Kinetics and mechanism of uncatalysed and ruthenium(III)-catalysed oxidation of D-panthenol by alkaline permanganate

Article abstract of DOI:10.1007/s11243-009-9319-4

The oxidation of D-panthenol by MnO₄-was studied in the absence and in the presence of ruthenium(III) catalyst in alkaline medium at 298 K and at constant ionic strength of 0.50 mol dm^-3 by spectrophotometry. The stoi-chiometry in both the cases was [panthenol]: [MnO₄^-] = 1:4. The oxidation products were identified by IR and GC-MS. The reaction was first-order with respect to both MnO₄^- and ruthenium(III), while the orders with respect to both panthenol and alkali varied from first order to zero order as the concentrations increased. The effects of added products, ionic strength and dielectric constant were studied. The reaction constants, activation parameters and thermodynamic quantities were calculated for both the uncatalysed and catalysed reactions. Springer Science+Business Media B.V. 2009.

Full text of DOI:10.1007/s11243-009-9319-4

Transition Met Chem (2010) 35:237–246
DOI 10.1007/s11243-009-9319-4
Kinetics and mechanism of uncatalysed and ruthenium(III)-
catalysed oxidation of D-panthenol by alkaline permanganate
•
Rajeshwari V. Hosahalli Anita P. Savanur
•
•
Sharanappa T. Nandibewoor
Shivamurti A. Chimatadar
Received: 15 October 2009 / Accepted: 25 November 2009 / Published online: 19 December 2009
Ó Springer Science+Business Media B.V. 2009
-
Abstract The oxidation of D-panthenol by MnO4 was
studied in the absence and in the presence of ruthenium(III)
catalyst in alkaline medium at 298 K and at constant ionic
strength of 0.50 mol dm-3 by spectrophotometry. The stoi-
chiometry in both the cases was [panthenol]: [MnO4-] =
1:4. The oxidation products were identified by IR and GC–
MS. The reaction was first-order with respect to both
MnO4- and ruthenium(III), while the orders with respect to
both panthenol and alkali varied from first order to zero
order as the concentrations increased. The effects of added
products, ionic strength and dielectric constant were stud-
ied. The reaction constants, activation parameters and
thermodynamic quantities were calculated for both the
uncatalysed and catalysed reactions.
acids in both aqueous [2] and nonaqueous media [3]. The
mechanism by which multivalent permanganate oxidizes a
substrate depends not only on the substrate but also on the
medium [4]. In strongly alkaline medium, the stable reduction
-
2-
2-
product of MnO4 is MnO4 [4]. However, the MnO4
-
appears only after the complete consumption of MnO4
.
The use of transition metal ions such as osmium, ruthe-
nium and iridium, either alone or as binary mixtures, as
catalysts in various redox processes has attracted consider-
able interest [5]. Ruthenium(III) acts as a catalyst in the
oxidation of many organic and inorganic substrates [6]. We
have observed that small amounts of ruthenium(III) catalyse
the oxidation of panthenol by alkaline permanganate. In
view of the potential pharmaceutical importance of panthe-
nol and the dearth of literature on the oxidation of this drug
[7], we have carried out a detailed study of the title reaction.
Introduction
D-panthenol (2,4-dihydroxy–N-(3-hydroxy-propyl)-3,3-
dimethyl–(R)-Butanamide) is an analogue of the pantothenic
acid (vitamin B5) and is thus a provitamin B5. It finds a
number ofapplications in pharmaceuticals, mainly for healing
wounds and treating inflammation. Permanganate ion oxi-
dizes a great variety of substrates and finds extensive appli-
cation in organic synthesis [1], especially after the advent of
phase transfer catalysis [2] which permits the use of solvents
such as ethylene chloride and benzene. Kinetic studies con-
stitute an important source of mechanistic information on
such reactions, as demonstrated by studies on unsaturated
Experimental
All chemicals were of analytical reagent grade, and double
distilled water was used throughout the work. An aqueous
solution of panthenol (AR; Scientific Research Laborato-
ries) was prepared and standardized against potassium
biphthalate solution [8]. Potassium permanganate solution
was standardized against oxalic acid [9]. Potassium man-
ganate solution was prepared as described by Carrington
and Symons [10]. Ruthenium(III) stock solution was made
by dissolving a known weight of RuCl3 (S.D fine chem) in
0.20 mol dm-3 HCl. Mercury was added to reduce any
ruthenium(IV), and the ruthenium(III) stock solution was
kept for one day. The ruthenium(III) concentration was
assayed by EDTA titration [11]. NaOH and NaClO4 (both
CDH, Analar) were employed to maintain the required
alkalinity and ionic strength, respectively.
R. V. Hosahalli ꢀ A. P. Savanur ꢀ S. T. Nandibewoor ꢀ
S. A. Chimatadar (&)
Department of Studies in Chemistry, Karnatak University,
Pavate Nagar, Dharwad 580003, India
e-mail: schimatadar@gmail.com
123

238
Transition Met Chem (2010) 35:237–246
Table 1 Effect of variation of MnVII, panthenol and OH- concen-
trations on the oxidation of panthenol by alkaline permanganate at
25 °C and I = 0.50 mol dm-3
Kinetic studies
The kinetic measurements were performed on a Varian
Cary 50 Bio UV–Vis spectrophotometer. The kinetics were
followed under pseudo first-order conditions where [pan-
thenol] [ [MnO4-] in both catalysed and uncatalysed
reactions at 25.0 0.1 °C. In the absence of catalyst, the
[MnVII] 9 104
[panthenol] 9 103
[OH-] 9 102
ku 9 103
(s-1
(mol dm-3
)
(mol dm-3
)
(mol dm-3
)
)
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
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
5.0
5.0
1.12
1.10
1.09
1.21
1.11
1.15
0.08
0.34
0.52
0.62
1.09
1.46
1.74
1.97
1.96
1.99
1.98
0.04
0.09
0.24
0.40
0.84
1.09
1.23
1.35
1.38
1.39
1.37
2.0
-
reaction was initiated by adding the MnO4 to panthenol
2.0
5.0
solution which also contained the required concentrations
of NaOH and NaClO4. The reaction in the presence of
-
catalyst was initiated by adding MnO4 to panthenol
2.0
5.0
2.0
5.0
2.0
5.0
solution which also contained the required concentrations
of NaOH, NaClO4 and ruthenium(III). The progress of the
reaction in both cases was followed by monitoring the
decrease in the absorbance of MnO4- at 526 nm in a 1-cm
cell placed in a thermostatted compartment of a Varian
Cary 50 Bio UV–Vis spectrophotometer. The applicability
of Beers law was verified at 526 nm under the reaction
conditions (e = 2,083 50 dm3 mol-1 cm-1). It was
verified that there was negligible interference from other
species present in the reaction mixture at this wavelength.
The reactions were followed for more than three half-
lives. It was observed that, in the absence of ruthenium(III)
0.25
0.50
0.75
1.00
2.00
3.00
4.00
5.00
6.00
7.00
8.00
2.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
-
0.25
0.50
0.75
1.00
3.00
5.00
7.00
9.00
10.0
20.0
30.0
catalyst, the oxidation of panthenol by MnO4 occurs in
2.0
measurable quantities. Hence, during calculation of the
pseudo first-order rate constants kc, the uncatalysed rate ku
has also taken into account. To allow for this in the cata-
lysed kinetic runs, a parallel kinetic runs under similar
conditions in the absence of catalyst were also carried out.
The pseudo first-order rate constants (ku or kc) in both the
cases were determined from the plots of log(At - A?)
versus time, where At refers to absorbance at time t and A?
at infinite time, which excludes the absorbance of any
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2-
MnO4 during the reaction. The plots were linear up to
2.0
75% completion of the reaction in both cases (see Fig. 1
for uncatalysed reaction). Thus, the total rate constant (kt)
is equal to the sum of the rate constants in the presence (kc)
and in the absence of (ku) of catalyst. The rate constants
were reproducible to within 5% and are the average of at
least three independent kinetic runs (Tables 1 and 2).
1.6
1
1.2
2
Results
3
4
5
0.8
0.4
0.0
Stoichiometry and product analysis
6
Different concentrations of reactants in 5.0 9 102 mol dm-3
OH- and constant ionic strength I = 0.50 mol dm-3 for
both uncatalysed and catalysed reactions were kept in closed
containers under nitrogen atmosphere at 25 °C. After 3 h, the
manganese(VII) concentration was assayed spectrophotomet-
rically by measuring the absorbance at 526 nm. Under the
conditions where [panthenol] [[MnO4-], the unreacted
panthenol was estimated using potassium biphthalate
0.0
1.0
2.0
3.0
4.0
5.0
6.0
Time (min)
Fig. 1 First-order plots of oxidation of panthenol by alkaline perman-
ganate at 25 °C [panthenol] = 2.0 9 10-3; [OH-] = 5.0 9 10-2
;
I = 0.50 mol dm-3, [MnVII] 9 104mol dm-3 = (1) 0.5, (2) 1.0, (3)
2.0, (4) 3.0, (5)4.0, (6) 5.0
123

Transition Met Chem (2010) 35:237–246
239
Table 2 Effect of variation of MnVII, panthenol, OH- and RuIII concentrations on the ruthenium(III)-catalysed oxidation of panthenol by
alkaline permanganate at 25 °C and I = 0.50 mol dm-3
[MnVII] 9 104
[panthenol] 9 103
[OH-] 9 102
[RuIII] 9 106
kt 9 103
(s-1
ku 9 103
(s-1
kc 9 103
(s-1
(mol dm-3
)
(mol dm-3
)
(mol dm-3
)
(mol dm-3
)
)
)
)
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
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
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
1.0
3.0
5.0
7.0
9.0
10.0
5.10
5.01
5.10
5.12
5.14
5.01
0.70
1.85
2.44
3.21
5.10
6.35
7.25
7.91
7.91
7.95
7.96
0.30
0.64
1.12
1.65
3.79
5.10
5.97
6.63
6.90
6.99
7.07
1.90
3.45
5.10
6.74
8.47
9.30
1.12
1.10
1.09
1.21
1.11
1.15
0.08
0.34
0.52
0.62
1.09
1.46
1.74
1.97
1.96
1.99
2.01
0.04
0.09
0.24
0.40
0.84
1.09
1.23
1.35
1.38
1.38
1.39
1.09
1.09
1.09
1.09
1.09
1.09
4.12
4.10
4.00
4.21
4.32
4.02
0.62
1.52
1.92
2.58
4.00
4.88
5.51
5.94
5.95
5.96
5.95
0.26
0.55
0.88
1.25
2.95
4.00
4.73
5.27
5.51
5.61
5.68
0.81
2.36
4.00
5.65
7.38
8.21
2.0
2.0
5.0
2.0
5.0
2.0
5.0
2.0
5.0
0.25
0.50
0.75
1.00
2.00
3.00
4.00
5.00
6.00
7.00
8.00
2.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
0.25
0.50
0.75
1.00
3.00
5.00
7.00
9.00
10.0
20.0
30.0
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
5.0
2.0
5.0
2.0
5.0
2.0
5.0
2.0
5.0
solution [8]. The results indicated that one mole of panthenol
reacted with four moles of manganese(VII) for both the un-
catalysed and catalysed reactions as given in Scheme 1.
The oxidation products in both the cases were isolated
using TLC. The stoichiometric ratio indicates that the main
products are manganese(VI), [(3-hydroxy-propyl)amino]
(oxo)acetic acid (A) and 2-methyl propane,1,2-diol (B).
The organic products were eluted using ethyl acetate and
submitted to spot tests [12] and chromatographic analysis.
Compound A was confirmed by its IR spectrum which
showed bands at 3,418, 1,746 and 1,642 cm-1 for OH
stretching, C=O stretching and amide stretching,
respectively. The presence of A was also confirmed by
GC–MS analysis, obtained on a Shimadzu 17A gas chro-
matograph with a Shimadzu XP- 5000A mass spectrometer
using EI ionization technique. The mass spectrum showed
the molecular ion peak at 145 amu consistent with loss of
two hydrogens from the molecular ion of 147 amu. All
other peaks observed in the GC–MS can be interpreted in
accordance with the structure of A. Another product B was
identified by GC–MS spectrum which showed the molec-
ular ion peak at 71 amu after a loss of one hydrogen from
its molecular ion of 72 amu.
123

240
Transition Met Chem (2010) 35:237–246
Scheme 1 Stoichiometric
equations for uncatalysed and
ruthenium(III) catalysed
oxidation of panthenol by
alkaline permanganate
O
O
OH
(B)
OH
OH
+
4[MnO4(OH)]2-
N
H
OH
OH
N
H
+
OH
OH
O
(A)
2-
4MnO4
+
+
H2O
Reaction orders
the order with respect to alkali varies from first to zero as
the alkali concentration increases (Tables 1 and 2) for both
the uncatalysed and catalysed reactions. The ruthenium(III)
concentration was varied in the range 1.0 9 10-6 to
1.0 9 10-5 mol dm-3 (Table 2). The order with respect to
ruthenium(III) concentration was found to be unity.
According to the experimental results, the derived rate
laws for the uncatalysed and catalysed reaction are as
follows.
The reaction orders in case of both uncatalysed and cata-
lysed reactions were determined from the slopes of the log
k versus log(concentration) plots by varying the concen-
trations of substrate, alkali and catalyst, in turn, while
keeping other conditions constant. In this way the order
-
with respect to MnO4 was found to be unity between
5.0 9 10-5 and 5.0 9 10-4 mol dm-3 (see Tables 1 and 2).
Rate
½MnO4ꢁꢂ
25:42 ½panthenolꢂ½OHꢁꢂ
¼ ku ¼
¼ kc ¼
1 þ 15½OHꢁꢂ þ f5:7 ꢃ 103½panthenolꢂ½OHꢁꢂg
Rate
½MnO4ꢁꢂ
2:78 ꢃ 107½panthenolꢂ½RuIIIꢂ½OHꢁꢂ
1 þ 16:8½OHꢁꢂ þ 420½panthenolꢂ þ 7:05 ꢃ 103½OHꢁꢂ½panthenolꢂ
Effects of dielectric constant and ionic strength
Similarly, the panthenol concentration was varied in the
range 0.25 9 10-3 to 8.0 9 10-3 mol dm-3. The order
with respect to panthenol varies from first to zero as its
concentration increases (Fig. 2). The effect of alkali was
studied in the range of 0.25 9 10-2 to 30.0 9 10-2
mol dm-3 at constant ionic strength of 0.50 mol dm-3 for
the uncatalysed reaction and at constant concentration of
ruthenium(III) for the catalysed reaction. As for panthenol,
The effect of dielectric constant on the rate was studied by
varying the t-butanol-water (v/v) content in the reaction
mixture with all other conditions kept constant in both
uncatalysed and catalysed reactions. The rate constants for
both reactions increase with decreasing dielectric constant.
Plots of log ku versus 1/D (Fig. 3) and log kc versus 1/D
4 + log[panthenol]
1/D x 10 2
0.0
0.4
0.8
1.2
1.6
2.0
2.0
1.6
1.2
0.8
0.4
0.0
4.0
3.0
2.0
1.0
0.0
1.2
1.3
1.4
1.5
1.6
1.7
1.2
0.9
0.6
0.3
0.0
3.0
2.0
1.0
0.0
0
2
4
6
8
0.0
0.5
1.0
1.5
2.0
4 + log[panthenol]
I 1/2
Fig. 2 Order with respect to panthenol concentrations for uncataly-
sed and catalysed reactions
Fig. 3 Effect of variation of dielectric constant (D) and ionic strength
(I) on the oxidation of panthenol by alkaline permanganate at 25 °C
123

Transition Met Chem (2010) 35:237–246
241
1/D x 102
K1
K2
-
-
[MnO4(OH)]2-
C1
MnO4
+
OH
1.2
1.3
1.4
1.5
1.6
1.7
2.0
1.5
1.0
0.5
0.0
1.5
1.2
0.9
0.6
0.3
0.0
[MnO4(OH)]2-
panthenol
+
H
N
k1
OH
O
+
+
OH-
OH
2-
C1
MnO4
+
.
slow
OH
OH
fast
fast
OH
OH
+
OH-
H
N
H
OH
OH
O
O
H
N
[MnO4(OH)]2-
2-
+
+
+
H2O
MnO4
.
0
2
4
I 1/2
6
8
O
OH
H
N
H
N
OH
O
OH
2MnO4
O
fast
2[MnO4(OH)]2-
2-
Fig. 4 Effect of variation of dielectric constant (D) and ionic strength
(I) on the ruthenium(III)-catalysed oxidation of panthenol by alkaline
permanganate at 25 °C
+
+
+
H2O
O
H
O
HO
Scheme 2
permanganate
Mechanism for oxidation of panthenol by alkaline
The effects of temperature on the catalysed reaction
were also studied. The rate constants (k2), of the slow step
of Scheme 3 were obtained from the slopes and intercepts
of the plots of [RuIII]/kc versus 1/[OH-] (Fig. 7) and
[RuIII]/kc versus 1/[panthenol] (Fig. 7) at four different
temperatures. The values are given in Table 4. The energy
of activation was obtained by the least squares method
from the plot of log k2 versus 1/T from which activation
parameters were calculated (Table 4).
(Fig. 4) were linear with positive slope. No reaction of the
solvent with the oxidant occurs under the experimental
conditions employed.
The effect of ionic strength was studied varying the
NaClO4 concentration, from 0.1 to 1.0 mol dm-3. The rate
of reaction increases with the increasing concentration of
NaClO4 for the uncatalysed reaction, whereas for the cat-
alysed reaction, the rate decreases with increasing NaClO4
‘
concentration. Thus, the plot of log ku versus I is linear
‘
with positive slope (Fig. 3) and a plot of log kc versus I is
linear with negative slope (Fig. 4).
Catalytic activity
Since the uncatalysed and catalysed reactions proceed
Test for free radicals
simultaneously, we may write [13]
x
kt ¼ ku þ KC½catalystꢂ
ð1Þ
To test for the participation of free radicals, the reaction
mixture was mixed with acrylonitrile monomer and kept
for 24 h under nitrogen. On diluting with methanol, a white
precipitate of polymer was obtained, indicating the pres-
ence of free radicals. Initially added acrylonitrile decreases
the rate of the reaction, indicating the participation of free
radicals in both the uncatalysed and catalysed reactions.
Here, kt and ku are the observed pseudo first-order rate
constants in the presence and absence of ruthenium(III),
respectively. KC is the catalytic constant and ‘x’ is the order
of the reaction with respect to ruthenium(III). Since in the
present investigation, x = 1. The values of KC at different
temperatures were calculated using Eq. 1, after suitable
arrangement. KC increases with temperature. Plots of log
KC versus 1/T were linear, and the activation parameters
with reference to the catalyst were computed. These results
are summarized in Table 5.
Effect of temperature
The influence of the temperature on the rate was studied
for the uncatalysed reaction at 15, 25, 35 and 45 °C. The
rate constants (k1) of the slow step of Scheme 2 were
obtained from the slopes and intercepts of plots of 1/ku
versus 1/[OH-] (Fig. 6) and 1/ku versus 1/[panthenol]
(Fig. 6) at four different temperatures. The values are
given in Table 3. The energy of activation was obtained by
the least squares method from the plot of log k1 versus 1/T,
from which the activation parameters were calculated
(Table 3).
Discussion
MnO4- is a powerful oxidant in an alkaline medium. As it
exhibits many oxidation states, the stoichiometric results
and pH of the medium are important considerations. Dur-
ing this study, the colour of the solution changed from
violet to blue and then green. The spectrum of the green
123

242
Transition Met Chem (2010) 35:237–246
Table 3 Thermodynamic activation parameter for the oxidation of panthenol by permanganate in alkaline medium with respect to slow step of
Scheme 2
Effect of temperature
Temperature (K)
Activation parameter
Parameter
k1 9 103 (dm3 mol-1 s-1
)
Value
288
298
308
318
1.24
4.43
8.45
12.4
Ea (kJ mol-1
DH# (kJ mol-1
DS# (JK-1 mol-1
)
56.8 2.2
54.3 2.1
)
)
-107
5
3
DG# (kJ mol-1
)
86
Log A
7.6 0.3
Effect of temperature on the first and second equilibrium step of Scheme 2
Temperature (K)
K1 (dm3 mol-1
)
K2 9 10-2 (dm3 mol-1
)
288
298
308
318
17.2 0.8
15.0 0.5
12.6 0.4
10.0 0.1
4.7 0.2
3.8 0.2
2.9 0.09
2.1 0.08
Thermodynamic quantity with respect to K1 and K2
Thermodynamic quantity
DH (kJ mol-1
DS (JK-1 mol-1
DG (kJ mol-1
Value from K1
Value from K2
)
-13.6 0.5
-23.3 1.1
-6.4 0.1
-20.2 0.8
-19.6 0.5
-14.1 0.4
)
)
K3
permanganate to give an alkali-permanganate species
[MnO4.OH]2- in a prior equilibrium step, which is in
accordance with the literature [14] and also the observed
order with respect to OH-. In the next step, [MnO4.OH]2-
combines with panthenol to form an intermediate complex
(C1). The fractional order with respect to panthenol at
intermediate concentrations can be attributed to this com-
plex formation prior to the slow step. Indeed, it is noted
that the plot of 1/ku versus 1/[panthenol] (Fig. 6) shows an
intercept, in agreement with complex formation. Further
evidence for complex formation was obtained from the
UV–Vis spectra of the oxidant, substrate and reaction
mixtures, in which a bathochromic shift from 315 to
322 nm and hyperchromicity at 322 nm was observed at
5.0 °C. The presence of the isobestic point is consistent
with complex formation (Fig. 5).
[Ru(H2O)5OH]2+
H
panthenol
+
C2 + H2O
OH
+
k2
O
N
[MnO4(OH)]2-
OH
2-
+
C2
+
MnO4
OH-
+
.
slow
OH
+ [Ru(H2O)4OH]2+
Scheme 3 Mechanism for ruthenium(III) catalysed oxidation of
panthenol by alkaline permanganate
solution was identical to that of MnO42-. It is probable that
-
the blue colour originated from the violet of MnO4 and
the green from manganate, MnVI, excluding the accumu-
lation of hypomanganate, MnV. The spectral changes dur-
ing the reaction are shown in Fig. 5. It is evident that the
signal from MnO4- decreases at 526 nm, while increases at
603 and 453 nm are due to MnVI. As the reaction proceeds,
a yellow turbidity slowly develops and after keeping for a
long time the solution turns to colourless with a brown
precipitate. This suggests that the initial products might
have undergone further reaction to give MnIV.
The decomposition of complex (C1) is considered to be
the slow step, forming a carbocation and a free radical from
panthenol and MnVI. In a subsequent fast step, the carbo-
cation combines with OH- to form 2-methyl propane-1,2-
diol. In the further fast step, the free radical reacts with
another mole of [MnO4.OH]2- to give an intermediate
product, N-(3-hydroxy-propyl)-2-oxo-acetamide and MnVI.
This aldehyde then reacts with two moles of [MnO4.OH] 2-
to give the final product [(3-hydroxy-propyl)amino](oxo)
acetic acid and MnVI. All the experimental results may be
interpreted in the form of Scheme 2.
Mechanism for the uncatalysed reaction
The reaction presents a 1:4, MnO4-:panthenol stoichiom-
etry with first-order dependence on MnO4-, while the order
with respect to both panthenol and alkali varies from first to
zero. These results imply that first the alkali combines with
123

Transition Met Chem (2010) 35:237–246
243
Table 4 Thermodynamic activation parameter for the ruthenium(III)-catalysed oxidation of panthenol by permanganate in alkaline medium
with respect to slow step of Scheme 3
Effect of temperature
Temperature (K)
Activation parameter
Parameter
k2 9 10-3 (dm3 mol-1 s-1
)
Value
288
298
308
318
2.89
3.94
Ea (kJ mol-1
DH# (kJ mol-1
DS# (JK-1 mol-1
)
37.9 2.0
35.4 2.2
)
7.11
)
-56
3
2
12.63
DG# (kJ mol-1
)
52
Log A
10.2 0.5
Effect of temperature on the first and second equilibrium step of Scheme 3
Temperature (K)
K1 (dm3 mol-1
)
K3 9 10-2 (dm3 mol-1
)
288
298
308
318
18.2 0.8
16.8 0.5
14.5 0.4
11.2 0.1
5.1 0.2
4.2 0.2
3.1 0.09
2.0 0.08
Thermodynamic quantity with respect to K1 and K3
Thermodynamic quantity
DH (kJ mol-1
DS (JK-1 mol-1
DG (kJ mol-1
Value from K1
Value from K3
)
-12.0 0.5
-17.5 1.1
-6.4 0.1
-23.0 0.8
-28.6 0.5
-14.0 0.4
)
)
Table 5 Value of catalytic constant (KC) at different temperatures
and activation parameter with respect to KC
KC 9 10-2
Temperature (K)
288
3.85
8.01
298
308
12.3
318
16.2
Fig. 5 Spectral changes during the oxidation of panthenol by alkaline
MnO4- at 25.0 °C; [MnVII] = 2.0 9 10-4, [panthenol] = 2.0 9 10-3
Activation parameter
,
Ea (kJ mol-1
DH#(kJ mol-1
DS#(JK-1 mol-1
)
36.3
33.8
[OH-] = 0.05 and I = 0.50/mol dm-3 (scanning time interval is
1.0 min)
)
)
-75.5
56.3
DG (kJ mol-1
)
1/[OH -] x 10 -2 dm3 mol-1
Log A
9.27
; ;
[panthenol] = 2.0 9 10-3 [OH-] = 0.05;
0.0
3.0
6.0
9.0
12.0
[MnVII] = 2.0 9 10-4
[Ru III] = 5.0 9 10-6 mol dm-3
6.0
4.5
3.0
1.5
0.0
3.0
2.0
1.0
0.0
For intermediate concentrations, from Scheme 2 the rate
law (2) can be written as
ꢁd½MnO4ꢁꢂ
k1K1K2½MnO4ꢁꢂ ½panthenolꢂ½OHꢁꢂ
¼
dt
1 þ K1½OHꢁꢂ þ fK1K2½panthenolꢂ½OHꢁꢂg
ð2Þ
0.0
0.5
1.0
1.5
2.0
2.5
1/[panthenol] x 10-3 dm3 mol-1
The rate law is consistent with all the observed orders
with respect to different species, which can be verified by
rearranging to Eq. 3.
Fig. 6 Verification of rate law (2) in the form of (3) (Conditions as in
Table 1)
123

244
1
Transition Met Chem (2010) 35:237–246
Mechanism of ruthenium(III)-catalysed reaction
1
1
1
¼
þ
þ
ku k1K1K2½OHꢁꢂ½panthenolꢂ k1K2½panthenolꢂ k1
It is interesting to identify the probable ruthenium(III)
chloride species in alkaline medium. Electronic spectral
studies [19] have confirmed that ruthenium chloride exists
in hydrated form as [Ru(H2O)5OH]2?. In the present
study, it is quite probable that for [Ru(III) (OH)x]3-x, the
value of x would 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 ligands. Hence, under the conditions employed,
i.e. [OH-] [[ [RuIII], ruthenium(III) is likely to be
present as the hydroxylated species, [Ru(H2O)5OH]2?
[20].
ð3Þ
According to Eq. 3, other conditions being constant,
plots of 1/ku versus 1/[panthenol] and 1/ku versus 1/[OH-]
should be linear and are found to be so (Fig. 6). From the
slopes and intercepts of such plots, the reaction constants
K1, K2 and k1 were calculated as (15.0 0.5) dm3 mol-1
,
(3.82 9 102 0.2) dm3 mol-1, and (4.44 0.2) 9 10-3
dm3 mol-1 s-1, respectively. The value of K1 is in good
agreement with that derived in earlier work [15]. The values
of K1 and K2 were calculated at four different temperatures
and are given in Table 3. The van’t Hoff plots were made
for variation of K1 and K2 with temperature. The values of
enthalpy, entropy and free energy of the reaction were
calculated for the first and second equilibrium steps of
Scheme 2. These values are also given in Table 3.
-
The reaction between MnO4 and panthenol in alkaline
medium in the presence of micro amounts of ruthe-
nium(III) is similar to the uncatalysed reaction with respect
to stoichiometry and reaction orders; in addition, the
reaction is first-order with respect to ruthenium(III).
The mechanism is therefore likely to be similar, except for
the participation of the catalyst. Thus we propose that a
hydroxylated species of ruthenium(III) reacts with pan-
thenol to give a complex (C2), which then reacts with
[MnO4.OH]2- in a slow step to form a carbocation, a free
radical derived from panthenol and MnVI. Such complex
formation between substrate and catalyst was also observed
in earlier work [6, 21]. Evidence for this was obtained from
UV–Vis spectra of a panthenol-ruthenium(III) mixtures in
which a bathochromic shift of from 324 to 329 nm and
hypochromicity at 329 nm were observed at 5.0 °C. In the
further fast step, carbocation combines with free OH- to
form 2-methyl propane, 1,2-diol. In the further fast step, free
radical formed reacts with another mole of [MnO4.OH]2- to
give an intermediate product, N-(3-hydroxy-propyl)-2-oxo-
acetamide. In the further fast step, aldehyde reacts with two
moles of [MnO4.OH]2- to give the final product an acid, [(3-
hydroxy-propyl)amino](oxo)acetic acid. All the experi-
mental results may be interpreted in the form of Scheme 3,
with subsequent fast steps as per Scheme 2.
The effect of ionic strength is consistent with the reac-
tion of two ions with like charges, as suggested in
Scheme 2. The general effects of solvent on the rate of the
reaction have been described in the detail in well-known
monographs of Laidler [16] and Amis [17]. For the limiting
case of zero-angle approach between two dipoles or an ion
dipole system, Amis [17] has shown that a plot of log ku
versus 1/D gives a straight line with negative slope for the
interaction between a negative ion and dipole or two
dipoles, while a positive slope results for positive ion and
dipole interaction. In the present study, an increase in the
rate with the decrease in the dielectric constant of the
medium has been observed, which cannot be explained by
Amis theory [17] as the presence of a positive ion is
unlikely in the alkaline medium employed. Perhaps this
effect is countered substantially by increased formation of
active reactant species at low dielectric constant, leading to
a net increase in the rate. The value of DS#, which is within
the range of radical reaction, has been ascribed to the
nature of the electron pairing processes and to the loss of
degrees of freedom formerly available to the reactants upon
the formation of rigid transition state. The observed modest
enthalpy of activation and relatively low value of the
For intermediate concentrations, from Scheme 3 the rate
law (4) can be written as
ꢁd½MnO4ꢁꢂ
k2K1K3½panthenolꢂ½MnO4ꢁꢂ½RuIIIꢂ½OHꢁꢂ
¼
ð4Þ
ð1 þ K3½panthenolꢂÞð1 þ K3½RuIIIꢂÞð1 þ K1½MnO4ꢁꢂÞð1 þ K1½OHꢁꢂÞ
dt
entropy of activation, together with the higher rate constant
of the slow step, indicate that the oxidation occurs via an
inner-sphere mechanism [18].
The terms (1 ? K1[MnO4-]) and (1 ? K3(Ru(III)]) in
the denominator of Eq. 4 approximate to unity in view of
the low concentrations of MnO4- and ruthenium(III) used.
123

Transition Met Chem (2010) 35:237–246
245
1/[OH-] x 10-2dm3 mol-1
may be due to the stabilization of the complex C2 at low
-
dielectric constant, which is less solvated than MnO4 at
0.0
0.3
0.6
0.9
1.2
4.0
3.0
2.0
1.0
0.0
10.0
7.5
5.0
2.5
0.0
higher dielectric constant because of its larger size. The
rate constant for the slow step suggests that oxidation
occurs via an inner-sphere mechanism. This conclusion is
supported by earlier observations [18].
Conclusion
The uncatalysed and ruthenium(III)-catalysed oxidation of
panthenol by MnO4- was studied in alkaline medium. The
0.0
0.5
1.0
1.5
2.0
2.5
-
MnO4 oxidant required a pH [ 12 below this value, the
1/[panthenol] x 10-3dm3 mol -1
reaction proceeds to MnIV which slowly develops a yellow
turbidity. The same oxidation products were identified for
both uncatalysed and catalysed reactions. The oxidant,
Fig. 7 Verification of rate law (5) in the form of (6) (Conditions as in
Table 2)
manganese(VII), exists in alkaline media as [MnO4.OH]2-
,
which takes part in the chemical reaction. The role of
hydroxyl ions is crucial to the chemical reaction. The given
mechanism is consistent with all the experimental results.
The active catalytic species of ruthenium(III) is believed to
Rate
¼ kc ¼ kt ꢁ ku
½MnO4ꢁꢂ
k2K1K3½panthenolꢂ½RuIIIꢂ½OHꢁꢂ
¼
be [Ru(H2O)5OH]2?
.
1 þ K1½OHꢁꢂ þ K3½panthenolꢂ þ K1K3½OHꢁꢂ½panthenolꢂ
ð5Þ
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1
1
þ
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ð6Þ
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