Kinetics and mechanism of the oxidation of some organic sulfides by morpholinium chlorochromate

Article abstract of DOI:10.1002/kin.20372

The oxidation of organic sulfides by morpholinium chlorochromate (MCC) resulted in the formation of the corresponding sulfoxides. The reaction is first order with respect to both MCC and the sulfide. The reaction is catalyzed by toluene-p-sulfonic acid (T

Full text of DOI:10.1002/kin.20372

Kinetics and Mechanism
of the Oxidation of Some
Organic Sulfides
by Morpholinium
Chlorochromate
NEHA MALANI, MANJU BAGHMAR, PRADEEP K. SHARMA
Department of Chemistry, J.N.V. University, Jodhpur 342 005, India
Received 11 January 2008; revised 13 June 2008; accepted 26 June 2008
DOI 10.1002/kin.20372
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: The oxidation of organic sulfides by morpholinium chlorochromate (MCC) resulted
in the formation of the corresponding sulfoxides. The reaction is first order with respect to both
MCC 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 the cation- and anion-solvating powers of the solvents play important roles.
The correlation analyses of the rate of oxidation 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 localized-delocalized-resonance and localized-delocalized-resonance-
steric 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 formation of a
sulfonium cation intermediate in the slow step has been proposed. ꢀ 2008 Wiley Periodicals,
C
Inc. Int J Chem Kinet 41: 65–72, 2009
seem to be only a few reports on the oxidation as-
pects, using MCC emanated from our laboratory [7,8].
However, the kinetics of oxidation of organic sulfides
by MCC has not been investigated. We have been in-
terested in the kinetic and mechanistic aspects of the
oxidation by complexed Cr(VI) species, and some re-
ports have already been published [9]. 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 34 organic sulfides by MCC
in dimethylsulfoxide (DMSO) as solvent. Attempts
have been made to correlate rate and structure in this
reaction. A probable mechanism has been proposed.
INTRODUCTION
Pyridinium and quinolinium halochromates have long
been used as mild and selective oxidizing reagents
in synthetic organic chemistry [1–5]. Morpholinium
chlorochromate (MCC) is one such compound used for
the oxidation of primary aliphatic alcohols [6]. There
Correspondence to: Pradeep K. Sharma; e-mail: drpkvs27@
yahoo.com.
Contract grant sponsor: Council of Scientific and Industrial Re-
search, New Delhi, India.
Contract grant number: 01/(2014)/05/EMR-II.
2008 Wiley Periodicals, Inc.
c
ꢀ

66
MALANI, BAGHMAR, AND SHARMA
EXPERIMENTAL
Materials
k2 = kobs/[sulfide]. Simple and multivariate regression
analyses were carried out by the least-squares method.
The sulfides were either commercial products or pre-
pared by known methods [11] and were purified by
distillation under reduced pressure or crystallization.
Their purity was checked by comparing their boiling or
melting point with the literature values. MCC was pre-
pared by the reported method [6]. Toluene-p-sulfonic
acid (TsOH) was used as a source of hydrogen ions.
RESULTS
The oxidation of organic sulfides by MCC resulted
in the formation of the corresponding sulfoxides. The
overall reaction may be represented as Eq. (1).
′
′
R−S−R + CrO2ClOMH
→
R−S−R + CrOClOMH (1)
O
Product Analysis
MCC undergoes a two-electron change. This is ac-
cording to the earlier observations with MCC [7,8]
and with structurally similar other halochromates [9].
It has already been shown that both pyridinium flu-
orochromate (PFC) [14] and pyridinium chlorochro-
mate (PCC) [15] act as two-electron oxidants and are
reduced to chromium(IV) species by determining the
oxidation state of chromium by magnetic susceptibil-
ity, electron spin resonance (ESR), and IR studies.
MeSPh or Me2S (0.1 mol) and MCC (0.01 mol) were
dissolved in DMSO (50 mL), and the mixture was al-
lowed to stand for ca. 20 h. Most of the solvent was re-
moved 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 evapo-
ration, and the residue was analyzed by IR and 1H NMR
spectroscopy. The spectra were identical to 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 ab-
sorption at 1050 cm−1. No band either at 1330 or
1135 cm−1, characteristic of sulfones [12] was seen.
In NMR spectroscopy studied in the case of Me2S,
the peak due to methyl protons shifted from 2.1 τ, in
the sulfide, to 2.6 τ in the product. In the corresponding
sulfone, the peak should have appeared at 3.0 τ [13].
Similar experiments were performed with the other
sulfides also. In all cases, the products were the corres-
ponding sulfoxides. The oxidation state of chromium
in completely reduced reaction mixtures, determined
by an iodometric method, was 3.95 0.1.
Rate Law
The reactions were found to be first order with respect
to MCC. The individual kinetic runs were strictly first
order to MCC. Furthermore, the first-order rate coef-
ficients did not vary with the initial concentration of
MCC. The order with respect to sulfide was also found
to be one (Table I).
Table I Rate Constants for the Oxidation of Methyl
Phenyl Sulfide by MCC at 298 K
103[MCC]
(mol dm−3
[MeSPh]
(mol dm−3
[H+]
(mol dm−3
106 kobs
(s−1
)
)
)
)
1.0
1.0
1.0
1.0
1.0
1.0
2.0
4.0
6.0
8.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
0.10
0.20
0.40
0.60
0.80
1.00
0.20
0.20
0.20
0.20
0.10
0.10
0.10
0.10
0.10
0.10
0.40a
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.1
0.2
0.4
0.6
0.8
1.0
0.0
2.31
4.70
9.45
14.4
18.9
23.6
4.95
4.32
5.04
4.50
2.59
3.02
3.75
4.41
5.29
6.16
9.81a
Kinetic Measurements
The reactions were studied under pseudo-first-order
conditions by keeping an excess (×15 or greater) of
the sulfide over MCC. 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 MCC
at 350 nm for up to 80% reaction extent. Pseudo-first-
order rate constants, kobs, were evaluated from linear
plots (r2 > 0.995) of log [MCC] against time. Du-
plicate 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
aContained 0.001 mol dm−3 acrylonitrile.
International Journal of Chemical Kinetics DOI 10.1002/kin

KINETICS AND MECHANISM OF THE OXIDATION OF SOME ORGANIC SULFIDES BY MCC
67
Test for Free Radicals. The oxidation of methyl
phenyl sulfide, in an atmosphere of nitrogen, failed
to induce the polymerization of acrylonitrile. Further-
more, the addition of acrylonitrile had no effect on
the rate of oxidation (Table I). To further confirm the
absence of free radicals in the reaction pathway, the re-
action was carried out in the presence of 0.05 mol dm−3
of 2,6-di-t-butyl-4-methylphenol (butylated hydrox-
ytoluene or BHT). It was observed that BHT was
recovered unchanged, almost quantitatively.
peratures, and the activation parameters were calcu-
lated (Table II).
Effect of Acidity. The reaction is catalyzed by
TsOH (Table I). The TsOH-dependence has the form
kobs = a + b [TsOH]. The values of a and b are
2.91 0.06 10−4 s−1 and 3.89 0.09 10−4 mol−1
dm3 s−1, respectively (r2 = 0.9976). Therefore, the
experimental rate law has the following form:
Effect of Substituents. The rates of oxidation of a
number of ortho-, meta-, and parasubstituted phenyl
methyl sulfides, alkyl phenyl sulfides, dialkyl sulfides,
and diphenyl sulfide were determined at different tem-
Rate =
k2[MCC][sulfide] + k3 [MCC][sulfide][TsOH] (2)
Table II Rate Constants and Activation Parameters of the Oxidation of Organic Sulfides by MCC
105 k2 (dm3 mol−1 s−1
)
ꢀH∗
ꢀS∗
ꢀG∗
Substance
288 K
298 K
308 K
318 K
(kJ mol−1
)
(J mol−1 K−1
)
(kJ mol−1
)
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
11.7
25.1
56.7
12.1
7.83
0.68
1.71
2.49
7.74
27.9
207
22.0
25.2
3.81
3.75
4.56
0.37
1.91
6.03
18.0
0.18
0.54
1.44
1.08
0.85
69.3
23.6
48.6
108
47.7
95.0
203
51.0
34.7
3.84
8.55
12.0
34.1
108
95.4
180
378
99.9
71.1
9.02
18.9
26.1
70.2
207
50.7 0.7
47.6 0.6
45.8 0.4
51.2 0.5
53.5 0.5
63.1 0.8
58.4 0.7
57.1 0.8
53.4 0.7
48.3 0.5
40.6 0.5
47.9 0.7
48.5 0.6
53.2 0.6
52.9 0.6
52.9 0.7
62.8 0.7
56.9 0.5
56.2 0.8
51.8 0.4
69.9 0.5
65.0 0.8
61.7 0.4
63.3 0.5
64.3 0.6
46.0 0.6
−125
−130
−129
−123
−119
−106
−115
−116
−119
−126
−137
−130
−136
−126
−127
−125
−112
−112
−112
−118
−93
2
2
1
1
2
3
2
2
2
2
2
2
2
2
2
2
2
2
3
1
87.9 0.5
86.2 0.4
84.2 0.3
87.8 0.4
88.8 0.4
94.6 0.6
92.5 0.6
91.6 0.6
88.9 0.5
85.9 0.4
81.2 0.4
86.5 0.5
86.2 0.5
90.6 0.4
90.7 0.5
90.2 0.5
96.1 0.6
96.1 0.6
89.4 0.6
86.8 0.4
97.6 0.4
95.1 0.6
92.7 0.3
93.4 0.4
93.9 0.5
83.7 0.5
24.7
16.5
1.60
3.83
5.47
16.2
55.2
365
42.3
47.7
8.01
7.83
9.45
0.88
4.23
13.2
37.5
0.49
1.32
3.42
2.61
2.07
131
647
1130
160
169
83.3
90.2
16.7
16.2
19.8
2.09
9.31
28.2
76.5
1.25
3.22
8.10
6.30
5.13
251
34.2
33.3
40.5
4.86
19.8
61.2
153
3.15
7.74
18.0
14.4
11.7
468
m-Br
m-I
m-NO2
m-CO2Me
o-Me
o-Ome
o-NO2
o-COOMe
o-Cl
o-Br
o-I
o-NH2
2
−101
−105
−101
−100
−127
3
1
2
2
2
Alkyl Phenyl Sulfides
Et
Pr
i-Pr
t-Bu
19.3
11.6
14.4
3.31
38.3
23.8
30.6
7.47
76.9
49.5
63.0
17.1
144
98.1
126
48.6 0.4
51.8 0.5
52.5 0.3
59.4 0.7
−128
−122
−117
−106
1
2
1
2
86.8 0.3
87.9 0.4
87.3 0.2
90.8 0.6
37.8
Other Sulfides
188
Me2S
Pr2S
Ph2S
25.6
44.1
5.67
48.6
81.9
12.1
96.3
153
25.2
48.2 0.9
44.4 0.6
54.6 0.7
−128
−136
−118
3
2
2
86.1 0.7
84.9 0.4
89.6 0.6
280
54.0
International Journal of Chemical Kinetics DOI 10.1002/kin

68
MALANI, BAGHMAR, AND SHARMA
polarity term, represented by π∗ (4)
Table III Solvent Effect on the Oxidation of MeSPh by
MCC at 308 K
log k2 = A0 + pπ∗ + bβ + aα
(3)
(4)
106 k2
(s−1
106 k2
(s−1
log k2 = −5.74 + 1.82 ( 0.20)π∗
R2 = 0.8312; SD = 0.20; n = 18; ψ = 0.31
Solvent
)
Solvent
)
Chloroform
17.8
Toluene
3.47
18.2
6.46
7.94
6.17
1,2-Dichloroethane 14.8
Dichloromethane 16.2
Dimethyl sulfoxide 47.7
Acetophenone
Tetrahydrofuran
t-Butyl alcohol
1,4-Dioxane
We have used the standard deviation (SD), the coeffi-
cient of multiple determination (R2), and Exner’s [19]
parameter, ψ, as the measures of goodness of fit. Here
n is the number of data points.
The data on the solvent effect were analyzed in
terms of Swain’s equation [20] of cation- and anion-
solvating concepts of the solvents as well.
Acetone
DMF
Butanone
Nitrobenzene
Benzene
12.6
22.9 1,2-Dimethoxyethane 3.89
10.5
19.5
4.27
0.44
Ethyl acetate
Carbon disulfide
Acetic acid
5.25
1.86
7.08
Cyclohexane
log k2 = aA + bB + C
(5)
Here A represents the anion-solvating power of the
solvent and B the cation-solvating power. C is the
intercept term. The rates in different solvents were an-
alyzed in terms of Eq. (5), separately with A and B and
with (A + B).
Effect of Solvent. The oxidation of methyl phenyl sul-
fide was studied in 19 different organic solvents. The
choice of solvent was limited by the solubility of MCC
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 III.
log k2 = 1.37( 0.03)A + 1.70( 0.02)B − 6.08 (6)
R2 = 0.9983; SD = 0.02; n = 19; ψ = 0.03
log k2 = 1.13( 0.56)A − 4.91 (7)
DISCUSSION
R2 = 0.1917; SD = 0.45; n = 19; ψ = 0.77
log k2 = 1.60( 0.24)B − 5.63 (8)
There is a fair correlation between the activation en-
thalpies and entropies of the oxidation of the 34 sulfides
(r2 = 0.9569), indicating the operation of a compensa-
tion effect [16]. A correlation between the calculated
values of enthalpies and entropies is often vitiated by
the experimental errors associated with them. The re-
action, however, exhibited an excellent isokinetic re-
lationship, as determined by Exner’s method [17]. An
Exner plot between log k2 at 288 K and at 318 K
was linear (r2 = 0.9989, SD = 0.01, slope = 0.8401).
The value of isokinetic temperature evaluated from the
Exner’s plot is 706 50 K. The linear isokinetic cor-
relation implies that all the sulfides are oxidized by the
same mechanism, and the change in the rate of oxida-
tion is governed by changes in both the enthalpy and
entropy of the activation.
R2 = 0.7186; SD = 0.27; n = 19; ψ = 0.40
log k2 = 1.59 0.05(A + B) − 6.07 (9)
R2 = 0.9863; SD = 0.06; n = 19; ψ = 0.09
The rates of oxidation of methyl phenyl sulfide in
the different solvents show an excellent correlation
with Swain’s equation with both the cation- and anion-
solvating powers playing significant roles, though the
contribution of the cation solvation is slightly more
than that of the anion solvation. However, the corre-
lations individually with A and B were poor. In view
of the fact that solvent polarity is able to account for
ca. 99% of the data, an attempt was made to correlate
the rate with the relative permittivity of the solvent.
However, a plot of log k2 against the inverse of the rel-
ative permittivity is not linear (r2 = 0.5983; SD = 0.32;
ψ = 0.49).
Solvent Effect
The correlation of rate constants for oxidation, k2,
in 18 solvents (CS2 was not considered, as the com-
plete range of solvent parameters was not available) in
terms of the linear solvation energy relationship (3) of
Kamlet et al. [18] is not much significant
(R2 = 0.8946). The major contribution is of solvent
Correlation Analysis of Reactivity
The data in Table II show that the oxidation of differ-
ent sulfides follows the order of their nucleophilicity:
Pr2S > Me2S > MeSPh > Ph2S.
International Journal of Chemical Kinetics DOI 10.1002/kin

KINETICS AND MECHANISM OF THE OXIDATION OF SOME ORGANIC SULFIDES BY MCC
69
Aryl Methyl Sulfides. The correlation of the effect
of substituents on the reactivity has been widely at-
tempted in terms of the Hammett equation [21] or with
dual substituent-parameter equations [22,23]. In the
late 1980s, Charton [24] introduced a triparametric
localized-delocalized-resonance (LDR) equation for
the quantitative description of structural effects on
chemical reactivities. This triparametric equation re-
sults from the fact that substituent types differ in their
mode of electron delocalization. This difference is re-
flected in a different sensitivity to the electronic de-
mand for the phenomenon being studied. It has the ad-
vantage 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 (10) to the
rate constants, k2.
delocalized electrical parameter of the diparametric LD
equation.
For orthosubstituted compounds, it is necessary to
account for the possibility of steric effects and Charton
[24], therefore, modified the LDR equation to gener-
ate the localized-delocalized-resonance-steric (LDRS)
equation (12).
log k2 = Lσl + Dσd + Rσe + Sυ + h
(12)
where υ is the well-known Charton’s steric parameter
based on Van der Waals radii [25].
The rates of oxidation of ortho-, meta-, and parasub-
stituted sulfides show excellent correlations in terms of
the LDR/LDRS equations (Table IV). The values of the
independent variables, σl, σd, σe, and υ were obtained
from the work of Charton and Charton [24]. Although
the number of data points is smaller than the optimum
number, the correlations are excellent as per Exner’s
[19] criterion also. Exner’s ψ parameter takes into ac-
count the number of data point 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 Eq. (12), reflecting the electron-donating power of
the substituent and its capacity to stabilize a cationic
species.
log k2 = Lσl + Dσd + Rσe + h
(10)
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 Eq. (11).
The negative value of S indicates that the reaction is
subjected to steric hindrance by the orthosubstituent.
This may be due to steric hindrance of the orthosub-
stituent to the approach of the oxidizing species.
The percent contribution [25] of the delocalized ef-
fect, PD is given by the following equation:
σD = ησe + σd
(11)
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, that is, η = R/D. σD represents the
PD = (|D| × 100)/(|L| + |D| + |R|)
(13)
Table IV Temperature Dependence for the Reaction Constants for the Oxidation of Organic Sulfides by MCC
T (K)
L
D
R
S
η
R2
SD
ψ
PD
PS
Parasubstituted
288
298
308
318
−1.53
−1.44
−1.35
−1.26
−1.89
−1.80
−1.71
−1.61
−1.69
−1.62
−1.52
−1.51
–
–
–
–
0.89
0.90
0.89
0.93
0.9999
0.9998
0.9998
0.9999
0.003
0.001
0.002
0.004
0.01
0.01
0.01
0.01
37.0
37.0
37.3
36.8
–
–
–
–
Metasubstituted
0.87
288
298
308
318
−1.98
−1.89
−1.81
−1.73
−1.53
−1.44
−1.35
−1.27
−1.33
−1.25
−1.23
−1.10
–
–
–
–
0.9999
0.9998
0.9999
0.9998
0.001
0.002
0.003
0.004
0.01
0.02
0.01
0.02
31.6
31.4
30.8
31.0
–
–
–
–
0.86
0.91
0.87
Orthosubstituted
288
298
308
318
−1.45
−1.40
−1.33
−1.26
−1.55
−1.49
−1.41
−1.33
−1.26
−1.20
−1.17
−1.10
−1.13
−1.05
−0.99
−0.96
0.81
0.81
0.83
0.87
0.9997
0.9999
0.9998
0.9997
0.016
0.002
0.003
0.015
0.02
0.01
0.01
0.02
36.4
36.4
36.1
36.0
21.0
20.4
20.2
20.6
International Journal of Chemical Kinetics DOI 10.1002/kin

70
MALANI, BAGHMAR, AND SHARMA
Similarly, the percent contribution of the steric pa-
rameter [25] to the total effect of the substituent, PS,
Mechanisms
The observed dependence on TsOH suggests that the
reaction follows two mechanistic pathways, one TsOH
catalyzed and the other uncatalyzed. The catalytic ef-
fect of TsOH can be attributed to a protonation of MCC
to give a stronger oxidant and electrophile (16).
was determined by using Eq. (14).
PS = (|S| × 100)/(|L| + |D| + |S| + |R|)
(14)
The values of PD and PS are also recorded in
Table IV. The value of PD for the oxidation of parasub-
stituted sulfides is ca. 52%, whereas the corresponding
values for the meta- and orthosubstituted aldehydes are
ca. 39% and 49%, respectively. The less pronounced
resonance effect from the orthoposition than from the
paraposition 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, in-
volving a direct oxygen transfer via an electrophilic
attack on the sulfide–sulfur, the reaction constants
were negative but of relatively small magnitude, for
example, by hydrogen peroxide (−1.13) [26], perio-
date (−1.40) [27], permanganate (−1.52) [28], and
peroxydisulfate (−0.56) [29]. Large negative reaction
constants were exhibited by oxidations involving for-
mation of halogeno-sulfonium cations, for example,
by chloramine-T (−4.25) [30], bromine (−3.2) [31],
and N-bromoacetamide (−3.75) [32]. In the oxidation
by N-chloroacetamide (NCA) [33], the values of field
(ρI) and resonance (ρR+) at 298 K are −1.3 and −1.7,
respectively.
bpyHOCrO2Cl + TsOH
+
−
→
←
[bpyHOCr(OH)OCl] [TsO]
(16)
TsOH is a strong acid and is expected to be highly
ionized in an aprotic polar solvent like DMSO. How-
ever, one cannot rule out the possibility of the for-
mation of an ion pair. Therefore, the reactive oxidiz-
ing species in the catalyzed reaction may well be the
ion pair. Formation of a complex between immida-
zolium dichromate (IDC) and TsOH has been sug-
gested [35]. It has been claimed that there are spec-
tral evidences, but no spectra have been included in
the paper. On the basis of only a threefold 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 = 1.78 0.16 × 10−3 s−1
,
b = 7.8 0.6 × 10−2 mol−1 dm3 s−1, and r2 = 0.9809.
The species could be either the protonated Cr(VI) com-
pound or an ion pair of the protonated Cr(VI) and
tosylate ion.
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 [34] dual
substituent-parameter (DSP) equation (15).
The analysis of the effect of solvents indicates that
the transition state is more polar than the reactants. Fur-
thermore, 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.
log k2 = ρ∗σ∗ + δEs + log K0
(15)
The experimental results can be accounted for in
terms of rate-determining electrophilic oxygen trans-
fer from MCC to the sulfide as Eqs. (17) and (18),
similar to that suggested for oxidations of sulfides and
iodide ions by periodate ion [36] and for the oxida-
tion of sulfides by hydrogen peroxide [26] and PFC
[6]. The nucleophilic attack of a sulfide–sulfur on an
MCC-oxygen may be viewed as an SN2 process. Low
magnitudes of the polar reaction constants are consis-
tent with the development of a polar transition state
rather than with the formation of an intermediate with
a positive sulfonium center as depicted in Eq. (19). Fur-
thermore, an electrophilic attack on the sulfide–sulfur
is confirmed by the positive value of η, which indi-
cates 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,
The correlations are excellent (Table V). Although
the number of compounds is small (five) for any anal-
ysis by a DSP equation, the results can be used quali-
tatively. The negative polar reaction constant confirms
that the electron-donating power of the alkyl group en-
hances the reaction rate. The steric effect plays a minor
inhibitory role.
Table V Correlation of Rate of Oxidation of Alkyl
Phenyl Sulfides with Pavelich-Taft Equationa
Temperature (K)
ρ∗
δ
R2
SD
288
298
308
318
−2.80 0.01 0.90 0.01 0.9999 0.01
−2.69 0.01 0.85 0.01 0.9998 0.02
−2.61 0.02 0.80 0.01 0.9999 0.01
−2.41 0.09 0.73 0.02 0.9993 0.03
aNo. of data points = 5.
International Journal of Chemical Kinetics DOI 10.1002/kin

KINETICS AND MECHANISM OF THE OXIDATION OF SOME ORGANIC SULFIDES BY MCC
71
indicates a less-pronounced charge separation in the
transition state. This militates against the formation of
a sulfonium cation and rather supports a mechanism in-
volving the formation of a polar transition state in the
rate-determining step. The formation of a positive sul-
fonium cation, in the oxidation of sulfides by Cl+, [37]
is more responsive to steric hindrance than observed
in the present study. The observed solvent effect also
supports an SN2-like transition state.
than those of reaction (17), and the observed small
magnitudes of steric reaction constants are thus con-
sistent with the proposed acyclic mechanism. The for-
mation of a cyclic transition state entails a more ex-
acting specificity of orientation and should result in
much larger negative entropy of activation than that ob-
served. The value of the entropy of activation obtained
in this reaction is close to the value observed in typical
#
R
OMH
′
R
+
CrO2ClOMH
HCr+O2ClOMH
OMH
S
O
Cr
R
S
′
R
O
Cl
+
CrO2ClOMH
′
R
(17)
R
S
O
R
OMH
Cr+
+
′
R
S
O
R
S
R′
HO
Cl
+
HCr+OClOMH
(18)
′
R
R
S
O
R
S+
O
Cr
reactions involving oxygen transfer, for example, ox-
idation of iodide ion by periodate [36] and that of
MeSPh by hydrogen peroxide [27], periodate [28], and
PFC [39] (ꢀS∗ = −96, −115, −113, and −89 J mol−1
′
R
+
CrO2ClOMH
R
S
(19)
′
R
O
Cl
The oxidation of sulfides by MCC may involve a
cyclic intermediate as has been suggested in many re-
actions of Cr(VI) [38]. The cyclic transition state will
be highly strained in view of the apical position of a
lone pair of electrons or an alkyl group as reaction (20).
The steric requirements of reaction (20) will be higher
K
−1, respectively). In the oxidation of vicinal-diols by
chromic acid, in which formation of a cyclic transi-
tion state has been proposed, Chatterjee and Mukherji
[40] obtained entropies of activation in the range from
−174 to −195 J mol−1 K−1
.
′
R
+
CrO2ClOMH
R
S
¥¥
′
R
R
R
S
O
S
O
R’
OMH
Cl
(20)
OMH
Cl
PRODUCTS
OR
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Cr
O
Cr
International Journal of Chemical Kinetics DOI 10.1002/kin

72
MALANI, BAGHMAR, AND SHARMA
It is of interest to compare here the mode of oxida-
13. Pouchart, C. J. The Aldrich Library of NMR Spectra,
1st and 2nd ed.; Aldrich Chemical Co.: Milwaukee, WI,
1983.
14. Brown, H. C.; Rao, C. G.; Kulkarni, S. U. J Org Chem
1979, 44, 2809.
tion of organic sulfides by PFC [39], PCC [41], pyri-
dinium bromochromate (PBC) [42], and MCC. The
oxidation by PFC, PBC, and MCC presented a similar
kinetic picture, that is, the reactions are of first order
with respect to the reductants. In the oxidation by PCC
a Michaelis-Menten type of kinetics was observed with
respect to the reductants. It is possible that the values of
the formation constants for the reductant-MCC com-
plexes are very low. This resulted in the observation
of second-order kinetics. No explanation of the dif-
ference 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 sim-
ilar nature in all these reactions, for which essentially
similar mechanisms have been proposed. The common
mechanism proposed [8] for the oxidation of diphenyl
sulfide by Cr(VI) reagents, in acetic acid, is essentially
similar to one proposed by us here.
15. Bhattacharjee, M. N.; Choudhuri, M. K.; Purakayastha,
S. Tetrahedron 1987, 43, 5389.
16. Liu, L.; Guo, Q-X. Chem Rev 2001, 101, 673.
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R. W. J Org Chem 1983, 48, 2877 and references cited
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International Journal of Chemical Kinetics DOI 10.1002/kin