Oxidation of Two α-Hydroxy Acids by Vanadium(V)

Article abstract of DOI:10.1007/s00706-003-0069-2

The pathways for the oxidation of tartaric acid and citric acid by vanadium(V) in perchloric acid medium have been studied at 303 K by following the formation of vanadium(IV) spectrophotometrically at 760 nm. The oxidation reaction displays fractional order dependence with respect to both the α-hydroxy acids concentrations as well as fractional (tartaric acid) and first order (citric acid) dependencies with respect to the perchloric acid concentration. It does not matter at all whether the ionic strength was varied or was maintained constant. The reaction rate remains essentially constant with increasing ionic strength in the tartaric acid oxidation, but decreases slightly in the citric acid case. A decrease of the dielectric constant of the reaction medium results in a rate decrease in the tartaric acid oxidation and in a rate enhancement in the citric acid reaction. The activation parameters were obtained from studies of the reactions at different temperatures (293 to 313 K). Based on the experimentally determined rate laws oxidation mechanisms and theoretical rate laws were proposed.

Full text of DOI:10.1007/s00706-003-0069-2

Monatshefte f €u r Chemie 134, 1321–1331 (2003)
DOI 10.1007/s00706-003-0069-2
Oxidation of Two a-Hydroxy Acids
by Vanadium(V)
R oˆ mulo A. Ando, Cristiano Raminelli, Wagner J. Barreto,
and Keiko Takashimaꢀ
Departamento de Qu ´ı mica, CCE, Universidade Estadual de Londrina, caixa postal 6001,
8
6051-990, Londrina, PR, Brazil
Received April 8, 2003; accepted May 10, 2003
Published online September 18, 2003 # Springer-Verlag 2003
Summary. The pathways for the oxidation of tartaric acid and citric acid by vanadium(V) in perchloric
acid medium have been studied at 303 K by following the formation of vanadium(IV) spectrophoto-
metrically at 760 nm. The oxidation reaction displays fractional order dependence with respect to both
the ꢀ-hydroxy acids concentrations as well as fractional (tartaric acid) and first order (citric acid)
dependencies with respect to the perchloric acid concentration. It does not matter at all whether the
ionic strength was varied or was maintained constant. The reaction rate remains essentially constant
with increasing ionic strength in the tartaric acid oxidation, but decreases slightly in the citric acid case.
A decrease of the dielectric constant of the reaction medium results in a rate decrease in the tartaric
acid oxidation and in a rate enhancement in the citric acid reaction. The activation parameters were
obtained from studies of the reactions at different temperatures (293 to 313K). Based on the experi-
mentally determined rate laws oxidation mechanisms and theoretical rate laws were proposed.
Keywords. Reaction kinetics; Aqueous solutions; Tartaric acid; Citric acid; Vanadium(V).
Introduction
The oxidation chemistry of aqueous acidic solutions of vanadium(V) is often com-
plicated because different species may be involved in equilibrium steps prior to
oxidation. For example, it has been established that vanadium(V) in acidic solution
þ
exists as pervanadyl ion ðVO Þ [1]. This ion is produced by the interaction of
2
ꢁ
vanadate ion ðVO Þ with water and yields more or less reactive species depend-
3
2
þ
ing on the nature of the acidic medium. Vanadium(V) species such as VO(OH)
and VðOHÞ
2
þ
are frequently involved in aqueous perchloric acid medium [2]
þ
3
whereas reactive species in sulfuric acid are attributed to VðOHÞ HSO and
4
3
þ
VðOHÞ ðHSO Þ [3]. In general, oxidation of ꢀ-hydroxy acids by vanadium(V)
4
2
has been shown to be faster in sulfuric acid under constant ionic strength
2
ꢀ Corresponding author. E-mail: keiko@uel.br

1
322
R. A. Ando et al.
conditions. As a part of our mechanistic studies [4–8], in this paper we report
the kinetic and mechanistic behavior of tartaric acid and citric acid oxidation by
vanadium(V) in aqueous perchloric acid medium. In these cases rates are faster
under conditions in which the ionic strength is varied.
Results
Effect of Reactants Concentration
The kinetic behavior at 303 K was studied at different substrate concentrations, [S],
maintaining constant the initial concentrations of V(V), [V(V)] , and of perchloric
o
ꢁ
5
acid, [HClO ] , as shown in Table 1 (r ꢂ 0.9990; sꢃ4.26ꢄ10 ). For both sub-
4
o
strates, HTa and HCi, the oxidation rate was observed to be faster in experiments
carried out at variable ionic strength. Thus, the rates are about 9% for HTa and
about 25% for HCi higher than those observed under conditions of constant ionic
ꢁ
2
strength. Fractional orders with respect to [HTa] (r ꢂ 0.9985; sꢃ1.43ꢄ10 ) and
ꢁ
2
[HCi] (r ꢂ 0.9863; sꢃ3.79ꢄ10 ) were found, if the ionic strength was varied or
was maintained constant. The first order dependence on the V(V) concentration,
Table 1. Pseudo-first order rate constants for the oxidation of tartaric acid and citric acid by V(V) at
3
03 K for variable and constant ionic strength (I)
ꢁ
3
ꢁ2
ꢁ3
ꢁ3
ꢁ3 ꢁ1
s
[S]=mol dm
[V(V)]=10 mol dm
I=mol dm
kobs=10
HTa
HCi
a
a
a
a
a
b
b
b
b
b
a
a
a
a
b
b
b
b
0
0
1
1
1
0
0
1
1
1
1
1
1
1
1
1
1
1
.50
.75
.00
.25
.50
.50
.75
.00
.25
.50
.00
.00
.00
.00
.00
.00
.00
.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
0.50
1.50
2.00
2.50
0.50
1.50
2.00
2.50
1.06
1.29
1.49
1.67
1.87
0.97
1.17
1.36
1.56
1.72
1.30
1.63
1.78
1.82
1.28
1.44
1.53
1.61
3.09
3.56
4.10
4.70
5.15
2.51
2.85
3.21
3.67
4.15
4.57
3.28
2.79
–
4.10
2.42
1.90
–
a
ꢁ3
þ
ꢁ3
for HTa and
Initial ionic strength is 0.16 mol dm
and [HClO ] ¼ [H ] ¼ 0.15 mol dm
4
.21 mol dm 3 and [H ] ¼ 0.20 mol dm for HCi, but changes in the course of the reaction; The
ꢁ
þ
ꢁ3
b
0
ꢁ
3
ꢁ3
ionic strength for HTa is 1.10 mol dm and 1.00 mol dm for HCi achieved by using NaClO as
4
added electrolyte

ꢀ-Hydroxy Acids Oxidation by Vanadium(V)
1323
[
(
V(V)], in perchloric acid can be inferred from the satisfactory linearity observed
ꢁ
4
r ꢂ 0.9988; sꢃ3.58ꢄ10 ) for the individual plots of ln [V(V)] as a function
ꢁ
2
of time at 303 K. For HTa oxidation, variation of [V(V)] from 0.50ꢄ10
o
ꢁ
2
ꢁ3
to 2.50ꢄ10 mol dm
increased the rate constant by approximately 40%
ꢁ
5
(
r ¼ 0.9815; s ¼ 4.56ꢄ10 ) in solutions at variable ionic strength, and by approxi-
ꢁ
6
mately 24% (r ¼ 0.9998; s ¼ 3.16ꢄ10 ) at constant ionic strength. By compari-
ꢁ
4
son, the HCi oxidation decreases by about 63% (r ¼ ꢁ0.9939; s ¼ 1.08ꢄ10 ) in
media with variable ionic strength, and by more than 100% (r ¼ ꢁ0.9935;
ꢁ
4
s ¼ 1.34ꢄ10 ) at constant ionic strength, when [V(V)] is varied in the range from
ꢁ
2
ꢁ2
ꢁ3
0
.50ꢄ10 to 2.00ꢄ10 mol dm as presented in Table 1.
Effect of Hydrogen Ion Concentration
The HTa oxidation rate was measured at different hydrogen ion concentrations
while maintaining the same initial reactant concentrations, [V(V)] and [S] at
o
o
þ
303 K. Results are shown in Table 2 for [H ] (perchloric acid) ranging from 0.05
to 1.00 mol dm . The reaction order with respect to [H ] is fractional whether
ꢁ
3
þ
ꢁ
3
the ionic strength was varied or was equal to 1.10 mol dm
þ
(r ꢂ 0.9978;
ꢁ
2
ꢁ3
sꢃ2.81ꢄ10 ). For the HCi oxidation, [H ] was varied from 0.10 to 0.90 mol dm
and a first order dependence for variable and constant ionic strength medium of
ꢁ
3
ꢁ2
1
.00 mol dm (r ꢂ 0.9978; sꢃ5.06ꢄ10 ) was obtained.
Effect of Ionic Strength
The ionic strength effect on the reaction rate was investigated by adding the proper
3
ꢁ
amount of sodium perchlorate to cover the range from 0.16 to 2.00 mol dm in
ꢁ
3
the HTa oxidation, and from 0.30 to 1.25 mol dm in the HCi case (Table 3).
Applying the Debye-H €u ckel equation by a plot of log kobs as a function of the
square root of ionic strength at 303 K [9, 10] yields practically zero (r ¼ 0.9888;
ꢁ
3
s ¼ 2.85ꢄ10 ) for z z the product of ionic charges for HTa, and ꢁ0.2
þ ꢁ,
ꢁ
3
(
r ¼ 0.9940; s ¼ 3.71ꢄ10 ) for HCi. A good linear relationship is observed in
both cases even though the activity coefficients of many electrolytes are known
to be fairly dependent on ionic strength. Moreover, the range of ionic strength
covered in the oxidation by V(V) is somewhat outside the usual range where an
extended Debye-H €u ckel law is valid.
Effect of Solvent
The reaction was studied in methanol=water mixtures of different composition
(
Table 4). For HTa oxidation, reducing the dielectric constant (") of the medium
by varying the methanol content (0–40% v=v) decreased the rate constant by
approximately 7% for variable ionic strength and by approximately 46% at con-
stant ionic strength. On the other hand, an increase by around 50% at variable ionic
strength and by around 30% at fixed ionic strength was observed for the HCi
oxidation rate constant by varying the methanol content (0–35% v=v). Plots of
ln kobs vs. the reciprocal of the medium dielectric constant, 1=", [11] of both
ꢁ
2
substrates resulted in straight lines with negative (r ꢂ ꢁ0.9920; sꢃ2.00ꢄ10 )

1
324
R. A. Ando et al.
Table 2. Perchloric acid concentration effect on the pseudo-first order rate constants for the oxidation
of tartaric acid and citric acid by V(V) at 303 K for variable and constant ionic strength (I);
2
[
V(V)] ¼ 1.00ꢄ10ꢁ mol dmꢁ3; [HTa] ¼ [HCi] ¼ 1.00 mol dmꢁ3
ꢁ
3
ꢁ3
ꢁ3 ꢁ1
[HClO ]=mol dm
4
I=mol dm
kobs=10
s
HTa
HCi
a
a
a
a
a
a
a
a
a
a
a
a
a
a
a
b
b
b
b
b
b
b
b
b
b
b
b
b
b
0.05
0.10
0.15
0.20
0.25
0.30
0.40
0.50
0.55
0.60
0.70
0.80
0.85
0.90
1.00
0.10
0.15
0.20
0.25
0.30
0.40
0.50
0.55
0.60
0.70
0.80
0.85
0.90
1.00
1.07
1.35
1.49
–
–
1.97
–
4.10
1.92
–
–
5.64
–
8.05
10.96
–
2.22
2.60
–
12.22
13.69
15.97
–
2.87
–
3.04
–
16.87
–
3.30
1.16
1.36
–
1.41
–
3.21
–
1.82
–
5.58
7.37
10.29
–
2.37
–
2.73
–
10.81
14.85
15.84
–
3.17
–
3.44
–
18.56
–
3.73
a
ꢁ3
þ
ꢁ3
b
For HTa, initial ionic strength is 0.16 mol dm and [HClO ] ¼ [H ] ¼ 0.15 mol dm ; for HCi,
4
.21 mol dm 3 and [H ] ¼ 0.20 mol dm , but changes in the course of the reaction; The ionic
ꢁ
þ
ꢁ3
0
ꢁ
3
ꢁ3
strength for HTa is 1.10 mol dm and 1.00 mol dm for HCi achieved by using NaClO as added
4
electrolyte
ꢁ
3
and positive (r ꢂ 0.9892; sꢃ9.79ꢄ10 ) slopes, respectively. Control experiments
showed negligible oxidation of methanol by V(V).
Effect of Temperature
The oxidation rates were measured at five different temperatures (293 to 313 K) by
3
ꢁ
varying the substrate concentration (0.50 to 1.50 mol dm ) while maintaining

ꢀ-Hydroxy Acids Oxidation by Vanadium(V)
1325
Table 3. Ionic strength (I) effect on the pseudo-first order rate constants for tartaric acid and citric
acid oxidation by V(V) at 303 K; [V(V)] ¼ 1.00ꢄ10ꢁ2 mol dmꢁ3; [HTa] ¼ [HCi] ¼ 1.00 mol dmꢁ3
ꢁ
3
ꢁ3 ꢁ1
I=mol dm
kobs=10
s
a
b
HTa
HCi
0
0
0
0
0
0
0
0
0
1
1
1
1
1
2
.16
.20
.30
.40
.50
.60
.70
.80
.90
.00
.10
.25
.40
.70
.00
1.26
1.29
–
–
–
3.90
3.72
3.61
3.57
3.40
3.37
3.31
3.21
–
–
1.31
–
–
1.33
–
–
1.36
–
3.10
–
1.38
1.39
1.42
–
–
a
ꢁ3
ꢁ3
þ
ꢁ3
b
Initial ionic strength is 0.16 mol dm and [H ] ¼ 0.15 mol dm
;
Initial ionic strength is
.21 mol dm 3 and [H ] ¼ 0.20 mol dm
ꢁ
þ
0
Table 4. Pseudo-first order rate constants for HTa and HCi oxidation by V(V) in aqueous methanol
ꢁ
2
ꢁ3
solutions at 303 K for variable and constant ionic strength (I); [V(V)] ¼ 1.00ꢄ10 mol dm
;
ꢁ
3
[HTa] ¼ [HCi] ¼ 1.00 mol dm
ꢁ
3
ꢁ3 ꢁ1
[
MeOH]=%
I=mol dm
kobs=10
HTa
s
HCi
a
a
a
a
a
a
a
b
b
b
b
b
b
b
0
1.49
–
4.10
4.62
4.91
5.08
5.46
5.83
–
15
20
25
30
35
40
0
5
0
5
0
5
0
1.45
1.43
1.42
1.41
1.39
1.36
–
3.21
3.40
3.55
3.63
3.70
3.77
–
1
2
2
3
3
4
1.19
1.10
1.03
0.96
0.93
a
ꢁ3
þ
ꢁ3
ꢁ3
For HTa initial ionic strength is 0.16 mol dm and [H ] ¼ 0.15 mol dm ; for HCi, 0.21 mol dm
þ
ꢁ3
b
and [H ] ¼ 0.20 mol dm , but changes in the course of the reaction; The ionic strength for HTa is
ꢁ ꢁ3
.10 mol dm and 1.00 mol dm for HCi achieved by using NaClO as added electrolyte
4
3
1

1
326
R. A. Ando et al.
Table 5. Temperature effect on the tartaric acid (HTa) and citric acid (HCi) oxidation by V(V) for
variable and constant ionic strength (I) ; [V(V)] ¼ 1.00ꢄ10 mol dmꢁ3; [S] ¼ 0.50ꢁ1.50 mol dmꢁ3
a
ꢁ2
ꢁ
3
ꢁ3 ꢁ1
T=K
I=mol dm
kobs=10 s
ꢁ
3
[
HTa]=mol dm
0.50
0.75
1.00
1.25
1.50
b
b
b
b
b
2
2
3
3
3
2
2
3
3
3
93
98
03
08
13
93
98
03
08
13
0.36
0.66
1.06
1.79
3.30
0.31
0.54
0.97
1.72
3.16
0.43
0.77
1.29
2.25
4.06
0.39
0.69
1.17
2.11
3.90
0.50
0.88
1.49
2.40
4.58
0.46
0.81
1.36
2.54
4.63
0.56
0.97
1.67
2.93
5.14
0.55
0.90
1.56
2.88
5.16
0.60
1.08
1.87
3.09
5.87
0.63
1.03
1.72
3.12
5.69
1.10
1.10
1.10
1.10
1.10
ꢁ
3
ꢁ3 ꢁ1
T=K
I=mol dm
kobs=10
s
ꢁ
3
[
HCi]=mol dm
0.50
0.75
1.00
1.25
1.50
c
c
c
c
c
2
2
3
3
3
2
2
3
3
3
93
98
03
08
13
93
98
03
08
13
1.00
1.92
3.09
4.65
7.54
0.75
1.57
2.51
3.90
5.49
1.20
2.23
3.56
5.80
8.95
0.87
1.69
2.85
4.29
6.83
1.35
1.54
1.75
2.47
4.10
6.72
10.33
0.99
1.96
3.21
5.19
7.35
2.68
4.70
7.31
11.01
1.10
2.17
3.67
5.81
8.24
2.99
5.15
8.17
12.55
1.28
2.44
4.15
6.38
8.97
1.00
1.00
1.00
1.00
1.00
a
b
Constant ionic strength by using NaClO4 as added electrolyte;
Initial ionic strength is
.16 mol dm 3 and [H ] ¼ 0.15 mol dm , but changes in the course of the reaction; Initial ionic
ꢁ
þ
ꢁ3
c
0
ꢁ
3
þ
ꢁ3
strength is 0.21 mol dm and [H ] ¼ 0.20 mol dm , but changes in the course of the reaction
constant the other experimental conditions. The results for HTa (r ꢂ 0.9962;
ꢁ
5
ꢁ5
sꢃ3.89ꢄ10 ) and for HCi (r ꢂ 0.9930; sꢃ4.65ꢄ10 ) are shown in Table 5.
The reaction order for both ꢀ-hydroxy acids remained around 0.5 in this tempera-
ture range. The apparent second order rate constants, k , were calculated from the
slopes of the linear plots of kobs against substrate concentration at different tem-
0
ꢁ
4
peratures (r ꢂ 0.9903; sꢃ2.14ꢄ10 ). Activation parameters were calculated
from the Eyring equation. For HTa oxidation, the enthalpies of activation,
DH , 86.4 and 77.3 kJ mol , and the activation entropies, DS , ꢁ19 and
z
ꢁ1
z
ꢁ
1
ꢁ1
ꢁ2
ꢁ
49 J mol
K
stant ionic strength, respectively. For HCi, the enthalpies of activation are, 73.2 and
(r ꢂ 0.9990; sꢃ7.52ꢄ10 ) were calculated for variable and con-
ꢁ
1
ꢁ1 ꢁ1
7
1.1 kJ mol , and the activation entropies, ꢁ56 and ꢁ65 J mol
K
(r ꢂ 0.9926;
ꢁ
2
sꢃ4.02ꢄ10 ) for variable and constant ionic strength, respectively.

ꢀ-Hydroxy Acids Oxidation by Vanadium(V)
1327
Discussion
The empirical rate law found for HTa oxidation, whether the ionic strength was
varied or was kept constant, obeys fractional order dependence with respect to
the HTa (Table 1) and to the hydrogen ion concentration (perchloric acid) from
ꢁ
3
0
.05 to 1.00 mol dm (Table 2). The HCi oxidation displays fractional order
dependence with respect to the substrate and first order dependence on the
hydrogen ion concentration. This is observed in the range of 0.10 to
ꢁ
3
0.90 mol dm regardless of the ionic strength being variable or constant. The
reaction is of first order with respect to the individual V(V) concentration, but
ꢁ
2
the rate constants increase for HTa oxidation between 0.50ꢄ10
and
.50ꢄ10 mol dm V(V) and decrease for HCi oxidation from 0.50ꢄ10 until
ꢁ
2
ꢁ3
ꢁ2
2
2
ꢁ ꢁ3
2
.00ꢄ10 mol dm V(V) (Table 1).
Unlike the HTa and HCi oxidation by V(V) in sulfuric acid medium [4, 5],
these two substrates are oxidized faster in perchloric acid when the ionic strength
is varied. In order to understand this behavior, it is relevant to know the reactivity
of V(V) species in perchloric acid medium under both conditions. With this in
þ
mind, geometrical optimization of V(V) hydrated species, namely VðOHÞ and
4
[1, 3], was performed through density functional theory (DFT) at the
2þ
VðOHÞ
3
SVWN=DN [12] level. These calculations yield a tetrahedral structure for
is predicted to be a planar
symmetric species with a dipole moment. Based on these results, the higher reac-
ꢀ
þ
2þ
VðOHÞ with no dipole moment, whereas VðOHÞ
4
3
2
þ
tivity can be attributed to VðOHÞ3 which is presumably the reactive species in
perchloric acid medium at variable ionic strength. Whereas the less reactive
species, VðOHÞ , would be found in a medium of constant ionic strength.
þ
4
A positive dielectric effect (Table 4) and the absence of a primary salt effect
(
plex, X , in the fast pre-equilibrium step. Furthermore, the activation entropies
Table 3) in the HTa oxidation suggest the production of a charged HTa-V(V) com-
ꢀ
2
þ
ꢁ
1
ꢁ1
found for HTa of ꢁ19 and ꢁ49 J mol
K at variable and constant ionic strength,
respectively, indicate that the activated complexes formed are more compact than
the reagents. The interaction of the activated species with a water molecule
yields the first CO through a C–C fission and a free radical, HOOCCH (OH)
2
2
C ꢅ CH COOH in the slow step. The formation of a polymeric product by addition
2
of acrylamide indicates that V(V) behaves as an one-equivalent oxidant [13]. Sub-
sequently, this radical could react with another V(V) to produce formaldehyde
according to the scheme presented for constant ionic strength:
2
þ
VðOHÞ4þ þ H þ HOOCCHðOHÞCHðOHÞCOOH
þ
Xꢀ
ð1Þ
ð2Þ
2
þ
k2
Xꢀ
ꢁꢁꢁ! HOOCCHðOHÞꢅCHðOHÞ þ VðOHÞ3þ
þ CO þ H O
þ
þ
2
3
k3
þ
HOOCCHðOHÞꢅCHðOHÞ þ VðOHÞ ꢁꢁꢁ! 2 HCOH þ CO þ VðOHÞ þ H O
ð3Þ
The mechanism under conditions of variable ionic strength is similar to the one
mentioned above, but VðOHÞ3 has to be taken into account as the oxidant. The
2
2
4
3
2
þ

1
328
R. A. Ando et al.
rate law, assuming k to represent the slow step, leads to
2
þ
þ
k K ½HTaꢆ½H ꢆ½VðVÞꢆ
k K ½HTaꢆ½H ꢆ
2
1
2
1
t
v ¼
v ¼ k ½VðVÞꢆ kobs
obs
¼
ð4Þ
þ
t
þ
1
þ K ½HTaꢆ½H ꢆ
1 þ K ½HTaꢆ½H ꢆ
1
1
where the total V(V) concentration at constant ionic strength is given by
ꢀ
þ
4
½
VðVÞꢆ ¼ ½ðX Þꢆ þ ½VðOHÞ ꢆ. The rate law (4) is in agreement with the experi-
t
ꢁ
1
ꢁ1
mental results since linearity is observed for the plot of k
vs. [HTa]
obs
ꢁ
3
ꢁ1
(
r ꢂ 0.9913; sꢃ27.3741). K is calculated to be 7.94 and k , 2.82ꢄ10
s
for
for fixed ionic
1
2
0
0
2
ꢁ3 ꢁ1
variable ionic strength, while K ¼ 7:53 and k ¼ 2:64ꢄ10
s
1
strength.
On the other hand, increasing the methanol content in the reaction mixture,
leads to an increase in the rate constant for the HCi oxidation (Table 4). This may
be attributed to a larger stabilization of the activated complex at low relative
permittivity, which is less solvated than V(V) at higher dielectric constant because
of its larger size. In addition, the activated complex is less polar than the reagents,
i.e., there is a negative charge delocalization of the carboxylate group through the
bonds involved in the reaction. The entropies of activation found for HCi are ꢁ56
ꢁ
1
ꢁ1
K for variable and constant ionic strength, respectively. These
and ꢁ65 J mol
values indicate that the activated complexes with HCi are more compact than those
with HTa. Except for the nature of the V(V) species, the mechanism is similar for
media of variable and constant ionic strength. Taking into account that one of the
reasons for the rate decrease is due to the lower solubility of HCi in perchloric acid
medium than in sulfuric acid, the oxidation rate as a function of the ionic strength
may be considered practically constant, although the ionic charges product is ꢁ0.2
(
Table 3). Hence, HCi should undergo two deprotonation steps prior to reaction
2
þ
ꢀ
with variable ionic strength. Subsequently, the activated species Y interacts with
with VðOHÞ
to provide a neutral species, Y , in the transition state in a medium
ꢀ
3
þ
H to produce the first CO and the radical HOOCCH (OH)C ꢅ CH COOH in the
2
2
2
rate limiting step, as described in the following scheme:
2
þ
ꢁ
ꢁ
Yꢀ
VðOHÞ3 þ HOOCCH ðOHÞCðCOO ÞCH COO
ð5Þ
ð6Þ
2
2
k5
ꢀ
þ
þ
3
Y þ H ꢁꢁꢁ! HOOCCH ðOHÞCꢅCH COOH þ CO þ VðOHÞ
2
2
2
By comparison, HCi should undergo only one deprotonation before interacting
ꢀ
þ
with VðOHÞ to produce a neutral activated species Z in a medium of constant
4
ionic strength:
VðOHÞ4þ þ HOOCCH ðOHÞCðCOO ÞCH COOH
ꢁ
Zꢀ
ð50Þ
ð60Þ
2
2
k5
ꢀ
þ
þ
3
Z þ H ꢁꢁꢁ! HOOCCH ðOHÞC ꢅ CH COOH þ CO þ VðOHÞ
2
2
2
In both cases, the subsequent rapid step would be the production of one more CO2,
the HOOCCH (OH)C ꢅ CH radical, and V(IV). In the final step, acetone is pro-
2
3
duced along with the release of a third CO molecule. Assuming the slow step as
2
0
the rate-determining one, k and k , these mechanisms lead to the rate laws (7) for
5
5

ꢀ-Hydroxy Acids Oxidation by Vanadium(V)
1329
the oxidation of HCi by V(V) in perchloric acid medium for variable and constant
ionic strength, respectively.
þ
þ
k K ½HCiꢆ½H ꢆ½VðVÞꢆ
k K ½HCiꢆ½H ꢆ
5
4
5
4
t
v ¼
v ¼ k ½VðVÞꢆ kobs
obs
¼
ð7Þ
t
1
þ K ½HCiꢆ
1 þ K ½HCiꢆ
3
4
ꢀ
2þ
Here the total concentrations of V(V) are given by ½VðVÞꢆ ¼ ½ðY Þꢆ þ ½VðOHÞ
ꢆ
t
3
ꢀ
þ
4
at variable ionic strength, and by ½VðVÞꢆ ¼ ½ðZ Þꢆ þ ½VðOHÞ ꢆ at constant ionic
t
strength. The rate law (7) is consistent with the experimental data, verified by the
excellent linear plots of k
ꢁ
1
þ ꢁ1
vs. [H ] (r ꢂ 0.9977; sꢃ15.3616) rather than
obs
ꢁ
1
ꢁ1
0
kobs vs. [HCi] (r ꢂ 0.9698; sꢃ17.7849). However, the numerical values of
0
5
K and k (or K and k ) cannot be calculated from the expression above because
4
5
4
þ
of the dependence between [H ] and [HCi].
So far our approach has been based entirely on the experimental results. The
values of DH and DS have been obtained from the Eyring equation and inter-
z
z
z
z
preted at a molecular level. The lower DH and the more negative DS account for
the reaction being faster in sulfuric acid medium [4–7]. However, the oxidation
takes place faster when the ionic strength is variable in perchloric acid medium and
surprisingly the activation parameters follow the same trend as that observed in
sulfuric acid medium. These values are presented in the section about the tempera-
ture effect and obtained from Table 5. This inversion suggests that occasionally it is
adequate to link the equilibrium properties to the kinetic process. Furthermore, it is
relevant to mention that there is a correlation between DH and DS values [14]
obtained from the oxidation of all the ꢀ-hydroxy acids by V(V) in both acidic
media already investigated [4–8] with tartaric acid and citric acid in this work.
Fig. 1 (r ¼ 0.9984; s ¼ 0.9111) shows the excellent linearity obtained for all these
substrates. This means that the mechanism is independent of the structure of these
z
z
ꢀ-hydroxy acids and that, the reactivity order may be reversed at a lower tempera-
ture than the isokinetic [14] one calculated to be 323 K.
z
z
Fig. 1. Relationship between enthalpy (DH ) and entropy of activation (DS ) obtained from some
ꢀ-hydroxy acids oxidation by V(V) in acidic medium; (a) malic acid; (b) lactic acid; (c) citric acid;
(d) tartaric acid

1
330
R. A. Ando et al.
In conclusion, the smaller V(V) reactivity in perchloric acid towards the HCi
and HTa has brought out one characteristic not found previously in sulfuric acid
medium, i.e., the reactions are more favorable at variable ionic strength. Besides,
the rate constant decrease with increasing V(V) concentration is more perceptible
for the HCi oxidation than for the HTa one. This is due to the fact that HCi
oxidations suffer more from steric hindrances than HTa oxidations do. In addition,
HCi deprotonates once and twice in media of constant and variable ionic strength,
respectively. Sulfuric acid behaves differently, it reacts in the molecular form prior
to the equilibrium step to yield the activated complex.
Experimental
Reagent grade chemicals and deionized water were employed in this work. Stock solutions of vana-
dium(V), V(V), were prepared as described previously [6]. NaClO was used to maintain the ionic
4
strength.
The kinetic measurements were carried out under pseudo-first order conditions with the sub-
strate and perchloric acid in excess over V(V). The appearance of vanadium(IV), V(IV), was
monitored spectrophotometrically (Hitachi U3000) at 760nm (molar absorption coefficient, 18.91ꢇ
ꢁ
3
ꢁ1
0
3
1
.83mol dm cm ). The bath water was recirculated and maintained at a temperature of
03.0ꢇ0.1 K (Microqu ´ı mica MQBTC99-20). The oxidation progress was followed in a thermostated
cm quartz cell for periods of up to four half-lives. First-order rate constants, kobs, were determined
from the slopes of linear plots of ln [V(V)] against time, and were reproducible within ꢇ5%. The
measurements were carried out in duplicate and, when necessary, in triplicate. The mediation of free
radicals in the oxidation, the production of CO , the characterization of formaldehyde produced from
2
tartaric acid, HTa, and acetone from citric acid, HCi, as well as the reaction stoichiometry have been
previously described [4, 5]. The overall reaction for HTa and HCi oxidation by vanadium(V) may be
represented by the following equations:
HOOCCHðOHÞCHðOHÞCOOH þ 2 VðVÞ ! 2 HCOH þ 2 CO2 þ 2 VðIVÞ þ H2O
HOOCCH2ðOHÞCðCOOHÞCH COOH þ 3 VðVÞ ! CH3COCH3 þ 3 CO2 þ 3 VðIVÞ þ H2O
2
2
þ
þ
Here, V(V) stands for VðOHÞ3 and VðOHÞ species in perchloric acid solutions for variable
4
þ
and constant ionic strength, respectively, while V(IV) represents the VðOHÞ species under both
3
conditions.
Acknowledgments
This work was possible through the financial support of PADCT=CAPES, CNPq=PIBIC (R.A.A.), and
CPG=UEL. We thank Prof. J. M. Riveros (University of S ~a o Paulo) for valuable suggestions concern-
ing this study.
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ꢀ-Hydroxy Acids Oxidation by Vanadium(V)
1331
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