Kinetics and mechanism of oxidation of aromatic sulfides and arylmercaptoacetic acids by N-chlorosuccinimide

Article abstract of DOI:10.1039/b206544d

Kinetic measurements of the oxidation of divalent organic sulfur compounds by N-chlorosuccinimide in acetonitrile-water mixture at constant [H⁺] show that the reaction is first order in both the oxidant and the organic sulfur compound. While th

Full text of DOI:10.1039/b206544d

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Kinetics and mechanism of oxidation of aromatic sulfides and
arylmercaptoacetic acids by N-chlorosuccinimide
Duraisamy Thenraja, Perumal Subramaniam and Chockalingam Srinivasan*
2
Department of Materials Science, Madurai Kamaraj University, Madurai 625 021, India.
E-mail: ceesri@yahoo.com
Received (in Cambridge, UK) 5th July 2002, Accepted 10th September 2002
First published as an Advance Article on the web 15th October 2002
Kinetic measurements of the oxidation of divalent organic sulfur compounds by N-chlorosuccinimide in
acetonitrile–water mixture at constant [Hϩ] show that the reaction is first order in both the oxidant and the
organic sulfur compound. While the rate of oxidation of methyl phenyl sulfide or diphenyl sulfide increases
with [Hϩ], that of phenylmercaptoacetic acid decreases. Therefore it has been concluded that protonated
N-chlorosuccinimide and N-chlorosuccinimide are the active oxidising species in the oxidation of aromatic
sulfides and N-chlorosuccinimide is the active species in the case of phenylmercaptoacetic acids. Structure–
reactivity correlations for the oxidation of aryl methyl sulfides, diaryl sulfides and arylmercaptoacetic acids
result in a high negative reaction constant, providing evidence for the formation of a chlorosulfonium ion
intermediate.
solvent. In the oxidation of diaryl sulfides the solvent was 60%
Introduction
acetonitrile–40% water (v/v) and with phenylmercaptoacetic
acids the solvent was 75% acetonitrile–25% water (v/v). All
these kinetic runs were carried out in acid medium, the acid
strength being maintained by the addition of perchloric acid
and the ionic strength by sodium perchlorate. The kinetics were
followed by estimating the unconsumed NCS. Aliquots (5 mL)
of the reaction mixture at regular intervals of time were added
to an iodine flask containing potassium iodide (10 mL, 5%) and
then added 2M hydrochloric acid (5 mL). Unconsumed NCS
liberated a proportionate amount of iodine and the latter was
estimated by standard sodium thiosulfate solution to a starch
end point. In all these oxidation the rates were followed for not
less than 70–80% completion. Reproducible results giving good
first-order plots (r > 0.997) were obtained for reactions run in
duplicate and the mean values were quoted in the discussions.
The sulfoxidation of sulfides is an area of attraction for many
groups.1–4 Although there are many reactions in which sulfides
or sulfoxides are oxidized, systematic studies are few.5 We have
been interested in the elucidation of the mechanism of oxid-
ation of organic sulfides by several oxidants for more than two
decades.4,6–11 Though the oxidation of dialkyl, diaryl and benzyl
phenyl sulfides by N-chlorosuccinimide (NCS) to give the
corresponding sulfoxides has been reported by Harville and
Reed,12 the kinetics of oxidation of divalent organic sulfur
compounds by NCS have not been reported. NCS is mainly
used as a chlorinating agent in the chlorination of organic
sulfides and sulfoxides in organic preparations. In aqueous
solution containing chloride ions the reactive species is molecu-
lar chlorine which results in chlorination followed by hydrolysis
to give the oxidised products. In this paper we report the results
of our extensive studies on the kinetics and mechanism
of reaction of aryl methyl sulfides, diaryl sulfides and aryl-
mercaptoacetic acids by NCS and this investigation includes
the influence of substituents in the organic divalent sulfur
compounds on the rate.
Product analysis
The reaction mixture from an actual kinetic run was evaporated
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(80%–20%). Methyl phenyl sulfoxide and diphenyl
sulfoxide were identified as final products in the oxidation of
methyl phenyl sulfide (MPS) and diphenyl sulfide (DPS)
respectively. GC analysis also indicated that only sulfoxides
were formed.
Experimental
Materials
All the aryl methyl sulfides,13 diaryl sulfides9 and aryl-
mercaptoacetic acids14 were prepared by known methods. The
sulfides and aryl mercaptoacetic acids were purified by distil-
lation or recrystallisation before the kinetic studies. The purity
of the compounds was established from their sharp melting
behaviour with solids, single spots on TLC and spectral studies.
AR grade NCS was used. Doubly distilled water was used
throughout, the second distillation being from permanganate.
A commercial sample of acetonitrile was purified by refluxing
with phosphorous pentoxide in all glass apparatus and then
distilled.15 All other chemicals used were of AR grade.
Results and discussion
Kinetic studies with aryl methyl and diaryl sulfides
MPS and DPS were selected as representatives respectively for
aryl methyl sulfides and diaryl sulfides. The kinetics were
carried out in 95% acetonitrile–5% water (v/v) for MPS and
60% acetonitrile–40% water (v/v) for DPS under pseudo-first-
order conditions keeping the substrate always in excess. The
acid strength was maintained at 0.001 M using perchloric
acid. The rate of the reaction was followed by estimating the
unconsumed NCS by iodometric method at different time
intervals.
Kinetic measurement
The kinetic runs for the oxidation of aryl methyl sulfides by
NCS were carried out in 95% acetonitrile–5% water (v/v) as
DOI: 10.1039/b206544d
J. Chem. Soc., Perkin Trans. 2, 2002, 2125–2129
This journal is © The Royal Society of Chemistry 2002
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Table 1 Pseudo-first-order and second-order rate constants for the
oxidation of MPS and DPS by NCS; [HClO4] = 0.001 M
It is found that added succinimide has no influence on the rate
of oxidation and this excludes the reversible nature of the rate-
determining step (Scheme 1) which involves succinimide as one
of the products. The operation of a one-electron oxidation
giving rise to free radicals is excluded as the rate of oxidation
is unaffected in the presence of acrylonitrile, a radical scaven-
ger. When the reaction has been studied in binary mixtures of
acetonitrile and water as solvent, the rate increases with
increase in polarity of the medium (Table 2). The solvent effect
supports the formation of a polar intermediate.
The activation parameters for both oxidations have been
evaluated from the slope and intercept of Eyring’s plot of
log(k2/T ) against 1/T , which is linear. The activation param-
eters (Tables 3 and 4) are of the order expected for a bimolecu-
lar nucleophilic reaction. The high negative entropy of
activation in both cases suggests a definite orientation in the
transition state and this may be in part due to the solvation
of the activated complex.
a
102[Sulfide]/M
103[NCS]/M
104k1/sϪ1 a
102k2
MPSb
1.0
2.0
3.0
4.0
5.0
2.0
2.0
2.0
1.0
1.0
1.0
1.0
1.0
0.50
1.5
2.0
4.13 0.03
8.01 0.21
12.6 0.14
17.6 0.34
20.8 0.46
8.11 0.08
8.74 0.15
8.55 0.09
4.13 0.03
4.00 0.10
4.20 0.05
4.40 0.08
4.15 0.09
4.06 0.04
4.37 0.08
4.27 0.04
DPSc
1.0
2.0
3.0
4.0
5.0
2.0
2.0
2.0
1.0
1.0
1.0
1.0
1.0
0.50
1.5
2.0
1.78 0.01
3.65 0.02
5.55 0.05
7.62 0.07
9.71 0.07
3.68 0.02
3.68 0.02
3.64 0.01
1.78 0.01
1.83 0.01
1.85 0.02
1.90 0.02
1.94 0.01
1.84 0.01
1.84 0.01
1.82 0.01
Methyl phenyl sulfoxide and diphenyl sulfoxide were isolated
as the products in the oxidation of MPS and DPS respectively.
The stoichiometry of the reaction was found to be 1 : 1 and the
oxidation of sulfides may be represented by
a The error quoted in k1 and k2 values in all tables is 95 % CL
of Student’s t (see ref. 30) b Solvent: 95% acetonitrile–5% water (v/v);
temperature = 20 ЊC c Solvent: 60% acetonitrile–40% water (v/v);
temperature = 45 ЊC
Ph–S–RЈ ϩ NCS ϩ H2O
PhSORЈ ϩ
succinimide ϩ Hϩ ϩ ClϪ (RЈ = CH3 or Ph) (1)
For both substrates, the plots of log(aϪx) against time are
linear (not shown) indicating the first-order dependence on
[NCS]. This is also confirmed by the constant values of k1
(Table 1) at different initial concentrations of NCS. The first-
order dependence on [sulfide] has been revealed by (i) the con-
stant second-order rate constants (k2 = k1/[sulfide]) at varying
initial concentrations of sulfide and (ii) the unit slope of logk1
vs. log[sulfide] (MPS: slope = 1.03 0.03, r = 0.999, s = 0.02;
DPS: slope = 1.05 0.01, r = 0.999, s = 0.01). The plots of 1/k1
vs. 1/[MPS] or 1/[DPS] yield excellent straight lines passing
through origin. Consequently a reaction pathway involving the
formation of an intermediate complex in a fast pre-equilibrium
and its slow decomposition can be excluded.
That the reaction proceeds through two paths, viz. acid-
dependent and acid-independent paths, is revealed by the fact
that k2 varies linearly with [Hϩ] with a positive intercept (Fig. 1.
MPS: r = 0.998; DPS: r = 0.979).
The rate is unaffected by the change in ionic strength of the
medium brought about by the addition of sodium perchlorate.
Scheme 1
Mechanism and rate law
The foregoing kinetic data show that the oxidation is found
to be first order with respect to NCS and MPS or DPS.
N-Halogeno-amides are known to undergo hydrolysis and/or
disproportionation in aqueous solution to give hypohalous
acids which have been proposed as the reactive species in many
reactions.16–18 However, the lack of any effect of succinimide on
the reaction rate rules out the involvement of any pre-equilibria
Fig. 1 Effect of [Hϩ] in NCS oxidation of MPS and DPS. [MPS] =
0.01 M; [NCS] = 0.001 M; [I] = 0.06 M; T = 20 ЊC; solvent 95%
acetonitrile–5% water (v/v). [DPS] = 0.02 M; [NCS] = 0.001 M; [I] =
0.10 M; T = 45 ЊC; solvent 60% acetonitrile–40% water (v/v).
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Table 2 Effect of solvent composition on the rate of oxidation of
MPS and DPS by NCS. [NCS] = 0.001 M. [HClO4] = 0.001 M. [I] =
0.02 M
substituent constants is excellent giving a ρ value of Ϫ3.26
0.10 at 60 ЊC (r = 0.997, s = 0.08, n = 7) which is analogous to
the reaction constant observed for aryl methyl sulfides. The plot
of logk2 values at 35 ЊC of substituted phenyl methyl sulfides
and diphenyl sulfides is linear with a slope of unity (slope =
0.984 0.09, r = 0.988). This confirms that diaryl sulfides and
aryl methyl sulfides follow the same mechanism in the NCS
oxidation.
H2O–CH3CN (%) (v/v)
102k2/MϪ1 sϪ1
MPSa
25–75
15–85
10–90
5–95
155 2.8
35.9 0.23
11.4 0.13
4.13 0.03
Isokinetic relationship
DPSb
Generally the Hammett correlation appears to apply to a
reaction series in which either ∆S‡ is constant or in which the
variation in ∆S‡ is linearly related to changes in ∆H‡.20–23 In
the latter case these two variables are correlated23–26 by eqn. (9).
60–40
50–50
40–60
30–70
20–80
10–90
8.15 0.17
5.98 0.05
1.83 0.01
0.542 0.01
0.146 0.001
0.059 0.001
∆H‡ = ∆H0‡ ϩ β∆S‡
(9)
a [MPS] = 0.01 M; T = 20 ЊC. b [DPS] = 0.02 M; T = 45 ЊC.
Though in both series of sulfides the Petersen error criterion
is satisfied, i.e., ∆∆H‡ > 2δ, the relation between ∆H‡ and ∆S‡
can be assumed to be valid27 in the present study. However, the
correlation between ∆H‡ and ∆S‡ is not very good. According
to Exner28 a good linear log–log plot of the rate constants at
two different temperatures is an indication of the existence of
an isokinetic relationship. The good correlation obtained in
Exner’s excellent plots (Fig. 2, r > 0.995) leads to the conclusion
in the present investigation. As the plot of k2 vs. [Hϩ] is linear
with a positive intercept (Fig. 1), it appears that NCS and its
protonated species are the oxidising species in the oxidation of
sulfides. The mechanism shown in Scheme 1 is proposed.
From Scheme 1, the rate law may be obtained
Ϫd[NCS]/dt = k1Ј[PhSR][NCS] ϩ k2Ј[PhSR][NCSHϩ] =
k1Ј[PhSR][NCS] ϩ Kk2Ј[PhSR][Hϩ][NCS]
as [NCSHϩ] = K [Hϩ][NCS].
Therefore,
Ϫd[NCS]/dt = {k1Ј ϩ Kk2Ј[Hϩ]}[PhSR][NCS]
(7)
Therefore the observed second-order rate constant is expressed
as
k2 = k1Ј ϩ Kk2Ј[Hϩ]
(8)
Eqn. (8) predicts that k2 should vary linearly with [Hϩ] with a
definite intercept. This has indeed been observed (Fig. 1).
Structure–reactivity correlation
(i) Substituent effect
As the study of influence of substituents on the rate of oxid-
ation often provides an insight into the nature of the trans-
ition state, the rates of oxidation of a number of meta- and
para-substituted phenyl methyl sulfides and 4-substituted and
4-nitro-4Ј-substituted diphenyl sulfides were determined in the
presence of perchloric acid at constant ionic strength (in most
cases at three temperatures) and the activation parameters were
evaluated (Tables 3 and 4). In both series of sulfides, electron-
releasing groups accelerate the rate of oxidation. The rates of
oxidation of aryl methyl sulfides correlate well with Hammett
substituent constants giving a negative reaction constant at
20 ЊC (ρ = Ϫ3.33 0.10, r = 0.995, s = 0.11, n = 13). The negative
reaction constant points to an electrophilic attack on the sulfur
atom by the oxidant and the high ρ value provides support for
the chlorosulfonium ion intermediate in the reaction. This con-
clusion is supported by the fact that the ρ values in the oxid-
ation of alkyl aryl sulfides by Brϩ and Clϩ proceeding through
RArSBrϩ and RArSCIϩ are Ϫ3.2016 and Ϫ4.2519 respectively.
As the reaction of NCS with 4-nitrodiphenyl sulfide, 4-nitro-
4Ј-chlorodiphenyl sulfide and 4,4Ј-dinitrodiphenyl sulfide are
too small to follow even at high substrate concentration and
at high temperature, their rates were not studied. The corre-
lation between logk2 values of diaryl sulfides and Hammett
Fig. 2 Isokinetic plots in NCS oxidation (Exner plots, numbers as in
Table 3 for ArSCH3 and Table 4 for ArSPh).
that the mechanism of the reaction NCS and all the aryl methyl
sulfides and also that of NCS and diaryl sulfides is one and the
same.
Kinetic studies with phenylmercaptoacetic acids
The rates of oxidation of phenylmercaptoacetic acid (PMAA)
and several substituted phenylmercaptoacetic acids (divalent
organic sulfur compounds) have been studied in 75% aceto-
nitrile–25% water (v/v) in the presence of 0.001 M perchloric
acid and at an ionic strength of 0.02 M. Detailed kinetic
investigations were made with PMAA as the representative of
the series under pseudo-first-order conditions of [PMAA] ӷ
[NCS]. The oxidation has been found to be first order both in
[PMAA] and in [NCS] from the following facts:
(i) The plots of log[NCS] vs. time are linear (not shown) and
the pseudo-first-order rate constant k1 values are independent
of initial [NCS] (see Table 5).
J. Chem. Soc., Perkin Trans. 2, 2002, 2125–2129
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Table 3 Second-order rate constants, enthalpies and entropies of activation for the oxidation of X–C6H4–SCH3 by NCS. [Ar–S–CH3] = 0.01 M.
[HClO4] = 0.001 M. [NCS] = 0.001 M. [I] = 0.02 M
Solvent: 95% acetonitrile–5% water (v/v)
102k2/M Ϫ1 sϪ1
X
20 ЊCa
35 ЊC
45 ЊC
∆H‡/kJ MϪ1
Ϫ∆S‡/J KϪ1MϪ1
1
2
3
4
5
6
7
8
9
10
11
12
13
H
4.06 0.06
46 0.44
8.45 0.05
93.1 1.9
23.9 0.32
27.9 0.22
26.4 0.42
5.33 0.04
1.38 0.01
1.67 0.02
16.9 0.19
7.47 0.07
0.948 0.02
—
15.8 0.05
29.6 0.47b
42.7 0.79
41.8 1.0
40.9 0.37
9.44 0.13
—
38.5 0.34
30.9 0.89
34.2 0.89
31.9 0.90
32.2 0.75
40.6 0.62
34.3 0.81
51.8 0.95
36.4 0.73
36.1 0.47
58.6 0.69
—
140 1.2
146 3.5
145 3.1
152 3.2
152 3.2
138 2.2
169 2.9
111 3.3
142 2.6
150 1.7
93.8 2.5
—
p-OMe
p-Me
p-Et
12.8 0.14
13.8 0.16
13.3 0.15
2.33 0.02
0.661 0.01
0.564 0.01
7.91 0.08
3.28 0.02
0.281 0.01
0.105 0.001c
0.011 0.001d
p-Pri
p-F
p-Cl
p-Br
m-Me
m-OMe
m-Cl
p-Ac
p-NO2
—
27.9 0.40
11.3 0.08
2.02 0.01
—
—
—
—
—
a Average k2 for two substrate concentrations; b temp. = 10 ЊC; c [ArSCH3] = 0.02 M; d [ArSCH3] = 0.05. The precisions of ∆H‡ and ∆S‡ values were
calculated using the method of Petersen et al.27
Table 4 Second-order rate constants, enthalpies and entropies of activation for the oxidation of X-C6H4–S–C6H4–Y by NCS [Ar–S–Ar] = 0.02 M.
[HClO4] = 0.001 M. [NCS] = 0.001 M. [I] = 0.02 M
Solvent: 60% acetonitrile–40% water (v/v)
102k2/M Ϫ1 sϪ1
X
Y
35 ЊCa
45 ЊC
60 ЊC
∆H‡ kJ M Ϫ1
Ϫ∆S‡ J KϪ1MϪ1
1
2
3
4
5
6
7
H
H
H
H
H
0.880 0.01
5.85 0.04
3.15 0.02
0.115 0.001
0.39 0.001
—
1.80 0.01
10.3 0.10
5.57 0.02
0.212 0.002
0.262 0.002
—
4.20 0.01
21.1 0.13b
12.1 0.11b
0.636 0.004
0.743 0.01
0.082 0.001b
0.028 0.001
50.4 0.31
41.0 0.52
43.2 0.43
57.1 0.55
54.8 0.55
—
121 1.0
136 1.7
134 1.5
117 1.9
122 1.9
—
4-OMe
4-Me
4-Cl
4-Br
4-NO2
4-NO2
H
4Ј-OMe
4Ј–Me
—
—
—
—
a Average k2 for two substrate concentrations b [ArSAr] = 0.01 M.
Table 5 Pseudo-first-order and second-order rate constants for the
oxidation of PMAA by NCS at 35 ЊC. [HClO4] = 0.001 M. [I] = 0.02 M
Table 6 Effect of temperature on the rate of oxidation of PMAA by
NCS and activation parameters. [PMAA] = 0.01 M. [HClO4] = 0.001 M.
[NCS] = 0.001M. [I] = 0.02 M
Solvent: 75% acetonitrile–25% water (v/v)
Solvent: 75% acetonitrile–25% water (v/v)
102[PMAA]/M
103[NCS]/M
104k1/sϪ1
102k2/MϪ1 sϪ1
Temperature/ЊC 102k2/MϪ1 sϪ1
∆H‡/kJ MϪ1
Ϫ∆S‡/J KϪ1 MϪ1
1.0
2.0
3.0
4.0
5.0
7.0
10
2.0
2.0
2.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
0.50
1.5
2.0
6.02 0.13
12.2 0.20
18.0 0.23
24.0 0.46
30.6 0.62
43.1 0.61
60.9 0.73
12.3 0.11
11.9 0.26
11.9 0.21
6.02 0.13
6.10 0.10
6.05 0.08
6.01 0.11
6.11 0.12
6.15 0.09
6.09 0.07
6.14 0.05
5.95 0.13
5.97 0.10
20
30
35
45
3.18 0.05
4.90 0.08
6.02 0.13
8.84 0.17
29.2 1.1
174 4.0
The increase in the ionic strength of the medium results in a
slight increase in the rate. The rate is also enhanced when the
water content is increased in the medium of the binary mixture.
From the measured rates at four temperatures (Table 6) and
from the slope and intercept of the plot of log(k2/T ) against
1/T , ∆H‡ and ∆S‡ have been evaluated.
(ii) The plot of k1 vs. [PMPAA] is linear and passes through
the origin (not shown).
(iii) The plot of logk1 vs. log[PMAA] is linear with a slope of
unity (not shown), and the second-order rate constants, k2, are
constant at various [PMAA] (Table 5).
Mechanism and rate law
Since added succinimide has not influenced the rate, an equi-
librium involving hypohalous acid is ruled out. The kinetic
results obtained in the present study can be rationalized if NCS
itself is considered to be the active species. The mechanism
shown in Scheme 2 is proposed for the NCS oxidation of
PMAA.
The mechanism accounts for the decrease in rate of oxidation
with increasing [HClO4]. Equilibrium (10) shifts to the right
with increasing [HClO4], decreasing the concentration of the
However, the rate of oxidation decreases with increase in
[Hϩ]. The second-order rate constants at 35 ЊC in 75 % aceto-
nitrile–25 % water at [HClO4] = 0.001, 0.002, 0.005, and 0.01 M
are 7.33 × 10Ϫ2, 6.01 × 10Ϫ2, 4.13 × 10Ϫ2, and 3.74 × 10Ϫ2 MϪ1
sϪ1 respectively ( [PMAA] = 0.01 M; [NCS] = 0.001 M; [I] =
0.05 M). In this respect the oxidation differs from those of aryl
methyl and diaryl sulfides. The rate is neither affected by the
addition of a vinyl monomer, acrylonitrile nor by succinimide.
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Table 7 Second-order rate constants for the oxidation of X–C6H4–S–
CH2COOH by NCS at 35 ЊC. [Ar–S–CH2COOH] = 0.02 M. [HClO4] =
0.001 M. [NCS] = 0.001 M. [I] = 0.02 M
Difference in reactivity of MPS, DPS and PMAA
Since the kinetics of aryl methyl sulfides, diaryl sulfides and
arylmercaptoacetic acids have been measured in different
solvent mixtures, a complete discussion for the difference in
behaviour involving all the substrates is not possible. However,
it is observed that the second-order rate constants for the NCS
Solvent: 75% acetonitrile–25% water (v/v)
X
102k2/MϪ1 sϪ1
oxidation of MPS and DPS are 15.8 × 10Ϫ2
M
Ϫ1sϪ1 (in 95%
Ϫ1sϪ1 (in 90 %
1
2
3
4
5
6
7
8
9
H
6.10 0.10
65.1 0.77
18.0 0.23
18.1 0.28
18.5 0.25
3.86 0.05
1.29 0.03
1.14 0.03
0.085 0.01
9.78 0.18
4.27 0.10
0.441 0.07
acetonitrile–5% water (v/v) and 5.9 × 10Ϫ4
M
p-OMe
p-Me
p-Et
acetonitrile–10 % water (v/v) respectively at 45 ЊC and at
[HClO4] = 0.001 M and [I] = 0.02 M. The considerable lower
reactivity of DPS may be attributed to a possible steric effect
offered by the additional phenyl group during the attack of the
oxidising species. The observed rate constants for the oxidation
of MPS and PMAA at 35 ЊC and [HClO4] = 0.001 M and [I] =
p-Pri
p-F
p-Cl
p-Br
p-NO2
m-Me
m-OMe
m-Cl
Ϫ2
0.02 M in 95 % acetonitrile–5 % water (v/v) are 8. 45 × 10
10
11
12
M
Ϫ1 sϪ1 and 1.67 × 10Ϫ3 MϪ1 sϪ1 respectively. The lower rate of
PMAA is not unexpected as the carboxyl group is electron-
attracting, the availability of electron density on the sulfur atom
in this substrate is somewhat reduced compared to MPS.
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Scheme 2
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active species of NCS. In the bromination of anisole by
N-bromosuccinimide (NBS),29 it has been concluded that NBS
is the oxidising species, since the rate decreases with increasing
[Hϩ].
The influence of substituents on the rate of oxidation has
been investigated by employing, several substituted phenyl-
mercaptoacetic acids. It is observed that the electron-releasing
groups in the phenyl ring favour the reaction (Table 7) and
electron-withdrawing substitutents retard the rate. A satisfac-
tory linear free-energy relationship is found between logk2 and
σ values (r = 0.986, s = 0.14, n = 12) with the reaction constant
(ρ) value of Ϫ2.73 0.14 at 35 ЊC. The negative ρ value indi-
cates an electron-deficient transition state and this value is
analogous to the one observed for the oxidation of aryl methyl
and diaryl sulfides (vide supra).
J. Chem. Soc., Perkin Trans. 2, 2002, 2125–2129
2129