Oxyhalogenâ€“sulfur chemistry: Kinetics and mechanism of the oxidation of a Bunte salt 2-aminoethanethiolsulfuric acid by chloritePart 4 (of 10) in the series of publications dedicated to the memory of Dr Cordelia R. Chinake, 1965â€“1998. Part 3: Int. J. Chem. Kinet., submitted.

Chinake, Cordelia R.; Mundoma, Claudius; Olojo, Rotimi; Chigwada, Tabitha R.; Simoyi, Reuben H.

doi:10.1039/b106167b

Cited by 8 publications

(15 citation statements)

References 14 publications

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“…This inhibitory effect of acid on the final concentration of ClO 2 is inconsistent with acid effects on typical oxyhalogen reactions. Nearly all oxyhalogen reactions are catalyzed by acid, suggesting that the observed inhibition in chlorine dioxide formation might be emanating from the chemistry of the substrate. However, changes in concentrations of the substrate produce greater effects on the dynamics of the reactions.…”

Section: Resultsmentioning

confidence: 99%

Oxyhalogen−Sulfur Chemistry: Oxidation of N-Acetylcysteine by Chlorite and Acidic Bromate

et al. 2003

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The kinetics and mechanism of the oxidation of an important organosulfur antioxidant, N-acetylcysteine, by chlorite and acidified bromate have been studied. In both cases, the final product is N-acetylcysteinesulfonic acid without cleavage of the C-S bond to form sulfate. There was also no evidence for the formation of N-chloramine nor N-bromamine as has been observed with other aminothiols such as taurine. N-Acetylcysteine was oxidized via a stepwise S-oxygenation process in which consecutively a sulfenic and a sulfinic acid were formed before formation of the cysteic acid product. The stoichiometry of the chlorite-N-acetylcysteine was experimentally deduced to be 3ClO 2 -+ 2(CH 3 CO)HNCH(CO 2 H)CH 2 SH f 3Cl -+ 2(CH 3 CO)HNCH-(CO 2 H)CH 2 SO 3 H. The reaction is characterized by an immediate and rapid production of chlorine dioxide without a measurable induction period. This is because the oxidation of N-acetylcysteine by chlorine dioxide is slow enough to allow for the chlorine dioxide to instantly accumulate without the induction period that characterizes most chlorite oxidations of organosulfur compounds. The global reaction dynamics for this reaction can be described fully by a truncated mechanism that utilizes only 8 reactions. The stoichiometry of the bromate-N-acetylcysteine reaction at stoichiometric ratios was deduced to be BrO 3 -+ (CH 3 CO)HNCH-(CO 2 H)CH 2 SH f Br -+ (CH 3 CO)HNCH(CO 2 H)CH 2 SO 3 H, while in excess bromate it was deduced to be 6BrOThis reaction proceeded with a prolonged induction period which gave way to a sudden formation of bromine. The rate of reaction between aqueous bromine and N-acetylcysteine is diffusion-limited which indicated that the end of the induction period coincided with a complete oxidation of N-acetylcysteine. The reaction was successfully modeled by the use of a reaction network made up of 12 elementary reactions. Despite their different physiological effects, both cysteine and N-acetylcysteine are oxidized by oxyhalogens via the same S-oxygenation pathway and gave the same oxidation metabolites and final product.

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Section: Resultsmentioning

confidence: 99%

Oxyhalogen−Sulfur Chemistry: Oxidation of N-Acetylcysteine by Chlorite and Acidic Bromate

et al. 2003

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“…The mechanism of the oxidation of chlorite by HOCl is also well known. Taube and Dodgen, using isotopic labeling, have concluded that this oxidation proceeds through the asymmetric intermediate Cl 2 O 2 : ,, Further reaction of the intermediate will yield ClO 2 : Both R17 and R18 are not elementary single steps although the kinetics of these reactions can be derived from their molecularities.…”

Section: Mechanismsmentioning

confidence: 99%

“…The mechanism of the oxidation of chlorite by HOCl is also well known. Taube and Dodgen, 37 using isotopic labeling, have concluded that this oxidation proceeds through the asymmetric intermediate Cl 2 O 2 : 26,35,39 Further reaction of the intermediate will yield ClO 2 :…”

Section: Mechanismsmentioning

confidence: 99%

S-Oxygenation of Thiocarbamides I: Oxidation of Phenylthiourea by Chlorite in Acidic Media

2005

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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.

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“…If we assume a nonradical pathway for the reduction of ClO 2 -, then a two-electron reduction of ClO 2should produce HOCl as the reactive species which is involved in further oxidations of the substrates. 29 Our indicator reaction is the formation of chlorine dioxide, R6. Reaction R6 is heavily dependent on reactions R5 and R7, and thus we can make correct predictions on the rate of reaction R5 by observing R6.…”

Section: Mechanismmentioning

confidence: 99%

Antioxidant Chemistry: Oxidation of l-Cysteine and Its Metabolites by Chlorite and Chlorine Dioxide

et al. 2004

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The oxidation of L-cysteine and its metabolites cystine and L-cysteinesulfinic acid by chlorite and chlorine dioxide has been studied in unbuffered neutral and slightly acidic media. The stoichiometry of the oxidation of L-cysteine was deduced to be 3ClO 2 -+ 2H 2 NCH(COOH)CH 2 SH f 3Cl -+ 2H 2 NCH(COOH)CH 2 SO 3 H with the final product as cysteic acid. The stoichiometry of the chlorite-cysteinesulfinic acid gave a ratio of 1:2, ClO 2 -+ 2H 2 NCH(COOH)CH 2 SO 2 H f Cl -+ 2H 2 NCH(COOH)CH 2 SO 3 H. There was no further oxidation past cysteic acid, and there was no evidence of sulfate formation which would have indicated the cleavage of the carbon-sulfur bond. The reaction is oligooscillatory in chlorine dioxide formation. In conditions of excess oxidant, the reaction is characterized by a short induction period followed by a rapid and autocatalytic formation of chlorine dioxide. Chlorine dioxide is formed by the reaction of intermediate HOCl with the excess chlorite: 2ClO 2 -+ 2HOCl + H + f 2ClO 2 (aq) + Cl -+ H 2 O. Oligooscillations observed in chlorine dioxide formation result from the competition between this pure oxyhalogen reaction and reactions that consume chlorine dioxide. The rate of the reaction of chlorine dioxide with cysteine and its metabolites is fast and is of comparable magnitude with the reactions that form chlorine dioxide. The reaction of chlorine dioxide with L-cysteine is first order in both oxidant and substrate, retarded by acid, and has a lower-limit bimolecular rate constant of 405 ( 50 M -1 s -1 , while for the reaction with L-cysteinesulfinic acid the rate constant is 210 ( 15 M -1 s -1 . It would appear that the existence of a zwitterion on the asymmetric carbon atom precludes the formation of N-chloramines as has been observed with taurine and aminomethanesulfonic acid. The mechanism for the reaction is satisfactorily described by a network of 28 elementary reactions which include autocatalysis by HOCl.

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Oxyhalogenâ€“sulfur chemistry: Kinetics and mechanism of the oxidation of a Bunte salt 2-aminoethanethiolsulfuric acid by chloritePart 4 (of 10) in the series of publications dedicated to the memory of Dr Cordelia R. Chinake, 1965â€“1998. Part 3: Int. J. Chem. Kinet., submitted.

Cited by 8 publications

References 14 publications

Oxyhalogen−Sulfur Chemistry: Oxidation of N-Acetylcysteine by Chlorite and Acidic Bromate

Oxyhalogen−Sulfur Chemistry: Oxidation of N-Acetylcysteine by Chlorite and Acidic Bromate

S-Oxygenation of Thiocarbamides I: Oxidation of Phenylthiourea by Chlorite in Acidic Media

Antioxidant Chemistry: Oxidation of l-Cysteine and Its Metabolites by Chlorite and Chlorine Dioxide

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