Kinetics of oxidation of L-valine by a copper(III) periodate complex in alkaline medium

Article abstract of DOI:10.1007/s10953-011-9772-5

The kinetics of oxidation of L-valine by a copper(III) periodate complex was studied spectrophotometrically. The inverse second-order dependency on [OH^-] was due to the formation of the protonated diperiodatocuprate(III) complex ([Cu(H₃IO₆) ₂]^-) from [Cu(H₂IO₆) ₂]^3-. The retarding effect of initially added periodate suggests that the dissociation of copper(III) periodate complex occurs in a pre-equilibrium step in which it loses one periodate ligand. Among the various forms of copper(III) periodate complex occurring in alkaline solutions, the monoperiodatocuprate(III) appears to be the active form of copper(III) periodate complex. The observed second-order dependency of [L-valine] on the rate of reaction appears to result from formation of a complex with monoperiodatocuprate(III) followed by oxidation in a slow step. A suitable mechanism consistent with experimental results was proposed. The rate law was derived as: (Equation presented) The temperature effect on the rate of reaction was also studied. The activation parameters of the reaction E_a, ΔH^#, ΔS^#, ΔG^#, and log ₁₀ A are 49 ± 2 kJ·mol^-1, 47.5 ± 2 kJ·mol^-1, -49 ± 2 J·K ^-1·mol^-1, 62 ± 3 kJ·mol^-1 and 6.0 ± 0.1, respectively. Springer Science+Business Media, LLC 2012.

Full text of DOI:10.1007/s10953-011-9772-5

J Solution Chem (2012) 41:187–199
DOI 10.1007/s10953-011-9772-5
Kinetics of Oxidation of L-Valine by a Copper(III)
Periodate Complex in Alkaline Medium
K. Sharanabasamma · Mahantesh A. Angadi ·
Manjalee S. Salunke · Suresh M. Tuwar
Received: 21 May 2010 / Accepted: 10 April 2011 / Published online: 2 March 2012
©
Springer Science+Business Media, LLC 2012
Abstract The kinetics of oxidation of L-valine by a copper(III) periodate complex was
−
studied spectrophotometrically. The inverse second-order dependency on [OH ] was due
−
to the formation of the protonated diperiodatocuprate(III) complex ([Cu(H3IO6)2] ) from
−
[
Cu(H2IO6)2]3 . The retarding effect of initially added periodate suggests that the disso-
ciation of copper(III) periodate complex occurs in a pre-equilibrium step in which it loses
one periodate ligand. Among the various forms of copper(III) periodate complex occurring
in alkaline solutions, the monoperiodatocuprate(III) appears to be the active form of cop-
per(III) periodate complex. The observed second-order dependency of [L-valine] on the rate
of reaction appears to result from formation of a complex with monoperiodatocuprate(III)
followed by oxidation in a slow step. A suitable mechanism consistent with experimental
results was proposed. The rate law was derived as:
3
2
f
d[DPC]
dt
kK1K2K3[Cu(H2IO6)2] −[L − Val]
f
−
=
.
2
6
−
− 2
[H IO ] [OH ]
3
f
f
The temperature effect on the rate of reaction was also studied. The activation parameters
#
#
#
−1
−1
,
of the reaction Ea, ꢀH , ꢀS , ꢀG , and log A are 49 ± 2 kJ·mol , 47.5 ± 2 kJ·mol
1
0
−
1
−1
−1
−
49 ± 2 J·K ·mol , 62 ± 3 kJ·mol and 6.0 ± 0.1, respectively.
Keywords Oxidation of L-valine · Reactivity of diperiodatocuprate(III) · Mechanism of
oxidation by diperiodatocuprate(III) · Kinetics of oxidation of L-valine · Mechanism ·
Oxidation · Kinetics
1
Introduction
Copper(III) complexes have occupied a major place in oxidation chemistry due to their
abundance in nature and stability of the complexes. The periodate and tellurate complexes
K. Sharanabasamma · M.A. Angadi · M.S. Salunke · S.M. Tuwar (ꢀ)
Department of Chemistry, Karnatak Science College, Dharwad 580001, Karnataka, India
e-mail: sureshtuwar@yahoo.co.in
S.M. Tuwar (ꢀ)
e-mail: sm.tuwar@gmail.com

188
J Solution Chem (2012) 41:187–199
of copper(III) have been found [1] to be relatively stable compared with its other complexes,
and are also found to be good oxidizing agents [2] in alkaline solutions. Hence, they have
been exploited in the analysis of some organic compounds [2], particularly alcohols, amines
and α-amino acids. They have also been used in the differential titration [3] of organic
mixtures, in the estimation of the amounts of chromium, calcium and magnesium from their
ores, and estimation of the amounts of antimony, arsenic and tin in their alloys.
In the recent past, diperiodatocuprate(III) (DPC) has been used to establish the reaction
mechanism for the oxidation of some alcohols [4, 5], amines and α-amino acids [6] as
it is comparatively more stable than the ditelluratocuprate(III) and copper(III) hydroxide
complexes. In such reactions, Cu(III) acts as an electron abstracting agent, occurring either
in a single step or through formation of an adduct with substrates in an equilibrium step
prior to the redox reactions [7].
Kumar et al. [8] have studied the oxidation of glycine and related substrates by diperi-
odatoargentate(III); they found that initially, glycine binds with silver(III) to give an adduct
and it then takes up another molecule of glycine to give the diglycine monoperiodatoargen-
tate(III) complex, followed by decomposition to give intermediates/products. In the present
investigation, the second-order dependency on [L-valine] can also be explained on the basis
8
of formation of an adduct with L-valine, because Cu(III) has a d electronic configuration,
similar to Ag(III) [9] and Au(III). The reactive form of DPC is not known as is the case for
Ni(IV) [10], Ag(III) [11, 12] and Au(III) [13] periodate complexes. However, we have made
an effort to establish the reactive form of DPC in alkaline solutions since Cu(III) can exist
in multiple equilibria with periodate in alkaline solutions at various pHs.
L-Valine (L-Val) is an essential α-amino acid that is usually found in the interior of pro-
teins. It is required as part of the nutrition of mammals. Its role is crucial in the development
of organs, especially in children. It also finds applications in medicines and pharmaceu-
ticals. The mechanism of oxidation of L-valine by various oxidants have been reported,
e.g., permanganate in neutral and basic solutions [14], cobalt(III) in acidic solutions [15],
chloramine-T in acidic solutions [16], and catalysis by Cr(III) [17] and Os(VIII) [18] with
permanganate in alkaline solutions, etc. In all such cases the order in L-valine concentration
was found to be either unity or fractional, and yielded aldehyde or keto acids as products.
A report [19] of the oxidation of L-valine by alkaline DPC is substantially different from
results of the present investigation. Moreover, the results obtained in this study are also un-
usual as compared to other studies [20] with DPC. Hence, the title study was undertaken to
investigate the oxidative path of L-valine with DPC in aqueous alkali.
2
Experimental Section
2.1 Chemicals Used
All of the compounds used, viz. L-valine, CuSO4, K2S2O8, KOH, KIO4, KNO3 and acry-
lonitrile, were of analytical grade. The purity of L-valine was further ensured by measuring
its melting point. Their reaction solutions were prepared in double distilled water, free from
dissolved oxygen and carbon dioxide. The acrylonitrile and K2S2O8 were purified by distil-
lation and re-crystallization, respectively, to remove any traces of impurities.
2.1.1 Synthesis of DPC
The DPC was prepared [21] by oxidizing CuSO4 in alkaline medium. CuSO4·5H2O (3.54 g),
3
KIO4 (6.8 g), K2S2O8 (2.2 g) and KOH (9.0 g) were added to 250 cm water. The mixture

J Solution Chem (2012) 41:187–199
189
was heated to boiling on a hot plate with constant stirring. The boiling mixture turned in-
tensely red and the boiling was continued for another 20 min to allow for completion of the
reaction. The mixture was then cooled, filtered through a sintered glass crucible (G-4), and
3
diluted to 250 cm with distilled water. The solution obtained was found to be fairly stable at
room temperature for several months. The complex was characterized [21] by its UV/visible
spectrum, which exhibits a broad absorption band at 415 nm. The aqueous solution of DPC
was standardized by iodometric titration and gravimetrically by the thiocyanate method.
The periodate solution was prepared by dissolving the required amount of sample in
hot water and was kept at that temperature for 24 h. Its concentration was ascertained [22]
iodometrically, at neutral pH maintained by a phosphate buffer. Since periodate is present
in excess in DPC, the possibility of oxidation of L-valine by periodate in alkaline medium
at 30 °C was tested by following the reaction iodometrically. It was found that there was no
significant reaction between them under the experimental conditions employed, compared
to the DPC oxidation of L-valine.
2.2 Kinetic Measurements
The oxidation of L-valine by DPC was followed under pseudo-first order conditions at 303 K
where [L-valine] was in excess over [DPC]. The reaction was initiated by mixing the re-
quired quantities of previously thermostated solution of L-valine and DPC, which also con-
−
tained required quantities of KOH, KNO3 and IO to maintain the required alkalinity, ionic
4
strength and periodate concentration, respectively. Here, the total concentration of hydroxide
was calculated by considering the KOH in the DPC solution as well as the KOH addition-
ally added. Similarly, the total metaperiodate concentration was calculated by considering
the amount of metaperiodate present in the solution of DPC and that additionally added.
The course of reaction was followed by measuring the absorbance of unreacted DPC in the
reaction solution using a 1 cm quartz cell in a thermostatted compartment of the spectropho-
tometer. The absorbance at its absorbance maximum, 415 nm, was followed as a function
of time. Earlier, it was verified that there is a negligible interference at this wavelength from
other species present in the reaction mixture. The obedience of absorbance by DPC to Beer’s
law at 415 nm was verified earlier and the molar absorbance coefficient, ε was found to be
3
−1
−1
6
250 ± 50 dm ·mol ·cm at this wavelength. The first-order rate constants, kobs, were ob-
tained from plots of log (a − x) versus time, where a and x are the initial concentration
1
0
and change in concentration of DPC at time t respectively. The plots were linear up to 80%
completion of the reaction.
2.3 Stoichiometry and Product Analysis
Different sets of reaction mixtures containing different concentrations of L-valine and DPC
at constant ionic strength and [alkali] were kept for 4 h at 303 K in a closed vessel. The
oxidative products of L-valine were identified as an aldehyde and ammonia by using [23]
Schiff’s and Nessler’s reagent, respectively. The aldehyde, 2-methyl propionaldehyde, was
quantified by its 2,4-dinitrophenyl hydrazone derivative. Further, the aldehyde was identified
−
1
[
24] by its IR spectrum which showed a band at 2919 cm due to an aldehydic –C–H
−
1
stretching and 1710 cm for the >C=O stretching.
When [L-valine] was present in excess over [DPC], the unreacted [L-valine] was esti-
mated quantitatively by using ninhydrin with a spectrophotometric method [25, 26]. Earlier,
the calibration curve for L-valine with ninhydrin was prepared from that of an authentic
sample. When the [DPC] was in excess over [L-valine], the unreacted [DPC] was estimated

1
90
J Solution Chem (2012) 41:187–199
3 3
Table 1 Effect of [DPC] and
L-valine] on the oxidation of
4
1
(
0 × [DPC]
10 × [L-Val]
−3
10 ×k
[
obs
−
3
−1
mol·dm
)
(mol·dm
)
(s
)
L-valine by
diperiodatocuprate(III) in
alkaline medium at 303 K:
0.2
0.5
1.0
1.0
1.0
1.0
1.0
1.0
4.15
4.14
4.13
4.15
4.13
4.16
−
−4
−3
−3
,
−3
[
IO ] = 2.5 × 10 mol·dm
4
−
[
OH ] = 2.0 × 10 mol·dm
,
0
1
1
2
.8
.0
.5
.0
−
3
and I = 0.01 mol·dm
1.0
1.0
1.0
1.0
1.0
1.0
0.3
0.5
0.8
1.0
2.0
3.0
0.46
1.20
2.63
4.15
13.2
31.6
spectrophotometrically by measuring its absorbance at 415 nm. Its product, Cu(II), was es-
timated quantitatively by a gravimetric method [22] as its DMG complex.
−
4
−3
−3
−4
For a reaction mixture containing 2.0 × 10 mol·dm of [DPC] and 1.0 × 10 mol·
−
3
−3
−
dm of [L-valine] at constant concentrations of 2.0 × 10 mol·dm [OH ] and 2.5 ×
−
4
−3
−
1
0
mol·dm [IO ], the concentration of the 2-methyl propionaldehyde product was es-
4
−
4
−3
timated to be 0.5 × 10 mol·dm , indicating that only half of the [L-valine] was oxidized
and the other half was remained with Cu(III), in its co-ordination sphere. This was also
−
4
−3
confirmed by the unconsumed [DPC] of 1.05 × 10 mol·dm . Similar results were also
obtained at different mole ratios of [DPC] to [L-valine].
3
Results and Discussion
3.1 Reaction Order
−
−
4
The orders with respect to [L-Val], [OH ] and [IO ] were determined from log kobs versus
10
log (concentration) plots. These orders were obtained by varying the concentration of
1
0
L-valine, periodate and alkali, in turn, while keeping the other concentrations constant.
3.2 Effect of [Diperiodatocuprate(III)]
−
5
−4
−3
The concentration of DPC was varied in the range, 2.0 × 10 to 2.0 × 10 mol·dm at
−
−
fixed [L-Val], [OH ], [IO ] and ionic strength. The lack of variation in the pseudo-first
4
order rate constants at various concentrations of DPC indicates that the order in [DPC] is
unity (Table 1). This was also confirmed from the linearity of plots of log10 (absorbance)
versus time to about 80% completion of the reaction.
3.3 Effect of [L-Valine]
−
4
The concentration of the substrate L-valine was varied in the range 3.0 × 10 to 3.0 ×
−
3
−3
1
0
mol·dm at 303 K by keeping all of other reactants’ concentrations constant (Table 1).

J Solution Chem (2012) 41:187–199
191
Fig. 1 Plot of k
versus
versus
obs
obs
[
L-valine] and k
2
[L-valine] (conditions as in
Table 1)
−
Table 2 Effect of [OH ] and
3
−
4
−
3
−
10 ×[OH ]
10 × [IO ]
10 ×kobs
(s
[
IO ] on the oxidation of
4
4
−3
−3
−1
)
(
mol·dm
)
(
mol·dm
)
L-valine by
diperiodatocuprate(III) in
alkaline medium at 303 K:
0.5
.8
1.0
2.5
2.5
2.5
2.5
2.5
2.5
46.8
20.3
13.8
−
4
−3
,
[
DPC] = 1.0 × 10 mol·dm
0
−
3
−3
,
[
L-Val] = 1.0 × 10 mol·dm
−
3
and I = 0.01 mol·dm
2
4
5
.0
.0
.0
4.10
1.29
0.89
2.0
2.0
2.0
2.0
2.0
0.4
0.8
1.0
1.5
3.0
7.94
5.71
4.98
3.98
3.16
The kobs values increased with increase in concentration of L-valine and its order was found
2
to be +2.0. This was also confirmed from linearity of the plot of kobs versus [L-Val] but not
for the plot of kobs versus [L-Val] (Fig. 1).
3.4 Effect of [Alkali]
The effect of concentration of alkali on the rate of reaction was studied at constant con-
−
centrations of L-valine, DPC, IO4 and ionic strength (Table 2). The rate constants for the
reaction decreased with increasing [alkali] with an order of −2.0. This was also confirmed
−
2
from the linear plot of kobs versus 1/[OH ] (Fig. 2).

192
J Solution Chem (2012) 41:187–199
Fig. 2 Plot of k
versus
versus
obs
−
1
/[OH ] and k
obs
−
2
1/[OH ] (conditions as in
Table 2)
Table 3 Effect of ionic strength
and dielectric constant of the
reaction medium on the oxidation
of L-valine by
Ionic strength (I)
D = 78.5
Dielectric constant (D)
−
3
I = 0.01 mol·dm
I (mol·dm−3)
103 × kobs (s−1)
103 × k
(s−1)
D
obs
diperiodatocuprate(III) in
alkaline medium at 303 K:
0.002
15.1
12.1
10.0
5.39
4.14
3.80
3.31
78.5
75.1
71.5
68.4
65.0
63.1
60.2
4.14
4.15
4.16
4.13
4.20
4.16
4.18
−
4
−3
,
[
[
[
DPC] = 1.0 × 10 mol·dm
−
3
−3
0.003
0.004
L-Val] = 1.0 × 10 mol·dm
,
−
and
−4
−3
IO ] = 2.5 × 10 mol·dm
,
4
0.008
0.010
0.020
0.040
−
−3
−3
[OH ] = 2.0 × 10 mol·dm
3.5 Effect of [Periodate]
−
The effect of [IO ] on the rate of reaction was also studied by varying its concentration from
4
−
5
−4
−3
4
.0 × 10 to 3.0 × 10 mol·dm at constant concentrations of DPC, L-valine, alkali and
−
ionic strength (Table 2). The rate constants decreased with increase in [IO ].
4
3.6 Effect of Ionic Strength and Solvent Polarity
The effect of ionic strength was studied by varying the potassium nitrate concentration in
the reaction mixture. The ionic strength of the reaction medium was varied from 2.0 ×
−
3
−2
−3
1
0
to 4.0 × 10 mol·dm with all other reactants’ concentrations and other conditions
being constant (Table 3). It was noticed that as ionic strength increased, the rate of reaction
√
decreased. The plot of log kobs versus I was linear with –ve slope (Fig. 3).
1
0

J Solution Chem (2012) 41:187–199
193
Fig. 3 Effect of ionic strength
on the oxidation of L-valine by
diperiodatocuprate(III) in
aqueous alkaline medium at
303 K (conditions as in Table 3)
The relative permittivity (εT ) effect was studied by varying the t-butyl alcohol–water
content in the reaction mixture with all other conditions being constant. The εT values were
computed [27] from the values of the pure liquids. Earlier, it was found that there was no
reaction between the solvent and oxidant under the experimental conditions used. Variation
of the t-butyl alcohol–water solvent composition of the reaction medium had no effect on
the rate of reaction (Table 3).
3.7 Effect of Initially Added Products
Initially added products, 2-methyl propionaldehyde and copper(II) sulfate, in the concentra-
−
5
−4
−3
tion range 1.0 × 10 to 1.0 × 10 mol·dm did not alter the rate of reaction.
.8 Polymerization Study
3
As DPC is a single equivalent oxidant, the oxidation of organic compounds by such an oxi-
dant could lead to a free radical generation from the substrate. Hence, to test for the possible
intervention of free radicals, the reaction mixture was mixed with acrylonitrile monomer and
kept for 24 h. On dilution with methanol, no white precipitate of polymer formed, indicating
that the reaction occurs without intervention of free radicals. This was also evidenced by the
lack of changes in the kobs values from the presence of initially added acrylonitrile.
3.9 Effect of Temperature
The rate of reaction was measured at different temperatures and at constant concentrations of
reactants with other conditions being constant (Tables 4a, 4b). The rate of reaction increased
with increasing temperature, as usual. The activation energy was calculated from the slope
of a plot of log kobs versus 1/T (r ≥ 0.9948), and was used to calculate other activation
1
0
parameters (Tables 4a, 4b).
The water soluble Cu(III) periodate is thought to exist [28] as Na7KH[Cu(IO6)2]
(
[
a sodium potassium hydrogen copper(III) periodate complex). However, periodate exists
29] in various chemical forms depending on the pH. The predominant periodate species in

194
J Solution Chem (2012) 41:187–199
Table 4a Effect of temperature on the oxidation of L-valine by diperi-
−
4
−3
−3
odatocuprate(III) in alkaline medium: [DPC] = 1.0 × 10 mol·dm
,
,
−
3
−3
−
−4
[
L-Val] = 1.0 × 10 mol·dm , [IO ] = 2.5 × 10 mol·dm
4
−
−3
−3
−3
[
OH ] = 2.0 × 10 mol·dm , and I = 0.01 mol·dm
3
−1
Temperature (K)
10 × k (s
)
obs
3
03
08
13
18
4.14
5.51
3
3
3
7.80
10.34
Table 4b Activation parameters
Ea (kJ·mol 1
−
)
ꢀH (kJ·mol
#
−1
)
ꢀS (kJ·mol
#
−1
)
ꢀG (kJ·mol
#
−1
)
log
A
10
49 ± 2
47 ± 2
−49 ± 2
62 ± 3
6.0 ± 0.1
2
−
3−
the pH range 8–12 are H3IO and H2IO6 . In alkaline medium, periodate dimerizes and
6
exists as H2I2O4 at higher concentrations, but this dimer can be neglected in the present ex-
−
1
0
perimental conditions as low concentration of periodate were used. Hence, the co-ordinated
3
6
−
periodate in DPC has been deduced to be H2IO and the structure for DPC can be as-
3
−
signed [20] as [Cu(H2IO6)2] . A similar degree of protonation has earlier been postulated
by Kirrschenbaum et al. for the silver(III) periodate complex [30].
−
The inverse second-order dependency on [OH ] suggests that periodate in DPC is further
−
protonated by releasing 2 OH as shown in the following equilibrium:
K1
Cu(H2IO6)2] − + 2H2O ꢀ[Cu(H3IO6)2] + 2OH .
3
−
−
(1)
[
The retarding effect of initially added periodate suggests the dissociation of this complex
occurs in a pre-equilibrium step in which DPC loses a periodate ligand:
K2
−
+
2−
[
Cu(H3IO6)2] + 2H2O ꢀ[Cu(H3IO6)(H2O)2] + H3IO
.
(2)
6
The above two equilibria are written as proposed by Lister [31]. However, he did not
consider the pH dependency of the periodate equilibria. Nevertheless, these equilibria have
been incorporated in recent studies. Hence, it can be concluded that, among the various
+
forms of DPC, the monoperiodatocuprate(III) MPC complex, [Cu(H3IO6)(H2O)2] , is the
reactive species.
The second-order dependency on L-valine indicates that there is an uptake of L-valine in
the Cu(III) co-ordination sphere that involves replacing one of the non-labile H2O ligands,
in a prior equilibrium step before undergoing oxidation:
K3
+
+
[
Cu(H3IO6)(H2O)2] + L-Val ꢀ[Cu(H3IO6)(H2O)(L-Val)] + H2O.
This [Cu(H3IO6)(H2O)(L-Val)] complex may either further take another molecule of
(3)
+
L-valine into in its co-ordination sphere by releasing the remaining H2O in a slow step to
+
give the monoperiodato divaline cuprate(III) complex [Cu(H3IO6)(L-Val)2] , or there is a
re-orientation of the mono L-valine Cu(III) periodate complex before taking up the second
ligand. It is likely that L-valine is initially attached at its axial position. This complex then
reorganizes to give a four co-ordination complex, which then adds another ligand in a rate-
determining step as shown below:

J Solution Chem (2012) 41:187–199
195
Fig. 4 Michaelis-Menten plot of
[
L-valine]/k
versus
obs
1
/[L-valine] and 1/k
versus
obs
2
1/[L-valine] (conditions as in
Table 1)
k
+
+
[
Cu(H3IO6)(H2O)(L-Val)] + L-Val −→ [Cu(H3IO6)(L-Val)2] + H2O.
(4)
slow
A Michaelis-Menton plot of [L-Val]/kobs versus 1/[L-Val] justifies the presence of com-
plex formation between DPC and a single molecule of L-valine. This provides evidence for
2
a complex: the plot of 1/kobs versus 1/[L-Val] being linear with zero intercept (Fig. 4). This
supports the conclusion that the monoperiodatodivaline Cu(III) complex forms with a fast
equilibrium, prior to the rate-determining step, in which another molecule of L-valine binds
to the monoperiodato divaline copper(III). This hypothesis is also supported by earlier re-
ports [8] of oxidation of glycine by the silver(III) periodate complex. In a similar [32] study
on the Cu(III) system, the formation of [Cu(gly)2] was reported.
L-valine has two functional groups, viz., –NH2 and –COOH. The pKa value of –COOH
is less than 7.0 and that of –NH2 is greater than 7, and in alkaline medium, –COOH exists
−
as –COO . It is known that at low pH, complexation might be taking place through both the
basic –NH2 and –COOH groups. On the contrary, in basic medium, complexation mainly
occurs through the basic –NH2 group. Hence, ‘N’ of the –NH2 moiety acts as a donor atom
for complexation with Cu(III) periodate (Eqs. 3 and 4) as shown in (Scheme I):

196
J Solution Chem (2012) 41:187–199
Thus, similar complexation [8] occurs with L-valine and related compounds having an
amino group in addition to a carbonyl group, which undergo complexation as well as redox
reactions.
The above scheme of oxidation of L-valine is noteworthy, as this scheme is substantially
different from the earlier report [19] of oxidation of L-valine by DPC in alkaline solution.
In that report, both –NH2 and –COOH groups were involved in formation of a complex with
Cu(III) periodate having six co-ordination (Scheme II),
which is quite unusual as Cu(III) normally can accommodate only four electron pairs, with
2
dsp hybridization being present with a square planar structure.
−
Apart from this, the orders in [L-Val] and [OH ] were reported to be 0.77 and −0.33,
respectively, and an oxidation mechanism of L-valine involving free radicals was proposed.
The results obtained by a thorough investigation in the present study are in contradiction to
the reported values [19] as described above. In this report, the order in [L-Val] was found
−
to second order and that of [OH ] is negative second order. Moreover, the polymerization
study revealed that oxidation occurs without an intervening free radical. Furthermore, the
results of the present investigations are different from that report.
Equations 1 to 4 result in the following rate expression for the formation of the monope-
riodato di(L-valine) copper(III) complex via a slow step:
kK1K2K3[Cu(H2IO6)2] −[L-Val]
3
2
f
d[DPC]
dt
f
−
= rate =
.
(5)
2
6
−
− 2
[H IO ] [OH ]
3
f
f
Equation 5 explains all the observed kinetic features except the order in periodate.
Further, this monoperiodato di(L-valine) copper(III) complex reacts with the active form
of DPC (that is MPC) in a fast step to yield the products, by oxidizing only one molecule
of L-valine, while the other molecule remains unoxidized as is evidenced in stoichiometric
studies:
+
+
−
[
Cu(H3IO6)(L-Val)2] + [Cu(H3IO6)(H2O)2] + 2OH
−
fast
2
+
2−
→ CO2 + 2Cu + L-Val + 2H3IO6 + NH3 + 3H2O + H3C2 –CH–CHO. (6)
|
CH3
Equation 6 strongly supports the fact that only 50% of the L-valine undergoes oxidation and
rest remained un-oxidized, which is in accordance with the experimental results. Apart from

J Solution Chem (2012) 41:187–199
197
Fig. 5 Verification of the rate
law (Eq. 8) for oxidation of
L-valine by
diperiodatocuprate(III) in
alkaline solutions (conditions as
in Tables 1 and 2)
this, the polymerization study also supports the non-intervention of free radicals during the
oxidation of L-valine.
−
−
On incorporating the [Cu(III)]f, [L-Val]f, [OH ]f and [IO ]f concentration dependences
4
in Eq. 5 from Eqs. 1 to 4 for their totals, Eq. 7 then results:
2
d[DPC]
dt
kK1K2K3[Cu(III)]T[L-Val]
T
−
= rate =
(7)
(8)
−
2
2−
2−
6
[OH ] [H IO ] + K [H IO ] + K K K [L-Val] + K K
3
1
3
1
2
3
1
2
6
2
kK1K2K3[L-Val]
T
kobs =
.
−
2
2−
2−
[
OH ] [H3IO ] + K1[H3IO ] + K1K2K3[L-Val] + K1K2
6
6
The rate constant ‘k’ and equilibrium constant K3 of multiple equilibria can be evaluated by
using Eq. 8 after omitting the subscripts ‘T ’. This gives Eq. 9:
−
2
2−
2−
[
L-Val]
[OH ] [H3IO
]
[H3IO
]
1
1
6
6
=
+
+
+ k
.
(9)
kobs
kK1K2K3[L-Val]
kK2K3[L-Val] kK3[L-Val]
−
2
According to Eq. 9, plots of [L-Val]/kobs versus [OH ] , [L-Val]/kobs versus [L-Val] and
2
6
−
[
L-Val]/kobs versus [H3IO ] should to be linear and are found be so as shown in Figs. 5
and 6. The slopes and intercepts of such plots lead to values of k and K3 of 5.55(± 0.05) ×
3
3
−1 −1
3
−1
1
0 dm ·mol ·s and 1.79(± 0.15) dm ·mol , respectively.
The decrease in rate with increasing ionic strength (Table 3) at lower ionic strengths is
in the expected direction [33], as the positively charged mono L-valine Cu(III) periodate
complex interacts with the anionic form of L-valine in a slow step.
#
−1
−1
The magnitude of ꢀS = −49 ± 2 J·K ·mol suggests [34] that two ionic species
combine in a rate determining step to give a single intermediate complex that is more ordered
than the reactants. The small value of log A (6.0) indicates that reaction presumably occurs
1
0
through an inner sphere mechanism (Eqs. 4 and 6), which is also supported by the small
#
−1
.
value of ꢀG = 62 ± 3 kJ·mol

198
J Solution Chem (2012) 41:187–199
Fig. 6 Verification of the rate
law (Eq. 8) for the oxidation of
L-valine by
diperiodatocuprate(III) in alkali
solutions (conditions as
in Table 2)
4
Conclusions
+
−3
In conclusion, [Cu(H3IO6)(H2O)2] is the reactive species of DPC in 0.002 mol·dm al-
kali solution. Initially, it binds with L-valine in a first step to produce the mono L-valine
copper(III) periodate complex. This intermediate takes up another ligand to yield the
+
[
Cu(H3IO6)(L-Val)2] complex in a slow step. This complexation reaction possibly takes
place through the basic amino group. This monoperiodato di(L-valine) copper(III) com-
plex decomposes by the transfer of an electron from the ligand to the Cu(III) center of two
molecules simultaneously, and eventually yields products by decarboxylation and deamina-
tion. However, in this study, it was not possible to distinguish between the mode of electron
transfer to monoperiodato L-valine copper(III) and to monoperiodato di L-valine copper(III)
complexes.
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