Oxidation of Toluidine Blue by Chlorite in Acid and Mechanisms of the

Nov 3, 2010 - ... (λ max = 626 nm) by acidic chlorite is elucidated by a kinetic approach. ... The overall reaction of toluidine blue and chlorite io...
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Oxidation of Toluidine Blue by Chlorite in Acid and Mechanisms of the Uncatalyzed and Ru(III)-Catalyzed Reactions: A Kinetic Approach Sreekanth B. Jonnalagadda* and Brijesh K. Pare School of Chemistry, UniVersity of KwaZulu-Natal, WestVille Campus, Chiltern Hills, P Bag X54001, Durban 4000, South Africa ReceiVed: July 7, 2010; ReVised Manuscript ReceiVed: October 8, 2010

The complex mechanism of the uncatalyzed and Ru(III)-catalyzed oxidation of toluidine blue [(7-amino-8methylphenothiazin-3-ylidene)dimethyl ammonium chloride, TB+Cl-] (λ max ) 626 nm) by acidic chlorite is elucidated by a kinetic approach. Both the uncatalyzed and catalyzed reactions had a first-order dependence on the initial ClO2- and H+ concentrations ([ClO2-]0 and [H+]0, respectively). The catalyzed reaction had a first-order dependence on the initial Ru(III) concentration ([Ru(III)]0). The overall reaction of toluidine blue and chlorite ion was as follows: TB+ + 5ClO2- + H+ ) P + 2ClO2 + 2HCOOH + 3Cl- + H2O, where P is (7-amino-8-methyl-5-sulfoxophenothiazin-3-ylidene)amine. Consistent with the experimental results, the pertinent reaction mechanisms are proposed. Introduction Reactions with chlorite ion have been of extensive interest in the past two decades, because of the bistability of this ion, leading to exotic temporal behavior, nonlinear kinetics, and spatial patterns exhibited by reactions in both batch reactors and continuous stirred-tank reactors (CSTRs).1-4 Because of its disinfectant properties, chlorite is also widely used in water treatment.5,6 Phenothiazine compounds such as methylene blue (MB+) are used as antimalarial drugs from ancient times.7 Toluidine blue and related phenothiazine dyes are water-soluble and have useful insecticidal and antithelminite properties.8 Other synonyms of toluidine blue are toluidine blue O, CI Basic Blue 17, and tolonium chloride. Toluidine blue (TB+Cl-, referred to as TB+) is used for dyeing cotton, staining tissue, and counterstaining tubercle and lepra bacilli mammalian tissue, as well as staining of DNA, RNA, and plant material. TB+ exhibits a broad absorption peak in visible region (λmax ) 626 nm) and experiences no peak shift due to pH changes. It is used as an internal indicator in the nonaqueous titrimetric determination of food preservatives and non-nutritive sweeteners. Although toluidine blue is nontoxic, many such heterocyclic dyes are toxic, water-soluble, and stable. Because of their stabilities, the presence of toxic dyes with long residence times is hazardous to aquatic systems.9,10 Thus, information about the degradation dynamics of water-soluble dyes has much bearing toward sustaining the environment. A literature survey shows the kinetics of reduction of heterocyclic dyes with possible reversal to oxidized compounds.11,12 Limited information is available in the literature on the oxidation reactions of phenothiazine dyes in general and toluidine blue in particular.8,13 Some results have been reported on the photocatalyzed mineralization of dyes, in particular, methylene blue.14-17 During oxidative degradation of MB+ using H2O2 on NiO2 catalyst, Oliveira et al. observed the hydroxylation of the outer rings of MB, prior to cleavage of the central hetero ring, finally leading to mineralization.18 In the recent past, reports were published on the efficiency of lignin * Corresponding author. E-mail: [email protected].

peroxidise in the decolorization of methylene blue19 and on the microbial degradation of toluidine blue by BreVibacillus sp. to CO2 and H2O, but no intermediates of the process were identified.20 Unlike other reported chlorite reactions, which exhibit nonlinear behavior,3,4 the TB+-acidic chlorite reaction exhibits relatively simple kinetics. This departure from the nonlinear behavior and the high observed sensitivity and selectivity to the presence of Ru(III) has generated interest in the detailed kinetic study of TB+-acidic chlorite reaction. In the present article, the detailed kinetics of both the uncatalyzed and Ru(III)catalyzed reactions of chlorite with TB+ and probable mechanisms are reported. Experimental Section Reagents. Sodium chlorite (BDH) was recrystallized before use.21 Toluidine blue (Aldrich) was used as received. All other reagents used were of Analar grade or high purity. All solutions were made in deionized distilled water. Ru(III) stock solution (0.02 M) was prepared by dissolving 0.207 g of ruthenium(III) chloride trihydrate (Aldrich) in 50 mL of 0.10 M sulfuric acid.22 Kinetic Measurements. In all experiments, pseudo-first-order kinetics with respect to toluidine blue was monitored at 626 nm (ε ) 1.9 × 104 M-1 cm-1) using a Cary II -Varian UV-vis double-beam spectrophotometer (Varian, Mulgrave, Victoria, Australia) interfaced for data storage and processing facilities. Kinetic data were analyzed with Microsoft Excel software. Beer’s law is valid for the measurements under the experimental conditions considered. No interference from the reagents, intermediates, or products was observed at 626 nm. The total initial volume of the reaction mixture was always kept at 10 mL, and the temerature was held at 25.0 ( 0.1 °C. Requisite volumes of reagent solutions were mixed in the following order: toluidine blue, sulfuric acid, water plus catalyst or other reagents, where necessary. A separately thermostatted solution of sodium chlorite was added to commence the reaction. After vigorous mixing, the reaction mixture was transferred to the thermostatted spectrophotometer cell. In all experiments, the reactions were followed for two half-lives.

10.1021/jp1062644  2010 American Chemical Society Published on Web 11/03/2010

Oxidation of Toluidine Blue by Chlorite in Acid The depletion kinetics of toluidine blue with HOCl was studied at 25 ( 0.1 °C and constant ionic strength (0.1 mol dm-3), using a Hi-Tech SF-61 double-mixing stopped-flow apparatus (Salisbury, England) equipped with photomultiplier/ diode array spectrometer. Requisite volumes of TB+, salt, and HOCl and acid solutions were separately thermostatted and transferred to the syringes of the stopped-flow equipment. The change in the absorbance due to depletion of TB+ was monitored at 626 nm. Factor analysis and global fitting were performed to obtain rate constants, using the computer program SPECFIT from Spectrum Software Associates (TgK Scientific Limited, Brasford on Avon, U.K.). Best fits were obtained with equations for a simple first-order reaction, with a single-exponential function [y ) -A exp(-kx) + C], which gave the computed pseudo-first-order rate coefficients. FT-IR Spectroscopy. FT-IR spectra of the major organic product were recorded on a Nicolet Impact 400 spectrophotometer and a Nicolet Impact model-420 spectrometer with 4 cm-1 resolution and 128 scans in the mid-IR (400-4000 cm-1) region using the KBr pellet technique. Gas Chromatography-Mass Spectrometry (GC-MS). For analysis of organics, an Agilent 6890 gas chromatograph equipped with a quadruple Agilent 5973n mass-selective detector was used. The column specifications are as follows: J&W DB5MS, 30-m length, 250-µm diameter, and 0.25-µm film thickness. The GC-MS analysis was carried out in electron impact (EI) mode, and the spectra were recorded in the interval 35-500 amu. Results and Discussion Product Analysis and Stoichiometry. For the product analysis, toluidine blue (250 mg of TB+ in 250 mL of water), 5.0 M sulfuric acid (100 mL), and (500 mg/150 mL) chlorite were mixed to make 500 mL. After 4 h of reaction, the reaction mixture was filtered, and the filtrate was neutralized with sodium bicarbonate, producing a precipitate that was filtered and collected. The completion of the reaction was confirmed with thin-layer chromatography (TLC) plates against the starting material using 7:3 ratio of hexane to ethyl acetate as the eluent. The same solvent system was used to separate the products using a column packed with silica gel 60 (0.063-0.2, Merck). The analysis of the product mixture by TLC using formic acid as the standard reference confirmed the presence of formic acid as one of the products.23 The IR spectra of the main product showed a sharp band at 1005-1070 cm-1, suggesting the formation of sulfoxide. The strong peaks in the ranges of 3000-3300 and 2700-3000 cm-1 indicated the presence of intact primary and secondary amino groups on the aromatic heterocyclic ring.24 Any strong absorption bands indicative of sulfone in the 1300-1135 cm-1 region were absent, and no intense peak in the range of 1300-1200 cm-1 indicative of N-O stretching was observed, which excludes the formation sulfone and nitrogen oxides in the heterocyclic ring. Further, the mass spectrometry data of the main product showed m/z 268 (60%) as the main peak, and the other significant peaks observed included m/z 252 (loss of oxygen atom from sulfoxide), m/z 235 (loss of amino group), and m/z 157 (fragmented peak of heterocyclic ring). The pattern of the fragmentation peaks resembles the mass spectra of methlylene blue oxidation products reported in the literature.25,26 Based the MS and IR data, the main oxidation product was identified as 2-amino-3-methyl-7amino phenothiazine sulfoxide.

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A survey of the literature also shows that phenothiazines are oxidized to their sulfoxides in the presence of sodium ethoxide27 and to the sulfones under stronger oxidizing conditions such as the presence of fuming nitric acid and hydrogen peroxide.8 The oxidation of phenothiazines with permanganate and hydrogen peroxide also reportedly gives sulfoxides and occasionally sulfones.8 The loss of color of the phenothiazines upon oxidation is attributed to the nonplanar nature of the structure of product at sulfur, upon oxidation to sulfoxide.28 The existence of protonated CsS+(sOH)sC species as an intermediate is reported during the oxidation of methylene blue by aqueous bromine.29 In the current studies, with strongly acidic media, the species remains potentially as CsS+(sOH)sC. During the photodegradation of MB+, Houas et al. observed that an electrophilic attack on the free doublet of heteroatom S, making its oxidation state pass from 2- to 0, is a probable step of the oxidative process. The passage from CsS+dC to CsS(dO)sC requires the conservation of the double bond conjugation, which induces the opening of the central ring containing both the hetero atoms.25 In the current study, no cleavage in the heterocyclic ring was observed. Gnaser et al., in similar studies on methylene blue oxidation, reported that the ring-opening is necessitated only when the attack takes place on the MB+ cation, with the charge concentrated on the S atom. However, if neutral S is attacked or if the cationic S is coordinated to a catalyst surface, no such ring-opening is required.26 Thus, sulfoxide formation without central ring rupture is possible. Yogi et al. reported the demethylation of methylene blue during photocatalyzed oxidation on titania.30 The stoichiometry of the reaction was determined using a 1:10 molar concentration ratio of TB+ to chlorite with excess of H+. After about 60 min of incubation, the residual TB+ was determined by measuring the absorbance at 626 nm, and the residual amount of chlorite was estimated iodometrically using sodium thiosulfate as a titrant and starch as an indicator.31 In agreement with the major reaction product, the overall reaction between toluidine blue and chlorite ion is

TB+ + 5ClO2- + H+ ) P + 2ClO2 + 2HCOOH + 3Cl- + H2O where P is (7-amino-8-methyl-5-sulfoxophenothiazin-3-ylidene)amine. For a fixed initial concentration of TB+ ([TB+]0), the stoichiometry and amount of chlorine dioxide formed varied and was observed to be primarily dependent on the initial concentration of ClO2- ([ClO2-]0) and marginally on the initial concentration of H+ ([H+]0). Chlorite ion in acidic conditions is known to decompose under catalysis by metal ions such as Fe(II).21 Reaction Kinetics. The kinetics of all of the reaction runs was conducted with excess concentrations of acid and chlorite and a low concentration of toluidine blue. Further, the initial concentration of acid was always kept equal or higher than the concentration of ClO2-. (a) Uncatalyzed Reaction and Dependence of Reaction Rate on ClO2- and H+ Concentrations. Figure 1 shows typical kinetic curves for the depletion of toluidine blue at varied initial

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Figure 1. Kinetic curves showing the effect of the variation in intial ClO2- concentration ([ClO2-]0) on TB+ depletion. Conditions: [TB+] ) 5.0 × 10-5 M; [H+] ) 0.05 M; and [ClO2-]0 ) (a) 0.02, (b) 0.03, (c) 0.04, (d) 0.05, and (e) 0.06. Inset: Plot of ln(absorbance) versus time.

Figure 2. Repeated spectra (300-700 nm) showing the depletion of toluidine blue and formation of chlorine dioxide for the Ru(III)catalyzed reaction. Conditions: [TB+] ) 5.0 × 10-5 M, [H+] ) 5.0 × 10-2 M, [ClO2-]0 ) 2.0 × 10-2 M, and [Ru(III)] ) 1.0 × 10-7 M.

TABLE 1: Toludine Blue Oxidation with Acidic Chloride: Effect of Chlorite and Hydrogen Ion Variation on the Reaction Ratea

TABLE 2: Toludine Blue Oxidation with Acidic Chloride: Effects of Chlorite and Hydrogen Ion Variations on the Reaction Rate in the Presence of a Fixed Concentration of Ru(III)a

[H+] (M)

[ClO2-] (M)

k′0b (10-3 s-1)

k0c (M-2 s-1)

[H+] (M)

[ClO2-] (M)

k′′ b (10-3 s-1)

kc (s-1)

0.05 0.05 0.05 0.05 0.05 0.02 0.03 0.04 0.05 0.06 0.08 0.10

0.015 0.02 0.03 0.04 0.05 0.02 0.02 0.02 0.02 0.02 0.02 0.02

1.33 1.71 2.47 3.47 4.38 0.67 1.07 1.31 1.71 1.95 2.73 3.33

1.77 1.71 1.65 1.73 1.75 1.66 1.78 1.63 1.71 1.62 1.71 1.67

0.05 0.05 0.05 0.05 0.05 0.05 0.02 0.03 0.05 0.06 0.08 0.1

0.008 0.01 0.02 0.03 0.04 0.05 0.02 0.02 0.02 0.02 0.02 0.02

1.45 1.95 3.80 5.43 7.89 10.58 1.50 2.29 3.80 4.45 6.23 7.61

3.64 3.90 3.80 3.62 3.94 4.23 3.74 3.81 3.80 3.71 3.89 3.80

a Conditions: [TB+] ) 5.0 × 10-5 M, ionic strength ) 0.17 M, temperature ) 25 ( 0.1 °C. b Means of duplicate experiments with < 5% standard deviation. c Third-order rate constant, k0 ) k′0/ ([H+][ClO2-]). Mean third-order constant ) 1.70 ( 0.05 M-2 s-1.

concentrations of chlorite, while other parameters were kept fixed. Plots of the natural logarithm of the absorbance versus time data gave good straight lines with R2 g 0.99 (see inset in Figure 1), suggesting that the reaction follows pseudo-first-order kinetics and that the order with respect to the TB+ concentration ([TB+]) is unity. Table 1 summarizes the k′0 values obtained at constant ionic strength with varied concentrations of chlorite and H+ ions, while the other parameters wer held constant. Plots of ln k′0 versus ln([chlorite]) and ln([H+]) versus ln k′0 gave straight lines with slopes of 0.99 each and R2 g 0.99 in both cases. This suggests that the reaction has first-order dependence on both the chlorite and H+ concentrations. The mean third-order rate constant for the uncatalyzed reaction was 1.70 ( 0.05 M-2 s-1. (b) Ru(III)-Catalyzed Reaction and Orders with Respect to ClO2- and H+ Ions. Whereas the presence of many cations had a marginal influence on the reaction, preliminary experiments distinctly recorded significant increases in the reaction rates in the presence of catalytic amounts of Ru(III). Hence, the kinetics of the reaction was investigated in detail. Figure 2 shows repeated absorption spectra of the reaction with [H+]0 ) 5.0 × 10-2 M, [chlorite]0 ) 2.0 × 10-2 M, [TB+]0 ) 5.0 × 10-5 M,

a Conditions: [TB+] ) 5.0 ×10-5 M, [Ru(III)] ) 1.0 × 10-7 M, ionic strength ) 0.17 M, temperature ) 25 ( 0.1 °C. b Means of duplicate experiments with < 5% standard deviation. c Third-order rate constant, k ) k′′/([H+][ClO2-]). Mean third-order constant ) 3.82 ( 0.16 s-1.

and [Ru(III)]0 ) 6.0 × 10-8 M in the range of 300-700 nm. With the depletion of toluidine blue, the progressive increase in the absorption peak at 360 nm and the characteristic wavelike spectra of ClO2 confirm the accumulation of chlorine dioxide (ε ) 1.26 × 103 M-1 cm-1) as the product of reductive decomposition of chlorite.19 Chlorine dioxide accumulation continued even after the decolorization of the dye blue, suggesting the continued decomposition and disproportionation of residual chlorite under acidic conditions. With fixed initial concentrations of catalyst and other reactants, in separate sets of experiments, the initial concentrations of chlorite or acid were varied, and the TB+ depletion kinetics was monitored. Table 2 summarizes the kinetic data at different concentrations of acid and chlorite. A perusal of the data shows that the k′′ values increased with increasing initial concentration of chlorite or acid, while other parameters were held constant. The plots of the natural logarithm of the absorbance versus time were linear (R2 g 0.99). Furthermore, the ln k′′ versus ln([ClO2-]) and ln k′′ versus [H+] graphs were straight lines (R2 g 0.99), with gradients of 1.04 and 1.01, respectively. The third-order rate constant for a fixed concentra-

Oxidation of Toluidine Blue by Chlorite in Acid

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TABLE 3: Catalyzed Oxidation of Toluidine Blue with Acidic Chloride: Effect of Ru(III) Variationa [Ru(III)] (10-7 M)

k′0b (10-2 s-1)

0 0.40 0.60 0.80 1.00 2.00 3.00 4.00 5.00 6.00

0.17 0.20 0.27 0.31 0.38 0.57 0.80 0.99 1.17 1.30

Conditions: [TB+] ) 5.0 × 10-5 M, [H+] ) 0.05 M, [ClO2-] ) 0.02 M, ionic strength ) 0.17 M, temperature ) 25 ( 0.1 °C. b Means of duplicate experiments with < 5% standard deviation. a

tion of catalyst (1.0 × 10-7 M) was 3.8 ( 0.2 M-2 s-1. This confirms that the catalyzed reaction also has a first-order dependence on both chlorite and H+ ion concentrations. (c) Kinetic Salt Effect. With reactant concentrations of [TB+] ) 5.0 × 10-5 M, [H+] ) 5.0 × 10-2 M, and [ClO2-] ) 2.0 × 10-2 M, the initial ionic strength of the reaction mixture was 0.17 M. An increase in ionic strength from 0.17 to 0.32 resulted in the decrease of k′0 from 1.70 × 10-3 to 1.20 × 10-3 s-1 for the uncatalyzed reaction and a decrease of k′′ from 3.80 × 10-3 to 2.72 × 10-3 s-1 for the catalyzed reaction. The plots of the logarithm of the rate constant versus the square root of the ionic strength for the uncatalyzed and catalyzed reactions were linear curves with negative slopes of -2.14 and -2.19, respectively (R2 g 0.98), suggesting that oppositely charged species are involved in the rate-limiting step. (d) Order with Respect to Catalyst. The catalytic effect of Ru(III) ion on the reaction was investigated using varied initial concentrations of the metal ion (Table 3). The k′′ versus [Ru(III)] plot gave a straight line (R2 ) 0.99) with a catalytic constant, k′C, with a high value of 1.98 × 104 M-3 s-1. This confirms that Ru(III) exhibits good catalytic efficiency toward the reaction and that the reaction order with respect to the catalyst is unity. The y intercept agreed well with the mean rate constant for the uncatalyzed reaction. (e) Kinetics of Reaction of TB+ with HOCl. Considering that hypochlorous acid is the stable reactive intermediate generated during the decomposition of chlorite under acidic conditions, the kinetics of TB+ with HOCl was also investigated. From the preliminary studies, the reaction was found to be fast and hence was studied using the stopped-flow technique. Figure 3 illustrates the kinetic curves depicting the effect of variations in the HOCl concentration on the depletion of toluidine blue. The plots the natural logarithm of the absorbance versus time data were straight lines (see inset in Figure 3), suggesting that the reaction follows pseudo-first-order kinetics and that the order with respect to TB+ is unity. Further, the plot of k versus [HOCl] was linear, confirming that the reaction has first-order dependence on HOCl (Table 4). The second-order rate constant was estimated to be 45.8 ( 2.1 M-2 s-1. Mechanism. Uncatalyzed Reaction. The observed negative salt effect suggests that the rate-determining step is the reaction involving oppositely charge species, possibly TB+, H+, and ClO2- ions. The rapid accumulation of ClO2 explains its inertness toward the reaction (Figure 1b). Under acidic conditions, the reaction of the following oxychlorine reactions are important to explain the chemistry of title reaction.4,32-34

Figure 3. Typical curves showing the effect of the initial HOCl concentration ([HOCl]0) on the toluidine blue depletion kinetics. Conditions: [TB+] ) 5.0 × 10-5 M; [H+] ) 0.05 M; and [HOCl]0 ) (a) 0.16, (b) 0.22, (c) 0.28, (d) 0.34, and (e) 0.40 × 10-2 M. Inset: Plot of ln absorbance versus time.

TABLE 4: Effect of HOCl Variation on the Oxidation of Toluidine Bluea [HOCl] (M)

k′ b (s-1)

kc (s-1)

0.0016 0.0022 0.0028 0.0034 0.0040

0.078 0.094 0.128 0.156 0.181

48.8 42.7 45.7 45.9 45.25

Conditions: [TB+] ) 5.0 × 10-5 M, [H+] ) 0.05 M, temperature ) 25 ( 0.1 °C. b Means of five replicate experiments with < 4% standard deviation. c Mean second-order rate constant, k ) 45.8 ( 2.1 M-2 s-1. a

TB+ + ClO2- + H+ f TBO+ + HOCl

(R1)

H+ + ClO2- T HClO2

(R2)

HClO2 + Cl- + H+ T 2HOCl

(R3)

HOCl + Cl- + H+ T Cl2 + H2O

(R4)

Cl2 + 2ClO2- f 2ClO2 + 2Cl-

(R5)

Cl2O2 + 2ClO2- + 2H+ T 2ClO2 + 2HOCl

(R6) HOCl + ClO2- + H+ T Cl2O2 + H2O

(R7)

The other possible reactions to be considered are the reactions of various chloro- and oxychlorine species with TB+ and the other reaction intermediates to give the final products. Those reactions can be expressed as follows

TB+ + HOCl f TBO+ + H+ + Cl-

(R8)

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TB+ + Cl2 + H2O f TBO+ + 2H+ + 2Cl-

(R9) TB+ + Cl2O2 + H2O f TBO+ + 2HOCl TBO+ + HOCl f I + HCHO + 2H+ + Cl-

(R10)

(R11)

TBO+ + H+ + ClO2- f I + HCHO + HOCl + H+ (R12) TBO+ + Cl2 + H2O f I + HCHO + 3H+ + 2Cl(R13) TBO+ + Cl2O2 + H2O f I + HCHO + 2HOCl + H+ (R14) I + HOCl f P + HCHO + H+ + Cl-

(R15)

I + ClO2- + H+ f P + HCHO + HOCl

(R16)

I + Cl2 + H2O f P + HCHO + 2H+ + 2Cl(R17) HCHO + HOCl f HCOOH + H+ + Cl-

(R18)

Because of the presence of strong oxidizing species such as Cl2, Cl2O2, and HOCl, the further oxidation and mineralization of some formic acid formed to CO2, achieving a complete mineralization, cannot be ruled out.34b The oxidative demethylation step from TBO+ to the intermediate (I, see Scheme 1) possibly involves more elementary steps. Upon oxidative attack by acidic chlorite, the initial abstraction of H atom from the amino group and hydroxylation, followed by the shift of electrons results in demethylation and formation of H+ and formaldehyde, as reported in the literature for the cytochrome P450 2B1 catalyzed oxidative demethylation of alkyl amines.35 Ru(III)-Catalyzed Reaction. Because ruthenium has a 4d75s1 electron configuration, it has the widest scope of oxidation states [from 2- in Ru(CO)42- to 8+ in RuO4] of all elements of the periodic table and various coordination geometries in each SCHEME 1: Substrate, Product, and Possible Intermediates

electron configuration, which is in contrast to the narrow scope of oxidation states and simple square-planar structure of palladium.36a As reviewed by Nioata et al., for ruthenium as a catalyst, organic syntheses involve a variety of reactions including reduction, oxidation, isomerization, carbon-carbon bond formation, and miscellaneous nucleophilic and electrophilic reactions.36a Ruthenium(III) trichloride was reported to catalyze the oxidation of primary amines to nitriles by oxygen. A new preparation method for the synthesis of iodylarenes using the RuCl3-catalyzed oxidation of iodoarenes with peracetic acid has also been reported.36b In the absence of acid and/or oxidant, Ru(III) was found to have no reactivity with TB+, confirming that, during the catalytic cycle, Ru(III) does not get reduced to a lower oxidation state. Ruthenium ion and its complexes are known to oxidize a variety of compounds. Lee et al. also reported the formation of Ru(V) during the RuCl3-catalyzed reactions of alkanes by peracetic acid.37,38 Although many stable Ru(V)-oxo complexes have been reported in the literature, some have been isolated and characterized.39 Thus, based on the observed reaction orders with respect to various reagents, for the Ru(III)-catalyzed oxidation, formation of a complex between TB+ and Ru(III) is envisaged, which readily combine with acid and chlorite ions or HOCl in a rapid reaction forming an activated complex. The decomposition of the activated complex is proposed. During this process, Ru(III) possibly gets oxidized to Ru(V) through an inner-sphere electron-transfer mechanism and, in turn, abstracts electrons from the substrate, reducing itself again to Ru(III), completing the catalytic cycle.37,38 It is possible that ruthenium ion acts as an electron mediator only within the activated complex.40-42

Ru(III) + TB+ f [binary complex] [binary complex] + ClO2- + H+ f [activated complex]

(R19)

slow (R20)

[activated complex] f Ru(III) + TBO+ + HOCl (R21) Ru(V) abstracts electrons from TB+ with the formation of sulfoxide intermediate (I+). Thus, within the activated complex, ruthenium ion probably acts as an electron mediator only.40,41 The chemistry of the remaining reaction scheme remains the same as for the uncatalyzed reaction. Rate Laws. The rate equation for the uncatalyzed reaction between TB+ and acidic chlorite can be represented by the equation

rate ) -d[TB+]/dt ) k0[ClO2-][H+][TB+]

(1)

Under excess concentration conditions of chlorite and acid, the equation reduces to

r ) k'0[TB+]

(2)

k'0 ) k0[ClO2-][H+]

(3)

where

and k′0 is the pseudo-first-order rate constant for uncatalyzed reaction.

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In the presence of the catalyst, the oxidation reaction proceeds through both uncatalyzed and catalyzed pathways. + -d[TB+]/dt ) {k0[ClO2 ][H ] + + + kC[ClO2 ][H ][Ru(III)]}[TB ]

(4)

Therefore, the following represents the rate of depletion of TB+ in the presence of catalyst under excess chlorite and acid concentrations

-d[TB+]/dt ) {k'0 + k'C[Ru(III)]}[TB+]

(5)

k'C ) kC[ClO2-][H+] ) k″[TB+]

(6)

k″ ) k'0 + k'C[Ru(III)]

(7)

where

where

where k′′ represents the pseudo-first-order constant in the presence of catalyst. If eq 5 holds well, then a plot of the observed pseudo-first-order rate constant in the presence of catalyst, k′′ versus [Ru(III)], should give a straight line with intercept equal to k′0. (The plot is provided in the Supporting Information.) The intercept of the plot (Table 3) indeed has a value comparable to the experimental pseudo-first-order rate constant for the uncatalyzed reaction, which validates the assumption. For the chosen reaction conditions, the k′C value for Ru(III) was (1.95 ( 0.3) × 104 M-1 s-1 and the catalytic constant, kC ) k′C/([ClO-2][H+]), was equivalent to 1.95 × 107 M-3 s-1. Conclusions Many aromatic and heterocyclic organic substrates exhibit nonlinear dynamics in reactions with chlorite, but interestingly, toluidine blue exhibits exponential decay during its oxidation process. Further, this study confirms that, for the oxidation of toluidine blue with acidic chlorite, Ru(III) is a good and selective catalyst, and both uncatalyzed and catalyzed conditions result in effective oxidation of toluidine blue to a demethylated sulfoxide, although the heterocyclic structure was retained. This reaction process also leads to disproportion of chlorite and accumulation of chlorine dioxide, and ClO2 has a negligible reactivity under acidic conditions. Acknowledgment. The authors thank the University of KwaZulu-Natal (UKZN), Durban, South Africa, and the National Research Foundation, Pretoria, South Africa, for financial assistance and Mr. S. Naidoo and Prof. F. O. Shode, UKZN, for their help in product characterization. Dr. Pare, who was a postdoctoral fellow from the Madhav Science College, Vikram University, Ujjain, India, thanks his institution for approval of the sabbatical leave. Supporting Information Available: Plot of k′′ versus [Ru(III)] and an explanation for Figure 3. This information is available free of charge via the Internet at http://pubs.acs.org.

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