Oxidation of organic sulfides by tetraethylammonium chlorochromate: A kinetic and mechanistic study

Article abstract of DOI:10.1080/17415993.2011.587519

The oxidation of organic sulfides by tetraethylammonium chlorochromate (TEACC) resulted in the formation of the corresponding sulfoxides. The reaction is first order with respect to both TEACC and the sulfide. The reaction is catalyzed by toluene-p-sulfon

Full text of DOI:10.1080/17415993.2011.587519

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Oxidation of organic sulfides by
tetraethylammonium chlorochromate:
a kinetic and mechanistic study
Deepika Sharma a , P. Pancharia a , K. Vadera a & Pradeep K.
Sharma a
a Department of Chemistry , J.N.V. University , Jodhpur, 342 005,
India
Published online: 08 Jun 2011.
To cite this article: Deepika Sharma , P. Pancharia , K. Vadera & Pradeep K. Sharma (2011)
Oxidation of organic sulfides by tetraethylammonium chlorochromate: a kinetic and mechanistic
study, Journal of Sulfur Chemistry, 32:4, 315-326, DOI: 10.1080/17415993.2011.587519
To link to this article: http://dx.doi.org/10.1080/17415993.2011.587519
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Journal of Sulfur Chemistry
Vol. 32, No. 4, August 2011, 315–326
Oxidation of organic sulfides by tetraethylammonium
chlorochromate: a kinetic and mechanistic study
Deepika Sharma, P. Pancharia, K. Vadera and Pradeep K. Sharma*
Department of Chemistry, J.N.V. University, Jodhpur 342 005, India
(Received 27 March 2011; final version received 9 May 2011)
The oxidation of organic sulfides by tetraethylammonium chlorochromate (TEACC) resulted in the for-
mation of the corresponding sulfoxides. The reaction is first order with respect to both TEACC and the
sulfide. The reaction is catalyzed by toluene-p-sulfonic acid (TsOH). The oxidation was studied in 19
different organic solvents. An analysis of the solvent effect by Swain’s equation showed that both cation-
and anion-solvating powers of the solvents play important roles. Correlation analyses of the rate of oxida-
tion of 34 sulfides were performed in terms of various single and multiparametric equations. For the aryl
methyl sulfides, the best correlation is obtained with Charton’s LDR and LDRS equations. The oxidation
of alkyl phenyl sulfides exhibited a very good correlation in terms of the Pavelich–Taft equation. The polar
reaction constants are negative indicating an electron-deficient sulfur center in the rate-determining step. A
mechanism involving the formation of a sulphenium cation intermediate in the slow step has been proposed.
R
O-N+Et4
Cl
#
R
O
O
O-N+Et4
Sδ+
Oδ− Cr
O
••
R'
Cr
S
+
R'
Cl
O
S
CrOClO-N+Et4
+
R
R'
Keywords: correlation analysis; halochromates; mechanism; oxidation kinetics; sulfides
1. Introduction
Pyridinium and quinolinium halochromates have long been used as mild and selective oxidizing
reagentsinthesyntheticorganicchemistry(1–5).Tetraethylammoniumchlorochromate(TEACC)
is one such compound used for the oxidation of primary aliphatic alcohols (6). There are very few
reports investigating the oxidation kinetics of sulfides with TEACC (7, 8). In contrast, there are
many reports on the oxidation of sulfides by Cr(VI) reagents (9). For example, Karunakaran et al.
(10) have reported a common mechanism for the oxidation of diphenyl sulfide by various Cr(VI)
reagents in acetic acid. In the present article, we report the kinetics of oxidation of thirty-four
*Corresponding author. Email: drpkvs27@yahoo.com
ISSN 1741-5993 print/ISSN 1741-6000 online
© 2011 Taylor & Francis
DOI: 10.1080/17415993.2011.587519
http://www.informaworld.com

316 D. Sharma et al.
organic sulfides by TEACC in dimethylformamide (DMF) as a solvent. The objectives of this
work are two-fold. First to find out if TEACC behave differently from other Cr(VI) reagents and
secondly to study the effect of structure and solvent on the rate.
2. Results
The oxidation of organic sulfides by TEACC resulted in the formation of the corresponding
sulfoxides. The overall reaction may be represented as Equation (1)
R − S − Rꢀ + O CrClO−N+Et −→ R − −Rꢀ + OCrClO−N+Et .
(1)
S
2
4
4
ꢁ
TEACC undergoes a two-electron change. This is in accord with the earlier observations with
TEACC (7, 8) and with structurally similar halochromates (9). It has already been shown that
both pyridinium fluorochromate (PFC) (11) and pyridinium chlorochromate (PCC) (12) act as
two electron oxidants and are reduced to chromium (IV) species by determining the oxidation
state of chromium by magnetic susceptibility, electron spin resonance and IR studies.
2.1. Rate law
Methyl phenyl sulfide was taken as a typical representative for detailed study. The reactions were
found to be first order with respect to TEACC. The individual kinetic runs were strictly first
order in TEACC (Figure 1). Further, the first-order rate coefficients did not vary with the initial
concentration of TEACC. The order with respect to sulfide was also found to be one (Table 1).
The experimental rate laws may be expressed as follows:
−d[TEACC]
= kobs[TEACC]
(2)
(3)
dt
−d[TEACC]
dt
= k2[TEACC][sulfide].
2.5
2
1.5
1
0.5
0
0
300
600
900
1200
Time (sec)
Figure 1. Oxidation of methyl phenyl sulfide by TEACC: a typical kinetic run.

Journal of Sulfur Chemistry 317
Table 1. Rate constants for the oxidation of methyl phenyl sulfide by TEACC at 298 K.
103[TEACC] (mol dm−3
)
[MeSPh] (mol dm−3
)
[TsOH] (mol dm−3
)
104 kobs (s−1
)
103 k2 (dm3 mol−1 s−1
)
1.0
1.0
1.0
1.0
1.0
1.0
2.0
4.0
6.0
8.0
1.0
0.10
0.20
0.40
0.60
0.80
1.00
0.20
0.20
0.20
0.20
0.40a
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
2.65
5.26
10.8
16.2
21.1
26.1
5.58
4.86
5.09
4.59
11.7a
2.65
2.63
2.70
2.70
2.64
2.61
2.79
2.43
2.55
2.29
2.92
Note: aContained 0.001 mol dm−3 acrylonitrile.
2.1.1. Test for free radicals
The oxidation of methyl phenyl sulfide, in an atmosphere of nitrogen, failed to induce the polymer-
ization of acrylonitrile. It was observed that added 2,6-di-t-butyl-4-methylphenol was recovered
Table 2. Rate constants and activation parameters of the oxidation of organic sulfides by TEACC.
104 k2 (dm3 mol−1 s−1
)
ꢀH∗
kJ mol−1
ꢀS∗
J mol−1 K−1
ꢀG∗
kJ mol−1
Substituents
288 K
298 K
308 K
318 K
(i) Aryl methyl sulfides
H
p-Me
p-Ome
p-F
p-Cl
p-NO2
p-COMe
p-COOMe
p-Br
p-NHAc
p-NH2
m-Me
m-Ome
m-Cl
9.45
26.1
51.3
108
65.7
126
252
73.8
49.5
5.94
12.6
17.1
48.3
135
171
306
612
189
133
18.0
36.0
48.6
130
342
1620
270
70.6 0.7
67.8 0.3
65.4 0.6
70.0 0.7
73.5 0.7
82.3 0.8
77.1 0.6
77.1 0.8
73.8 0.7
68.1 0.8
60.3 0.5
67.1 0.7
66.7 0.5
73.7 0.8
73.9 0.6
72.7 0.6
82.2 0.8
77.0 0.3
56.2 0.8
75.8 0.9
85.5 0.5
84.5 0.8
82.2 0.8
83.2 0.7
84.8 0.8
66.1 0.7
−58
−62
−64
−59
−51
−40
−45
−48
−50
−60
−72
−65
−65
−56
−55
−58
−44
−50
−46
−54
−37
−34
−35
−34
−30
−62
2
1
2
2
2
3
2
3
2
3
1
2
1
3
2
2
3
1
3
2
2
3
1
2
2
2
87.8 0.5
86.1 0.2
84.3 0.5
87.5 0.5
88.6 0.5
94.1 0.7
92.1 0.5
91.3 0.6
88.6 0.5
85.9 0.6
81.5 0.4
86.4 0.5
86.1 0.5
90.1 0.4
90.2 0.5
89.7 0.5
89.7 0.6
95.1 0.6
89.4 0.7
87.2 0.4
96.4 0.4
94.3 0.6
92.5 0.4
93.1 0.5
93.7 0.6
84.5 0.6
19.1
41.4
10.8
28.8
18.9
2.00
4.50
6.30
18.0
56.7
324
45.0
52.2
10.4
9.90
11.7
1.31
5.40
13.5
32.4
0.79
1.80
3.87
2.97
2.38
99.9
6.57
0.63
1.44
2.07
6.39
20.7
135
17.1
19.8
3.45
3.42
4.14
0.41
1.80
4.41
720
108
126
306
26.4
26.1
30.6
3.87
15.3
35.1
83.7
2.43
5.53
11.7
8.91
7.20
234
71.1
70.2
81.0
11.7
41.4
99.0
216
7.47
17.1
34.2
27.0
22.5
576
m-Br
m-I
m-NO2
m-CO2Me
o-Me
o-Ome
o-NO2
o-COOMe
o-Cl
o-Br
o-I
o-NH2
11.7
0.23
0.55
1.21
0.91
0.71
37.8
(ii) Alkyl phenyl sulfides
Et
Pr
i-Pr
t-Bu
14.4
9.27
11.7
2.88
40.5
27.0
33.3
10.2
99.0
70.2
89.1
33.3
252
189
243
108
69.7 0.7
73.6 0.7
74.2 0.8
72.5 0.9
−58
−48
−44
−45
2
2
3
2
86.7 0.5
87.9 0.5
87.1 0.6
88.2 0.7
(iii) Other sulfides
Me2S
Pr2S
Ph2S
30.6
47.7
5.67
77.4
117
16.2
198
297
48.6
486
693
126
67.8 0.8
65.7 0.7
76.7 0.8
−58
−62
−41
2
2
3
85.0 0.6
83.9 0.5
88.8 0.6

318 D. Sharma et al.
unchanged, almost quantitatively. This further confirmed the absence of a free radical in the
reaction pathway.
2.1.2. Effect of substituents
The rates of oxidation of a number of ortho-, meta- and para-substituted phenyl methyl sul-
fides, alkyl phenyl sulfides, dialkyl sulfides and diphenyl sulfide were determined at different
temperatures and the activation parameters were calculated (Table 2) (Figure 2).
2.1.3. Effect of acidity
The reaction is catalyzed by toluene-p-sulfonic acid (TsOH) (Table 3). The TsOH dependence
has the following form:
kobs = k2 + k3 [TsOH].
(4)
The values of k2 and k3 are 2.53 0.09 10−4 s−1 and 4.46 0.15 10−4 mol−1 dm3 s−1
respectively (r2 = 0.9955).
,
2.1.4. Effect of solvent
The oxidation of methyl phenyl sulfide was studied in 19 different organic solvents. The choice of
the solvent was limited by the solubility of TEACC and its reaction with primary and secondary
alcohols. There was no reaction with the solvent chosen. The kinetics were similar in all the
solvents. The values of k2 are recorded in Table 4.
5
4
H
m-Cl
3
2
1
p-Ome
o-I
3
3.1
3.2
10–3 ( 1/T )
Figure 2. Oxidation of sulfides by TEACC: effect of temperature.
3.3
3.4
3.5
Table 3. Dependence of the reaction rate on hydrogen-ion concentration.
[Sulfide] 0.10 (mol dm−3 [TEACC] 0.001 (mol dm−3
[TsOH] (mol dm−3
)
)
Temperature 298 K
)
0.10
2.97
0.20
3.51
0.40
4.32
0.60
5.04
0.80
6.03
1.00
7.11
105 kobs (s−1
)

Journal of Sulfur Chemistry 319
Table 4. Solvent effect on the oxidation of MeSPh by TEACC at 308 K.
Solvent
104k2 s−1
Solvent
Toluene
Acetophenone
Tetrahydrofuran
t-Butyl alcohol
1,4-Dioxane
1,2-Dimethoxyethane
Ethyl acetate
Carbon disulfide
Acetic acid
104k2 s−1
Chloroform
1,2-Dichloroethane
Dichloromethane
DMSO
Acetone
DMF
Butanone
Nitrobenzene
Benzene
47.9
38.9
44.7
145
33.9
65.7
22.4
51.3
9.77
0.66
7.94
42.7
14.8
18.2
12.0
9.12
13.2
3.47
20.9
Cyclohexane
3. Discussion
There is a fair correlation between the activation enthalpies and entropies of the oxidation of the
34 sulfides (r2 = 0.8819), indicating the operation of a compensation effect (13). An Exner’s
plot (14) between log k2 at 288 and 318 K was linear (r2 = 0.9934, slope = 0.8336 0.0123),
indicating an excellent isokinetic relationship. The value of the isokinetic temperature evaluated
from Exner’s plot is 672 66 K. The free energy of activation ranges from 81 to 96 kJ mol−1
.
This variation is due to ca. 200 times variation in the reaction rate (Figure 3).
3.1. Solvent effect
The rate constants for oxidation, k2, in 18 solvents (CS2 was not considered, as the complete range
of solvent parameters was not available) did not exhibit a very good correlation (R2 = 0.8950)
in terms of the linear solvation energy relationship (5) of Kamlet et al. (15).
log k2 = A0 + pπ∗ + bβ + aα.
(5)
5.5
5
4.5
4
3.5
3
2.5
2
1.5
1
1.5
2
2.5
3
3.5
4
4.5
2 + logk2 at 288K
Figure 3. Exner’s isokinetic relationship in the oxidation of sulfides by TEACC.

320 D. Sharma et al.
The data on the solvent effect were analyzed in terms of Swain’s equation (16) using the cation-
and anion-solvating power concept of the solvents as well.
log k2 = aA + bB + C.
(6)
Here A represents the anion-solvating power of the solvent, B the cation-solvating power and
C is the intercept term. The rates in different solvents were analyzed in terms of Equation (6),
separately with A and B and with (A + B).
log k2 = 1.50 ( 0.05)A + 1.80 ( 0.04)B − 3.92,
R2 = 0.9952; sd = 0.04; n = 19; ψ = 0.07,
log k2 = 1.70 0.05 (A + B) − 3.71,
(7)
(8)
r2 = 0.9870; sd = 0.06; n = 19; ψ = 0.12.
The rates of oxidation of methyl phenyl sulfide in different solvents show an excellent correlation
with Swain’s equation with both the cation- and anion-solvating powers playing significant roles,
although the contribution of the cation solvation is slightly more than that of the anion solvation.
However, the correlations individually with A and B were poor.
3.2. Correlation analysis of reactivity
The data in Table 3 show that the oxidation of different sulfides follows the order of their
nucleophilicity: Pr2S > Me2S > MeSPh > Ph2S.
3.2.1. Aryl methyl sulfides
The correlation of the effect of substituents on the reactivity has been widely attempted in terms
of the Hammett equation (17) or with dual-substituent parameter (DSP) equations (18, 19). In the
late 1980s, Charton and Charton (20) introduced a triparametric LDR equation for the quantitative
description of structural effects on chemical reactivities. This triparametric equation results from
the fact that substituent types differ in their mode of electron delocalization. This difference is
reflected in a different sensitivity to the electronic demand for the phenomenon being studied. It
has the advantage of not requiring a choice of parameters as the same three substituent constants
are reported to cover the entire range of electrical effects of substituents. We have, therefore,
begun a study of structural effects on reactivity by means of the LDR equation. In this work, we
have applied the LDR equation (9) to the rate constants, k2.
log k2 = Lσl + Dσd + Rσe + h.
(9)
Here, σl is a localized (field and/or inductive) effect parameter, σd is the intrinsic delocalized
(resonance) electrical effect parameter when active site electronic demand is minimal and σe
represents the sensitivity of the substituent to changes in electronic demand by the active site. The
latter two substituent parameters are related by Equation (10).
σD = ησe + σd.
(10)
Here η represents the electronic demand of the reaction site and represents the ratio of regression
coefficient of the sensitivity parameter, σe, and that of resonance parameter, σd, i.e. η = R/D. σD
represents the delocalized electrical parameter of the diparametric LD equation.

Journal of Sulfur Chemistry 321
Table 5. Correlation of rate of oxidation of alkyl phenyl sulfides with Pavelich–Taft equation.a
Temperature (K)
ρ∗
δ
R2
sd
288
298
308
318
−2.80 0.01
−2.69 0.01
−2.61 0.02
−2.41 0.09
0.90 0.01
0.85 0.01
0.80 0.01
0.73 0.02
0.9999
0.9998
0.9999
0.9993
0.01
0.02
0.01
0.03
Note: aNumber of data points = 5.
For ortho-substituted compounds, it is necessary to account for the possibility of steric effects,
and Charton and Charton (20) therefore modified the LDR equation to generate the LDRS
equation (11).
log k2 = Lσl + Dσd + Rσe + Sυ + h,
(11)
where υ is the well-known Charton’s steric parameter based on Van der Waals radii (21).
The rates of oxidation of ortho-, meta- and para-substituted sulfides show excellent correlations
in terms of the LDR/LDRS equations (Table 5). The values of the independent variables, σl, σd,
σe and υ, were obtained from the work of Charton and Charton (20). Though the number of data
points is less than the optimum number, the correlations are excellent as per Exner’s (16) criterion
also. Exner’s y parameter takes into account the number of data points also.
All three regression coefficients, L, D and R, are negative indicating an electron-deficient sulfur
center in the transition state of the rate-determining step. The positive value of η adds a negative
increment to σd as in Equation (11), reflecting the electron-donating power of the substituent and
its capacity to stabilize a cationic species.
The negative value of S indicates that the reaction is subjected to steric hindrance. This may
be due to steric hindrance of the ortho-substituent to the approach of the oxidizing species.
To test the significance of localized, delocalized and steric effects in the ortho-substituted
sulfides, multiple linear regression analyses were carried out with (i) σl, σd and σe, (ii) σd, σe and
υ and (iii) σl, σe and υ. The absence of significant correlations [Equations (12)–(14)] showed that
all the four substituent constants are significant.
Log k2 = −1.97( 0.43)σl − 1.34( 0.35)σd − 1.93( 2.50)σe − 2.90,
R2 = 0.9089, sd = 0.26, n = 10, ψ = 0.38.
(12)
(13)
(14)
log k2 = −1.88( 0.45)σd + 0.55( 0.22)σe − 1.74( 0.56)υ − 2.71,
R2 = 0.8425, sd = 0.35, n = 10, ψ = 0.47.
log k2 = −2.15( 0.87)σl − 5.36( 4.37)σe − 0.60( 0.88)υ − 2.48,
R2 = 0.6781, sd = 0.50, n = 10, ψ = 0.72.
Similarly, in the cases of the oxidation of para- and meta-substituted sulfides, multiple regression
analyses indicated that both localization and delocalization effects are significant. There is no
significant colinearity between the various substituent constants for the three series.
The percent contribution (21 of the delocalized effect, PD, is given by the following equation:
|D| × 100
PD =
.
(15)
|L| + |D| + |R|

322 D. Sharma et al.
Similarly, the percent contribution of the steric parameter (21) to the total effect of the
substituent, PS, was determined by using the following equation
|S| × 100
PS =
.
(16)
|L| + |D| + |S| + |R|
The values of PD and PS are also recorded in Table 4. The value of PD for the oxidation of
para-substituted sulfides is ca. 56%, whereas the corresponding values for the meta- and ortho-
substituted sulfides are ca. 44% and 51%, respectively. The less pronounced resonance effect from
the ortho-position than from the para-position may be due to the twisting away of the methylthio
group from the plane of the benzene ring.
In earlier studies on the oxidations of sulfides, involving a direct oxygen transfer via an
electrophilic attack on the sulfide sulfur, the reaction constants were negative but of relatively
small magnitude, e.g. by hydrogen peroxide (−1.13) (22), periodate (−1.40) (23), permanganate
(−1.52) (24) and peroxydisulfate (−0.56) (25). Large negative reaction constants were exhib-
ited by oxidations involving the formation of halogeno-sulfonium cations, e.g. by chloramine-T
(−4.25) (26), bromine (−3.2) (27) and N-bromoacetamide (−3.75) (28). In the oxidation by
N-chloroacetamide (29), the values of field (ρI) and resonance (ρR+), at 298 K are −1.3 and −1.7,
respectively.
3.2.2. Alkyl phenyl sulfides
The rates of oxidation of alkyl phenyl sulfides did not yield any significant correlation separately
with Taft’s σ∗ or Es values. The rates were therefore analyzed in terms of Pavelich–Taft’s (30)
DSP equation (17):
log k2 = ρ∗σ∗ + δEs + log k0.
(17)
The correlations are excellent (Table 6). Though the number of compounds is small (five) for
any analysis by a DSP equation, the results can be used qualitatively. The negative polar reaction
constant confirms that the electron-donating power of the alkyl group enhances the reaction rate.
The steric effect plays a minor inhibitory role.
Table 6. Temperature dependence for the reaction constants for the oxidation of organic sulfides by TEACC.
T/K
L
R
D
S
η
R2
sd
ψ
PD
PS
Para-substituted
288 −1.42 0.01 −1.81 0.01 −1.35 0.06
298 −1.35 0.01 −1.71 0.01 −1.25 0.03
308 −1.26 0.01 −1.62 0.01 −1.14 0.03
318 −1.17 0.01 −1.52 0.01 −1.10 0.03
–
–
–
–
0.69 0.9998 0.008 0.02 56.0
0.73 0.9999 0.005 0.01 55.9
0.70 0.9989 0.004 0.04 56.3
0.72 0.9999 0.004 0.01 56.5
–
–
–
–
Meta-substituted
288 −1.80 0.01 −1.42 0.01 −1.29 0.07
298 −1.70 0.01 −1.35 0.01 −1.16 0.08
308 −1.62 0.01 −1.26 0.01 −1.04 0.10
318 −1.52 0.01 −1.18 0.01 −0.96 0.07
–
–
–
–
0.91 0.9998 0.004 0.02 44.1
0.87 0.9999 0.005 0.01 44.3
0.92 0.9998 0.007 0.02 43.8
0.81 0.9999 0.004 0.01 43.7
–
–
–
–
Ortho-substituted
288 −1.52 0.01 −1.61 0.01 −1.35 0.01 −1.17 0.02 0.84 0.9999 0.001 0.01 51.4 27.2
298 −1.45 0.01 −1.52 0.01 −1.27 0.01 −1.07 0.02 0.84 0.9998 0.005 0.02 51.2 26.5
308 −1.34 0.01 −1.45 0.05 −1.15 0.07 −0.99 0.08 0.79 0.9989 0.008 0.05 52.0 26.2
318 −1.30 0.02 −1.37 0.01 −0.98 0.01 −0.88 0.02 0.72 0.9998 0.009 0.02 51.3 24.8

Journal of Sulfur Chemistry 323
3.3. Mechanisms
The observed dependence on TsOH suggests that the reaction follows two mechanistic pathways,
one TsOH-catalyzed and the other uncatalyzed. The catalytic effect of TsOH can be attributed to
a protonation of TEACC to give a stronger oxidant and electrophile (6)
O2CrClO−N+Et4 + TsOH ꢀ [OCr(OH)ClO−N+Et4]+[TsO]−.
(18)
TsOH is a strong acid and is expected to be highly ionized in an aprotic polar solvent such as
dimethyl sulfoxide (DMSO). However, one cannot rule out the possibility of the formation of
an ion pair. Therefore, the reactive oxidizing species in the catalyzed reaction may well be the
ion pair. The formation of a complex between immidazolium dichromate and TsOH has been
suggested (31). It has been claimed that there is spectral evidence but no spectra have been
included in the paper. On the basis of only a three-fold variation in [TsOH], the authors have
claimed a Michaelis–Menten type of kinetics with respect to TsOH. However, a least-squares
analysis of their data indicated that in that case also, the dependence on TsOH has the form
kobs = a + b [TsOH] with a = 2.53 0.09 × 10−4 s−1, b = 4.46 0.15 × 10−4 mol−1 dm3 s−1
and r2 = 0.9955. The species could be either the protonated Cr(VI) compound or an ion pair of
the protonated Cr(VI) and tosylate ion.
The analysis of the effect of solvents indicates that the transition state is more polar than
the reactants. Further, the fact that both cation- and anion-solvating powers of the solvents play
important roles suggests that a moderate degree of charge separation take place during the rate-
determining step.
The experimental results can be accounted for in terms of rate-determining electrophilic oxygen
transfer from TEACC to the sulfide:
Low magnitudes of the polar reaction constants are consistent with the development of a polar
transition state rather than with the formation of an intermediate with a positive sulfonium center

324 D. Sharma et al.
as depicted as follows:
Further, an electrophilic attack on the sulfide sulfur is confirmed by the positive value of η
which indicates that the substituent is able to stabilize a cationic or electron-deficient site. The
low magnitude of η, which represents the electronic demand of the reaction, indicates a less-
pronounced charge separation in the transition state. This militates against the formation of a
sulfonium cation and rather supports a mechanism involving the formation of a polar transition
state in the rate-determining step. The formation of a positive sulfonium cation, in the oxidation
of sulfides by Cl+ (32), is more responsive to steric hindrance than observed in the present
study. The observed solvent effect also supports a transition state that is more polar than the
reactants.
The oxidation of sulfides by TEACC may involve a cyclic intermediate as has been suggested
in many reactions of Cr(VI) (33). The cyclic transition state will be highly strained in view of the
apical position of a lone pair of electrons or an alkyl group (22).
The steric requirements of reaction (22) will be higher when compared with those of reaction
(19) and the observed small magnitudes of steric reaction constants are thus consistent with the
proposed acyclic mechanism. The formation of a cyclic transition state entails a more exacting
specificity of orientation and should result in much larger negative entropy of activation than
that observed. The value of the entropy of activation obtained in this reaction is close to the
value observed in typical reactions involving oxygen transfer, e.g. oxidation of MeSPh by hydro-
gen peroxide (23), periodate (24) and PFC (9) (ꢀS∗ = −96, −115, −113 and −89 J mol−1 K−1
,
respectively). In the oxidation of vicinal diols by chromic acid, where formation of a cyclic tran-
sition state has been proposed, Chatterjee and Mukherji (34) obtained entropies of activation in
the range of −174 to −195 J mol−1 K−1
.
It is of interest to compare here the mode of oxidation of organic sulfides by PFC (9), PCC (9),
PBC (9) and TEACC. The oxidations by PFC, PBC and TEACC presented similar kinetic pictures,
i.e. the reactions are of first order with respect to the reductants, whereas in the oxidation by PCC,
Michaelis–Menten type kinetics was observed with respect to the reductants. It is possible that the
values of the formation constants for the reductant–TEACC complexes are very low. This resulted
in the observation of second-order kinetics. No explanation of the difference is available presently.
Solvent effects (in the oxidation by PCC, the effect of different solvents was not studied) and the
dependence on TsOH are of similar nature in all these reactions, for which essentially similar
mechanisms have been proposed.

Journal of Sulfur Chemistry 325
4. Experimental
4.1. Materials
The sulfides were either commercial products or prepared by known methods (35) and were
purified by distillation under reduced pressure or crystallization. Their purity was checked by
comparing their boiling or melting points with the literature values. TEACC was prepared by the
reported method (6). TsOH was used as a source of hydrogen ions.
4.2. Product analysis
MeSPh or Me2S (0.1 mol) andTEACC (0.01 mol) was dissolved in DMSO (50 ml) and the mixture
was allowed to stand for ca. 20 h. Most of the solvent was removed under reduced pressure. The
residue was diluted with water and extracted with chloroform (3 × 50 ml). The chloroform layer
was dried over anhydrous magnesium sulfate, the solvent was removed by evaporation and the
residue was analyzed by IR and 1H NMR spectroscopy. The spectra were identical with those of
the corresponding sulfoxides. Peaks characteristic of the sulfide and sulfone could not be detected.
In IR spectra, the product showed a strong and broad absorption at 1050 cm−1. No band either
at 1330 or 1135 cm−1, characteristic of sulfones (36), was seen. In NMR spectroscopy, studied
in the case of Me2S, the peak due to methyl protons shifted from 2.1 ppm, in the sulfide, to
2.6 ppm in the product. In the corresponding sulfone, the peak should have appeared at 3.0 ppm
(37). Similar experiments were performed with other sulfides also. In all cases, the products were
the corresponding sulfoxides. The oxidation state of chromium in completely reduced reaction
mixtures, determined by an iodometric method, was 3.90 0.15.
4.3. Kinetic measurements
The solvent was DMSO, unless mentioned otherwise. The reactions were studied at constant
temperature ( 0.1 K) and were followed by monitoring the decrease in the concentration of
TEACC at 370 nm for up to 80% of the extent of the reaction. Duplicate kinetic runs showed
that the rate constants are reproducible to within 3%. All kinetic runs, except those for studying
the effect of acidity, were studied in the absence of TsOH. The values of the second-order rate
constants were computed from the relation k2 = kobs/[sulfide]. Simple and multivariate regression
analyses were carried out by the least-squares method.
Acknowledgements
Thanks are due to the University Grants Commission, New Delhi, India, for financial support in the form of a Major
Research project and to Professor Kalyan Banerji, Dean, Sciences, NLU, Mandore, Jodhpur (India).
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