S-oxygenation of thiocarbamides IV: Kinetics of oxidation of tetramethylthiourea by aqueous bromine and acidic bromate

The kinetics and mechanism of oxidation of tetramethylthiourea (TTTU) by bromine and acidic bromate has been studied in aqueous media. The kinetics of reaction of bromate with TTTU was characterized by an induction period followed by formation of bromine. The reaction stoichiometry was determined to be 4BrO(3)(-) + 3(R)(2)C═S + 3H(2)O → 4Br(-) + 3(R)(2)C═O + 3SO(4)(2-) + 6H(+). For the reaction of TTTU with bromine, a 4:1 stoichiometric ratio of bromine to TTTU was obtained with 4Br(2) + (R)(2)C═S + 5H(2)O → 8Br(-) + SO(4)(2-) + (R)(2)C═O + 10H(+). The oxidation pathway went through the formation of tetramethythiourea sulfenic acid as evidenced by the electrospray ionization mass spectrum of the dynamic reaction solution. This S-oxide was then oxidized to produce tetramethylurea and sulfate as final products of reaction. There was no evidence for the formation of the sulfinic and sulfonic acids in the oxidation pathway. This implicates the sulfoxylate anion as a precursor to formation of sulfate. In aerobic conditions, this anion can unleash a series of genotoxic reactive oxygen species which can explain TTTU's observed toxicity. A bimolecular rate constant of 5.33 ± 0.32 M(-1) s(-1) for the direct reaction of TTTU with bromine was obtained.     

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.     

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.     

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 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.     

The oxidation of 3,3′-dimethoxy benzidine with potassium bromate in acidic solutions

The kinetics and mechanism of the oxidation of 3,3′-dimethoxybenzidine (oda, o-dianisidine) by potassium bromate in aqueous acidic medium were studied by monitoring the formation rate of the reaction product, 3,3′-dimethoxy 4,4′-diphenoquinone at 447 nm. The reaction is, first order with respect to both the substrate and oxidant, and second order with respect to H⁺. The oda: bromate stoichiometric ratio is 1:1. Plausible mechanism and rate laws are proposed accounting the experimental findings. Computer simulations were done using the proposed mechanism.     

Kinetics and mechanism of the oxidation of 4‐methyl‐3‐thiosemicarbazide by acidic bromate

The oxidation of 4‐methyl‐3‐thiosemicarbazide (MTSC) by bromate and bromine was studied in acidic medium. The stoichiometry of the reaction is extremely complex, and is dependent on the ratio of the initial concentrations of the oxidant to reductant. In excess MTSC and after prolonged standing, the stoichiometry was determined to be H₃CN(H)CSN(H)NH₂ + 3BrO₃^? → 2CO₂ + NH₄⁺ + SO₄^2? + N₂ + 3Br^? + H⁺ (A). An interim stoichiometry is also obtained in which one of the CO₂ molecules is replaced by HCOOH with an overall stoichiometry of 3H₃CN(H)CSN(H)NH₂ + 8BrO₃^? → CO₂ + NH₄⁺ + SO₄^2? + HCOOH + N₂ + 3Br^? + 3H⁺ (B). Stoichiometry A and B are not very different, and so mixtures of the two were obtained. Compared to other oxidations of thiourea‐based compounds, this reaction is moderately fast and is first order in both bromate and substrate. It is autocatalytic in HOBr. The reaction is characterized by an autocatalytic sigmoidal decay in the consumption of MTSC, while in excess bromate conditions the reaction shows an induction period before autocatalytic formation of bromine. In both cases, oxybromine chemistry, which involves the initial formation of the reactive species HOBr and Br₂, is dominant. The reactions of MTSC with both HOBr and Br₂ are fast, and so the overall rate of oxidation is dependent upon the rates of formation of these reactive species from bromate. Our proposed mechanism involves the initial cleavage of the C? N bond on the azo‐side of the molecule to release nitrogen and an activated sulfur species that quickly and rapidly rearranges to give a series of thiourea acids. These thiourea acids are then oxidized to the sulfonic acid before cleavage of the C? S bond to give SO₄^2?, CO₂, and NH₄⁺. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 237–247, 2002     

Uncatalysed oscillatory chemical reactions. Effect of different “contraits” during the oxidation reaction of 1,4-cyclohexanedione by acidic bromate

The effect of temperature, oxygen, mechanical stirring and one-electron redox couples (Belousov—Zhabotinskii catalyst) on the 1,4-cyclohexanedione/BrO^?₃/H⁺ system have been investigated.     

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     

Kinetic salt effects in the bromide oxidation by bromate

The kinetic salt effect on the oxidation of bromide ion by bromate have been studied. It is concluded that the salt effects are the result of ion-solvent interactions and seem to be connected not only to the strength of these interactions, as measured by the lowering of solvent activity, but also by the structural characteristics of ion-solvent interactions. This conclusion is based on the use of Pitzer's treatment to estimate the activity coefficients.     

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.     

Complex dynamical behavior in the oxidation of hydroxylamine by bromate

Complex dynamical behavior has been observed in the oxidation of hydroxylamine by bromate in acidic sulfate medium. The reaction shows clock type kinetics in closed conditions and an aperiodic oscillations if gaseous products are removed from the system with a constant flow-rate. The reduction kinetics of bromate ions with excess hydroxylamine has been studied in the presence of allyl alcohol. The observed pseudo-first-order rate constant k_obs has been found to follow the expression where [hydroxylamine] is total initial hydroxylamine concentration, K₁ = 0.5 M^?1, K₂ = 10⁶ M^?1, and k = 2.57 × 10³ M^?1 s^?1 at 298.15 K and I = 2.0 M. The rate constant for the bromine oxidation of hydroxylamine in sulfuric aqueous solution has been determined. © 1994 John Wiley & Sons, Inc.     

Oscillatory behavior in the reaction of 3-alizarin-sulfonic acid sodium salt with acidic bromate

Uncatalyzed and catalyzed oscillatory behavior in the redox potential in the oxidation of 3-alizarin-sulfonic acid sodium salt with acidid (H₂SO₄) bromate is reported. Optimum and boundary conditions for each reactant exhibiting oscillatory behavior have been studied and a probable mechanism is suggested.

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- 3-- (H₂SO₄) . , .

     

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), - , .

     

Study of the bromide oxidation by bromate in zwitterionic micellar solutions

The redox reaction Br⁻ + BrO₃⁻ has been studied in aqueous zwitterionic micellar solutions of N‐tetradecyl‐N, N‐dimethyl‐3‐ammonio‐1‐propanesulfonate, SB3‐14, and N‐hexadecyl‐N,N‐dimethyl‐3‐ammonio‐1‐propanesulfonate, SB3‐16. A simple expression for the observed rate constant, k_obs, based on the pseudophase model, could explain the influences of changes in the surfactant concentration on k_obs. The kinetic effect of added NaClO₄ on the reaction rate in SB3‐14 micellar solutions has also been studied. They were rationalized by considering the binding of the perchlorate anions to the sulfobetaine micelles and their competition with the reactive bromide ions for the micellar surface. © 2000 John Wiley & Sons, Inc. Int J Chem Kinet 32: 388–394, 2000     

Study on the kinetics of the reaction of thymol blue with acidic bromate catalysed by Ru(III) or V(V)

The kinetics of the reaction between thymol blue and acidic bromate catalysed by Ru(III) or V(V) have been studied by monitoring the absorbance at 544 nm. The reaction showed a complex kinetic behaviour. The possible application to the catalytic determination of Ru(III) and V(V) is discussed.     

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