Oxidative cleavage of S-Arylmercaptoacetic acids by pyridinium chlorochromate: Kinetic and correlation analysis

Article abstract of DOI:10.1002/(SICI)1097-4601(1999)31:10<683::AID-JCK1>3.0.CO;2-9

Kinetics of oxidation of 24 S-Arylmercaptoacetic acids (SAMA) by pyridinium chlorochromate (PCC) have been studied in acid medium. The product of oxidation is the corresponding thiophenol. The rate data of meta- and para-substituted acids have been correlated well with σ_I, σ°_R values and the meta-compounds correlate well with F,R values. The reaction constants are negative and of smaller magnitudes. Further, the ortho-substituted acids show a good correlation with triparametric equation involving Taft's σ_I and σ°_R and charton's steric parameter v. There is no considerable steric contribution to the total orthosubstituent effect. Based on these observations, the mechanism involving the formation of protonated arylsulfinylacetic acid intermediate, followed by an intramolecular rearrangement leading to the product thiophenol has been proposed.

Full text of DOI:10.1002/(SICI)1097-4601(1999)31:10<683::AID-JCK1>3.0.CO;2-9

Oxidative Cleavage of
S-Arylmercaptoacetic
Acids by Pyridinium
Chlorochromate: Kinetic
and Correlation Analysis
S. KABILAN, R. GIRIJA, V. RAJAGOPAL
Department of Chemistry Annamalai University, Annamalai Nagar -608 002, India
Received 7 May 1998; accepted 24 May 1999
ABSTRACT: Kinetics of oxidation of 24 S-Arylmercaptoacetic acids (SAMA) by pyridinium chlo-
rochromate (PCC) have been studied in acidmedium. The product of oxidation is the corre-
sponding thiophenol. The rate data of meta- and para-substituted acids have been correlated
o
well with ␴ , ␴ values and the meta-compounds correlate well with F,R values. The reaction
I
R
constants are negative and of smaller magnitudes. Further, the ortho-substituted acids show
o
a good correlation with triparametric equation involving Taft’s ␴ and ␴ and charton’s steric
I
R
parameter v. There is no considerable steric contribution to the total orthosubstituent effect.
Based on these observations, the mechanism involving the formation of protonated arylsul-
finylacetic acid intermediate, followed by an intramolecular rearrangement leading to the
product thiophenol has been proposed. ᭧ 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31:
6
83–688, 1999
INTRODUCTION
mate as an oxidant catalyzed by perchloric acid in
aqueous acetic acid medium. Furthermore, eight each
meta- para- and seven ortho-substituted S-Arylmer-
captoacetic acids have been subjected to this reaction
in similar conditions employed for the parent system,
and the results are discussed through multiple regres-
sion analysis.
Oxidation of S-Arylmercaptoacetic acid (SAMA) in
the presence of aqueous mineral acid undergoes a
pummerrer [1,2] type of rearrangement leading to the
product thiophenol and glyoxalic acid. Synthetic util-
ity of this reaction has been suggested for the prepa-
ration of substituted thiophenols from thiophenol [3–
6
]. Studies on the reaction reveals that the reaction
proceeds through the formation of phenylsulphinyl-
acetic acid intermediate [3–6]. Though there are some
kinetic studies [7–13], on the oxidation of S-Aryl-
mercaptoacetic acid by various oxidants, this reaction
has not yet been studied using Pyridinium chlorochro-
EXPERIMENTAL
S-Arylmercaptoacetic acid were prepared as described
in [14] and purified by repeated recrystallization from
suitable solvents. Pyridinium chlorochromate was pre-
pared by the method of Corey et al. [15]. The product
thiophenol was isolated as its disulphide and charac-
terized by its melting point (mp. 62ЊC).
Correspondence to: S. Kabilan
᭧
1999 John Wiley & Sons, Inc.
CCC 0538-8066/99/100683-06

6
84
KABILAN, GIRIJA, AND RAJAGOPAL
Table I Effect of Varying [PCC] and [SAMA] on the
Table II Effect of Varying [Perchloric Acid] on the
Reaction Rate
Reaction Rate
[SAMA] ϭ 1.0 ϫ 10
[PCC] ϭ 1.0 ϫ 10
Ϫ
2
[
HClO ] ϭ 0.1625 M
M
AcoH Ϫ H O ϭ 50-50% (v/v)
4
2
Ϫ3
ACOH Ϫ H O ϭ 50-50% (v/v)
Temperature: 308 K
M
Temperature: 308 K
2
3
2
4
102k
moldm
1
3
[
PCC]10
[SAMA]10
kobs 10
(s )
[HClO ] 10
kobs 10
s
2
4
Ϫ3
Ϫ3
Ϫ
1
Ϫ3
Ϫ1
Ϫ3
Ϫ1
(
mol dm )
(mol dm )
s
(mol dm )
0
1
1
2
1
1
1
1
1
1
1
.50
.00
.50
.00
.00
.00
.00
.00
.00
.00
.00
2.00
2.00
2.00
2.00
1.00
1.50
2.00
2.50
3.00
3.50
4.00
2.21
2.41
2.42
2.39
1.33
1.97
2.34
3.15
3.99
4.40
5.32
—
—
—
—
1.33
1.31
1.17
1.26
1.33
1.25
1.33
0.810
1.625
2.240
3.250
0.60
1.33
1.94
2.49
PCC. The reaction was first order with respect to PCC
as evidenced from linearity of the plot of [PCC] vs
time.
The pseudo-first-order rate constants were deter-
mined in all the cases. The plot of log kobs vs log
[SAMA] gave a good straight line in all the cases, with
KINETIC MEASUREMENT
almost unit slope indicating first-order dependence for
SAMA. The rate constant k were calculated using
2
All the kinetic measurements were carried out in glass
stoppered iodine flasks protected from light and ther-
mostatted to within Ϯ0.01ЊC with pseudo-first-order
conditions maintained throughout the study. Reactions
were carried out in 1:1 aqueous acetic acid at 308 K
in the presence of 0.1625 M perchloric acid unless
otherwise stated. The course of the reaction was fol-
lowed by monitoring the decrease in absorption of pyr-
idinium chlorochromate at 350 nm employing a
JASCO UVIDEC340 spectrophotometer with a vari-
able temperature accessory.
k1
k
ϭ
(1)
2
[
SAMA]
The results of varying [PCC] and [SAMA] are given
in Table I.
The kinetics of oxidation of SAMA was studied at
various H concentrations and the results are given in
ϩ
ϩ
Table II. It is seen that the reaction is catalyzed by H
ions, thus it may be concluded that the protonated form
of PCC may be the effective oxidizing species in this
reaction.
Decrease in dielectric constant of the medium in-
creases the rate of the reaction (Table III).
PRODUCT ANALYSIS
This suggests that in the transition state there is a
charge dispersal. The variation in the ionic strength of
the medium had no effect on the reaction rate. Also,
formation of free radical in the rate determining step
was ruled out by noticing no appreciable change in
S-Phenylmercaptoacetic acid (1.68 g) 10 m.mol was
added to 50% v/v of acetic acid water mixture con-
taining 0.1625 M perchloric acid at 40ЊC. Pyridinium
chlorochromate (2.15 g) 10 mmol was added in por-
tions with constant stirring over a period of 30 min-
utes. The resulting mixture was subjected to steam dis-
tillation. The steam distillated mixture was extracted
with ether and dried over anhydrous sodium sulphate
and evaporated. The distinct odor of thiophenol con-
firmed the product was thiophenol, which was also
identified by IR spectra.
Table III Effect of Varying Dielectric Constant on the
Reaction Rate
Ϫ
2
Ϫ1
[SAMA] ϭ 1.00 ϫ 10
M
HClO ϭ 1.625 ϫ 10
Temperature: 308 K
M
4
Ϫ3
[PCC] ϭ 1.00 ϫ 10
M
4
Acetic acid
%
kobs 10
(s )
Ϫ
1
RESULTS AND DISCUSSION
4
5
6
7
0
0
0
0
0.946
1.336
2.060
3.780
The kinetic measurements were made using different
initial concentration of PCC and SAMA at 308 K. The
concentration of SAMA was 10–20 times that of

OXIDATIVE CLEAVAGE OF S-ARYLMERCAPTOACETIC ACIDS
685
rate by adding different initial concentrations of ac-
rylonitrile.
In light of the above observations a possible mech-
anism has been proposed. Initially, the protonated
form of PCC makes the electrophilic attack on sulphur
atom of the thioacid giving the product sulfoxide
gression analysis in terms of Taft [16] and Swain’s
[17] dual substituent parameter (DSP) equation. Since
the rate data failed to show satisfactory correlation
with the Hammett’s values. Table V provides the re-
sults of correlation analysis for the meta- and para-
substituted compounds along with statistical parame-
ters standard deviation (SD), coefficient of multiple
correlation (R), and the parameter ‘f’ and ‘␺’ as the
measure of goodness of fit [18,19].
(in slow step) which undergoes an intramolecular
rearrangement in the subsequent step leading to the
products (Scheme I). The rate law of scheme by
equation (2):
Comparison of the coefficient of the multiple cor-
relation (R) shows that the para-substituted com-
pounds correlate better with the ␴ and ␴ Њ values than
Mechanism
I
R
to the other dual substituent constants (viz. ␴ and
I
K1
Ϫ
(BA)
ϩ
ϩ
ϩ
␴R , ␴ and ␴R
␴I and ␴R and F and R parameters).
PCC ϩ H
[PCCH ]
I
However the meta- compounds correlate well with F
and R values. The reaction constants and statistical
data for different temperatures are recorded in Table
VI. The magnitudes of p and p for both meta- and
k2
ϩ
[
PCCH ] ϩ C H SCH CO H
6
5
2
2
Slow
OH
I
R
para-compounds are small and negative supporting the
involvement of direct oxygen transfer via an electro-
philic attack on the sulphur atom [20]. In oxidation by
reagents which involve electrophilic attack on sulphur
atom, the reaction constants are negative and of
smaller magnitudes [19–22]. On the other hand, the
reaction involving halosulphonium cations exhibit
large reaction constants [23–25]. In this investigation
the total effect of the meta- and para-substituents have
been separated into contributions by inductive and res-
onance effects.
The values of ␭ for para-compounds are in the
range of 1.31–1.15 whereas for meta-substituted com-
pounds ␭ ranges from 0.24 to 0.55. This suggests that
resonance effect is much more prononced in para-sub-
stituted compounds than in meta- substituted com-
pounds. The percentage contributions of resonance ef-
fect to the total polar effects for para-compounds have
been calculated using equation (3).
C H 9S9CH CO H ϩ Cr(IV)*
6
5
2
2
(ϩ)
OH
Rearrangement
C H 9S9CH CO H
6
5
2
2
(ϩ)
ϩ
OH2
C H S9CH9CO H
6
5
2
ϩ
OH2
C H S9CH9CO H
C H SH ϩ OHCCO H
6 5 2
6
5
2
*
[Cr(IV) ϩ Cr(IV)
2Cr(V)]
2
(C H SCH CO H ϩ Cr(V)
6
5
2
2
OH
C H 9S9CH CO H ϩ Cr(III)]
6
5
2
2
(ϩ)
Scheme I
Rate Law
[
[
pR] ϫ 100
pI] ϩ [pR]
PR ϭ
(3)
Ϫd[PCC]
ϩ
ϭ K k [PCC][H ][SAMA]
(2)
1
2
dt
For para-compounds the resonance contribution is
about 56% of the total polar effect. This suggests the
field as well as resonance effects of the substituents
have almost equal influence on these oxidation reac-
tions. However, in meta-compounds the percentage of
resonance character is 30–35 percent of total polar
effects and the field contribution predominates.
The rate constant at five different temperatures
along with the activation parameters for the ortho-,
meta-, and para-substituted compounds are given in
Table IV. A linear isokinetic relationship was ob-
#
#
served by plotting ⌬H Vs ⌬S . Also, a plot of log k
obs
at 298 K against log kobs at 318 K gave a straight line
suggesting a common mechanism for the oxidation of
all the substituted mercapto acids under investigation.
Ortho- Effect
Correlation Analysis of Reactivity
The rate data of ortho-substituted S-Arylmercapto-
acetic acids were analyzed by Charton’s method [26]
using ␴ ␴ and v values compiled by Chapman et al.
The oxidation of meta- and para-substituted S-aryl-
mercaptoacetic acids were subjected to multiple re-
I
R

6
86
KABILAN, GIRIJA, AND RAJAGOPAL
Table IV Rate Constant at Different Temperatures and Activation Parameters of the Oxidation of
S-Arylmercaptoacetic Acids by PCC
0 k mol dmϪ3sϪ
2
1
1
2
H#
kJmol
᭝S#
JK mol
᭝
Ϫ1
Ϫ
1
Ϫ1
Substituent
298 K
303 K
308 K
313 K
318 K
H
7.96
18.36
12.14
3.16
3.20
2.50
0.45
0.91
4.71
14.22
0.98
0.62
0.55
0.90
0.49
0.76
7.30
13.86
1.29
1.94
1.63
1.07
0.17
10.10
22.14
17.28
3.97
4.00
3.16
0.65
1.31
6.98
17.28
1.60
0.99
0.89
1.45
0.74
1.16
9.75
17.62
1.81
2.65
2.26
1.44
0.25
13.40
28.45
21.88
5.10
5.10
4.00
0.95
1.77
10.44
21.83
2.50
1.42
1.62
2.00
1.44
1.48
13.84
22.00
2.51
3.67
3.09
2.01
0.40
16.30
32.50
25.60
6.50
6.41
5.48
1.34
2.42
14.90
25.59
4.00
2.28
2.34
3.00
2.09
1.88
17.49
27.70
3.43
4.87
4.20
2.59
0.54
20.76
39.64
30.20
7.88
7.15
7.11
1.90
3.30
21.52
31.21
6.16
3.32
3.80
4.35
3.00
2.56
23.54
34.28
4.65
6.54
5.64
3.45
0.80
35.30
27.77
32.46
34.10
29.16
40.20
54.20
48.12
57.40
28.44
69.70
63.10
73.30
59.50
68.80
46.34
43.60
33.00
47.90
45.30
46.35
43.5
Ϫ147.70
Ϫ165.91
Ϫ153.10
Ϫ159.50
Ϫ175.67
Ϫ141.10
Ϫ108.10
Ϫ122.60
Ϫ77.80
Ϫ165.70
Ϫ50.00
Ϫ75.20
Ϫ42.00
Ϫ84.64
p-OCH3
p-F
p-Cl
p-Br
p-I
p-NO2
p-COCH3
m-OCH3
m-CH3
m-F
m-Cl
m-Br
m-I
m-NO2
m-COCH3
o-OCH3
o-CH3
o-F
o-Cl
o-Br
o-I
o-NO2
Ϫ57.93
Ϫ133.41
Ϫ120.40
Ϫ150.00
Ϫ120.20
Ϫ125.60
Ϫ123.70
Ϫ136.00
Ϫ102.10
58.34
[
27]. The results of the correlation are given in Table
VII it is clear that the rate data for ortho-substituted
S-Arylmercaptoacetic acids correlate well with Tafts
␴ , ␴ Њ and Charton’s v values in which the -NO
VII. The correlation coefficient obtained in the mul-
tiple linear regression using only ␴ and ␴ are poor
I
R
I
R
2
indicating that electrical effects alone are not sufficient
to account for the observed ortho-effect. From Table
group is considered as planar. The regression coeffi-
cients of ␴ , ␴ Њ are negative indicating that electron-
I
R
Table V Correlation of the Rates of Oxidation of Para- and Meta-Substituted S-Arylmercaptoacetic Acids by PCC at
08 K
3
Substituent
Constant
pI
pR
R
SD
fa
␺a
para-substituted
⅙
␴I␴R
␴I␴ (
␴I␴R
␴I␴R
Ϫ1.47
Ϫ1.48
Ϫ1.26
Ϫ1.41
Ϫ0.89
Ϫ1.72
Ϫ1.40
Ϫ1.04
Ϫ0.94
Ϫ0.47
0.991
0.983
0.974
0.970
0.989
0.0106
0.0281
0.0346
0.0373
0.0226
0.1097
0.1835
0.2259
0.2436
0.1476
0.2256
0.2082
0.2569
0.2757
0.677
BA)
R
Ϫ
ϩ
F, R
meta-substituted
⅙
␴I␴R
␴I␴ (
␴I␴R
␴I␴R
Ϫ1.815
Ϫ1.826
Ϫ1.750
Ϫ1.799
Ϫ1.080
Ϫ0.596
Ϫ0.542
Ϫ0.342
Ϫ0.389
Ϫ0.216
0.900
0.903
0.890
0.920
0.930
0.0492
0.0484
0.0515
0.0442
0.0442
0.4358
0.4287
0.4562
0.3915
0.3915
0.4942
0.4862
0.5170
0.4444
0.4444
BA)
R
Ϫ
ϩ
F, R
a
Statistical parameters for goodness of fit ref [18, 19].

OXIDATIVE CLEAVAGE OF S-ARYLMERCAPTOACETIC ACIDS
687
Table VI Temperature Dependance of the Reaction Constants
T/K
pI
pR
␭
R
SD
f
␺
PR
para-substituted
2
3
3
3
3
98
03
08
13
18
Ϫ1.512
Ϫ1.511
Ϫ1.470
Ϫ1.370
Ϫ1.310
Ϫ1.87
Ϫ1.76
Ϫ1.72
Ϫ1.80
Ϫ1.51
1.23
1.16
1.17
1.31
1.15
0.990
0.985
0.990
0.982
0.976
0.0242
0.0287
0.0324
0.0270
0.0310
0.1411
0.1726
0.1989
0.1889
0.2178
0.1595
0.1952
0.2251
0.2131
0.2464
55.32
53.82
53.91
56.78
53.54
meta-substituted
2
3
3
3
3
98
03
08
13
18
Ϫ2.20
Ϫ2.03
Ϫ1.81
Ϫ1.63
Ϫ1.50
Ϫ0.546
Ϫ0.590
Ϫ0.596
Ϫ0.700
Ϫ0.777
0.245
0.295
0.329
0.430
0.511
0.925
0.926
0.900
0.900
0.910
0.0702
0.0645
0.0695
0.0649
0.0584
0.3799
0.3774
0.4356
0.4359
0.4146
0.4308
0.4281
0.4942
0.4942
0.4701
19.70
22.52
24.58
30.04
33.92
releasing groups accelerate the reaction. Also, the neg-
ative regression coefficient for the steric term v points
out that the reaction is also subjected to steric retar-
dation by the ortho-substituents.
tions, the resonance effects being less pronounced in
ortho-substituted compounds. Moreover, the contri-
bution from steric effects to the total polar effects are
considerably lower in this reaction.
The contribution of the resonance parameters (PR)
to the total polar effects [26] and the contributions of
the steric parameters [PS] to the total effects of the
ortho-substituents [26] have also been determined at
five different temperatures. These values are given in
CONCLUSION
Oxidation of S-Arylmercaptoacetic acid by pyridi-
nium chlorochromate in acid medium leads to an acid
catalyzed cleavage of the thio acids into the corre-
sponding thiophenol and glyoxalic acid. The reaction
proceeds through an intramolecular Pummerer type re-
arrangement. The field and resonance effects almost
Table VIII. While the values of P for para-substitu-
R
ents are in the range 48 of 55%, for ortho-substituents
they are in the range of 31 to 34% of the total polar
effects. This shows that the balance of resonance and
field effects is different for the ortho- and para-posi-
Table VII Correlation of the Rates of Oxidation of Ortho-Substituted S-Arylmercaptoacetic Acid by Pyridinium
Chlorochromate at 308 K
Substituent
Constant
h
pI
pR
␾
R
SE
f
␺
␴I, ␴R
␴I, ␴v
␴R, ␴v
Ϫ0.86
Ϫ0.55
Ϫ0.77
Ϫ0.86
Ϫ0.68
Ϫ1.31
Ϫ0.86
Ϫ0.86
Ϫ0.83
Ϫ0.86
Ϫ0.82
Ϫ0.88
Ϫ0.83
Ϫ0.84
Ϫ0.79
Ϫ0.86
Ϫ2.12
Ϫ1.47
—
Ϫ2.22
Ϫ2.05
—
Ϫ2.13
Ϫ2.08
Ϫ2.09
Ϫ2.06
Ϫ1.91
Ϫ2.24
Ϫ2.04
Ϫ2.10
Ϫ1.89
Ϫ2.04
Ϫ1.06
—
0.05
Ϫ1.15
—
Ϫ0.75
Ϫ1.06
Ϫ1.30
Ϫ0.98
Ϫ0.94
Ϫ0.60
Ϫ1.15
Ϫ1.30
Ϫ0.98
Ϫ0.81
Ϫ0.67
—
0.985
0.929
0.781
0.991
0.884
0.350
0.986
0.984
0.982
0.979
0.985
0.984
0.985
0.982
0.980
0.984
0.036
0.043
0.061
0.035
0.039
0.090
0.031
0.021
0.033
0.038
0.032
0.040
0.038
0.043
0.045
0.040
0.16
0.19
0.31
0.15
0.20
0.56
0.16
0.10
0.18
0.20
0.17
0.17
0.16
0.18
0.19
0.17
0.22
0.28
0.38
0.21
0.28
0.66
0.21
0.18
0.23
0.25
0.21
0.30
0.23
0.26
0.28
0.30
Ϫ0.516
Ϫ1.080
—
Ϫ0.090
Ϫ0.560
Ϫ0.008
0.010
a
␴I, ␴
R
a
␴I, v
␴R, va
␴I, ␴ v
R
⅙
␴I, ␴ v
R
ϩ
␴I, ␴ v
R
0.001
Ϫ
␴I, ␴ v
R
Ϫ0.009
Ϫ0.120
0.068
Ϫ0.083
0.047
␴I, ␴(
BA)
v
R
a
␴I, ␴ v
R
⅙
a
␴I, ␴ v
R
ϩ
a
␴IR
v
Ϫ
R
a
␴I, ␴ v
␴I, ␴(BA)va
0.045
Ϫ0.014
R
a
ϭ -NO , -COOCH groups are assumed to be orthogonal
2
3

6
88
KABILAN, GIRIJA, AND RAJAGOPAL
Table VIII Temperature Dependance of the Reaction Constants for the Oxidation of Ortho-Substituted
S-Arylmercaptoacetic Acids by PCC
T/K
pI
pR
␾
R
SD
f
␺
PR
PS
298
303
308
313
318
Ϫ2.28
Ϫ2.20
Ϫ2.13
Ϫ2.05
Ϫ1.97
Ϫ1.10
Ϫ1.02
Ϫ1.06
Ϫ1.06
Ϫ1.04
Ϫ0.067
Ϫ0.018
Ϫ0.008
Ϫ0.004
Ϫ0.001
0.986
0.985
0.986
0.983
0.982
0.0335
0.0338
0.0314
0.0337
0.0334
0.1667
0.1725
0.1600
0.1836
0.1890
0.1923
0.1989
0.2100
0.2117
0.2178
31.91
31.50
33.14
34.03
34.54
1.94
0.55
0.25
0.12
0.03
equally influence the rate of the reaction in para-sub-
stituted compounds, whereas in meta-substituted com-
pounds the field effect predominates. However, in or-
tho-substituted compounds, the resonance effects are
lesser than those in para-compounds. Also, the steric
effects are considerably lower in ortho-compounds.
11. Ganapathy, K.; Kabilan, S. Int J Chem Kinet 1989, 21,
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3. Kabilan, S.; Pandiarajan, K.; Krishnasamy, K.; Sankar,
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1
1
1
The authors are thankful to Prof. R. Gurumurthy for the kind
provision of necessary facilities to complete this work.
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M. S. J Am Chem Soc 1983, 105, 492.
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