Die Bildung von freien Radikalen während der Reaktion konnte bestätigt werden. Es wird ein Reaktionsmechanismus vorgeschlagen.

     

Kinetics of oxidation of manganese(II) by peroxomonosulfuric acid in aqueous acidic solution

Summary Oxidation of Mn _aq ²⁺ by HSO ₅ ^– in acetate buffer to manganese(IV) is autocatalytic, and obeys a rate expression of the general form -d[Mn^II]/dt = k₀[Mn^II] + k₁[Mn^II][MnO_x]. The first-order (k₀) and heterogenetic (k₁) rate constants show first-order dependences on [HSO ₅ ^– ] and on 1/[H⁺]. The reaction is catalyzed by the addition of the chelating ligand glycine; k₁ shows a first-order dependence on [glycine] at a fixed pH. This catalysis is ascribed to complexation, whereby the redox potential for Mn(gly) _n ^(2–n)+ is lower than that for Mn _aq ²⁺ , facilitating oxidation. The stoichiometry of the reaction is Mn²⁺: HSO ₅ ^– = 11, and the manganese(IV) oxide formed is of battery-active grade. Purity of the recovered product is not affected by the presence of high concentrations of natural sugars in the initial solution.     

Kinetics and mechanism of oxidation of some aryl alcohols by acid bromate

The kinetics of oxidation of some substituted benzyl alcohols as well as the unsubstituted one by bromate ion in hydrochloric acid medium has been suggested. The results indicate that the reaction takes place by way of intermediate ester formation. Methoxy compounds react at much faster rates than the corresponding nitro substituted derivatives. The thermodynamic values associated with the equilibrium step and also for the slow step have been evaluated. A mechanism consistent with the experimental observations has been suggested.     

Kinetics of the oxidation of octacyanomolybdate(IV) by periodate in aqueous acid

Summary The kinetics of oxidation of [Mo(CN)₈]^4– by IO ₄ ^– in aqueous acid is described by the equation: d[{Mo(CN)₈}^3–]/ dt=2k₃[{Mo(CN)₈}^4–][IO ₄ ^– ][H⁺]. Unlike IO ₄ ^– oxidations of [Fe(CN)₆]^4– and [W(CN)₈]^4–, no [H⁺] independent term exists in the [Mo(CN)₈]^4– reaction, which indicates that, in neutral and alkaline solutions, oxidation of [Mo(CN)₈]^4– is thermodynamically unfavourable. An inner-sphere mechanism, consistent with the rate law, is proposed. This conclusion is based, in the absence of direct evidence, on the observed behaviour of IO ₄ ^– as an inner-sphere oxidant.     

Kinetics and mechanism of reaction of toluidine blue with acidic bromate

The detailed kinetics of the reaction of toluidine blue {phenothiazine-5-ium, 3-amino-7(dimethylamino)-2-methyl chloride, tolonium chloride, TB⁺Cl⁻} with potassium bromate and with aqueous bromine reaction were studied. In most of the experiments, the kinetics were monitored by following the rate of consumption of TB⁺ at 590 nm with excess acid and bromate. The reaction exhibited complex kinetic behavior. Initial reaction was slow and after an induction time, the TB⁺ concentration decreased fast. It had first-order dependence on both TB⁺ and bromate, and second-order dependence on H⁺. Under excess bromate conditions, the stoichiometric ratio of TB⁺ to bromate was 1:1. Demethylated sulfoxides were found at the reaction products. Sharp increase in the overall potential synchronized with the increase in bromine levels and the fast depletion of [TB⁺]. The role of bromide ion and bromine in the reaction was established. A multi-step reaction mechanism is proposed consistent with the experimental results. © 1998 John Wiley & Sons, Inc. Int J Chem Kinet: 30: 111–120, 1998.