Oxidation of vanillin by diperiodatocuprate(III) in aqueous alkaline medium: A kinetic and mechanistic study by stopped flow technique

Article abstract of DOI:10.1002/kin.20233

The kinetics of oxidation of vanillin (VAN) by diperiodatocuprate(III) (DPC) in alkaline medium at a constant ionic strength of 0.50 mol dm ^-3 was studied spectrophotometrically. The reaction between DPC and vanillin in alkaline medium exhibits 1:2 stoichiometry (vanillin: DPC). The reaction is of first order in [DPC] and has less than unit order in both [VAN] and [alkali]. Intervention of free radicals was observed in the reaction. Increase in periodate concentration decreases the rate. The oxidation reaction in alkaline medium has been shown to proceed via a monoperiodatocuprate(III)- vanillin complex, which decomposes slowly in a rate-determining step followed by other fast steps to give the products. The main products were identified by spot test, IR, and MS studies. The reaction constants involved in the different steps of the mechanism are calculated. The activation parameters with respect to slow step of the mechanism are computed and discussed, and thermodynamic quantities are also determined.

Full text of DOI:10.1002/kin.20233

Oxidation of Vanillin by
Diperiodatocuprate(III) in
Aqueous Alkaline Medium:
A Kinetic and Mechanistic
Study by Stopped Flow
Technique
D. C. HIREMATH, T. S. KIRAN, S. T. NANDIBEWOOR
P. G. Department of Studies in Chemistry, Karnatak University, Dharwad 580 003, India
Received 19 June 2006; revised 18 August 2006, 1 October 2006, 10 November 2006, 23 November 2006;
accepted 25 November 2006
DOI 10.1002/kin.20233
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: The kinetics of oxidation of vanillin (VAN) by diperiodatocuprate(III) (DPC) in alka-
line medium at a constant ionic strength of 0.50 mol dm−3 was studied spectrophotometrically.
The reaction between DPC and vanillin in alkaline medium exhibits 1:2 stoichiometry (vanillin:
DPC). The reaction is of first order in [DPC] and has less than unit order in both [VAN] and
[alkali]. Intervention of free radicals was observed in the reaction. Increase in periodate con-
centration decreases the rate. The oxidation reaction in alkaline medium has been shown to
proceed via a monoperiodatocuprate(III)–vanillin complex, which decomposes slowly in a rate-
determining step followed by other fast steps to give the products. The main products were
identified by spot test, IR, and MS studies. The reaction constants involved in the different steps
of the mechanism are calculated. The activation parameters with respect to slow step of the
mechanism are computed and discussed, and thermodynamic quantities are also determined.
ꢀ
C
2007 Wiley Periodicals, Inc. Int J Chem Kinet 39: 236–244, 2007
ands. Metal chelates such as diperiodatocuprate(III)
[1] diperiodatoargentate(III) [2], and diperiodato-
nickelate(IV) [3] are good oxidants in a medium with
an appropriate pH value. Copper(III) is shown to be
an intermediate in the copper(II)-catalyzed oxidation
of amino acids by peroxydisulfate [4]. The oxidation
reaction usually involves the copper(II)–copper(I) cou-
ple, and such aspects are detailed in different reviews
[5, 6]. The use of diperiodatocuprate(III) (DPC) as an
oxidant in alkaline medium is new and restricted to a
INTRODUCTION
In recent years, the study of the highest oxidation state
of transition metals has intrigued many researchers.
Transition metals in a higher oxidation state can be
stabilized by chelation with suitable polydentate lig-
Correspondence to: S. T. Nandibewoor; e-mail: stnandibewoor@
yahoo.com.
c
ꢀ
2007 Wiley Periodicals, Inc.

OXIDATION OF VANILLIN BY DIPERIODATOCUPRATE(III) IN AQUEOUS ALKALINE MEDIUM
237
few cases because of its limited solubility and stability
in aqueous medium. Copper complexes have occupied
a major place in oxidation chemistry because of their
abundance and relevance in biological chemistry [7–
11]. Copper(III) is involved in many biological electron
transfer reactions [12]. When the copper(III) periodate
complex is oxidant and multiple equilibria between dif-
ferent copper(III) species [13] are involved, it would
be interesting to know which of the species is the active
oxidant.
solution of copper(III) was standardized by iodometric
titration and gravimetrically by the thiocyanate [21]
method. The copper(II) and vanillic acid solutions
were prepared by dissolving the known amount of
copper sulfate (BDH, Mumbai, India) and vanillic
acid crystals (s.d-Fine, Mumbai, India), respectively,
in distilled water. Periodate solution was prepared by
weighing the required amount of sample in hot water
and used after keeping it for 24 h. Its concentration was
ascertained iodometrically [22] at neutral pH by phos-
phate buffer. KOH and KNO3 (BDH, AR, Mumbai,
India) were employed to maintain the required
alkalinity and ionic strength, respectively, in reaction
solutions.
3-Methoxy-4-hydroxy benzaldehyde, commer-
cially called p-Vanillin and in general vanillin (VAN),
occurs in nature as a glucoside, which hydrolyzes to
vanillin and sugar. It is a very popular flavoring reagent
in the food industry, and it is also useful in the syn-
thesis of drugs. It is also used in the preparation of
perfume and as a catalyst to polymerize methylacry-
late [14]. Vanillin has both phenolic and aldehydic
groups and is capable of undergoing three different
types of reactions: those of aldehydic group, the phe-
nolic hydroxyl, and aromatic nucleus. However, as a
p-hydroxybenzaldehyde, vanillin does not undergo
some very common aldehyde reactions, such as the
Cannizzaro’s reaction and the Benzoin condensation.
If the hydroxyl group in vanillin is protected, it un-
dergoes oxidation [15] to vanillic acid. As a phenol,
vanillin forms esters and ethers, and the nucleus is eas-
ily substituted by halogen and nitro groups. In com-
parison with most other aldehydes, vanillin is notable
for its stability. The kinetics of oxidation of vanillin by
some oxidants is reported in the literature [16–19]. In
view of this, we have undertaken the study of the oxi-
dation of vanillin by diperiodatocuprate(III) in alkaline
medium to understand the suitable mechanism and ac-
tive species in such a medium.
KINETIC MEASUREMENTS
The kinetic measurements were performed on a
Hitachi 150-20 UV–Visible spectrophotometer attached
to a rapid kinetic accessory (HI-TECH SFA-12). The
kinetics was followed under pseudo-first-order con-
dition, where [VAN] > [DPC] at 25
0.1◦C, unless
specified. The reaction was initiated by mixing the
DPC with VAN solution, which also contained the re-
quired concentration of KNO3, KOH, and KIO4, and
the progress of the reaction was followed spectro-
photometrically at 415 nm by monitoring the decrease
in absorbance due to DPC with the molar absorbancy
index, ε to be 6230 100 dm3 mol−1 cm−1. It was
verified that there is a negligible interference from
other species present in the reaction mixture at this
wavelength.
The pseudo-first-order rate constants, kobs, were de-
termined from the log (absorbance) versus time plots.
The plots were linear up to 85% completion of reaction
under the range of [OH−] used. The orders for various
species were determined from the slopes of plots of log
kobs versus respective concentration of species except
for [DPC] in which nonvariation of kobs was observed
as expected to the reaction condition. During the ki-
netics, a constant concentration viz. 1.0 × 10−4 mol
dm−3 of KIO4 was used throughout the study unless
otherwise stated. Since periodate is present in the ex-
cess in DPC, the possibility of oxidation of vanillin by
periodate in alkaline medium at 25◦C was tested. The
progress of the reaction was followed iodometrically.
However, it was found that there was no significant
reaction under the experimental conditions employed
compared to the DPC oxidation of vanillin. The total
concentration of periodate and OH− was calculated by
considering the amount present in the DPC solution
and that additionally added. Kinetics runs were also
EXPERIMENTAL
Materials and Reagents
All chemicals used were of reagent grade, and double
distilled water was used throughout the work. A solu-
tion of vanillin (s.d-Fine, Mumbai, India) was prepared
by dissolving an appropriate amount of recrystallized
sample in double distilled water. The purity of vanillin
sample was checked by comparing its IR spectrum
with the literature data and with its mp 81–83◦C.
The required concentration of vanillin was prepared
from its stock solution. The copper(III) periodate
complex was prepared by standard procedure [20].
Existence of copper(III) complex was verified by its
UV–Vis spectrum, which showed an absorption band
with maximum absorption at 415 nm. The aqueous
International Journal of Chemical Kinetics DOI 10.1002/kin

238
HIREMATH, KIRAN, AND NANDIBEWOOR
centration of DPC was estimated by spectrophoto-
metrically at 415 nm. The results indicated 1:2 stoi-
chiometry as given in Eq. (1).
The main product was isolated by acidifying the re-
action mixture followed by ether and ethyl acetate ex-
traction. Each extract was dehydrated with anhydrous
Na2SO4 and decanted; the solvent was removed by
evaporation. The residue was recrystallized from warm
glacial acetic acid. The main reaction product was iden-
tified as vanillic acid. The nature of vanillic acid was
confirmed by its spot test [23] and also by IR spec-
trum (1686 cm−1 due to C O stretching, 3486 cm−1
due to O H stretching). It was confirmed by its melt-
ing point 206◦C (literature mp 208◦C). Further, vanillic
acid was subjected to mass spectral analysis. MS data
were obtained on a QP-5050A Shimadzu mass spec-
trometer using the EI ionization technique. The mass
spectrum showed a molecular ion peak at 168 amu con-
firming vanillic acid. The product, Cu(II), was iden-
tified by UV–Vis spectra. The reaction products do
not undergo further oxidation under the present kinetic
conditions. Regression analysis of experimental data to
obtain the regression coefficient r and standard devia-
tion S of points from the regression line was performed
using Microsoft 2003 Excel program.
Figure 1 Spectroscopic changes occurring in the oxidation
of vanillin by diperiodatocuprate(III) at 25◦C, [DPC] = 1 ×
10−4, [VAN] = 1 × 10−3, [OH−] = 0.10, and I = 0.50 mol
dm−3 with scanning time interval = 1 min.
carried out in N2 atmosphere in order to understand
the effect of dissolved oxygen on the rate of reaction.
No significant difference in the results was obtained
under a N2 atmosphere and in the presence of air. 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 reac-
tion rates. The spectral changes during the reaction
are shown in Fig. 1. It is evident from the figure that
the concentration of DPC decreases at 415 nm. As
shown in the figure, the small absorption peak at 390
nm could be due to charge transfer from oxygen to
iodine, which disappears by time because of oxidative
reaction.
Reaction Orders
The reaction orders were determined from the slope of
log kobs versus log (concentration) plots by varying the
concentrations of vanillin, alkali, and periodate in turn,
while keeping all other concentrations and conditions
constant.
Effect of [Diperiodatocuprate(III)]
RESULTS
The oxidant DPC concentration was varied in the range
5.0 × 10−5 to 5.0 × 10−4 mol dm−3, and the fairly
constant kobs values indicate that order with respect
to [DPC] was unity (Table I). This was also confirmed
by linearity of the plots of log [absorbance] versus time
(r ≥ 0.986, S ≤ 0.013) up to 80% completion of the
reaction (Fig. 2).
Stoichiometry and Product Analysis
Different sets of reaction mixtures containing various
ratios of DPC to vanillin in the presence of constant
amounts of OH− and KNO3 were kept for 2 h in closed
vessel under nitrogen atmosphere. The remaining con-
International Journal of Chemical Kinetics DOI 10.1002/kin

OXIDATION OF VANILLIN BY DIPERIODATOCUPRATE(III) IN AQUEOUS ALKALINE MEDIUM
239
Table I Effect of [DPC], [VAN], [OH−], and [IO4−] on Diperiodatocuprate(III) Oxidation of Vanillin in Alkaline Medium
at 25◦C, I = 0.50 mol dm−3
k
obs× 102 (s−1
)
[DPC] × 104
[VAN] × 103
[IO−4 ] × 104
[OH−]
(mol dm−3
(mol dm−3
)
(mol dm−3
)
(mol dm−3
)
)
Found
Calculated
0.5
0.8
1.0
2.0
3.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
0.5
0.8
1.0
3.0
5.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
0.1
0.3
0.5
0.8
1.0
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.05
0.08
0.1
0.3
0.5
0.1
0.1
0.1
0.1
0.1
1.20
1.19
1.20
1.18
1.20
0.68
0.99
1.20
2.31
3.13
0.89
1.09
1.20
1.78
1.93
2.79
2.19
1.79
1.41
1.20
1.21
1.20
1.24
1.20
1.20
0.71
1.04
1.24
2.45
3.04
0.87
1.12
1.24
1.73
1.88
2.84
2.21
1.81
1.42
1.24
substrate, vanillin, was varied in the range 5.0 × 10−4
to 5.0 × 10−3 mol dm−3. The kobs values increased
with increase in concentration of vanillin. The order
with respect to [vanillin] was found to be less than
unity (Table I) (r ≥ 0.995, S ≤ 0.007).
Effect of [Vanillin]
The effect of vanillin on the rate of reaction was studied
at constant concentrations of alkali, DPC, and perio-
date at a constant ionic strength of 0.50 mol dm−3. The
[Diperiodatocuprate (III)] X104 (mol dm−3
)
Effect of [Alkali]
1) 0.5 2) 0.8 3) 1.0
4) 2.0
5) 3.0
The effect of increase in concentration of alkali on
the reaction was studied at constant concentrations of
vanillin, DPC, and periodate at a constant ionic strength
of 0.50 mol dm−3 at 25◦C. The rate constants increased
with increase in alkali concentration (Table I), indi-
cating fractional order dependence of rate on alkali
concentration (r ≥ 0.998, S ≤ 0.006).
2.5
2.0
1.5
1.0
0.5
(5)
(4)
(3)
(2)
Effect of [Periodate]
(1)
The effect of increasing concentration of periodate
was studied by varying the periodate concentration
from 1.0 × 10−5 to 1.0 × 10−4 mol dm−3 keep-
ing all other reactant concentrations constant. It was
found that the added periodate had a retarding effect
on the rate of reaction, the order with respect to pe-
riodate concentration being negative less than unity
(Table I).
0
0.5
1.0
1.5
2.0
2.5
Time (min)
Figure 2 First-order plots on the oxidation of vanillin by
diperiodatocuprate(III) in aqueous alkaline medium. [Diperi-
odatocuprate (III)] 1: 0.5 × 104 mol dm−3, 2: 0.8 × 104 mol
dm−3, 3: 1.0 × 104 mol dm−3, 4: 2.0 × 104 mol dm−3, and
5: 3.0 × 104 mol dm−3
.
International Journal of Chemical Kinetics DOI 10.1002/kin

240
HIREMATH, KIRAN, AND NANDIBEWOOR
known quantity of acrylonitrile monomer was initially
added, was kept for 2 h in an inert atmosphere. On
diluting the reaction mixture with methanol, a white
precipitate was formed, indicating the intervention of
free radicals in the reaction. The blank experiments of
either DPC or vanillin alone with acrylonitrile did not
induce any polymerization under the same condition as
those induced for reaction mixture. Initially, the added
acrylonitrile decreases the rate of reaction indicating
free-radical intervention, which is the case in earlier
work [24].
Effect of Ionic Strength (I) and Dielectric
Constant on the Medium (D)
The addition of KNO3 at constant [DPC], [VAN],
[OH−], and [IO−4 ] was found that increasing ionic
strength had negligible effect on the rate of the
reaction.
Dielectric constant of the medium, D, was varied by
varying t-butyl alcohol and water percentage. The D
values were calculated from the equation D = DwVw+
DBVB, where Dw and DB are dielectric constants of
pure water and t-butyl alcohol, respectively, and Vw and
VB are the volume fractions of components water and
t-butyl alcohol, respectively, in the total mixture. The
decrease in dielectric constant of the reaction medium
has no effect on the rate of reaction.
Effect of Temperature (T)
The kinetics was studied at four different tempera-
tures under varying concentrations of vanillin, alkali,
and periodate, keeping other conditions constant. The
rate constants were found to increase with increase in
temperature. The rate constants (k) of the slow step of
Scheme 1 were obtained from the slopes and intercepts
of 1/kobs versus 1/[VAN], 1/kobs versus [H3IO26−], and
1/kobs versus 1/[OH−] plots at four different tempera-
tures and were used to calculate the activation param-
eters. The energy of activation corresponding to these
constants was evaluated from the Arrhenius plot of log
k versus 1/T (r ≥ 0.9987, S ≤ 0.011), and other acti-
vation parameters obtained are tabulated in Table II.
Effect of Initially Added Products
The externally added products, vanillic acid and
copper(II) (CuSO4) in the range 1.0 × 10−4 to
1.0 × 10−3 mol dm−3 did not have any significant
effect on the rate of the reaction.
Polymerization Study
The intervention of free radicals in the reaction was
examined as follows. The reaction mixture, to which a
Table II Thermodynamic Activation Parameters for the Oxidation of Vanillin by DPC in Aqueous Alkaline Medium
with Respect to the Slow Step of Scheme 1
(A) Effect of temperature
Temperature (K)
(B) Activation parameters (Scheme 1)
k × 102 (s−1
)
Parameters
Values
298
303
308
313
3.7
4.2
4.6
5.1
ꢀH# (kJ mol−1
)
13.1 0.5
−227 15
ꢀS#(JK−1 mol−1
)
ꢀG# (kJ mol−1
)
81
2
(C) Effect of temperature on equilibrium constants of Scheme 1
for the oxidation of vanillin by diperiodatocuprate(III) in alkaline medium
K × 10
K × 105
K × 10−3
1
2
3
Temperature (K)
(dm3 mol−1
)
(mol dm−3
)
(dm3 mol−1
)
298
303
308
313
4.6 0.2
6.5 0.4
8.1 0.6
11.1 0.7
5.7 0.3
3.8 0.1
6.0 0.3
8.8 0.4
9.9 0.5
2.4 0.06
1.8 0.03
0.88 0.02
(D) Thermodynamic quantities using K , K , and K
3
1
2
Thermodynamic quantities
Values from K
Values from K
Values from K
3
1
2
ꢀH (k J mol−1
)
45.4 0.6
−91.4 2.0
−390 20
24.2 0.9
49.9 0.7
237 15
−20.4 0.8
ꢀS (J K−1 mol−1
ꢀG (k J mol−1
)
)
166
4
−3.8 0.1
298
International Journal of Chemical Kinetics DOI 10.1002/kin

OXIDATION OF VANILLIN BY DIPERIODATOCUPRATE(III) IN AQUEOUS ALKALINE MEDIUM
241
K1
DISCUSSION
[Cu(H2IO6)2]3−
[Cu(H2IO6)(H2O)2]
H2IO63− H3O+
H2O
+
[Cu(H2IO6)(H3IO6)]2−
[Cu(H2IO6)2]3−
+
OH−
2H2O
K2
K
+
[H2IO6]3−
+
The water-soluble copper(III) periodate complex is re-
ported [25]to be [Cu(HIO6)2]5−. However, in an aque-
ous alkaline medium and at a high pH range as em-
ployed in the study, periodate is unlikely to exist as
HIO46−(as present in the complex) as is evident from
its involvement in the multiple equilibria [26] ((2)–(4))
depending on the pH of the solution.
H3IO62−
+
H2O
H
+
O
C
K3
Complex (C)
[Cu(H2IO6)(H2O)2]
+
OCH3
O
−
O
C
.
−
+
ꢁ
ꢂ
H5IO6
H IO + H
3
(2)
(3)
(4)
4
6
k
Cu2+
+
H2IO6
+
2H2O + H+
3−
Complex (C)
+
H4IO−
H IO2− + H+
slow
ꢁ
ꢂ
6
6
OCH3
O
−
H3IO2−
H IO3− + H+
ꢁ
ꢂ
O
C
2
6
O
C
6
−
.
O
fast
Periodic acid exists in acid medium as H5IO6 and
as H4IO−6 around pH 7. Thus, under the conditions
employed in alkaline medium, the main species are
expected to be H3IO26− and H2IO36−. At higher concen-
trations, periodate also tends to dimerize [27]. How-
ever, formation of this species is negligible under
conditions employed for kinetic study. Hence, at the
pH employed in this study, the soluble copper(III)
periodate complex exists as diperiodatocuprate (III),
[Cu(H3IO6)(H2IO6)]2−, a conclusion also supported
by earlier work [28].
−
+
[Cu(H2IO6)(H2O)2]
+
OH
OCH3
OCH3
O
O
−
−
3−
+
Cu2+
fast
+
H2IO6
+
2H2O + H+
−
2H+
+ 2OH
2H2O
Scheme 1
[Cu(H2IO6)2]3− + 2H2O
3−
6
ꢁ
ꢂ
[Cu(H IO )(H O) ] + [H IO ]
(6)
2
6
2
2
2
The reaction between the diperiodatocuprate(III)
complex and vanillin in alkaline medium has the
stoichiometry 1:2 (VAN:DPC) with a first-order depen-
dence on [DPC] and an apparent order of less than unity
in [substrate], [alkali] and negative fractional order
dependence on the periodate. No effect of added prod-
ucts was observed. Based on the experimental results,
a mechanism is proposed for which all the observed or-
ders in each constituent such as [oxidant], [reductant],
[OH−], and [IO−4 ] may be well accommodated.
Lister [29] proposed three forms of copper(III) peri-
odate in alkaline medium as diperiodatocuprate(III)
(DPC), monoperiodatocuprate(III) (MPC), and
tetrahydroxocuprate(III). The latter is ruled out as its
equilibrium constant is 8.0 × 10−11 at 40◦C. Hence, in
the present study, DPC and MPC are to be considered
as active forms of copper(III) periodate complex. It
may be expected that a lower periodate complex such
as MPC is more important in the reaction than the
DPC. The results of increase in the rate with increase
in alkali concentration and decrease in the rate with
increase in periodate concentration suggest that
equilibria of different copper(III) periodate complexes
are possible as in Eqs. (5) and (6).
The inverse fractional order in [IO−4 ] might also
be due to this reason. Vanillin is known to exist as
deprotonated form in alkaline medium because of its
phenolic group. The less than unit order in [VAN] pre-
sumably results from formation of a complex (C) be-
tween the oxidant and vanillin prior to the formation of
the products. K3 is the composite equilibrium constant
comprising the equilibrium to bind active species of
vanillin to MPC species to form a complex (C). Then,
this complex (C) decomposes in a slow step to form
a free radical derived from vanillin. This free-radical
species further reacts with another molecule of MPC
species in a fast step to yield the products such as
vanillic acid, Cu(II), and periodate.
So, the detailed mechanistic scheme for the oxida-
tion of vanillin by diperiodatocuprate(III) is presented
in Scheme 1.
The probable structure of the complex (C) is given
by
OH2
H2O
Cu
I
O
C
H
O
O
O
O
H3CO
O
[Cu(H2IO6)(H3IO6)]2− + OH−
HO
OH
3−
ꢁ
ꢂ
[Cu(H IO ) ] + H2O
(5)
2
6 2
International Journal of Chemical Kinetics DOI 10.1002/kin

242
HIREMATH, KIRAN, AND NANDIBEWOOR
4
−3
3−
Since Scheme 1 is in accordance with the generally
well-accepted principle of noncomplementary oxida-
tions taking place in sequence of one-electron steps,
the reaction between the substrate and oxidant would
afford a radical intermediate. A free-radical scaveng-
ing experiment revealed such a possibility (as given in
the Results section). This type of radical intermediate
has also been observed in earlier work [30].
Spectroscopic evidence for the complex formation
between oxidant and substrate was obtained from UV–
Vis spectra of vanillin (1.0 × 10−3), DPC (1.0 ×
10−4), [OH−] = 0.1 mol dm−3 and mixture of both. A
bathochromic shift of about 7 nm from 342 to 349 nm
in the spectra of DPC was observed. The Michaelis–
Menten plot also proved the complex formation be-
tween DPC and vanillin, which explains the less than
unit order dependence on [VAN]. Such a complex be-
tween a substrate and an oxidant has been observed in
other studies [31].
[H2IO6 ] × 10 mol dm
(
)
1.0
0.8
0.6
0.4
0.2
0
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
0.3
0.4
0.5
0.6
0.7
0.8
1.5
1.3
1.1
0.9
0.7
0.5
•
0.9
1.0
1.0
0
0
0.5
1.5
2.0
1/[VAN] × 10−3dm3 mol
−1
(
)
5
10
15
20
25
1/[OH] dm3 mol
(
)
−1
•
Figure 3 Verification of rate law (8) for the of oxidation of
vanillin by diperiodatocuprate(III) at 25◦C.
Scheme 1 leads to the rate law (7)
d[DPC]
2kK1K2K3[DPC] [VAN] [OH−]
rate = −
=
(7)
[H2IO36−] + K1[OH−][H2IO63−] + K1K2[OH−] + K1K2K3[OH−][VAN]
dt
Rate
2kK1K2K3 [VAN] [OH−]
kobs
=
=
(8)
[H2IO36−] + K1[OH−] [H2 IO63−] + K1K2[OH−] + K1K2K3 [OH−] [VAN]
[DPC]
0.1) × 10−2
s
−1, respectively. The equilibrium
which explains all the observed kinetic orders of dif-
ferent species.
The rate law (8) can be rearranged into the following
form, which is suitable for verification:
constant K1 is far greater than K2. This may be
attributed to the greater tendency of DPC to undergo
hydrolysis compared to the dissociation of hydrolyzed
species in alkaline medium.
The negligible small effect of ionic strength and di-
electric constant of medium on the rate explains qual-
itatively the reaction between neutral and negatively
charged ions, as seen in Scheme 1.
1
kobs
[H2IO36−
]
[H2IO36−
]
=
+
+
2kK1K2K3 [VAN] [OH−] 2kK2K3 [VAN]
1
1
+
(9)
The thermodynamic quantities for the first, second,
and third equilibrium steps of Scheme 1 can be evalu-
ated as follows. The [H2IO36−], [VAN], and [OH−] (as
in Table I) were varied at four different temperatures.
The plots of 1/kobs versus 1/[OH−], 1/kobs versus
1/[VAN], and 1/kobs versus [H2IO36−] should be linear
(Fig. 3). From the slopes and intercepts, the values of
K1 were calculated at different temperatures and these
values are given in Table II. A van’t Hoff’s plot was
made for variation of K1 with temperature (log K1 vs.
1/T (r ≥ 0.9604, S ≤ 0.006) and the values of enthalpy
of reaction ꢀH, entropy of reaction ꢀS, and free en-
ergy of reaction ꢀG were calculated for the first equi-
librium step. These values are given in Table II. A com-
parison of the latter values with those obtained for the
slow step of the reaction shows that these values mainly
refer to the rate-limiting step, supporting the fact that
2kK3 [VAN] 2k
According to Eq. (9), other conditions being
constant, plots of 1/kobs versus 1/[OH−] (r ≥ 0.995,
S ≤ 0.029), 1/kobs versus 1/[VAN] (r ≥ 0.997, S ≤
0.016), and 1/kobs versus [H2IO36−] (r ≥ 0.998, S ≤
0.011)should be linear and are found to be so (Fig. 3).
Since pK for the third dissociation of periodate has a
magnitude of about 12, at high concentrations of alkali,
the concentration of H3IO26− can be neglected and since
periodate in the complex existed prior to the addition
of periodate externally, the added periodate can be
approximated to [H2IO63−]. The slopes and intercepts
of such plots lead to the values of K1, K2, K3, and k as
(4.6 0.2) × 10−1 mol dm−3, (5.7 0.3) × 10−5 mol
dm−3, (3.8
0.1) × 103 dm3 mol−1, and (3.7
International Journal of Chemical Kinetics DOI 10.1002/kin

OXIDATION OF VANILLIN BY DIPERIODATOCUPRATE(III) IN AQUEOUS ALKALINE MEDIUM
243
the reaction before rate-determining step is fairly fast
CONCLUSION
and involves low-activation energy [32]. In the same
manner, K2 and K3 values were calculated at differ-
ent temperatures and their corresponding values of the
thermodynamic quantities are given in the Table II.
The values of ꢀH#and ꢀS#were both favorable for
electron transfer processes. The negative value of ꢀS#
indicates that the complex (C) is more ordered than the
reactants [33]. The value of ꢀS#within the range for
radical reaction has been ascribed [34] to the nature
of electron pairing and unpairing processes and to the
loss of degrees of freedom formerly available to the re-
actants upon the formation of rigid transition state. The
observed modest enthalpy of activation and a relatively
low value of the entropy of activation as well as a higher
rate constant of the slow step indicate that the oxidation
presumably occurs via inner-sphere mechanism. This
conclusion is supported by earlier observation [35].
Among various species of DPC in alkaline medium,
MPC is considered as active species for the title reac-
tion. The results demonstrate that in carrying out this
reaction, the role of pH in the reaction medium is cru-
cial. Rate constant of slow step and other equilibrium
constants involved in the mechanism are evaluated, and
activation parameters with respect to slow step of reac-
tion were computed. The overall mechanistic sequence
described here is consistent with product studies and
kinetic studies.
APPENDIX
According to Scheme 1
−d[DPC]
2kK1K2K3 [DPC][OH−][VAN]
Rate =
= k[C] =
[H2 IO6]3−
dt
[DPC]T = [DPC]f + [Cu(H2IO6)2]3− + [Cu(H2 IO6)(H2O)2] + C
(I)
where T and f refer to total and free concentrations
ꢀ
ꢁ
K1K2[OH−] K1K2K3[OH−][VAN]
= [DPC]f 1 + K1[OH−] +
+
[H2IO36−
]
[H2IO36−
]
[DPC]T[H2IO36−
]
[DPC]f =
(II)
[H2IO36−] + K1[OH−][H2IO63−] + K1K2[OH−] + K1K2K3[OH−][VAN]
Similarly, [OH−]T = [OH−]f + [Cu(H2IO6)2]3− + [Cu(H2IO6)(H2O)2]
K1K2[DPC][OH−]f
= [OH]f + K1[DPC][OH−]f +
[H2IO36−
]
In view of the low concentration of DPC and H2IO36− used
[OH]T = [OH]f
[VAN]T = [VAN]f + [C]
K1K2K3[DPC][OH−][VAN]
= [VAN]f +
(III)
(IV)
[H2IO36−
]
In view of the low concentration of DPC, OH−, and H2 IO63− used,
[VAN]T = [VAN]f
International Journal of Chemical Kinetics DOI 10.1002/kin

244
HIREMATH, KIRAN, AND NANDIBEWOOR
Substituting (II), (III), and (IV) in (I) and omitting the subscripts T and f, we get
2kK1K2K3[DPC][VAN][OH−]
Rate =
[H2IO36−] + K1[OH−][H2IO63−] + K1K2[OH−] + K1K2K3[OH−][VAN]
21. Jeffery, G. H.; Bassett, J.; Mendham, J.; Denny, R. C.
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International Journal of Chemical Kinetics DOI 10.1002/kin