Picolinic acid promoted oxidative decarboxylation of phenylsulfinylacetic acid by Cr(VI)

Article abstract of DOI:10.4314/bcse.v30i1.13

The kinetics and mechanism of picolinic acid promoted reaction of phenylsulfinylacetic acid (PSAA) with Cr(VI) was carried out in aqueous acetonitrile medium under pseudo first order conditions. The reaction follows Michaelis-Menten type of kinetics with respect to PSAA. The catalytic activity by picolinic acid can be interpreted on the basis of the formation of a highly active oxidizing species, Cr(VI)-PA complex. The mechanism involves the formation of a termolecular complex, Cr(VI)-PA-PSAA by the nucleophilic attack of the sulfur atom of PSAA on chromium of Cr(VI)-PA complex in an equilibrium step followed by ligand coupling in a slow step. Electron releasing substituents in the phenyl ring of PSAA accelerate while electron withdrawing groups retard the reaction rate. The overall rate constants for the para- and meta-substituted PSAAs are found to correlate excellently with Hammett ó constants with a very low reaction constant, ρ.

Full text of DOI:10.4314/bcse.v30i1.13

Bull. Chem. Soc. Ethiop. 2016, 30(1), 137-146.
Printed in Ethiopia
DOI: http://dx.doi.org/10.4314/bcse.v30i1.13
ISSN 1011-3924
 2016 Chemical Society of Ethiopia
PICOLINIC ACID PROMOTED OXIDATIVE DECARBOXYLATION OF
PHENYLSULFINYLACETIC ACID BY Cr(VI)
Perumal Subramaniam1* and Natesan Thamil Selvi2
1Research Department of Chemistry, Aditanar College of Arts and Science,
Tiruchendur-628 216, India
2Govindammal Aditanar College for Women, Tiruchendur, India
(Received August 22, 2013; revised October 29, 2015)
ABSTRACT. The kinetics and mechanism of picolinic acid promoted reaction of phenylsulfinylacetic acid
(PSAA) with Cr(VI) was carried out in aqueous acetonitrile medium under pseudo first order conditions. The
reaction follows Michaelis-Menten type of kinetics with respect to PSAA. The catalytic activity by picolinic acid
can be interpreted on the basis of the formation of a highly active oxidizing species, Cr(VI)-PA complex. The
mechanism involves the formation of a termolecular complex, Cr(VI)-PA-PSAA by the nucleophilic attack of the
sulfur atom of PSAA on chromium of Cr(VI)-PA complex in an equilibrium step followed by ligand coupling in a
slow step. Electron releasing substituents in the phenyl ring of PSAA accelerate while electron withdrawing
groups retard the reaction rate. The overall rate constants for the para- and meta-substituted PSAAs are found to
correlate excellently with Hammett σ constants with a very low reaction constant, ρ.
.
KEY WORDS: Phenylsulfinylacetic acid, Cr(VI), Picolinic acid, Oxidative decarboxylation, Substituent effect,
Catalysis
INTRODUCTION
Cr(VI) is an excellent oxidizing agent for both preparative and analytical purposes. During
oxidation reactions, the main co-ordination site of Cr binding involves alcoholato, carboxylato
and thiolato groups [1, 2]. The pyridine bases and other ligands containing hetero nitrogen
atoms were found to facilitate the Cr(VI) oxidation reactions to a greater extent and the various
chelating agents explored for this purpose are picolinic acid, pyridine-2,6-dicarboxylic acid,
2,2’-bipyridyl, 1,10-phenanthroline, EDTA, etc. In the presence of chelating agents the redox
activity of Cr(VI) has been found to change drastically. The complexing agents influence the
nature of Cr(VI) species in the reaction mixture and even affect the mechanism of Cr(VI)
oxidation in some cases.
Picolinic acid (PA), an endogenous metabolite of L-tryptophan that has been detected in a
variety of biological media including cell culture supernatants, blood serum [3], cerebrospinal
fluid [4], pancreatic juice and intestinal homogenates [5], possesses a wide range of
neuroprotective, immunological and anti-proliferative effects [6]. PA is an efficient chelating
agent which forms mono and bis complexes through pyridine nitrogen and carboxylate oxygen
[7] with a range of metals including Tl, Mo and W [8, 9]. Capitalizing on its chelation
properties, PA-metal complexes are widely used as means of introducing bioactive metals into
biological systems [10, 11]. In the case of PA catalyzed Cr(VI) oxidation reactions, PA is not
itself oxidized but lost during the reaction due to the formation of an inert Cr(III)-PA complex.
Thus PA is not a true catalyst and it is better described as a promoter. Picolinic acid promoted
Cr(VI) oxidation of several organic substrates [12-16] including organic sulfur compounds [12,
17-19] have been extensively studied. The catalytic effect was attributed to the activation of
oxidizing species of chromium through a precursor complex formation.
Though phenylsulfinylacetic acid (PSAA) is a versatile compound finding utility in
pharmaceutical and synthetic fields [20-26], literature survey clearly reveals that there is no
__________
*Corresponding author. E-mail: subramaniam.perumal@gmail.com

138
Perumal Subramaniam and Natesan Thamil Selvi
systematic mechanistic study on the oxidation of PSAA except our recent publications [27-31].
Hence, in continuation of our work it is decided to carry out a systematic investigation on the
PA promoted Cr(VI) reaction with PSAA with a view to proposing a suitable mechanistic path.
EXPERIMENTAL
Phenylsulfinylacetic acid and several para- and meta-substituted PSAAs were synthesized from
the corresponding phenylthioacetic acids via controlled oxidation using equimolar amount of
H2O2 [32] and recrystallised from suitable solvents [33]. The purity of the samples was checked
by LCMS and comparing their melting points with the literature values [33]. Potassium
dichromate (Merck), sodium perchlorate (Merck), HClO4 (Merck) and picolinic acid (SDS) were
of AnalaR grade and were used as received. The solvents, acetonitrile and water were purified
by literature methods.
Kinetic measurements
The kinetic measurements were performed by monitoring absorbance changes using Elico
Double beam UV-vis Bio-spectrophotometer with an inbuilt thermostat, under pseudo first order
conditions by maintaining [PSAA] » [Cr(VI)] in 40% acetonitrile - 60% water (v/v) medium in
the presence of PA. The disappearance of Cr(VI) was followed at 351 nm until 75% of Cr(VI)
was consumed and the spectral changes observed at different time intervals during the reaction
are given in Figure 1. Sodium perchlorate and perchloric acid were used to maintain the ionic
strength and acidity of the medium, respectively.
Figure 1. UV-visible spectra for the kinetic run. [PSAA] = 5.0 x 10-2 mol dm-3; [Cr(VI)] = 5.0 x
10-4 mol dm-3; [PA] = 0.3 mol dm-3; [H+] = 0.75 mol dm-3.
The pseudo-first order rate constant (k1) for each kinetic run was evaluated from the slope of
log OD vs time by the method of least squares. The precision of k values is given in terms of
95% confidence limits of Student’s t test.
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Picolinic acid promoted oxidative decarboxylation of phenylsulfinylacetic acid by Cr(VI) 139
Product analysis
The reaction mixture with excess of [Cr(VI)] over [PSAA] was kept for 48 hours to ensure
completion of the reaction and the solution was then extracted with ether. The ether layer was
collected, dried over anhydrous sodium sulfate and the solvent was removed by evaporation.
The GC-MS chromatogram of the product (Figure 2) with 100% abundance eluted at a retention
time of 12.47 AV gave the base parent peak at m/z = 156. The value shows the molar mass of
methyl phenyl sulfone. The peak position and fragmentation pattern exactly matches with that of
the reference, methyl phenyl sulfone (Figure 2 - inset). This is further confirmed by IR
spectroscopy (Figure 3) which shows strong bands at 1148 cm-1 and 1290 cm-1 characteristic of
symmetric and asymmetric stretching respectively of >SO2 group [34, 35]. The other
characteristic absorption frequencies (cm-1) of methyl phenyl sulfone are sp2 C-H stretch (3024),
sp2 C-H bend (744), sp3 C-H stretch (2922), sp3 C-H bend (1451), aromatic C=C (1647) and
aromatic C-C stretch (1402).
Figure 2. GC-MS spectrogram of the product.
The UV-visible spectroscopy aids in the determination of the final fate of Cr(VI) in the
reaction mixture. The UV-visible spectra of Cr(III) ion shows two peaks at 421 nm and 592 nm
4
4
which are attributed to the octahedral transitions, A2g(F) 4T1g(F) and A2g(F)
4T2g(F) [36-
→
→
38] whereas the spectrum of the product mixture exhibits two peaks at 408 nm and 542 nm
(Figure 4). This clearly demonstrates that Cr(VI) is reduced to Cr(III) ion but the blue shift
observed in λmax values for the product mixture is due to existence of Cr(III) in the form of
complex, probably the Cr(III)-PA complex as reported in the PA catalysed Cr(VI) oxidation
reactions [12-14].
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Perumal Subramaniam and Natesan Thamil Selvi
Figure 3. IR spectrum of the product.
Figure 4. UV-visible spectra of Cr(III) ion and product mixture.
RESULTS
That the reaction exhibits first order dependence on Cr(VI) in the presence of PA is obvious
from the linear log OD vs time plots even upto 75% completion of the reaction and from
constant k1 values obtained at different initial [Cr(VI)] (Table 1). The second order rate
constants obtained using the relation k2 = k1/[PSAA] are not found to be constant. Further, the
Bull. Chem. Soc. Ethiop. 2016, 30(1)

Picolinic acid promoted oxidative decarboxylation of phenylsulfinylacetic acid by Cr(VI) 141
double logarithmic plot of k1 vs [PSAA] is linear (r = 0.999) with slope = 0.61 ± 0.01 indicating
that the reaction is of fractional order (0.61) with respect to PSAA. Hence, the overall rate of the
-1
reaction, kov is obtained from the relation, kov = k1 / [PSAA]0.61. The plot of k1 vs [PSAA]-1 is
linear (r = 0.998) with an intercept on the rate axis indicating the Michaelis-Menten dependence
on PSAA [39].
Table 1. Pseudo-first order and overall rate constants for the reaction.
102[PSAA] (mol dm-3)
104[Cr(VI)] (mol dm-3)
104 k1 (s-1)
3.70 ± 0.13
7.65 ± 0.07
10.0± 0.11
12.3 ± 0.02
15.3 ± 0.05
9.30 ± 0.46
9.96 ± 0.03
9.54 ± 0.39
103kov (dm3mol-1)0.61s-1
6.15± 0.13
1.0
3.0
5.0
7.0
10
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
1.0
3.0
7.0
6.49± 0.02
6.22± 0.02
6.21± 0.01
6.23± 0.01
5.78± 0.09
6.19± 0.01
5.93± 0.08
[PA] = 0.3 mol dm-3; [H+] = 0.75 mol dm-3; I = 0.80 mol dm-3; solvent = 40% acetonitrile - 60% water (v/v) ; T =
30 °C.
The reaction is greatly influenced by the acidity of the medium (Table 2) and the order in
[H+] is unity which is evident from the linear log-log plot of k1 against [H+] (r = 0.999) with
slope = 1.0 ± 0.02. The rate increases significantly with increasing acetonitrile content of the
medium (Table 2) suggesting a facile reactivity in a medium of low dielectric constant.
The effect of picolinic acid on the reaction rate indicates a slight increase in rate with
increase in the concentration of PA initially up to 0.15 mol dm-3 and after that an appreciable
linear enhancement of rate is noted with increase in [PA]. The effect of PA is depicted in Table
2. The double inverse plot of k1 vs [PA] in the 0.15-0.5 mol dm-3 range shows an intercept in the
rate axis which confirms the reversible complex formation of PA with any one of the reactants.
Table 2. Effect of [H+], [PA], solvent composition and temperature on rate.
a
b
d
102 PA
104k1
(s-1)
101[H+]
104k1
(s-1)
CH3CN-H2O
(%,v/v)
104k1c
(s-1)
T
(ºC)
103kov
(mol dm-3)
(mol dm-3)
(dm3mol-1)0.61s-1
0
0.50
5.0
8.0
15
25
30
40
50
6.49 ± 0.25
6.94 ± 0.10
7.92 ± 0.11
7.93 ± 0.11
7.93 ± 0.03
9.97 ± 0.06
10.0 ± 0.11
12.1 ± 0.04
13.8 ± 0.06
3.0
5.0
7.5
9.0
10
4.10 ± 0.13
6.98 ± 0.12
10.2 ± 0.09
12.3 ± 0.05
14.0 ± 0.52
20 - 40
40 - 60
60 - 80
80 - 20
8.23 ± 0.12
10.0 ± 0.11
12.7 ± 0.08
16.1 ± 0.11
20
25
30
35
2.75 ± 0.09
4.12 ± 0.01
6.22 ± 0.12
8.23 ± 0.17
[PSAA] = 5.0 x 10 -2 mol dm-3; [Cr(VI)] = 5.0 x 10-4 mol dm-3; [H+] = 0.75 mol dm-3; CH3CN-H2O (40-60% , v/v);
a,c,dI = 0.8 mol dm-3 ; bI = 1.1 mol dm-3.
The reaction is carried out at four different temperatures viz., 20, 25, 30 and 35 ºC and the
kov values are presented in Table 2. The thermodynamic parameters are evaluated from the
intercept and slope of the Eyring plot of log kov/T vs 1/T. The entropy of activation, ∆S≠ and
enthalpy of activation, ∆H≠ calculated for the PA promoted Cr(VI) reaction of PSAA are
-112.38 ± 6.6 J K-1 mol-1 and 53.17 ± 1.9 KJ mol-1 respectively whereas for the uncatalysed
reaction the values are -24.49 ± 0.09 J K-1 mol-1 and 75.58 ± 2.5 KJ mol-1 respectively which
follow the trend expected for the catalysed reaction.
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Perumal Subramaniam and Natesan Thamil Selvi
As the study of substituent effect gives positive evidences on the nature of transition state
and mechanism, the effect of substituents on the PA catalysed oxidative decarboxylation rate is
carried out with several para- and meta- substituted phenylsulfinylacetic acids at 30 ºC. The plot
of log kov vs Hammett constants, σ gives an excellent correlation (Figure 5, r = 0.997) with low
negative reaction constant, ρ (ρ = -0.322 ± 0.01).
Figure 5. Hammett plot at 30 ºC.
DISCUSSION
The active oxidizing species involved in the reaction may be Cr(VI) itself or Cr(V) formed as a
result of one-electron transfer or Cr(IV) due to two-electron transfer. The absence of absorption
at 750 nm, where Cr(V) is the only absorbing species [40], rules out the involvement of Cr(V)
as an active species. Besides, the insensitivity of rate to added radical scavenger, acrylamide,
clearly rules out the single-electron transfer mechanism and hence the involvement of Cr(V)
species in the reaction mechanism. The added Mn2+ ion, scavenger for Cr(IV), has failed to
produce any noticeable effect on the reaction rate ruling out the participation of Cr(IV) as the
active species. Thus, the active species in this reaction is Cr(VI) itself, which exists in aqueous
acidic solution in a variety of forms such as CrO42−, HCrO4 and Cr2O72− besides other protonated
‾
+
‾
forms like HCrO3 , H2CrO4, H2Cr2O7 and HCr2O7 depending on [Cr(VI)] and the pH of the
medium [41-44]. If the concentration is above 0.05 mol dm-3, Cr(VI) mainly exists as dimeric
forms while at lower concentrations it exists predominantly in monomeric forms [41, 45, 46]. At
+
lower concentrations of H+, HCrO4− is the main species whereas HCrO3 is the major moiety at
higher [H+] [45, 47, 48]. Thus, under the present experimental conditions of high [H+] and low
[Cr(VI)], HCrO3+ species is assumed to be the active form of Cr(VI). The first order dependence
of the reaction rate on [H+] and [Cr(VI)] are in agreement with the existence of HCrO3+ species.
In earlier reports [12, 14, 49] it has been observed that the redox potential of Cr(VI)
increases in the presence of chelating agents and believed that Cr(VI) forms complexes with the
chelating agents. Thus in the present case, the increase in reaction rate with increase in [PA]
may be attributed to the formation of a bimolecular cyclic complex (C1, Scheme 1) between
Cr(VI) and PA which is assumed to be the kinetically active oxidizing species [12, 17-19].
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Picolinic acid promoted oxidative decarboxylation of phenylsulfinylacetic acid by Cr(VI) 143
HCr+O3
H2CrO4
+
H2O
+
(I)
+
H+
+
O
Cr OH
+
+
H2O
(II)
O
C
O
N
N
COOH
..
O
O
Cr
O
(C1)
O
+
O
C
N
O
+
:
S
O
C
N
O
-
Cr
S+
(III)
CH2
O
+
O
O
Cr
O
C
slow
O
CH2
HO
O
C
O
HO
(C2)
(C1)
+
O
O
C
N
Ligand coupling
S
O
Cr
C2
(IV)
CH2
O
O
C
O
O
H
(C3)
O
S
fast
C3
+
CH3
+
(V)
O
C
O
N
-CO2
O
Cr
O
(Cr(IV-PA)
fast
fast
(VI)
2Cr(V)-PA
Ph-SO2CH3
+ Cr(VI)-PA
Cr(IV)-PA
Cr(V)-PA
(VII)
Cr(III)-PA
+
+
PSAA
Scheme 1
The spectral evidence for the formation of Cr(VI)-PA complex is obtained from the
hyperchromic shift and broadening of the UV absorption peak at 263 nm after the addition of
PA to Cr(VI) (Figure 6).
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Perumal Subramaniam and Natesan Thamil Selvi
Figure 6. UV-vis absorption spectra. [PSAA] = 5.0 x 10-2 mol dm-3; [Cr(VI)] = 5.0 x 10-4 mol
dm-3; [PA] = 0.3 mol dm-3; [H+] = 0.75 mol dm-3.
The high reactivity in the presence of PA, confirms that the cationic Cr(VI)-PA complex
+
(C1) is a more efficient electrophile than HCrO3 itself. The involvement of positive active
species in the reaction is proved from the study of dielectric constant of the medium, i.e. a
positive slope is obtained when log k1 is plotted against 1/dielectric constant of the medium. The
appreciably high value of Michaelis constant, Km = 0.204 mol dm-3, obtained from the plot of
1/k1 vs 1/[PA] in the range 0.15-0.5 mol dm-3 illustrate that only weak binding exists between
PA and Cr(VI) during the formation of Cr(VI)-PA complex.
The chromium atom of Cr(VI)-PA complex, C1 then receives a nucleophilic attack by the
sulfur atom of PSAA to form a termolecular complex, C2 in a slow step. As the reaction has a
non-integral kinetic order for PSAA it is reasonable to envisage that the reaction follows
Michaelis-Menten kinetics:
1/k1 = 1/k + Km/k[PSAA]
By applying Michaelis-Menten kinetics, the values of k and Km were evaluated from the
slope and intercept of the double inverse plot of k1 against [PSAA]. The values of k and Km
obtained are 2.04 x 10-3 s-1 and 4.54 x 10-2 mol dm-3. The nature of binding of PSAA with the
active species is found to be moderately high as inferred from the low Km value. The spectral
evidence for the formation of termolecular complex is obtained from the noticeable
hypochromic shift at 351 nm (Figure 7c) in the UV-visible spectrum of the reaction mixture
containing PSAA.
As a result of nucleophilic attack of PSAA on C1, a positive charge is developed on the
sulfur atom of PSAA in the intermediate complex C2. The trend of the reaction with different
substituents indicates that the electron releasing substituents in the para- and meta- positions
accelerate the reaction whereas electron withdrawing groups retard the reaction rate. This can be
explained on the basis of stabilization of the positive sulfur centre in complex C2 by electron
releasing substituents and destabilization by electron withdrawing substituents.
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Picolinic acid promoted oxidative decarboxylation of phenylsulfinylacetic acid by Cr(VI) 145
The ternary complex then undergoes ligand coupling in a subsequent step to form the
intermediate, C3. The observed low negative reaction constant, ρ may be visualized due to the
neutralization of positive charge on sulfur atom as a result of ligand coupling with oxygen atom.
Such a ligand coupling pathway has already been suggested by the researchers in the PA
catalysed Cr(VI) oxidation of organic sulfur compounds [12, 50]. The intermediate, C3 then
cleaves at α-β position to eliminate CO2 followed by oxygen atom transfer to sulfur in fast steps
leading to the formation of the product, methylphenyl sulfone. Thus, Cr(IV)-PA species is
formed as a result of two electron transfer which participates in the faster steps with Cr(VI) to
give Cr(V) [51] and consequently, Cr(V) oxidizes another PSAA molecule and itself gets
reduced to Cr(III)-PA species. The Cr(V)-Cr(III) couple has a potential of 1.75 V, which would
enable the rapid conversion of Cr(V) to Cr(III) after the reaction with the substrate [52].
CONCLUSION
The observed spectral changes in the picolinic acid promoted reaction of phenylsulfinylacetic
acid (PSAA) with Cr(VI) in aqueous acetonitrile media evidence the formation of Cr(VI)-PA
complex and the termolecular complex formed between Cr(VI), PA and PSAA. The reaction
follows Michaelis-Menten kinetics with respect to PSAA. The ternary complex then undergoes
ligand coupling followed by several fast steps to form the products. The trend of the reaction
with different meta- and para-substituted PSAAs indicates that the electron releasing
substituents accelerate the reaction whereas electron withdrawing groups retard the reaction rate.
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