et al. world journal of pharmaceutical research volume 3

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910

Nandibewoor et al. World Journal of Pharmaceutical Research

MECHANISTIC INVESTIGATIONS OF UNCATALYSED AND

RUTHENIUM(III) CATALYSED OXIDATION OF

PHARMACEUTICALLY IMPORTANT D-SORBITOL BY

PERIODATE IN AQUEOUS ALKALINE MEDIUM

Sharanappa Totappa Nandibewoor* and Prashant Ashok Magdum

P. G. Department of studies in Chemistry, Karnatak University, Dharwad –580003, India.

ABSTRACT

The kinetics of oxidation of pharmaceutically important D-Sorbitol (D-

Sorb) by periodate both in the absence and presence of ruthenium(III)

catalyst in alkaline medium at 298 K and at a constant ionic strength of

0.21 mol dm-3 was studied. The reaction exhibits 1:1 stoichiometry

([D-Sorb]: [periodate]). The reaction shows first order kinetics in

[periodate], [Ru(III)] and less than unit order both in [D-Sorb] and

[OH-]. The ionic strength and dielectic constant of the medium did not

affect the rate significantly. The main products were identified by spot

tests, FT-IR and LC-MS spectral studies. Based on the experimental

results, the possible mechanisms were proposed. The reaction

constants involved in the different steps of the mechanisms were

evaluated. The catalytic constant (Kc) was also calculated for Ru(III)

catalysis at different temperatures. The activation parameters with respect to the catalyst and

slow step of the mechanisms were computed and also thermodynamic quantities determined.

Kinetic studies suggest that the active species of periodate and Ru(III) were found to be

[H2IO63-] and [Ru(H2O)5OH]2+ respectively.

Key words: Periodate, D-Sorbitol, Oxidation, Ruthenium, Uncatalysed and Catalysed,

Mechanism.

INTRODUCTION

Periodic acid is widely used as specific diol cleaving reagent. Periodate as periodic acid in

acid medium is extensively used as an oxidant due to the fact that it has specific action

towards α-dicarbonyl compounds which is useful in elucidating the structure. In alkaline

World Journal of Pharmaceutical research

Volume 3, Issue 1, 910-931. Research Article ISSN 2277 – 7105

Article Received on 20 October2013 Revised on 22 November 2013, Accepted on 17December 2013

*Correspondence for

Author:

Prof. Sharanappa Totappa

Nandibewoor

P.G. Department of Chemistry,

Karnatak University, Dharwad,

India.

[email protected],

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911


medium it is relatively weak oxidant and is used rarely. In alkaline medium, periodate is

known to exist as different species involving multiple equilibria (Cotton and Wilkinson,

1988) and it needs to know the active form of the oxidant in the reaction.

Sorbitol (D-Sorb) is a sugar alcohol, which the human body metabolizes slowly. Sorbitol is

referred to as a nutritive sweetener because it provides dietary energy. It also occurs naturally

in many stone fruits and berries from trees of the genus Sorbus (Nelson and Cox, 2004).

Sorbitol can be used as a non-stimulant laxative via an oral suspension or enema. It is also

used frequently in "sugar free" chewing gum. It is thought that these agents may help to

prevent the accumulation of intracellular sorbitol that leads to cellular damage in diabetics.

In recent years, the use of transition metal ions such as ruthenium , osmium, palladium,

chromium and iridium, either alone or as binary mixtures, as catalysts in various redox

processes has attracted considerable interest (Das, 2001). Ruthenium(III) acts as an efficient

catalyst in many redox reactions, particularly in an alkaline medium (Swarnalaxmi et al.,

1987). The catalyzed mechanism can be quite complicated due to formation of different

intermediate complexes, and different oxidation states of ruthenium(III). Although the

mechanism of catalysis depends on the nature of the substrate, oxidant, and on experimental

conditions, it has been shown (Singh et al., 2007) 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 reports (Sandu et al.,

1983), it has been observed that Ru(III) forms a complex with the substrate, which gets

oxidized by the oxidant to form Ru(IV)–substrate complex followed by the rapid redox

decomposition to regenerate Ru(III). In another report (Panda and Sahu, 1989), it was

observed that there involves the formation of a Ru(III)–substrate complex with further

cleavage in a concerted manner giving rise to a Ru(I) species, which gets rapidly oxidized by

the oxidant to regenerate the catalyst. In some other reports (Tegginamath et al., 2007), it was

observed that Ru(III) forms a complex with substrate and is oxidized by the oxidant with the

regeneration of the catalyst. Hence, understanding the role of Ru(III) in catalyzed reaction is

important.

Some work on oxidation of sorbitol by permanganate and hexachloroiridate(IV) has been

documented (Odebunmi and Marufu, 1999). Literature survey reveals that there is no reports

either on uncatalysed or ruthenium(III) catalysed oxidation of D-sorbitol by periodate in

alkaline medium. In order to investigate the redox chemistry and in view of potential

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912


pharmaceutical importance of D-Sorb and to know the active species of oxidant and catalyst

in such media and to propose the appropriate mechanisms of the reactions on the basis of

kinetic and spectral results, the title reaction is investigated in detail.

AIMS AND OBJECTIVES

The study was aimed to identify the reactive species of periodate and propose a reasonable

and convincing reaction mechanism for the title reaction.

EXPERIMENTAL

MATERIALS AND METHODS

All chemicals used were of reagent grade and double distilled water was used throughout

this study. The stock solution of periodate (0.02 mol dm-3) was prepared by dissolving 2.30 g

of potassium metaperiodate (S.D.Fine Chem.) in 500 cm3 water and the solution was used

after keeping for 24 hours. The concentration of the solution was verified (Panigrahi and

Misro, 1977) by titration with standard sodium thiosulfate iodometrically at neutral pH

maintained by adding 5 cm3 5% potassium dihydrogen phosphate and 5 cm3 of 5%

dipotasssium hydrogen phosphate solution, using starch solution as an indicator.

A solution of D- Sorbitol (Himedia) was prepared by dissolving an appropriate amount of

recrystallised sample in double distilled water. The purity of D-Sorb sample was checked by

comparing its melting point (94 0C) with the literature data (95 0C).The required

concentration of D-Sorb was obtained from its stock solution. A standard solution of Ru(III)

was prepared dissolving RuCl3 (S.D. Fine Chem.) in 0.20 mol dm-3 HCl. The concentration

was determined (Reddy and Vijaykumar, 1995; Kamble et al., 1996) by EDTA.

Sodium thiosulfate (Thomos Baker Chemicals Ltd.) solution was prepared in water. It was

standardized (Kamble et al., 1996) against potassium iodate as follows- to the potassium

iodide solution containing 1.0 mol dm-3 sulfuric acid in iodine flask, a known volume of

standard potassium iodate solution was added. The liberated iodine was titrated against

sodium thiosulfate solution using starch indicator. Potassium hydroxide and potassium nitrate

were employed to maintain the required alkalinity and ionic strength respectively. Potassium

dihydrogen phosphate (Thomas Baker Chemicals Ltd.) and potassium iodide (S.D.Fine

Chem.) were used in iodometric determination of periodate at neutral pH.

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Instruments used

For product analysis, the LC/ESI/MS instrument using a Finnigan (Thermo Finnigan, San

Jos´e, CA, USA) LCQ DECA ION trap instrument equipped with Xcal-ibur software and

Nicolet 5700- FT-IR spectrometer (Thermo, USA) were used. For pH measurements ELICO

pH meter model LI 120 (Hyderabad, India) was used.

Kinetic measurement

The kinetics was followed under pseudo first order condition where [D-Sorb] > [Periodate] in

both uncatalysed and catalysed reactions at 25 ± 0.1 0C, unless specified. The reaction was

initiated by mixing periodate with the D-sorbitol solution which contained required

concentrations of KNO3 and KOH. The reaction in the presence of catalyst Ru(III) was

initiated by mixing periodate with the D-sorbitol solution which also contained required

concentrations of KNO3, KOH and Ru(III) catalyst. The reaction was followed by measuring

the decrease in concentration of periodate titrimetrically using sodium thiosulfate, at regular

intervals of time. In view of the modest concentration of OH− used in the reaction medium,

attention was also directed to the effect of the reaction vessel surface on the kinetics. Use of

polythene/acrylic wares gave the same results, indicating that the surface did not have any

significant effect on the reaction rates.

In view of the ubiquitous contamination of carbonate in the basic medium, the effect of

carbonate was also studied. Added carbonate had no effect on the reaction rates. However,

fresh solutions were nevertheless used for carrying out each kinetic run. 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 2003 program.

RESULTS

Stoichiometry and Product analysis

Different sets of mixtures containing varying ratios of periodate to D-Sorb in presence of

constant amount of OH- (0.2 mol dm-3) and KNO3(0.01 mol dm-3) in an uncatalysed reaction

and with a constant amount of Ru(III) 1.5 x10-5 mol dm-3 in a catalysed reaction with

constant ionic strength of 0.21 mol dm-3 , were kept for 8 hours in a closed vessel under

nitrogen atmosphere at 25 0C and then analysed. Under the condition [IO-4] > [D-Sorb], the

periodate in the rection was estimated iodometrically at neutral pH. The results shows 1:1

stoichiometry for both uncatalysed and catalysed reactions as given in Eq.(1).

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The main oxidation product, D- glucose was identified by spot tests (Fiegl, 1975). The

product D-glucose was also confirmed by IR and LC-MS spectra. The nature of the glucose

was confirmed by the IR spectrum, which has shown a carbonyl (>C=O) stretch at 1737 cm−1

and O-H stretching of the glucose at 3470cm−1 (Fig. 1). The product D- glucose was also

confirmed by LC-MS analysis data. The mass spectrum showed molecular ion peak at 181

amu, which confirms the product, D-glucose (Fig. 2). It was observed that D-glucose did not

undergo further oxidation under the present kinetic conditions.

Figure.1. FTIR Spectrum of D-Glucose

Figure.2.LC-MS of D-Glucose with its molecular ionic peak at 181 m/z

CC

C

CCH2OH

CH2OH

OHH

HHO

OHH

OHH

+ CC

C

CCH2OH

CHO

OHH

HHO

OHH

OHH

+ IO3- + H2O (1)

Ru(III) IO4

-

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915


Reaction orders

As the periodate oxidation of D-sorbitol in alkaline medium proceeds with a measurable rate

in the absence of Ru(III), the catalysed reaction is understood to occur in parallel paths with

contributions from both the catalysed and uncatalysed paths. Thus, the total rate constants

(kT) is equal to the sum of the rate constants of the catalysed (kC) and uncatalysed (kU)

reactions, so kC = kT - kU. Hence the orders for various species were determined from the

slopes of plots of log (kU or kC) versus respective concentration of species except for

[periodate] in which nonvariation of kU or kC was observed as expected to the reaction

condition. The reaction orders have been determined from the slopes of log kC versus log

(concentration) plots by varying the concentrations of D-Sorb, OH−, and catalyst Ru(III), in

turn while keeping others constant.

Effect of [Periodate]

In absence and presence of Ru(III) catalyst, the periodate concentratrion was varied in the

range of 1.0 x 10-3 to 1.2 x 10-2 mol dm-3 at fixed [D-Sorb], [OH] and ionic strength. The

non variation in the pseudo-first order rate constants at various concentration of KIO4

indicates the order in [IO-4] as unity (Table 1) for kU and (Table 2) for kC. This was also

confirmed from the linearity of log (concentration) versus time up to 80% completion of the

uncatalysed and Ru(III) catalysed reactions.

Effect of [D-Sorbitol]

In both the cases [D-Sorb] was varied in the range of 5.0 x 10-2 to 5.0 x 10-1 mol dm-3 at 25 0C keeping all other reactants concentrations and conditions constant. The kU and kC values

increased with the increase in concentration of D-sorbitol indicating an apparent less than

unit order dependence on [D-Sorb] under the conditions of experiment and concentration

range used. (Table 1, uncatalyzed; Table 2, Ru(III) catalyzed). This was also confirmed by

the plots of kU versus [D-Sorb]0.2 and kC versus [D-Sorb]0.2, which were linear rather than the

direct plot of kU versus [D-Sorb] and kC versus [D-Sorb] (Figure 3; r ≥ 0.980, S ≤0.001 for

uncatalyzed; r ≥ 0.975, S ≤0.005 for Ru(III) catalyzed).

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916


Figure.3. Plot of kU versus [D-Sorb]0.2 and kU versus [D-Sorb]

Effect of [OH-]

The effect of OH- in absence and presence of Ru(III) catalyst reaction was studied in the

range of 0.01 to 0.2 mol dm-3 at constant concentrations of D-Sorb, IO-4 and ionic strength of

0.21 mol dm-3. The rate increased with increase in concentration of alkali. The order with

respect to [alkali] was found to be less than unit order. (ie. Table.1 for kU and Table.2 for kC).

Effect of [Ru(III)]

The ruthenium(III) concentration was varied from 3.0 x10-6 to 3.0 x10-5 mol dm-3 range, at

constant concentrations of IO-4, D-Sorb, OH- and ionic strength. The order in [Ru(III)] was

found to be unity from the linearity of the plot kC versus [Ru(III)] (r≥0.997, S≥0.025) (Table

2).

Effect of initially added products

In both the cases initially added products, D-glucose and IO3- did not have any significant

effect on the rate of reaction. Thus, from the observed experimental results, the experimental

rate law for uncatalysed reaction is given as Rate = kU [IO4-]1.0 [D-Sorb]0.2 [OH-]0.2. The rate

law for Ru(III) catalysed reaction is given as Rate = kC [IO4-]1.0 [D-Sorb]0.2 [OH-]0.6

[Ru(III)]1.0.

Effect of ionic strength (I) and Dielectric constant of the medium (D).

The ionic strength of the uncatalysed and Ru(III) catalysed reaction medium was varied from

0.25 to 0.6 mol dm-3 at constant [IO-4] ,[D-Sorb] and [OH-] .It was found that increasing

ionic strength had no significant effect on the rate of reaction in both the cases of uncatalysed

and catalysed reactions.

[D-Sorb]0.2 (mol dm-3)

k U X

104 (s

-1)

k U X

104 (s

-1)

[D-Sorb] (mol dm-3)

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The relative permittivity (ɛT) effect was studied by varying the t-butyl alcohol – water content

in the reaction mixture with all other conditions being maintained constant. Attempts to

measure the relative permitivities were not successful. However, they were computed from

the values of pure liquids (Lide, 1992 )the dielectric constants of the reaction medium at

various composition of t- butyl alcohol (V/V)-water were calculated by using the followimg

equation,

D= DwVw + DBVB

where Vw and VB are volume fractions and Dw and are DB are dielectric constants of water and

t-butyl alcohol. There was no reaction of the solvent with the oxidant under the experimental

conditions. The rate constant, kU and kC did not change with increase in the dielectric

constant of the medium.

Test for free radicals (polymerization)

The intervention of free radicals in both uncatalysed and catalysed reactions was examined

as follows: The reaction mixture was mixed with acrylonitrile monomer and kept for 2 h and

1 h respectively, under nitrogen atmosphere. On dilution with methanol, a white precipitate

was formed, indicating the intervention of free radicals in the reactions. The blank

experiments of either periodate or D-Sorbitol alone with acrylonitrile did not induce any

polymerization under the same conditions as those induced for the reaction mixture. Initially

added acrylonitrile decreased the rate of reaction indicating free radical intervention, which is

the case in earlier work (Jagadeesh and Puttaswamy, 2008)

Table 1: Effect of variation of [Periodate], [D-Sorb] and [OH-] concentrations on the

oxidation of D-sorbitol by periodate in aqueous alkaline medium at 25 0C and I= 0.21

mol dm-3

[IO4-]x103

(mol dm-3)

[D-Sorb] x101

(mol dm-3)

[OH-]x101

(mol dm-3)

kU x104 (s-1)

Found Calculated

1.0

2.0

4.0

6.0

8.0

12.0

2.0

2.0

2.0

2.0

2.0

2.0

2.0

2.0

2.0

2.0

2.0

2.0

7.67 7.91

7.29 7.91

8.04 7.91

7.67 7.91

7.29 7.91

7.67 7.91

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4.0

4.0

4.0

4.0

4.0

4.0

4.0

4.0

4.0

4.0

4.0

4.0

0.5

1.0

2.0

3.0

4.0

5.0

2.0

2.0

2.0

2.0

2.0

2.0

2.0

2.0

2.0

2.0

2.0

2.0

0.1

0.2

0.5

1.0

1.5

2.0

5.96 5.60

7.12 7.02

8.04 8.02

8.47 8.43

8.84 8.65

9.09 8.79

4.0 4.02

5.52 5.54

6.89 6.93

7.56 7.63

7.96 7.89

8.04 8.02

Table 2: Effect of variation of [Periodate], [D-Sorb], [OH-] and [Ru(III)] concentrations

on the Ru(III) catalysed oxidation of D-sorbitol by periodate in aqueous alkaline

medium at 25 0C and I = 0.21 mol dm-3

[IO4-]103

(mol dm-3)

[OH-]101

(mol dm-3)

[D-Sorb]101

(mol dm-3)

Ru(III)] 105

(mol dm-3)

kT x 102

(s-1)

kU x104

(s-1) kC x102(s-1)

Found Calculated

1.0

2.0

4.0

6.0

8.0

4.0

4.0

4.0

4.0

4.0

4.0

4.0

2.0

2.0

2.0

2.0

2.0

2.0

2.0

2.0

2.0

2.0

0.1

0.2

2.0

2.0

2.0

2.0

2.0

0.5

1.0

2.0

3.0

4.0

2.0

2.0

1.5

1.5

1.5

1.5

1.5

1.5

1.5

1.5

1.5

1.5

1.5

1.5

3.68

3.60

3.76

3.53

3.72

2.68

3.24

3.76

3.84

4.02

0.70

1.24

8.04

8.04

8.04

8.04

8.04

5.96

7.12

8.04

8.47

8.84

4.0

5.52

3.59 3.27

3.51 3.27

3.67 3.27

3.44 3.27

3.63 3.27

2.62 2.35

3.16 2.89

3.67 3.27

3.75 3.42

3.93 3.50

0.66 0.66

1.18 1.14

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Effect of temperature (T)

The influence of temperature on the rate of reaction was studied for both uncatalysed and

catalysed reaction at different temperatures (25, 30, 35 and 40 0C) under varying

concentrations of D-Sorbitol and OH- keeping other conditions constant. The rate constants

were found to increase with increase in temperature. The rate constant (k1) of the slow step of

the uncatalysed reaction was obtained from the slopes and intercept of plots of 1/kU versus

1/[D-Sorb] and 1/kU versus 1/[OH-] at four different temperatures and were used to calculate

the activation parameters. The energy of activation corresponding to these constants was

evaluated from Arrhenius plot of log k1 versus 1/T (r≥0.998, S ≤0.004) and other activation

parameters obtained are tabulated in Table 3.

Similarly, the rate constant (k2) of the slow step of catalysed reaction mechanism was

obtained from the intercept of the plots of [Ru(III)]/kC versus 1/[D-Sorb] and [Ru(III)]/kC

versus 1/[OH-] at different temperatures. The values are given in Table 3. The energy of

activation for the rate determining step was obtained by the plot of log k2 versus 1/T

(r≥0.988, S ≤0.007) and other activation parameters calculated for reaction are presented in

Table 3.

Catalytic activity

It has been pointed out by Moelwyn-Hughes (Moelwyn-Hughes, 1961) that in presence of the

catalyst, the uncatalysed and catalysed reactions proceed simultaneously, so that,

kT = kU + Kc [Ru(III)]x

4.0

4.0

4.0

4.0

4.0

4.0

4.0

4.0

4.0

4.0

0.5

1.0

1.5

2.0

2.0

2.0

2.0

2.0

2.0

2.0

2.0

2.0

2.0

2.0

2.0

2.0

2.0

2.0

2.0

2.0

1.5

1.5

1.5

1.5

0.3

0.5

0.8

1.0

1.5

3.0

1.98

2.56

3.16

3.76

0.89

1.46

2.28

2.79

3.76

8.06

6.89

7.56

7.96

8.04

8.04

8.04

8.04

8.04

8.04

8.04

1.91 2.02

2.48 2.71

3.08 3.06

3.67 3.27

0.81 0.81

1.37 1.37

2.19 2.19

2.70 2.70

3.67 3.67

7.93 7.93

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920


Here ‘kT’ is observed pseudo first-order rate constant, ‘kU’ is the pseudo first-order rate

constant for the uncatalysed reaction, kC is for catalyzed ‘Kc’ is the catalytic constant and ‘x’

the order of the reaction with respect to [Ru(III)]. In the present investigation, the x value for

standard run was found to be unity. Then, the value of Kc was calculated by using the

equation,

Kc = kT - kU

[Ru(III)]x=

kC

[Ru(III)]xwhere kT - kU = kC

The values of Kc were evaluated at different temperatures and found to vary at different

temperatures. Further, the plot of log KC versus 1/T was linear with value of energy of

activation and other activation parameters with reference to catalyst were computed, and are

summarized in Table 4.

Table 3: Activation parameters and thermodynamic quantities for the oxidation of D-

Sorb by periodate in aqueous alkaline medium with respect to the slow step of Scheme 1

and Scheme 2 (uncatalyzed and catalyzed): (A) effect of temperature; (B) activation

parameters; (C) effect of temperature to calculate K1 and K2; and (D) thermodynamic

quantities using K1, K2 and K3.

(A)

Temperature (K)

k1 x 104 (s-1)

Uncatalysed

k2 x 10-3 (dm3 mol-1s-1)

Catalysed

298

303

308

313

(B)

9.38

12.0

15.3

18.8

3.16

3.44

4.01

4.56

Parameters

Uncatalysed

values

Catalysed

Values

Ea (k J mol-1)

ΔH# (k J mol-1)

ΔS# (J K-1 mol-1)

ΔG# (298K) (k J mol-1)

log A

37.8 ± 1.5

35.3 ± 1.2

-184.5 ± 10

90 ± 3

3.6 ± 0.2

20.8 ± 1.5

18.3 ± 0.8

-116.7 ± 6

53.0 ± 2

7.2 ± 0.4

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(C)

Temperature(K) K1x10-1( dm3 mol-1)

Uncatalysed Catalysed

0.86 1.92

2.51 2.36

4.61 3.93

7.28 5.22

K2x10-1( dm3 mol-1)

Uncatalysed

K3x10-1(dm3 mol-1)

Catalysed

298

303

308

313

(D)

4.71

1.92

1.17

0.96

3.35

2.64

1.74

1.26

Thermodynamic

quantities

Values from K1

Uncatalysed Catalysed

96.9 ± 2.5 74.6 ± 2.8

344 ± 8 274 ± 10

-5.6 ± 0.2 7.0 ± 0.3

Values from K2

Uncatalysed

Values from K3

Catalysed

ΔH (k J mol-1)

ΔS (J K-1 mol-1)

ΔG298 (k J mol-1)

-86.4 ± 3.2

-258.8 ± 10

-9.3 ± 0.4

-54.1 ± 2.8

-151.9 ± 3

-8.9 ± 0.4

[Periodate]=4.0 x 10-3, [D-Sorb]= 0.2, [OH-]=0.2, [Ru(III)]=1.5 x 10-5 , I= 0.21 /mol dm-3.

Table 4: Values of catalytic constant (Kc) at different temperatures and activation

parameters calculated using kC values. [IO4-] = 4.0x 10-3; [D-Sorb] = 0.2 ; [OH-] = 0.2 ;

[Ru(III)] = 1.5 x 10-5 ; I = 0.21/mol dm-3;

DISCUSSION

The activity of periodate as an oxidizing agent varies greatly as a function of pH and is

capable of subtle control. In acidic solution it is one of the most powerful oxidizing agents

Temperature (K) Kc x 102 298 303 308 313

1.97 3.58 3.70 3.78

Parameters Values

Ea (kJmol-1)

ΔH# (kJmol-1)

ΔS# (JK-1mol-1)

ΔG# (298K)(kJmol-1)

log A

32.6 ± 1.5

30.2 ± 1.2

-99.6 ± 3.2

59.8 ± 2.1

8.0 ± 0.4

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922


known, whereas in alkaline solution it is slightly less so. However in aqueous alkaline

medium and in the pH ranges employed in the present study, periodate cannot exist as H4IO6-

because in aqueous solution periodate is involved (Crouthamel et al., 1951; Bailar et al.,

1975) in the following equillibria, depending on the pH of the solution.

H5IO6 H4IO6- + H+

H3IO62- + H+

H3IO62- H2IO6

3-+ H+

H4IO6-

(2)

(3)

(4) The species H4IO6

- exists near pH 7.0. Hence under the alkaline conditions employed in the

present system, the main species expected to be trihydrogenparaperiodate(H3IO62-). The

observed fractional order in alkali concentration may be understood in terms of H2IO63- as the

main species in alkaline medium with the following equilibrium, which is also supported by

earlier work (Tuwar et al., 1992).

H3IO62- + OH- H2IO6

3- + H2O (5)K1

Mechanism for uncatalysed reaction

The reaction between periodate and D-Sorbitol in alkaline medium presents a 1:1

stoichiometry of oxidant to reductant. Since the reaction was first order dependence in [IO-4]

and an apparent order of less than unit order in [D-sorb] and [OH-]. No effect of added

products was observed. Based on the experimental results, a mechanism is proposed for

which all the observed orders in each constituent such as [IO-4], [D-sorb] and [OH-] may be

well accommodated (Scheme 1). The less than unit order in [D-sorb] presumably results from

formation of a complex (C1) between periodate species and D-Sorb prior to the formation of

the products. This complex (C1) decomposes in a slow step to give a free radical of D-

Sorbitol and intermediate I(VI) species. Further this free radical species reacts with I(VI)

species in a fast step to give the products Such type of free radical is observed in literature in

the oxidation of D-Sorbitol (SenGupta et al., 1983). Free radical experiments also revealed

such a possibility. All these reactions can be accommodated in Scheme 1.

The direct plot of kU versus [D-Sorb] was drawn to know the parallel reaction if any along

with interaction of oxidant and reductant. However the plot of kU versus [D-Sorb] was not

linear. Thus, in Scheme 1, the parallel reaction and involvement of two molecules of D-

Sorbitol in the complex are excluded.

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923


H 2I O 63 -

+ H 2O

CC

C

CC H 2O H

C H 2O H

O HH

HH O

O HH

O HH

H 3 IO 62 - O H -+

+ H 2 IO 63 -

K 1

K 2 C o m p le x (C 1)

H 2 IO 64 -

+ CC

C

CC H 2 O H

C H 2 OO HH

HH O

O HH

O HH.

+ H +

CC

C

CC H 2 O H

C H 2OO HH

HH O

O HH

O HH

.

+ H 2IO 64 - C

C

C

CC H 2 O H

C H OO HH

HH O

O HH

O HH

+ H 2 IO 65 -

+ H +f a s t

k 1

s lo w I(V I)

I (V )

C o m p le x (C 1)

Scheme 1.Detailed Scheme for oxidation of D-Sorbitol by alkaline periodate.

The Michaelis–Menten plot provides the complex formation between periodate and D-sorb,

which explains less than unit order dependence on [D-Sorb]. Scheme 1 leads to the rate law

(Eq. 6)

k U =R ate

[H 3IO 62 -]

= [O H -]T[so rb ]Tk 1 K 1 K 2

1 + K 1[O H -] + K 1K 2 [so rb ][O H -] (6 )

which explains all the observed kinetic orders of different species.

The rate law (6) can be arranged in the following form, which is suitable for verification.

1kU

=1

[sorb]k1 K1 K2+

1

[sorb]k1+

1

k1 K2[OH -] (7)

According to equation (7), other conditions being constant, plot of 1/kU versus 1/ [D-Sorb]

( r≥0.990, S≤0.001),1/kU versus 1/[OH-] (r≥0.999, S≤0.001) should be linear with an

intercept supporting the D-Sorb complex, which is verified in Fig 4. From the intercepts and

slopes of such plots lead to the values of K1, K2 and k1 were calculated as 8.60 dm3 mol-

1,47.17 dm3 mol-1 and 9.38 x10-4 s-1 respectively. These reaction constants are in good

agreement with the earlier work (Koli and Nandibewoor, 2009).

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

due to the presence of a neutral species as shown in Scheme 1. A high negative value of

ΔS# (-184.5 J K-1 mol-1) suggests that intermediate complex (C1) is more ordered than the

reactants (Weissberger, 1974). These constants were used to calculate the rate constants and

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924


compared with the experimental values and found to be reasonable agreement with each other

given in Table 1, which fortities Scheme 1.

The thermodynamic quantities for the different equilibrium steps in Scheme 1 can be

evaluated as follows. The [D-Sorbitol] and [OH-] concentrations as in Table 1 were varied at

different temperatures. From the slopes and intercepts, the values of K1 and K2 were

calculated at different temperatures and these values are given in Table 3. The van’t Hoff’s

plots were made for the variation of K1 and K2 with temperature [i.e. log K1 versus 1/T

(r≥0.948, S≤ 0.018)] and [log K2 versus 1/T (r≥0.934, S≤0.017)] and the values of the

enthalpy of reaction ΔH, entropy of reaction ΔS and free energy of reaction ΔG were

calculated for first and second equilibrium steps. These values are given in Table 3. A

comparison of the ΔH value of first step (96.9 k J mol-1) with ΔH# value from Scheme 1(35.3

k J 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 the rate determining step is

fairly slow and involves high activation energy (Rangappa et al., 1998; Bilehal et al., 2001).

In the same manner, K2 values were calculated at different temperatures and the

corresponding values of thermodynamic quantities are given in Table 3.

(A)

Figure. 4. Verification of rate law (Eq. 7) for the uncatalyzed oxidation of D- Sorbitol by periodate. Plots of (A) 1/kU versus1/[D-Sorb], (B) 1/kU versus [OH−] at four different temperatures (conditions as given in Table 1).

1/ [D-Sorb] (dm3 mol-1)

1/k U

X 1

03 (s)

298 K

303 K

308 K

313 K

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925


(B)

Mechanism for Ru(III) catalysed reaction

Ruthenium(III) chloride acts as an efficient catalyst in many redox reactions, particularly in

an alkaline medium (Jagadeesh and Puttaswamy, 2008). It is interesting to identify the

probable ruthenium(III) chloride species in alkaline media. In the present study it is quite

probable that the [Ru(H2O)5OH]2+ species might assume the general form [Ru(III)(OH)x]3–x.

The x value would always be less than six because there are no definite reports of any

hexahydroxy ruthenium species. The remainder of the coordination sphere would be filled by

water molecules. At higher pH, the electronic spectra studies have confirmed (Connick and

Fine, 1960) that the ruthenium(III) chloride exists in the hydrated form as [Ru(H2O)6]3+.

Metal ions of the form [Ru(H2O)6]3+ are also known to exist as [Ru(H2O)5OH]2+ in an

alkaline medium and are most likely mononuclear species. Hence, under the conditions

employed (e.g., [OH−] >> [Ru(III)]) ruthenium(III) is mostly present as the hydroxylated

species, [Ru(H2O)5OH]2+. Similar species have been reported between Ru(III) catalyzed

oxidation of several other substrates with various oxidants in OH− medium (Cotton et al.,

1999) In earlier reports of Ru(III) catalyzed oxidation, it has been observed that (Sandu et al.,

1983) there is a fractional order dependence with respect to [substrate] and [Ru(III)] and unit

order with respect to [oxidant], Ru(III) forms a complex with the substrate. It gets oxidized

by the oxidant to form Ru(IV)-substrate complex followed by the rapid redox decomposition

with regeneration of Ru(III). In another case (Panda and Sahu, 1989), if the process shows a

zeroth order dependence with respect to [oxidant], first order with respect to [Ru(III)] and a

fractional order with respect to [substrate], there involves the formation of a Ru(III)-substrate

complex. It undergoes further cleavage in a concerted manner giving rise to a Ru(I) species,

298 K

303 K

308 K

1/ [OH-] (dm3 mol-1)

1/k U

X 1

03 ( s)

313 K

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926


which is rapidly oxidized by the oxidant to regenerate the catalyst. In some other reports

(Tegginamath et al., 2007), it is observed that Ru(III) forms a complex with substrate and is

oxidized by the oxidant with the regeneration of the catalyst. Hence, the study of behavior of

Ru(III) in catalyzed reaction becomes significant. The equilibrium step 1 and the

stoichiometry were same as in the case of uncatalyzed reaction. In the Ru(III) catalyzed

reaction, [Periodate] was first order dependence, an apparent order of less than unit order in

[D-Sorb] and [OH-] and order with respect to Ru(III) was found to be unity. No effect of

added products was observed. Based on the experimental results, a mechanism is proposed in

Scheme 2 for which all the observed orders in each constituent such as [Periodate], D-Sorb],

[OH-] and [Ru(III)] may be well accommodated.

In the prior equilibrium step 1, in the view of the relative less than unit order in OH-

concentration, the main oxidant species is likely to be [H2IO63-] and its formation by the

above equilibrium is important in the present study. The less than unit order in [D-Sorb]

presumably results from formation of a complex (C2) between the Ru(III) species and D-

sorbitol. This complex (C2) reacts with one mole of periodate in a slow step to give the free

radical species of D-Sorb,I (VI), with the regeneration of catalyst, Ru(III). Further this free

radical of D-Sorb reacts with I (VI) species in a fast step to form the final product such as D-

glucose and I (V) as shown in Scheme 2. The reduction of Ru(III) to lower oxidation state

and then regeneration of Ru(III) by oxidant was not possible under the experimental

conditions due to observed orders in different constituents of reaction.

H2IO63-

+ H2O

CC

C

CCH2OH

CH2OH

OHH

HHO

OHH

OHH

H3IO62- OH-+

K1

K3 Complex (C2)

Complex(C2)H2IO6

4-+ C

C

C

CCH2OH

CH2OOHH

HHO

OHH

OHH

.

+

+ H2IO64- C

C

C

CCH2OH

CHOOHH

HHO

OHH

OHH

+ H2IO65- + H+fast

k2

slow I (VI)

[Ru(H2O)5OH]2+

+ H+

I (V)

+

[Ru(H2O)5OH]2+

CC

C

CCH2OH

CH2OOHHHHO

OHH

OHH

.

Scheme 2. Detailed Scheme for Ru(III) catalysed oxidation of D-Sorbitol by alkaline periodate.

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927


The Michaelis-Menten plot proved the complex formation between catalyst and substrate,

which explains less than unit order in [D-Sorb]. Such a complex between a catalyst and

substrate has also been observed in other studies(Cotton et al., 1999). The rate law (Eq. 8) for

the scheme 2 could be derived as

Rate[H 3IO 6

2-]=

k2K 1K 3[sorb]T [OH -]T [Ru(III)]T

1+ K 3[sorb] + K 1[OH -] + K1K 3[sorb][OH -] (8)

This explains all the observed kinetic orders of different species. The rate law (Eq. 8) can

be rearranged to be (Eq. 9), which is suitable for verification.

[Ru(III)]

kC

=1

k2K1K3[sorb]T [OH-]T

1

k2K1[OH-]T

1

k2K3[sorb]T

+1

k2

(9)+ +

According to (Eq.9), other conditions being constant, the plots of [Ru(III)]/kC versus [OH−]

( r≥0.996, S≤ 0.007) and [Ru(III)]/kC versus 1/[D-Sorb] (r ≥0.995, S ≤0.007), should be linear

and found to be so as in Figure 5. From the intercepts and slopes of such plots, the reaction

constants K1, K3, and k2 were calculated as (19.26 dm3mol-1), (33.53 dm3mol-1) and (3.16

x103 dm3mol−1s−1) respectively. These constants were used to calculate the rate constants and

compared with the experimental kC values and found to be in reasonable agreement with each

other, which fortifies the Scheme 2.

The thermodynamic quantities for the different equilibrium steps in Scheme 2 can be

evaluated as follows. The [D-Sorbitol] and [OH-] concentrations as in Table 2 were varied at

different temperatures. From the slopes and intercepts, the values of K1 and K3 were

calculated at different temperatures. A van ’t Hoff’s plot was made for the variation of K1

and K3 with temperature [i.e. log K1 versus 1/T (r ≥0.980, S ≤0.024) ; log K3 versus 1/T (r

≥0.989, S≤ 0.018)] and the values of the enthalpy of reaction ΔH, entropy of reaction ΔS and

free energy of reaction ΔG were calculated. These values are also given in Table 3. A

comparison of the ΔH value of first step (74.6 k J mol-1) with ΔH# value from Scheme 2 (18.3

k J 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 the rate determining step is

fairly slow and involves high activation energy (Rangappa et al., 1998; Bilehal et al., 2001).

In the same manner, K3 values were calculated at different temperatures and the

corresponding values of thermodynamic quantities are given in Table 3.

The negligible effect of ionic strength might be due to the presence of various ions and the

negligible effect of dielectric constant in both uncatalysed and catalysed reactions (Schemes

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928


1 and 2). The observed higher rate constant for the slow step indicates that the oxidation

presumably occurs via an innersphere mechanism. This conclusion is supported by earlier

observation (Martinez et al., 1996; Farokhi and Nandibewoor, 2003). The activation

parameters evaluated for the uncatalysed and catalysed reactions explain the catalytic effect

on the reaction. The catalyst Ru(III) forms the complex (C2) with substrate which enhances

the reducing property of the substrate than that without catalyst. Further, the catalyst, Ru(III)

modifies the reaction path by lowering the energy of activation. In the same manner, K1 and

K3 values were calculated at different temperatures and the corresponding values of

thermodynamic quantities are also given in Table 3.

Figure. 5. Verification of rate law ( Eq.8) for the Ru(III) catalyzed oxidation of D-

sorbitol by periodate. Plots of (A) [Ru(III)]/kC versus 1/[D-Sorb] and (B) [Ru(III)/kC

versus [OH−] at four different temperatures (conditions as given in Table 2).

(A)

(B)

1/ [D-Sorb] (dm3 mol-1)

[Ru(

III)]/

k C X

104 (m

ol d

m-3

s)

298 K

303 K

308 K

313 K

298 K

303 K

308 K

313 K

1/ [OH-] (dm3 mol-1)

[Ru(

III)]/

k C X

104

(mol

dm

-3 s)

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929


Appendix

Derivation of rate law for uncatalyzed reaction

According to Scheme 1:

The rate law for ruthenium (III) catalysed reaction was derived similarly.

CONCLUSION

A comparative study of uncatalyzed and ruthenium(III) catalyzed oxidation of D-sorbitol by

periodate was studied. Oxidation products were identified and found to be same for both

cases. Among the various species of periodate in alkaline medium, [H2IO63-] is considered as

active species for the title reaction. Active species of Ru(III) is found to be [Ru(H2O)5OH]2+.

The catalyzed reaction was about hundred fold faster than uncatalyzed reaction. It becomes

apparent that the role of pH in the reaction medium is crucial. Thermodynamic activation

parameters of individual steps in the mechanisms were evaluated for uncatalyzed and Ru(III)

catalyzed reactions at different temperatures, respectively. The activation parameters with

R ate =d[H 2IO 6

3-]d t

K 2 =

= k1[C 1] [A .1]

[C 1]

[sorb] [H 2IO 63-]

[C 1] = K 2 [sorb ] [H 2IO 63-] [A .2]

K 1 =[H 2IO 6

3 -]

[H 3IO 62-] [O H -]

[H 2 IO 63 -] [H 3IO 6

2-] [O H -]=

[C 1] = K 1 K 2 [H 3IO 62-] [O H -][sorb]

R ate = K 1 K 2 [H 3IO 62-] [O H -][so rb]k 1 [A .3]

[sorb]T = [sorb]f + C 1

w here T and f refer to to tal and free concentra tions.

[sorb]T= [sorb]f + K 1K 2 [sorb] f [H 3 IO 62-][O H -]

= [sorb]f (1+ K 1K 2[H 3IO 62-][O H -])

[sorb]f =

K 1

_

T he to tal concentra tion of [D -So rb]T is given b y ,

[sorb]T

1 +K 1 K 2 [H 3IO 62-] [O H -]

[A .4]

[H 3IO 63-]T = [H 3IO 6

2 -]f + [H 2 IO 63 -]f + C 1

= [H 3IO 62 -]f + K 1[H 3 IO 6

2 -]f [O H -] +

K 1K 2[sorb ][O H -][H 3IO 62-] f

[H 3 IO 62-]f =

(1+ K 1[O H -]+ K 1K 2[sorb][O H -])

[H 3IO 63-]T

Sim ilarly ,

Su bstitu ting equ ation s (A .4),(A .5) an d (A .6) in equ ation (A .3) w e get,

R ate = [H 3IO 6

2-]T [O H -]T[so rb]Tk1 K 1 K 2

1 + K 1 [O H -] + K 1K 2[so rb][O H -]

kU =R ate

[H 3IO 62-]

= [O H -]T[so rb]Tk1 K 1 K 2

1 + K 1 [O H -] + K 1K 2[so rb][O H -]

= [H 3 IO 62 -]f(1+K 1 [O H -]+K 1K 2 [sorb][O H -])

[A .7]

[A .6]

Sim ilarly , the con cen tration O H - is

[O H -] f = [O H -]T [A .5]

In v iew of low concentra tion of [H 3 IO 62 -] u sed ,

the second term in den om inator is neglec ted .

[sorb ]f = [sorb ]T

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930


reference to catalyst were also computed. The description of the mechanisms was consistent

with all the experimental evidences including kinetic, spectral and product studies.

ACKNOWLEDGEMENT

One of the authors (PAM) thanks Department of Science and Technology, New Delhi for the

award of INSPIRE fellowship.

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