Kinetics and mechanism of oxygenation of aromatic sulfides and arylmercaptoacetic acids by peroxomonophosphoric acid

Article abstract of DOI:10.1016/S0040-4020(02)00358-7

The oxygenation of aryl methyl sulfides and diaryl sulfides by peroxomonophosphoric acid in acetonitrile-water mixture follows an overall second-order kinetics, first-order in each reactant. The study of the influence of [H⁺] on the oxidation of phenyl methyl and diphenyl sulfides reveals that H₂PO₅^- and HPO₅^2- are the oxidising species in the oxidation process. In substituted phenyl methyl sulfides and diphenyl sulfides, the rate of oxygenation is accelerated by electron-donating substituents and retarded by electron-withdrawing groups, indicating the nucleophilic displacement by the sulfide sulfur on the peroxide oxygen. The negative ρ value obtained in the correlation analysis of rate constants with σ constants also shows the formation of a more positively charged sulfur species in the rate-limiting step. Studies with different alkyl phenyl sulfides (C₆H₅SR; R=Me, Et, Prⁱ or Bu^t) indicate that the rate is retarded by bulky R groups. Similar kinetic results are also obtained in the peroxomonophosphoric acid oxidation of sodium phenylmercaptoacetate. On the basis of the kinetic studies, a common mechanism has been proposed.

Full text of DOI:10.1016/S0040-4020(02)00358-7

TETRAHEDRON
Pergamon
Tetrahedron 58 42002) 4283±4290
Kinetics and mechanism of oxygenation of aromatic sul®des and
arylmercaptoacetic acids by peroxomonophosphoric acid
Duraisamy Thenraja, Perumal Subramaniam and Chockalingam Srinivasanp
Department of Materials Science, Madurai Kamaraj University, Madurai 625 021, India
Received 20 November 2001; revised 7March 2002; accepted 28 March 2002
AbstractÐThe oxygenation of aryl methyl sul®des and diaryl sul®des by peroxomonophosphoric acid in acetonitrile±water mixture follows
an overall second-order kinetics, ®rst-order in each reactant. The study of the in¯uence of [H1] on the oxidation of phenyl methyl and
diphenyl sul®des reveals that H2PO52 and HPO522 are the oxidising species in the oxidation process. In substituted phenyl methyl sul®des
and diphenyl sul®des, the rate of oxygenation is accelerated by electron-donating substituents and retarded by electron-withdrawing groups,
indicating the nucleophilic displacement by the sul®de sulfur on the peroxide oxygen. The negative r value obtained in the correlation
analysis of rate constants with s constants also shows the formation of a more positively charged sulfur species in the rate-limiting step.
Studies with different alkyl phenyl sul®des 4C6H5SR; RˆMe, Et, Pri or But) indicate that the rate is retarded by bulky R groups. Similar
kinetic results are also obtained in the peroxomonophosphoric acid oxidation of sodium phenylmercaptoacetate. On the basis of the kinetic
studies, a common mechanism has been proposed. q 2002 Elsevier Science Ltd. All rights reserved.
1. Introduction
the nucleophilic attack of the sulfoxide sulfur on the peroxo-
oxygen of H3PO5 in the rate-limiting step. In this paper we
report our results on the oxygenation of alkyl aryl sul®des,
diaryl sul®des and aryl mercaptoacetic acids by PMPA and
this study includes the in¯uence of substituents in the
organic divalent sulfur compounds on the rate.
We have been interested in the elucidation of the mechan-
ism of oxidation of organic sul®des by several oxidants for
more than two decades.1 During the course of a systematic
study of the oxidations of organo-sulfur compounds by
peroxodiphosphate ion 4PP) in this laboratory, it has been
found that the oxidation of all alkyl aryl sul®des follows
second-order kinetics and proceeds through a rate-determin-
ing nucleophilic displacement of the sul®de sulfur on the
peroxide oxygen of PP.2a In the oxidation of diphenyl sul®de
4DPS), however, the rate of the reaction is independent of
[DPS] and also independent of the substituent in the
substrate revealing that the redox reaction is essentially
proceeding via hydrolysis of the protonated peroxo-anions
to give peroxomonophosphoric acid, followed by a fast step
involving the oxidation of DPS.2b Therefore, we felt it
necessary to study in detail the kinetics of oxidation of
aromatic sul®des by peroxomonophosphoric acid 4PMPA).
Panigrahi has reported the PMPA oxidation of dialkyl
sul®des3 and phenylmercaptoacetic acid and p-nitrophenyl-
mercaptoacetic acid.4 There is no report on the mechanism
of PMPA oxidation of aryl methyl sul®des. Recently we
have reported the mechanism of oxidation of aryl methyl
and diaryl sulfoxides by PMPA.5 Based on an analysis of the
effect of [H1] on the oxidation rate and structure±reactivity
studies, it has been proposed that the mechanism involves
2. Results and discussion
2.1. Kinetic studies with aryl methyl and diaryl sul®des
Methyl phenyl sul®de 4MPS) and DPS were selected as
representatives for aryl methyl sul®des and diaryl sul®des,
respectively. The kinetics were carried out in 60% aceto-
nitrile±40% water 4v/v) under pseudo ®rst-order conditions
keeping the substrate always in excess. The pH of the
medium was maintained by using buffer solutions. The
PMPA oxidations of MPS and DPS were carried out at pH
of 8.55 and 4.68, respectively for convenience. The rate of
the reaction was followed by estimating the unconsumed
PMPA iodometrically after quenching the aliquots with a
mixture of 0.2 M solution of sodium acetate±acetic acid
buffer.
The oxidation is ®rst-order each in [MPS] or [DPS] and in
[PMPA]. The plots of log 4a2x) vs time 4not shown) are
linear indicating ®rst-order dependence on the oxidant. The
constancy of second-order rate constants in the variation of
[MPS] or [DPS] 4Table 1) and also the unit slope of the plot
of log k1 vs log [MPS] 4slopeˆ1.00^0.03, rˆ0.999,
sˆ0.01), or of log k1 vs log [DPS] 4slopeˆ0.973^0.01,
Keywords: sul®de oxygenation; peroxomonophosphoric acid; aryl methyl
and diaryl sul®des; phenylmercaptoacetate.
Corresponding author. Tel.: 191-452-458247; fax: 191-452-459181;
e-mail: ceesri@yahoo.com
p
0040±4020/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4020402)00358-7

4284
D. Thenraja et al. / Tetrahedron 58 '2002) 4283±4290
Table 1. Pseudo ®rst-order and second-order rate constants for the oxida-
tion of MPS and DPS by PMPA at 358C; solvent: 60% acetonitrile±40%
water 4v/v)
Table 2. Effect of solvent composition on the rate of oxidation of MPS and
DPS by PMPA at 358C; [sul®de]ˆ0.01 M; [PMPA]ˆ0.001 M
H2O±CH3CN % 4v/v)
k2£102 4M21 s21
)
[Sul®de] 4M£102) [PMPA] 4M£103) k1£104 4s21
)
k2£102 4M21 s21
)
MPS 4pHˆ8.55)
70±30
60±40
50±50
40±60
30±70
MPSa
6.62^0.15
4.46^0.06
3.04^0.08
1.92^0.07
1.53^0.03
1.0
2.0
3.0
4.0
5.0
3.0
3.0
3.0
3.0
1.0
1.0
1.0
1.0
1.0
1.5
2.0
2.5
3.0
1.92^0.071.92
^0.07
1.82^0.07
1.96^0.04
1.86^0.05
1.91^0.06
1.70^0.04
1.58^0.03
1.43^0.03
^0.02
3.66^0.14
5.89^0.13
7.45^0.18
9.55^0.30
5.10^0.12
4.73^0.08
4.30^0.08
3.71^0.071.24
DPS 4pHˆ4.68)
60±40
50±50
40±60
30±70
20±80
4.87^0.10
3.22^0.08
2.36^0.04
2.07^0.02
1.60^0.03
DPSb
1.0
2.0
3.0
4.0
5.0
3.0
3.0
3.0
3.0
1.0
1.0
1.0
1.0
1.0
1.5
2.0
2.5
3.0
2.36^0.04
4.61^0.10
6.78^0.21
8.98^0.36
11.4^0.05
4.50^0.10
4.17^0.09
3.55^0.071.18
3.03^0.04
2.36^0.04
2.31^0.05
2.26^0.07
2.25^0.09
2.27^0.01
1.50^0.03
1.39^0.03
^0.02
al.6 that one of the dimeric forms of peroxomonophosphoric
acid will be less reactive and we presume that such a dimer
is formed in our experimental conditions. Oxidation of both
substrates is appreciably fast in acidic conditions and there-
fore the pH that could be employed to study the kinetics was
8.55 and 4.68 for MPS and DPS, respectively. The plot of k2
vs [H1] is linear 4Fig. 1) with a de®nite intercept in the two
cases. This implies participation of more than one species of
the oxidant. Addition of a neutral salt, sodium perchlorate to
vary the ionic strength of the medium had no effect on the
rate. This presumably points out that the rate-limiting step
involves a neutral molecule and an ion. Added acrylonitrile
has no retarding effect on the rate. Therefore, radical forma-
tion in the rate-determining step may be excluded.
1.01^0.01
The error quoted in k1 and k2 values in all tables is 95% CL of the student's t
4see Ref. 30).
a
pHˆ8.55.
b
pHˆ4.68.
rˆ0.999, sˆ0.01) 4not shown) clearly indicate the ®rst-
order dependence on sul®de concentration. In both oxida-
tions the pseudo ®rst-order rate constants decrease with an
increase in the initial concentration of PMPA 4cf. Table 1).
This may be due to the dimerisation of oxidant at higher
concentrations of PMPA. It has been suggested by Ogata et
The results in Table 2 show that the rate of PMPA oxidation
of sul®des increased with increasing water content in the
medium i.e. the rate is strongly affected by the dielectric
Figure 1. Variation of [H1] in PMPA oxidation of 4a) MPS and 4b) DPS at 358C. [Sul®de]ˆ0.01 M; [PMPA]ˆ0.001 M; solvent: 60% acetonitrile±40% water
4v/v).

D. Thenraja et al. / Tetrahedron 58 '2002) 4283±4290
4285
Table 3. Second-order rate constants, enthalpies and entropies of activation for the oxidation of X±C6H4±S±CH3 by PMPA; [sul®de]ˆ0.01 M;
[PMPA]ˆ0.001 M; pHˆ8.55; solvent: 60% acetonitrile±40% water 4v/v)
S.No.
X
k2£102 4M21 s21
)
DH± 4kJ M21
)
2DS± 4J K21 M21
)
208C
358C
458C
^0.11
10.0^0.29
5.38^0.18
4.93^0.23
5.05^0.1748.3
^0.11
2.55^0.13
2.68^0.10
1.29^0.03
0.661^0.02
4.19^0.06
3.50^0.10
1.67^0.02
1
2
3
4
5
6
7
8
H
0.712^0.01
1.78^0.03
1.12^0.02
1.04^0.03
0.983^0.01
0.851^ 0.04
0.531^0.01
0.513^0.01
0.191^0.01
0.107^0.01
0.950^0.01
0.724^0.01
0.320^0.01
1.92^0.073.60
6.26^0.08
3.12^0.08
2.73^0.09
2.58^0.08
2.50^0.074.45
1.39^0.05
1.62^0.04
0.643^0.03
0.323^0.01
2.48^0.04
2.11^0.04
0.900^0.02
47.3^1.9
51.9^1.2
46.4^1.6
45.9^2.3
^1.5
49.1^2.2
46.3^2.2
49.2^1.6
49.1^1.6
49.3^1.5
43.8^0.8
46.7^1.3
48.9^1.1
123^6.3
101^4.2
124^5.5
126^8.2
118^5.4
117^7.8
46.3^2.2
120^5.8
127^5.7
132^5.2
134^3.0
126^4.5
126^3.9
p-OMe
p-Me
p-Et
p-Pri
p-F
p-Cl
p-Br
p-Ac
p-NO2
m-Me
m-OMe
m-Cl
9
10
11
12
13
The precision of DH± and DS± values were calculated using the method of Petersen et al.20
constant of the medium. A polar activated complex is there-
fore likely to be formed in the transition state, which is more
stabilised as a result of solvation by an increase in the
dielectric constant of the medium. As sulfoxides are
strongly solvated and sul®des and sulfones are poorly
solvated in proton-donating solvents,7 solvents which
could solvate the sulfoxide should increase the rate of oxida-
tion of a sul®de to sulfoxide and the same solvents should
decrease the rate of oxidation of a sulfoxide to sulfone.
Several authors have con®rmed the above fact.5,8±10
Peroxomonophosphoric acid dissociates as shown in Eqs.
41)±43).
K1
H3PO5 O H2PO25 1 H1
ꢀ1†
ꢀ2†
ꢀ3†
K2
H2PO25 O HPO252 1 H1
K3
HPO252 O PO352 1 H1
The three acid dissociation constants of H3PO5 were deter-
mined
spectrophotometrically
by
Battaglia
and
By measuring the rate constants at three temperatures
4Tables 3 and 4), the activation parameters were evaluated
from the Eyring's plot. The fairly high negative value of
DS± in both cases is as expected for a bimolecular nucleo-
philic reaction.
Edwards11 and reported to be K1ˆ9.0£1022 mol dm23
,
.
K2ˆ3.0£1026 mol dm23 and K3ˆ1.6£10213 mol dm23
Hence in the pH range of 7±11 employed for the oxidation
of MPS and in the range of 3.3±8.6 used for DPS, the
predominant species would be H2PO5 and HPO5 and
2
22
32
the other two species H3PO5 and PO5
would have
Both sul®des on PMPA oxidation gave the corresponding
sulfoxides and the stoichiometry was found to be 1:1.
negligible contribution. Therefore, the total concentration
of PMPA is given by
‰PMPAŠ ˆ ‰HPO252Š 1 ‰H2PO52Š
The rate expression is given by
ꢀ4†
T
2.2. Mechanism and rate law
Peroxomonophosphate is trinegative and is protonated in
acid medium. The various species present are PO5
32
2d‰PMPAŠ
T
ˆ k01‰HPO252Š‰sulfideŠ 1 k20 ‰H2PO52Š‰sulfideŠ
ꢀ5†
,
2
HPO522, H2PO5 and H3PO5. The concentration of these
species varies with the changing pH of the medium.
dt
Table 4. Second-order rate constants, enthalpies and entropies of activation for the oxidation of X±C6H4±S±C6H4±Y by PMPA; [sul®de]ˆ0.01 M;
[PMPA]ˆ0.001 M; pHˆ4.68; solvent: 60% acetonitrile±40% water 4v/v)
S.No.
X
Y
k2£102 4M21 s21
458C
)
DH± 4kJ M21
)
2DS± 4J K21 M21
)
358C
608C
1
2
3
4
H
H
H
H
H
H
H
2.36^0.04
4.41^0.14
3.17^ 0.06
1.25^0.03
1.57^0.03
0.401^0.01
0.651^0.01
0.487^0.01
0.335^0.01
4.42^0.09
9.08^0.011
6.20^0.14
3.14^0.11
3.62^0.03
0.794^0.02
1.43^0.02
0.946^0.24
0.682^0.01
11.3^0.34
24.9^1.1
54.4^1.9
56.6^2.1
55.7^1.5
62.3^1.9
57.7^1.2
59.2^1.6
58.1^1.3
66.2^1.8
^1.3
96.9^6.3
87.4^7.1
93.1^4.9
79.1^6.5
92.2^4.1
99.0^5.5
98.2^ 4.3
75.2^6.2
75.4^4.5
4-OMe
4-Me
4-Cl
4-Br
4-NO2
17.6^0.49
8.67^0.19
9.47^0.28
2.49^0.06
3.89^0.12
3.60^0.29
2.55.^00.0767
5
6
74-NO
8
9
40-OMe
40-Me
40-Cl
2
4-NO2
4-NO2
The precision of DH± and DS± values were calculated using the method of Petersen et al.20

4286
D. Thenraja et al. / Tetrahedron 58 '2002) 4283±4290
where k01 and k02 are the rate constants for the disappearance
and 20.641 at 358C in the oxidation of aryl methyl sul®des
by peroxodisulfate13 and peroxodiphosphate,2a respectively.
A satisfactory correlation exists between log k2 with s1/s2
values with a r value of 20.796 at 358C in the phenyliodo-
sodiacetate14 oxidation of aryl methyl sul®des. The
Hammett correlation gives a r value of 20.990 at 208C in
the oxidation of these sul®des by peroxomonosulfate.15
Correlation analysis of the rate data yields a reaction
constant of 21.80 in the uncatalysed Cr4VI) oxidation of
aryl methyl sul®des16 and 21.07in the picolinic acid cata-
lysed Cr4VI) oxidation.17 In the selective oxidation of
several para-substituted phenyl methyl sul®des with
oxo4salen)chromium4V) complexes, the rate data correlate
linearly with Hammett s constants and the r values are in
the range of 21.30 to 22.70 with different substituted
oxo4salen)chromium4V) complexes.1 Higher values are,
however, observed in a few cases by Ruff and Kucsman18
and Miotti et al.19 in the oxidation of these sul®des with
chloramine-T 4rˆ24.25 and 23.56, respectively with
TsNHCl and TsNCl2) and with bromine 4rˆ23.2). The
large negative value in these cases may be attributed to
the strong electrophilic character of the attacking species
Cl1 or Br1 and also due to the differences in the mechanistic
22
of the respective species. Expressing [H2PO5] and [HPO5
in terms of [PMPA]T in Eq. 45), we obtain Eq. 46).
]
ꢀk02 2 k2†
k2 ˆ
‰H1Š 1 K01
ꢀ6†
K2
where k2 is the observed second-order rate constant. The
above equation predicts a linear variation of k2 with [H1]
with an intercept and this is indeed observed 4see Fig. 1).
Panigrahi and Panda3 also concluded in their kinetic and
mechanistic study of oxidation of dialkyl sul®des that
2
22
H2PO5 and HPO5 are the active species in the pH
range 4±7.
Having established the active oxidising species, on the basis
of the other kinetic data discussed earlier, a polar
mechanism shown in Scheme 1 has been proposed. The
oxygenation of sul®des can be visualised as involving the
nucleophilic attack of the sul®de sulfur atom on the peroxo-
oxygen of the oxidising species leading to the formation of a
sulfonium ion I 4Scheme 1). The sulfonium ion I may
decompose to give the sulfoxide by loss of proton in the
presence of water 4Scheme 1, see 410)).
22
pathways. As the two oxidising species viz HPO5 and
H2PO52 involved in the oxygenation of aryl methyl sul®des
are not likely to be typical electrophilic reagents, we
observe a reaction constant of ,21.0 as reported with
other peroxo agents.
2.3. Structure±reactivity correlation
2.3.1. Substituent effect. The study of the in¯uence of
substituents on the rate of PMPA oxidation also lends
further support to the proposed mechanism. The rate
constants of several meta- and para-substituted phenyl
methyl sul®des were determined at pH 8.55 at three
temperatures 4Table 3). The kinetic data for 4-substituted
diphenyl sul®des and 40-substituted-4-nitrodiphenyl sul®des
at three temperatures at a constant pH 44.68) are presented in
Table 4. The rate of the reaction is accelerated by electron-
releasing groups and retarded by electron-withdrawing
substituents present in the aryl ring. The rate variation is
essentially in agreement with the Hammett's substituent
constants s; the plot of log k2 vs s is linear in each series
giving a negative slope of about 21. The statistical results
of the correlation analysis are shown in Table 5. The nega-
tive r value is indicative of the development of positive
charge in the transition state and thereby supports the forma-
tion of the sulfonium ion I in Scheme 1. Such small r values
are reported in the oxidation of organic sulfur compounds.
Modena and Maioli12 have reported a r value of 21.13 in
the oxidation of alkyl aryl sul®des by hydrogen peroxide.
Srinivasan and co-workers observed r1 values of 20.563
That both the series of sul®des follow the same type of
mechanism is manifested in the linear plot of log k2 of
para-substituted phenyl methyl sul®des and 4-substituted
diphenyl sul®des with a slope of about unity.
2.3.2. Isokinetic relationship. Though in both series
Petersen's error criterion20 is satis®ed i.e. DDH±.2d, to
apply the isokinetic relationship 4Eq. 47)),21 the correlation
of DH± with DS±
DH± ˆ DH0± 1 bD±
ꢀ7†
is poor. Therefore, the isokinetic relationship has been
tested by the method of Exner. A good Exner's plot22
4Fig. 2) is obtained for each series implying that all substi-
tuted compounds in a given series of a sul®de follow the
same mechanism in oxygenation.
2.3.3. Steric effectsÐoxidation of alkyl phenyl sul®des.
The oxygenation of alkyl phenyl sul®des with PMPA was
also investigated. The rate data in Table 6 show that the rate
decreases in the order PhSMe.PhSEt.PhSPri.PhSBut
and this could be attributed to the steric factor rather than
to polarity effects. This conclusion is also supported by a
linear correlation between the logarithms of rate constants
and the Taft's ES constants 4rˆ0.998). If the 1I effect of the
alkyl groups predominates over the steric effect of the alkyl
group, one would expect a reverse order in the rate
mentioned above.
Table 5. Results of correlations of log k2 with s
8C
r
r
s
n
MPS
20
35
21.09^0.06
21.06^0.08
20.991^0.06
0.983
0.969
0.979
0.065
0.085
0.979
13
13
13
45
DPS
35
45
60
20.931^0.06
20.932^0.070.983
20.8259^0.070.97 0.080
00.9870.07
0.081
9
9
2.4. Kinetic studies with phenylmercaptoacetic acids
9
We wanted to extend our study to the oxygenation of
several substituted phenylmercaptoacetic acids 4divalent
organic sulfur compounds) by PMPA. Since the pKa of
rˆreaction constant; rˆcorrelation coef®cient; sˆstandard deviation;
nˆnumber of data points.

D. Thenraja et al. / Tetrahedron 58 '2002) 4283±4290
4287
Scheme 1.
Figure 2. Exner plot 4isokinetic plot) 4a) aryl methyl sul®des at pHˆ8.55 and 4b) diaryl sul®des at pHˆ4.68. Numbers 1±13 as in Table 3 4for aryl methyl
sul®des) and 1±9 as in Table 4 4for diaryl sul®des).
phenylmercaptoacetic acid is 4.74,23 it was decided to study
its oxidation in water at a pH of 7. The kinetic studies were
carried out at a pH of 7.0 under pseudo ®rst-order conditions
with a substrate to oxidant ratio of 10:1. Sodium phenyl-
mercaptoacetate 4PMA) was used for the kinetic runs. PMA
was obtained by mixing equimolar quantities of phenyl-
mercaptoacetic acid and sodium hydroxide in doubly
distilled water.
The reaction is ®rst-order in PMPA as evidenced by good
linear plots 4r.0.995) of log [PMPA] vs time 4not shown).
The constant second-order rate constants at different initial
concentrations of PMA and the linear plot of log k1 vs
log [PMA] 4Fig. 3, rˆ0.996, sˆ0.004) with a slope of
1.02^0.01 indicate ®rst-order dependence on [PMA]. The
pseudo ®rst-order rate constants at various initial [PMPA]
slightly decreases with increasing [PMPA]. This may be due
to the dimerisation of the oxidant at high [PMPA].6
Table 6. Effect of side chain substitution on the rate of oxidation of C6H5±
S±R by PMPA at 358C; [sul®de]ˆ0.01 M; pHˆ8.55; [PMPA]ˆ0.001 M;
solvent: 60% acetonitrile±40% water 4v/v)
R
k2£102 4M21 s21
)
Me
Et
1.92^0.07
1.76^0.02
1.47^0.07
0.839^0.01
Pri
But

4288
D. Thenraja et al. / Tetrahedron 58 '2002) 4283±4290
Figure 3. Plot of log k1 vs log [PMA] at 208C. [PMPA]ˆ0.001 M; pHˆ7.0.
The rate of oxidation of PMA by PMPA was studied at
different pH 4KH2PO4 and Na2HPO4 buffer). The rate
increases with increasing [H1]. In the pH range of 6.04±
7.69 employed here, the value of k1 increases with decrease
in pH as observed in the cases of aryl methyl sul®des and
diphenyl sul®des. The rate of oxidation increases with
increase in ionic strength of the medium 4maintained by
the addition of sodium perchlorate). Further the plot of
log k1 vs I is linear 4Fig. 4, rˆ0.999) with a positive
slope. These facts reveal that in the rate-determining step,
the phenylmercaptoacetate ion and another anion would be
involved. However, the slope in Fig. 4 does not correlate
with the product of the charges. This may presumably be
due to two reasons: 4i) the Bronsted±Bjerrum equation is
applicable only in very dilute solution 4as the Debye equa-
tion is valid only in dilute solution), but we have employed
very high ionic strength and 4ii) the value of the constant
4usually denoted A) in this equation is taken only for
aqueous solution, while our kinetic measurements were
carried out in acetonitrile±water mixture. The participation
of any free radical in the slow step is ruled out as the rate is
unaffected by the introduction of acrylonitrile. When the
reaction has been investigated in a binary mixture of
water±acetonitrile, the rate increases with the increase in
water content. From the kinetic data at four temperatures
and from the plot of log 4k2/T ) vs 1/T, the activation
Figure 4. Effect of ionic strength on the rate of oxidation of phenylmercaptoacetate by PMPA at 208C. [PMA]ˆO.O1 M; [PMPA]ˆ0.001 M; pHˆ7.0.

D. Thenraja et al. / Tetrahedron 58 '2002) 4283±4290
4289
Table 7. Second-order rate constants for the oxidation of X±C6H4±S±
CH2COO2 by PMPA at 208C; [ArSCH2COO2]ˆ0.001 M; pHˆ7.0;
3.2. Kinetic measurement
[PMPA]ˆ0.001 M
The kinetic runs of oxidation of aryl methyl sulphides with
PMPA were carried out in 60% acetonitrile±40% water
4v/v) at pH of 8.55 under pseudo ®rst-order conditions
with a substrate, oxidant ratio of at least 10:1. As the oxida-
tion of diaryl sulphides was very slow under these condi-
tions, a pH of 4.68 was selected for convenience. The
oxidation of arylmercaptoacetic acids was carried out at a
convenient pH of 7.0 in doubly distilled water. The pH was
maintained by KH2PO4 and Na2HPO4 buffer. The ionic
strength of the medium in all kinetic runs with diaryl
sul®des was maintained as 0.05 M with sodium perchlorate.
The reactions were followed by measuring the disappear-
ance of PMPA. Aliquots withdrawn at appropriate time
intervals were added to acetic acid±sodium acetate buffer
to maintain the pH of the medium between 4 and 5 with a
few drops of 5% ammonium molybdate solution 4to avoid
the diffused end point due to the presence of 2±3% H2O2
formed during the course of hydrolysis) and estimated by
the usual iodometric procedure.
S.No.
X
k2£102 4M21 s21
)
1
2
3
4
5
6
7
8
H
9.06^0.06
15.7^0.24
11.9^0.18
10.2^0.19
10.3^0.11
5.50^0.05
4.95^0.05
1.32^0.02
9.20^0.24
5.92^0.06
3.51^0.05
p-Ome
p-Me
p-Et
p-F
p-Cl
p-Br
p-NO2
m-Me
m-Ome
m-Cl
9
10
11
parameters
have
been
evaluated.
The
DH±
441.8^0.4 kJ M21) and DS±42123^1.5 J K21M21) values
are of the magnitude expected for a bimolecular nucleo-
philic reaction and analogous to those of aryl methyl
sul®des 4Table 3).
3.3. Product analysis
The effect of substituents on the oxidation was studied by
using eleven substituted phenylmercaptoacetic acids
4Table 7). A fair Hammett correlation is found between
log k2 and s 4rˆ20.996^0.03 at 208C, rˆ0.982, sˆ0.03,
nˆ11). The r value is very close to 21.09 as observed for
the oxidation of aryl methyl sul®des 4vide supra).
The reaction mixture from an actual kinetic run was evapo-
rated under reduced pressure to remove the solvent and
extracted with chloroform and dried over anhydrous sodium
sulfate. The solvent was removed under reduced pressure
and the solution was analysed by TLC in a solvent system of
benzene±ethyl acetate 480:20). Methyl phenyl sulfoxide and
diphenyl sulfoxide were identi®ed as ®nal products in the
oxidation of MPS and DPS, respectively. The residue
obtained after the removal of the solvent was also analysed
by gas chromatography 4NETEL, India) ®tted with SE-30
410%) and FID in the cases of methyl phenyl, p-methoxy-
phenyl methyl and p-chlorophenyl methyl sul®des. In all the
cases sulfoxide was the only product along with unreacted
sul®de. Overoxidation to sulfone has not taken place under
our experimental conditions, as the oxidant concentration
was low. In another set of independent experiments we
oxidised a number of aryl methyl sul®des using PMPA in
the preparative scale and we observed only the formation of
sulfoxide as the sole product 4.90% yield) in each case and
no sulfone was formed.29 Further it is well established that
the oxidation of sulfoxide to sulfone is extremely slow in
comparison to the oxidation of sul®de.2a
The in¯uence of pH on the rate, indicates that more than one
species of PMPA participates. As the pH employed is
2
between 6 and 7.7, the active species will be H2PO5 and
HPO522 as found for the oxidation of sul®des. Therefore, it
is concluded that a mechanism described under Scheme 1 is
also applicable to PMA oxygenation 4R0ˆCH2CO22). That
a similar mechanism operates in the PMPA oxidation of aryl
methyl sul®des and phenylmercaptoacetic acids is also
con®rmed by the linear plot between log k2 values of aryl
methyl sul®des and phenylmercaptoacetates at 208C with
almost a unit slope.
3. Experimental
3.1. Materials
All the aryl methyl24 and diaryl sul®des16 and aryl mer-
captoacetic acids25 were prepared by known methods. The
sul®des and aryl mercaptoacetic acids were puri®ed by
distillation or recrystallisation before kinetic studies. The
purities of the compounds were established by means of
sharp melting behaviour as solids, single spot by TLC and
by spectral studies. PMPA was prepared by the acid
hydrolysis26,27 of tetra potassium peroxodiphosphate 4FMC
Corporation) which was employed after repeated recrystal-
lisation from methanol±water mixture. Doubly distilled
water was used throughout, the second distillation being
from permanganate. A commercial sample of acetonitrile
was puri®ed by re¯uxing with phosphorous pentoxide in
an all glass apparatus and then distilled.28 All other chemi-
cals used were of AR grade.
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