Kinetic Investigations of Osmium(VIII) Catalysed Oxidation of Thiamine Hydrochloride by Diperiodatocuprate(III) in Aqueous Alkaline Medium

London Journal of Research in Science: Natural and Formal
Volume | Issue | Compilation
Authored by Dr. Sharanappa T. Nandibewoor , SHREEKANT M PATIL,ATMANAND M BAGOJI & SANTOSH B KONNUR
Classification: NA
Keywords: Thiamine(vitamin B 1 ); diperiodatocuprate(III); kinetics; mechanism; oxidation.
Language: English

The osmium (VIII) (Os(VIII)) catalysed oxidation of thiamine (TA) by diperiodatocuprate (III) (DPC) in aqueous alkaline medium at a constant ionic strength of 0.18 mol dm -3 was investigated spectrophotometrically. 1:3 stoichiometry (thaimine:DPC) exhibited between the reaction of thiamine and DPC was observed in aqueous alkaline medium. The orders of the reaction with respect to [DPC], [Os(VIII)] and [thiamine] were unity, unity and less than unity respectively, in the concentrations range studied. The rate of the reaction increased as the concentration of alkali increased and periodate had retarding effect on the rate of reaction. Ionic strength had a negligible effect on the reaction rate. The main reaction products were identified by spot test and spectroscopic analysis. A mechanism involving reaction paths via free radical was proposed. The activation parameters for the slow step of the mechanism and also the thermodynamic quantities for different steps of mechanism were determined and discussed. 

               

Kinetic Investigations of Osmium(VIII) Catalysed Oxidation of Thiamine Hydrochloride by Diperiodatocuprate(III) in Aqueous Alkaline Medium

Shreekant M Patilα, Atmanand M Bagojiα, Santosh B Konnura

 & Sharanappa T Nandibewoora*

ABSTRACT

The osmium (VIII) (Os(VIII)) catalysed oxidation of thiamine (TA) by diperiodatocuprate (III) (DPC) in aqueous alkaline medium at a constant ionic strength of 0.18 mol dm-3 was investigated spectrophotometrically. 1:3 stoichiometry (thiamine:DPC) exhibited between the reaction of thiamine and DPC was observed in aqueous alkaline medium. The orders of the reaction with respect to [DPC], [Os(VIII)] and [thiamine] were unity, unity and less than unity respectively, in the concentrations range studied. The rate of the reaction increased as the concentration of alkali increased and periodate had a retarding effect on the rate of reaction. Ionic strength had a negligible effect on the reaction rate. The main reaction products were identified by spot test and spectroscopic analysis. A mechanism involving reaction paths via free radical was proposed. The activation parameters for the slow step of the mechanism and also the thermodynamic quantities for different steps of mechanism were determined and discussed.  

Keywords. Thiamine (vitamin B1); diperio- datocuprate (III); kinetics; mechanism; oxidation.

Author α: P. G. Department of Studies in Chemistry, Karnatak University, Dharwad-580003, India.

e-mail: stnandibewoor@yahoo.com

  1. INTRODUCTION

Thiamine (TA), thio-vitamin (Scheme 1) is a colourless organosulfur, soluble in water and its structure consists of an aminopyrimidine and thiazole ring linked by a methylene bridge. All living organisms use TA and it is synthesized only in bacteria, fungi, protozoans and plants1,2. Diabetes, beriberi, optic neuropathy and polyneutrics are the results of thiamine deficiency3,4.

An important characteristic of transition metals is that they exhibit multiple oxidation states and at higher oxidation states they can form stable complex with suitable polydentate ligands namely, diperiodatocuprate(III) (DPC)5, diperiodatoargentate(III) (DPA)6 and diperiodatonickelate(IV)(DPN)7, which are well known oxidizing agents in a certain buffer medium with a suitable pH value.  Diperiodatocuprate (III) (DPC) is a flexible one-electron oxidant8 and due to its poor solubility and stability in aqueous medium, the involvement of DPC in oxidation reactions is inadequate or scanty. Its use as an analytical reagent is now well recognized9.

Scheme 1: Chemical structure of Thiamine

An extraordinary role of copper complexes in the biological system is explained by several studies. Ongoing fascination in copper complexes is due to their uses as antimicrobial, antiviral, anti-inflammatory, antitumor agents10, etc. There are multiple equilibria between different copper (III) species. When the copper (III) periodate complex is the oxidant, it would be interesting to know which of the species is the active oxidant.

Kinetically, catalytic reactions are particular reactions which involve lower activation energy than the corresponding uncatalysed reactions resulting in a higher reaction rate at the same temperature and for the same reactant concentrations. The role of osmium(VIII) as a catalyst in some redox reactions has been reviewed11,12. Although the mechanism of catalysis depends on the nature of the substrate, oxidant and on experimental conditions, it has been shown13 that metal ions act as catalysts by one of these different paths such as the formation of complexes with reactants or oxidation of the substrate itself or through the formation of free radicals. In earlier report14, it has been investigated that Os(VIII) forms a complex with substrate, which is reduced to Os(VI) then to osmium(VII) species, followed by the rapid reaction of Os(VII) with one mole of oxidant to regenerate Os(VIII). In another report15, it has been observed that oxidant-substrate complex reacts with Os(VIII) to form Os(VI) species, which

again reacts with oxidant in a fast step to regenerate Os(VIII). In some other reports16, it is observed that Os(VIII) forms a complex with substrate which is oxidized by the oxidant with the regeneration of Os(VIII). Hence understanding the role of Os(VIII) in the catalysed reaction is important. Catalysis by osmium(VIII) in redox reactions involves different degrees of complexity, due to the formation of different intermediate complexes and different oxidation states of osmium.

As per the literature survey, the osmium (VIII) catalyzed oxidation of thiamine with alkaline copper(III) complex has not been reported in the literature from a kinetic and mechanistic point of view. Such studies are of much importance in understanding the mechanism of oxidation of TA and also it is helpful in getting the information of copper metal ions interaction with the biologically active compounds. Hence, the present investigation is aimed to unveil the Os(VIII) catalysed oxidation of TA by DPC and to arrive at a plausible mechanism and to understand the reactive species of Os(VIII) and DPC.

  1. EXPERIMENTAL

2.1 Chemicals and solutions

Thiamine was purchased from Sigma Aldrich, India. The analytical grade reagents were used in the experiment and Millipore water was used throughout the work. A stock solution of TA was prepared by dissolving a known amount of the TA in Millipore water. The required concentration of TA was obtained from its stock solution during the experiment. A stock solution of copper sulfate (BDH) was prepared by dissolving appropriate amounts of the copper sulfate in Millipore water. The osmium(VIII) solution was prepared by dissolving OsO4 (Johnson Matthey) in 0.50 mol dm-3 in NaOH. The concentration was ascertained16 by determining the unreacted [Fe(CN)6]4-with standard Ce(IV) solution in an acidic medium. A stock solution of IO4- was prepared by dissolving a known weight of KIO4 (Riedel-de Haen) in hot water; the stock solution used after keeping for 24 hr and its concentration was deduced iodometrically17 at neutral pH maintained using a phosphate buffer. The temperature was maintained constant to within ±0.10° C.

The diperiodatocuprate(III) was prepared by known procedure5,18-19 and standardized by a standard procedure20. Copper sulphate (3.54g), potassium periodate (6.80g), potassium persulphate (2.20g) and potassium hydroxide (9.0g) were added to 250mL of water. The mixture was shaken thoroughly and heated on a hot plate. The mixture was turned intense red after 3 hrs and boiling was continued for 20 min more to complete the reaction. The mixture was filtered through sintered crucible (G4), cooled and diluted to 250mL. The UV-vis spectrum of copper (III) complex exhibited three absorption bands at 211, 263 and 419 nm which are characteristic of the DPC. The ionic strength was maintained by adding KNO3 (AR) solution and the pH value of the medium was maintained with KOH (BDH) solution.

2.2 Instrumentation and kinetic measurements

The kinetic measurements were carried out on Varian CARY 50 Bio UV–vis spectrophotometer (Varian, Victoria-3170, Australia) attached with a Peltier Accessory (temperature control). The product analysis was carried out using ESI-MS. ESI–MS data was obtained on a 17A Shimadzu gas chromatograph with a QP-5050A Shimadzu mass spectrometer using the EI ionisation technique. For pH measurement an Elico pH meter model LI 120 was used.

The kinetics of oxidation of thiamine was followed under pseudo-first order condition where, [TA] > [DPC] at 298 ± 0.1 K. The reaction was initiated by mixing thermally equilibrated DPC with TA solution, with required concentrations of KOH, KNO3 and KIO4 and progress of the reaction was followed spectrophotometrically at 419 nm by monitoring the decrease in the absorbance due to DPC with molar absorption index, ε = 6231 ± 100 dm3 mol-1 cm-1. It was also verified that interference from other species in the reaction mixture at the wavelength 419 nm was negligible.

Regression analysis of experimental data to obtain regression coefficient ‘r’ and the standard deviation ‘S’, of points from the regression line, was performed with the Microsoft office Excel 2007 programmer.

The rate constants, kobs were obtained from the plot of log (absorbance) versus time plots and were reproducible within ± 5% error and were the average of three experiments. All the kinetic runs were followed by more than 80% reaction completion. During the kinetics, a constant concentration, viz., 1.0×10-4 mol dm-3 of KIO4 was used throughout the experiment. Since periodate is present in the excess in DPC, the possibility of oxidation of TA by periodate in alkaline medium at 298 K was tested. The progress of the reaction was followed iodometrically. However, it was found that there was no significant reaction under the experimental conditions employed compared to the DPC oxidation of TA. The total concentration of periodate and OH was calculated by considering the amount present in the DPC solution and that additionally added. The spectral changes during the reaction are shown in Figure 1. It is evident from the Figure that the concentration of DPC decreases at 419 nm.

Figure 1:  Spectroscopic changes occurring in the Os(VIII) catalyzed oxidation of thiamine by diperiodatocuprate(III) at 298 K, [DPC]= 5.0 × 10-4 mol dm-3, [TA] = 5.0 × 10-3 mol dm-3, [OH-] = 0.08, [IO4-] = 1.0× 10-4 mol dm-3  and I = 0.18 mol dm-3 with time interval of 1 min (curves a-f).

  1.  RESULTS AND DISCUSSION

3.1 Stoichiometry and product analysis 

Different sets of reaction mixtures containing different ratios of DPC to thiamine at constant ionic strength, [KOH], [Os(VIII)] were kept for 4 hrs at 298 K in a vessel under inert atmosphere. The remaining concentration of DPC was estimated spectrophotometrically at 419 nm. The results indicated 1:3 stoichiometry as given in equation (1). After completion of reaction, the reaction mixture was acidified, concentrated and extracted with ether. The reaction product was further recrystallized from aqueous alcohol.

The main reaction product for Os(VIII) catalysed reactions was identified as 2- (4-meth- ylthiazol-5-yl) ethanol, which was confirmed by ESI–mass spectral analysis (Figure S1 Supplementary Information). The mass spectrum showed a molecular ion peak at 142.96 amu confirming the presence of 2-(4-methyl- thiazol-5-yl)ethanol.

3.2 Reaction order

As the diperiodatocuprate(III) oxidation of thiamine in alkaline medium proceeds with a measurable rate in the absence of osmium(VIII), the catalysed reaction is understood to occur in parallel paths with contributions from both the catalysed and uncatalyzed paths.

Thus, the total rate constant (kT) is equal to the sum of the rate constants of the catalysed (kC) and uncatalysed (kU) reactions, so kC = kT - kU. Hence the reaction orders have been determined from the slopes of log kC versus log(concentration) plots by varying the concentrations of thiamine, Os(VIII), alkali and periodate in turn while keeping the others constant. The uncatalysed reaction was followed under the conditions [TA] = 5.0 × 10-3; [DPC] = 5.0 × 10-4, [OH-] = 0.08; I = 0.18 / mol dm-3. The rate constant of uncatalyzed reaction (kU) was obtained by the plot of log(absorbance) versus time by following the progress of the reaction spectrophotometrically at 419 nm. 

3.3 Effect of [diperiodatocuprate(III)]

The DPC concentration was varied in the range of 1.0 × 10-5 to 1.0 × 10-4 mol dm-3 for Os(VIII) catalysed reaction. The linearity of the plots of log absorbance versus time up to 80% completion of the reaction indicates a reaction order of unity in [DPC]. This was also confirmed by varying [DPC], which did not result in any change in the pseudo-first- order rate constants, kC (Table 1).

3.4 Effect of [thiamine] 

The thiamine concentration was varied in the range 1.0 × 10-3-1.0 × 10-2 mol dm-3 at 298 K while keeping other reactant concentrations and conditions constant in the presence of catalyst. The kC values increased with the increase in concentration of thiamine, indicating an apparent less than unit order dependence on [TA]. This was also confirmed by the plot of kC versus [TA]0.7784 which was linear rather than the direct plot of kC versus [TA] (Figure 2) ( r=0.9906, S < 0.0052 for catalysed) (Table 1).

C:\Users\STN\Desktop\ATM papers\Thiamine\JCSc Thaimine\THaimine images\thiamine2.jpg

Figure 2:  Plots of kC vs [TA]0.7784 and kC vs [TA] (conditions as in Table 1).

3.5 Effect of [alkali] and [periodate]

The effect of alkali on the reaction was studied at constant concentrations of thiamine and DPC and at a constant ionic strength of 0.18 mol dm-3 for catalyzed reaction at 298 K. The rate constants increased with increase in [alkali] in the presence of catalyst which gives less than unit order dependence on [alkali] as given in Table 1. The effect of [IO4-] was observed by varying the concentration from 5.0 × 10-5 to 5.0 ×10-4 mol dm-3 while keeping all other reactants concentrations constant. It was observed that the rate constants decreased by increasing [IO4-] (Table 1). 

3.6 Effect of [osmium(VIII)] 

The osmium(VIII) concentration varied from 0.5 × 10-8 to 5.0 × 10-8 mol dm-3 range, at constant concentrations of diperiodatocuprate(III)], thiamine, and alkali and at constant ionic strength. The order in [Os(VIII)] was found to be unity from the linearity of the plot of  kC versus  [Os(VIII)] (r=0.9848, S ≤ 0.0031) (Table 1).

3.7 Effect of dielectric constant of the medium (D)

The dielectric constant of the medium, D, was varied by varying the t-butyl alcohol-water percentage. The dielectric constants of the reaction medium at various composition of t-butyl alcohol and water (v/v) were calculated from the equation, D = V1D1 + V2D2, where D1 and D2 are dielectric constants of pure water and t-butyl alcohol, i.e., 78.5 and 10.9 at 298 K respectively, and, V1 and V2 are the volume fractions of the components, water and t-butyl alcohol respectively, in the total volume of mixture. It was found that the decreasing polarity had no effect on the rate for Os(VIII) catalysed reaction.

Table 1:  Effect of variation of [DPC], [TA], [OH-], [IO4-] and [Os(VIII)] on the osmium(VIII) catalyzed oxidation of thiamine by DPC in aqueous alkaline medium at 298 K [I = 0.18 mol dm-3]

 [DPC] × 104

(mol dm-3)

[TA] × 103

(mol dm-3)

[KIO4] ×104

(mol dm-3)

[OH] ×102

(mol dm-3)

[Os(VIII)] ×108

(mol dm-3)

kT ×102

(s-1)

ku ×102

(s-1)

kc (102)

(s-1)

1

5

1

8

1

7.9

0.66

7.24

3

5

1

8

1

8.1

0.69

7.41

5

5

1

8

1

8.3

0.62

7.68

8

5

1

8

1

8.15

0.61

7.54

10

5

1

8

1

8.27

0.58

7.69

5

1

1

8

1

3.83

0.29

3.54

5

3

1

8

1

5.41

0.43

4.98

5

5

1

8

1

8.3

0.62

7.68

5

8

1

8

1

18.2

1.38

16.82

5

10

1

8

1

22.1

1.7

20.4

5

5

0.5

8

1

13.8

1.02

12.78

5

5

0.8

8

1

10.7

0.81

9.89

5

5

1

8

1

8.3

0.62

7.68

5

5

3

8

1

4.65

0.36

4.29

5

5

5

8

1

3.2

0.25

2.95

5

5

1

3

1

4.2

0.35

3.85

5

5

1

5

1

6.3

0.52

5.78

5

5

1

8

1

8.3

0.62

7.68

5

5

1

10

1

11.2

0.82

10.38

5

5

1

30

1

17.7

1.31

16.39

5

5

1

8

0.5

3.29

0.62

2.67

5

5

1

8

0.8

6.2

0.62

5.58

5

5

1

8

1

8.3

0.62

7.68

5

5

1

8

3

21.9

0.62

21.28

5

5

1

8

5

47.3

0.62

46.68

3.8 Effect of ionic strength (I) 

The addition of KNO3 at constant [DPC], [Os(VIII)], [TA], [OH-] and [IO4-] was found that increasing ionic strength had  no significant effect on the rate of reaction.

3.9 Effect of initially added products

The initially added products, Cu(II) and 2-(4-methylthiazol-5-yl)ethanol did not have any significant effect on the rate of reaction in Os(VIII) catalysed reaction.

3.10 Test for free radicals

The involvement of free radicals in the reaction was examined as follows. A known quantity of acrylonitrile monomer was initially added to the reaction mixture and was kept for 2 hr in an inert atmosphere. A white precipitate was formed on diluting the reaction mixture with methanol, indicating the involvement of free radicals in the reaction21. The blank experiments of either DPC or thiamine alone with acrylonitrile did not induce any polymerization under the same conditions.

3.11 Effect of temperature

The influence of temperature on the rate of reaction was studied for catalyzed reaction at four different temperatures (288, 298, 308 and 318 K) under varying concentrations of thiamine, alkali, [Os(VIII)] and periodate keeping other conditions constant. The rate constants increased with increase in temperature. The rate constants (k) of the slow step and equilibrium constants of Scheme 2 for the catalyzed reaction were obtained from the slopes and the intercepts of the plots of [Os(VIII)]/kC versus 1/[TA], [Os(VIII))]/kC versus 1/[OH¯] and  [Os(VIII)]/kC versus [IO4¯] at four different temperatures. The values are given in Table 2. The energy of activation for the rate determining step was obtained by the least square method of plot of log k2 versus 1/T (r=0.9968, S ≤ 0.014) and other activation parameters calculated for the reaction are presented in Table 2. The thermodynamic quantities with respect to K1, K2 and K4 were calculated from Van’t Hoff’s equation and are presented in Table 2.

3.12 Catalytic activity

It has been pointed out by Moelwyn-Hughes22 that, in the presence of the catalyst, the uncatalysed and catalysed reactions proceed simultaneously, so that

                kT = kU + KC [catalyst]x                             (2)

 Here kT is the observed pseudo-first order rate constant in the presence of Os(VIII) catalyst, kU the pseudo-first order rate constant for the uncatalysed reaction, KC the catalytic constant and ‘x’ is the order of the reaction with respect to [Os(VIII)]. In the present investigation, x values for the standard run were found to be 1.0 for Os(VIII). Then the value of KC can be calculated using the equation (3)

The values of KC were evaluated with respect to the catalyst at different temperatures and found to vary at different temperatures. The values of KC and the corresponding activation parameters with respect to Os(VIII) catalysts are presented in        Table 3.

3.13 Mechanism of reaction

Due to versatile behavior of one-electron oxidant, the oxidation of many organic and inorganic compounds by Cu(III) species have been carried out. The literature survey reveals that the water soluble copper(III) periodate complex is reported23 to be  [Cu(HIO6)2(OH)2]7-. However, in aqueous alkaline medium and at the high pH range as employed in the study, periodate is unlikely to exist as HIO64- (as present in the complex) as is evident from its involvement in the multiple equilibria24, (4) to (6) depending on the pH of the solution, as given below.

Periodic acid exists in the acid medium as H5IO6 and as H4IO6- at around pH 7. So under the conditions employed, in the alkaline medium the main species are expected to be H3IO62- and H2IO63-. At higher concentrations, periodate also tends to dimerise25. However, formation of this species is negligible under conditions employed for kinetic study. Hence, at the pH employed in this study, the soluble copper(III) periodate complex exists as diperiodatocuprate(III), [Cu(H3IO6)(H2IO6)]2-, a conclusion also supported by earlier work25.

Lister26 proposed three forms of copper(III) periodate in alkaline medium, viz., diperiodato- cuprate (III) (DPC), monoperiodatocuprate(III) (MPC), and tetrahydroxy cuprate (III). The last one is ruled out, as its equilibrium constant is 8.0 × 10-11 at 313 K. Hence, in the present study, DPC and MPC are considered as the active forms of copper(III) periodate complex. It may be expected that a lower periodate complex such as MPC is more important in the reaction than DPC. The results of increase in the rate with increase in alkali concentration and decrease in rate with increase in periodate concentration (Table 1) suggest an equilibrium of the copper(III) periodate complex to form a monoperiodato- cuptrate(III) (MPC) species as shown in equations (7) and (8). Similar results have been well reported in literature27.

At different  concentrations osmium(VIII) is known to form different complexes34, i.e., [OsO4(OH)2]2- and [OsO5(OH)]3- are the complexes at lower concentrations and higher concentrations of. Since the rate of oxidation increased with increase in [] at lower range of concentrations of  In the present study, it is convenient that [OsO4(OH)2]2- was considered and that its formation is important in the reaction30. It may be expected that lower Cu(III) periodate species such as MPC are more important in the reaction than the DPC. The inverse fractional order in [H3IO62-] might also be due to this reason. The first two equilibrium steps were similar as given in equations (7) and (8). Thiamine reacts with osmium(VIII) species to form a complex (C1) which reacts with one mole of the MPC in a slow step to give a free radical species (A) and intermediate species (B) with the regeneration of catalyst Os(VIII) species. The free-radical species (A) formed further reacts with one mole OH- to give (4-amino-2-methylpyrimidin-5-yl)methanol, which is converted into byproduct of the reaction i.e., 4-amino-2-methylpyrimidine-5-carbaldehyde upon reacting with two molecules of MPC. The intermediate species (B) reacts with one mole of OH- to give the main product 2 -(4-meth- ylthiazol-5-yl) ethanol, which was identified by its ESI-MS (Figure S1). The detailed mechanism for the catalyzed oxidation of thiamine by diperiodatocuprate(III) is represented as given in Scheme 2.

Scheme 2. Mechanism of Os(VIII) catalysed oxidation of thiamine by diperiodatocuprate(III)

Spectroscopic evidence for the complex formation between catalyst and substrate was obtained from UV–vis spectra of thiamine (5.0 ×10-3), Os(VIII) (1.0 × 10-8 , [OH-] = 0.08 mol dm-3 ) and a mixture of both. A hypsochromic shift of about 7 nm from 263 to 256 nm of Os(VIII) to Os(VIII)-TA complex was observed. However, the Michaelis-Menten plot also proved the complex formation between Os(VIII)  and TA, which explains the less than unit order dependence on [TA]. Such a complex between a substrate and a catalyst has been observed in other studies30. The rate law (9) for Scheme 2 was derived as

which explains all the observed kinetic orders of different species.

The rate law (9) can be rearranged into the following form suitable for verification:

The plots of [Os(VIII)]/kC versus 1/[TA], [H3IO62-] and 1/ [OH-] were linear with an intercept supporting the Os(VIII)-TA complex, as is verified in Figure 3. From the intercepts and slopes of such plots, the reaction constants K1, K2, K4 and k were calculated as 1.55 × 10-1 dm3 mol-3, 0.39 × 10-3 mol dm-3, 6.4 × 103 dm3 mol-1 and 0.6 × 105 dm3 mol-1 s-1, respectively (Table 2). The values of K1 and K2 obtained are in good agreement with previously reported values28.

The thermodynamic quantities for the different equilibrium steps in Scheme 2 were evaluated as follows. The thiamine, hydroxide ion and periodate concentrations were varied at different temperatures. A Vant Hoff’s plot was made for the variation of K1, K2, K3 with temperature [(logK1 versus 1/T (r = 0.9972, S ≤ 0.009), logK2 versus 1/T (r=0.9789, S ≤0.11) and logK3 versus 1/T (r=0.9878, S ≤ 0.15)] and the values of the enthalpy of reaction ΔH, entropy of reaction ΔS and free energy of reaction ΔG, were calculated. These values are given in Table 2. A comparison of the ΔH value of second step (25.8 kJ mol-1) of Scheme 2 with that of ΔH# (44.6 kJ mol-1) obtained for the slow step of the reaction shows that these values mainly refer to the rate limiting step, supporting the fact that the reaction before rate determining step is fairly fast and involves low activation energy. The values of ΔH# and ΔS# were both favourable for electron transfer processes. The favourable enthalpy was due to release of energy on solutions changes in the transition state. The negative value of ΔS# suggests that the intermediate complex is more ordered than the reactants28. The observed modest enthalpy of activation and a higher rate constant for the slow step indicates that the oxidation presumably occurs via an inner-sphere mechanism which was supported by earlier work29

Figure 3: Verification of rate law (9) for the Os(VIII) catalyzed oxidation of TA by diperiodatocuprate(III). Plots of (A) [Os(VIII)]/kc v/s 1/[TA], (B) [Os(VIII)]/kc v/s 1/[OH-], (C) [Os(VIII)]/kc v/s [H3IO62-], at four different temperatures (conditions as in Table 1)

The negligible effect of ionic strength and dielectric constant of medium on the rate explains

qualitatively the reaction between neutral and negatively charged ions, as seen in Schemes 2.

Table 2:  (A) Activation parameters with respect to the slow step of Scheme 2 and thermodynamic quantities for the Os(VIII) catalyzed oxidation of thiamine by diperiodatocuprate(III) in aqueous alkaline medium. {[DPC] =5.0 ×10-4 mol dm-

Temperature(K)

k (x10-5 ) dm3 mol-1 s-1

(A) Effect of temperature

288

0.35

0.60

1.27

1.97

298

308

318

Activation of parameters

Ea(kJ mol-1)

47.1±2.4

44.6±2.2

-194±8

102±5

∆H#(kJ mol-1)

∆S#(J K-1 mol-1)

∆G#(J mol-1)

(B) Effect of temperature to calculate K1, K2 and K3 for the oxidation of thiamine by

diperiodatocuprate(III) in alkaline medium.

Temperature(K)

K1 x101 (dm3 mol-1)

K2 x103 (mol dm-3)

K3 x10-3(dm3 mol-1)

288

1.13

1.7

2.6

298

1.55

0.39

6.4

308

2.29

0.11

18.2

318

3.16

0.06

30.2

(C) Thermodynamic quantities using K1, K2, and K3

Thermodynamic quantities

Values from K1

Values from K2

Values from K3

∆H (kJ mol-1)

25.8

-100

70.4

∆S (J K-1 mol-1)

109

-323

309

∆G298(kJ mol-1)

-6.9

3.7

-22.1

The activation parameters evaluated for the catalysed reaction explain the catalytic effect on the reaction. The catalyst Os(VIII) forms the complex (C1) with the substrate, which enhances the reducing property of the substrate than that without the Os(VIII) catalyst. Further, the Os(VIII) catalyst modifies the reaction path by lowering the energy of activation. Further, plots of log KC versus 1/T were linear and the values of energy of activation and other activation parameters with reference to catalyst were computed. These results are summarised in Table 3. The value of KC at 298 K is 7.68 ×103.

Table 3: Values of catalytic constant (KC) at different temperatures and activation parameters calculated using kC values. [DPC] = 1.0 × 10-4; [TA] = 5.0 × 10-3; [OH-] = 0.08; [IO4-] = 1.0 ×10-4; [Os(VIII)] = 1.0× 10-8, I = 0.18/mol dm-3.

Temperature

KC × 10-3

288

1.97

298

7.68

308

21.2

318

41.7

Activation parameters

Ea (kJ mol-1)

78.1 ± 1.6

ΔH# (kJ mol-1)

75.6 ± 1.4

ΔS# (J K-1 mol-1)

138 ± 0.88

ΔG#298 (kJ mol-1)

34.2 ± 1.23

  1. CONCLUSIONS

The osmium(VIII) catalyzed oxidation of thiamine by diperiodatocuprate(III) was studied. Oxidation products were identified. Among the various species of Cu(III) in alkaline medium, [Cu(H2IO6)(H2O)2] was considered as an active species for Os(VIII) catalysed reactions. Active species of Os(VIII) was found to be [OsO4(OH)2]2-. Based on experimental results the probable mechanism was proposed for the reaction. Thermodynamic quantities and activation parameters of individual steps in the mechanism were evaluated for Os(VIII) catalyzed reaction at different temperatures. The catalytic constants and the activation parameters with reference to catalyst were also computed. The description of the mechanism was consistent with all the experimental evidence including kinetic, spectral and product studies.

4.1 Supplementary Information (SI)

Additional information pertaining to the main reaction product (Figure S1) is given in the supporting information.

ACKNOWLEDGEMENTS

The corresponding author thanks the UGC, New Dehli for the award of UGC-BSR faculty fellowship to Dr. S. T. Nandibewoor. Authors Thanks to University Scientific Instruments Center (USIC) and Department of Chemistry Karnatak University, Dharwad, for providing necessary laboratory facilities.

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Supplementary Information

C:\Users\atmanand\Desktop\SHREEKANT\Kinetics\2. Kinetics\paper5\spectra.png

Figure S1. ESI-Mass spectrum of 2-(4-methylthiazol-5-yl)ethanol with its molecular ion peak at m/z = 142.96.



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