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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.
stnandibewoor@yahoo.com,
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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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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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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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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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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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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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Nandibewoor et al. World Journal of Pharmaceutical Research
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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Nandibewoor et al. World Journal of Pharmaceutical Research
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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Nandibewoor et al. World Journal of Pharmaceutical Research
(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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Nandibewoor et al. World Journal of Pharmaceutical Research
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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Nandibewoor et al. World Journal of Pharmaceutical Research
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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Nandibewoor et al. World Journal of Pharmaceutical Research
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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Nandibewoor et al. World Journal of Pharmaceutical Research
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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Nandibewoor et al. World Journal of Pharmaceutical Research
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.
REFERENCES
1. Connick RE, Fine DA. J. Am. Chem. Soc. 1960; 82: 4187- 4191.
2. Cotton FA, and Wilkinson G. Advanced Inorganic Chemistry, A Comperhensive Text 5th
edn, Wiley Interscience Publication New York 1988; 569.
3. Cotton FA, Wilkinson G, Murillo CA, Bochmann M. Advanced Inorganic Chemistry, 6th
edn. Wiley, New York 1999; 1012.
4. Crouthamel CE, Hayes AM, Martin DS. J. Am. Chem.Soc. 1951; 73: 82-87: Bailar Jr
JC, Emeleus HJ, Nyholm SR, Trotman-Dikenson AF. Comprehensive Inorganic
Chemistry, Pergamon Press. Oxford 1975; 2: 1456.
5. DasAK. Coord. Chem. Rev. 2001;213: 307-325.
6. Fiegl F. Spot Tests in Organic Analysis, Elsevier Scientific Publishing Company, New
York 1975; 195.
7. Jagadeesh RV, Puttaswamy. J. Phys. Org. Chem. 2008; 21: 844-858.
8. Kamble DL, Chougale RB, Nandibewoor ST. Indian J.Chem. 1996; 35A, 865-869.
9. Koli BI, Nandibewoor ST. Indian J. Chem. 2009; 48(A): 958-963.
10. Lide DR.(Ed.),CRC Hand Book of Chemistry and Physics 73rd edn. CRC press; London
1992; 8.
11. Martinez M, Pitarque MA, Eldik RV. J. Chem. Soc. Dalton Trans. 1996; 2665-2671:
Farokhi SA, Nandibewoor ST. Tetrahedron, 2003; 59: 7595-7602.
12. Moelwyn-Hughes EA. Kinetics of Reactions in Solutions, physical chemistry, Pergamon
Press, New York, 2nd edn. 1961; 297-299.
13. Nelson DL and Cox MM. Lehninger, Principles of Biochemistry, 4th edn, Freeman WH
and Compony Publication 2004; 238-272.
14. Odebunmi EO, Marufu H. Nijerian J.Sci. 1999; 33: 133-143.
15. Panda HP, Sahu BD. Indian J. Chem. 1989, 28A: 323–324.
16. Panigrahi GP, Misro PK. Indian J. Chem. 1977; 15A: 1066-1069.
www.wjpr.net
931
Nandibewoor et al. World Journal of Pharmaceutical Research
17. Rangappa KS, Raghavendra MP, Mahadevappa DS, Channegouda D. J. Org. Chem.,
1998; 63: 531-536 : Bilehal DC, Kulkarni RM, Nandibewoor ST. Can. J. Chem. 2001; 79:
1926-1933.
18. Reddy CS, Vijaykumar T. Indian J.Chem. 1995; 34A: 615-620.
19. Sandu S, Sethuram B, Rao TN. J. Indian Chem. Soc. 1983; 60: 198–200.
20. SenGupta KK, SenGupta S, Chatterjee U, Tarafdar A, Samanta T. Carbohydrate Research
1983; 117: 81-87.
21. Singh AK, Srivastava S, Srivastava J, Srivastava R, Singh PJ. J Mol Catal. A: Chem.
2007; 278: 72–81.
22. Swarnalaxmi N, Uma V, Sethuram B, Navaneeth Rao T. Indian J. Chem. 1987; 26A: 592-
595.
23. Tegginamath V, Hiremath CV, Nandibewoor ST. J. Phys. Org. Chem. 2007; 20: 55–64.
24. Tuwar SM, Nandibewoor ST, Raju JR. J. Indian Chem.Soc. 1992; 69: 651-653.
25. Weissberger A. Techniques of Chemistry ed. Lewis ES. Investigation of Rates and
Mechanism of Reactions in Techniques of Chemistry, Wiley, New York 1974; 6: 421-
427.
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