Oxidation of organic sulfides by imidazolium fluorochromate: A kinetic and mechanistic approach

Article abstract of DOI:10.14233/ajchem.2014.15753

The oxidation of organic sulfides by imidazolium fluorochromate resulted in the formation of the corresponding sulfoxides. The reaction is first order with respect to imidazolium fluorochromate. A Michaelis-Menten type kinetics was observed with respect t

Full text of DOI:10.14233/ajchem.2014.15753

Asian Journal of Chemistry; Vol. 26, No. 9 (2014), 2597-2603
ASIAN JOURNAL OF CHEMISTRY
http://dx.doi.org/10.14233/ajchem.2014.15753
Oxidation of Organic Sulfides by Imidazolium
Fluorochromate: A Kinetic and Mechanistic Approach
*
LOKESH MATHUR, A. CHOUDHARY, OM PRAKASH and PRADEEP K. SHARMA
Department of Chemistry, J.N.V. University, Jodhpur-342 005, India
*Corresponding author: E-mail: drpkvs27@yahoo.com
Received: 10 May 2013; Accepted: 6 August 2013;
Published online: 28 April 2014;
AJC-15077
The oxidation of organic sulfides by imidazolium fluorochromate resulted in the formation of the corresponding sulfoxides. The reaction
is first order with respect to imidazolium fluorochromate. A Michaelis-Menten type kinetics was observed with respect to the reactants.
The reaction is catalyzed by toluene-p-sulfonic acid. The oxidation was studied in nineteen different organic solvents. An analysis of the
solvent effect by Swain's equation showed that the both cation- and anion-solvating powers of the solvents play important roles. The
correlation analyses of the rate of oxidation of thirty four 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 good correlation in terms of Pavelich-Taft equation. The polar reaction constants are negative indicating an electron-
deficient sulfur centre in the rate-determining step. A mechanism involving formation of a sulphenium cation intermediate in the slow step
has been proposed.
Keywords: Correlation analysis, Halochromate, Kinetics, Mechanism, Oxidation.
Product analysis: MeSPh or Me2S (0.1 mol) and IFC
(0.01 mol) was dissolved in DMSO (50 mL) and the mixture
was allowed to stand for 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 analysed by
IR and 1H NMR spectroscopy. The spectra were identical with
those of the corrsponding 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
sulfones9 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 ppm10. Similar
experiments were performed with the 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.
INTRODUCTION
Various halochromates have been used as mild and
selective oxidizing reagents in synthetic organic chemistry1.
Imidazolium fluorochromate (IFC) is also one of such com-
pounds used for the oxidation of benzylic alcohols2. We have
been interested in the kinetic and mechanistic aspects of the
oxidation by complexed Cr(VI) species and several reports on
halochromates have already reported from our laboratory3-6.
It is, known that mode of oxidation depends upon the nature
of counter-ion attached to the chromium anion. Karunakaran
et. al.7 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 organic sulfides by IFC in dimethyformamide as
solvent.Attempts have been made to correlate rate and structure
in this reaction. A probable mechanism has been proposed.
EXPERIMENTAL
The sulfides were either commercial products or prepared
by known methods8 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. Imidazolium fluorochromate was prepared by the
reported method2. Toluene-p-sulfonic acid (TsOH) was used
as a source of hydrogen ions.
Kinetic measurements: The reactions were studied under
pseudo-first-order conditions by keeping an excess (× 15 or
greater) of the sulfide over IFC. The solvent was DMSO, unless
mentioned otherwise. The reactions were studied at constant

2598 Mathur et al.
Asian J. Chem.
0.20
0.18
0.16
0.14
0.12
0.10
0.08
0.06
0.04
0.02
0
temperature ( 0.1 K) and were followed by monitoring the
decrease in the concentration of IFC at 370 nm for up to 80 %
reaction extent. Pseudo-first-order rate constants, kobs, were
evaluated from linear plots (r2 > 0.995) of log [IFC] against
time. 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.
RESULTS AND DISCUSSION
0
5
10
15
1/Sulphide
The oxidation of organic sulfides by IFC resulted in the
formation of the corresponding sulfoxides. The overall reaction
may be represented as eqn (1).
Fig. 2. 1/kobs vs. 1/[Sulfide]: A double reciprocal plot
TABLE–1
RATE CONSTANTS FOR THE OXIDATION OF
METHYL PHENYL SULFIDE BY IFC AT 298 K
R
S
R'
+ O2CrClOImH
R
S
R' +
OCrClOImH (1)
O
103 [IFC]
(mol dm-3) (s-1)
[MeSPh]
(mol dm-3)
[TsOH]
(mol dm-3)
104 kobs
(s-1)
Rate law: The reactions are of first order with respect to
1.0
1.0
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
1.50
3.00
0.20
0.20
0.20
0.20
0.40*
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
5.49
8.02
10.5
11.7
12.5
12.9
13.6
14.4
8.19
7.83
8.01
7.92
12.6*
IFC. Fig. 1 depicts a typical kinetic run. Further, the pseudo-
first order rate constant, kobs is independent of the initial concen-
tration of IFC. The reaction rate increases with increase in the
concentration of the sulfide but not linearly (Table-1). A plot
of 1/kobs against 1/[Sulfide] is linear (r > 0.995) with an intercept
on the rate-ordinate (Fig. 2). Thus, Michaelis-Menten type
kinetics are observed with respect to the sulfide. This leads to
the postulation of following overall mechanism (2) and (3)
and rate law (4).
K
Sulfide + IFC
[complex]
(2)
(3)
k2
* contained 0.001 mol dm-3 acrylonitrile
[Complex] → Products
Rate = k2 K [Sulfide] [IFC]/(1 + K [Sulfide])
(4)
Test for free radicals: The oxidation of methyl phenyl
sulfide, in an atmosphere of nitrogen, failed to induce the poly-
merization of acrylonitrile. Further, the addition of acrylonitrile
had no effect on the rate of oxidation (Table-1). To further
confirm the absence of free radicals in the reaction pathway,
the reaction was carried out in the presence of 0.05 mol dm-3
of 2,6-di-t-butyl-4-methylphenol (butylated hydroxytoluene
or BHT). It was observed that BHT was recovered unchanged,
almost quantitatively.
The dependence of reaction rate on the reductant concen-
tration was studied at different temperatures and the values of
K and k2 were evaluated from the double reciprocal plots. The
thermodynamic parameters of the complex formation and acti-
vation parameters of the decomposition of the complexes were
calculated from the values of K and k2 respectively at different
temperatures (Tables 3 and 4). Fig. 2 depict a typical kinetic run.
2.5
2.0
1.5
Effect of substituents: The rates of oxidation of a number
of ortho-, meta- and para-substituted phenyl methyl sulfides,
alkyl phenyl sulfides, dialkyl sulfides and diphenyl sulfide were
determined at different temperatures and the activation para-
meters were calculated (Table-3).
Effect of acidity:The reaction is catalyzed byTsOH (Table-
2). The TsOH-dependence has the form kobs = a + b [TsOH]. The
values of a and b for methyl phenyl sulfide are 5.21 0.12 10-4
s-1 and 9.25 0.20 10-4 mol-1 dm3 s-1 respectively (r2 = 0.9980).
Therefore, the experimental rate law has the following form:
1.0
0.5
0
Rate = k2 [IFC] [sulfide] + k3 [IFC] [sulfide] [TsOH] (5)
Effect of solvent: The oxidation of methyl phenyl sulfide
was studied in nineteen different organic solvents. The choice
of solvent was limited by the solubility of IFC and its reaction
with primary and secondary alcohols. There was no reaction
0
500
1000
1500
2000
Time (s)
Fig. 1. Oxidation of methyl phenyl sulphite by IFC: A typical kinetic run

Vol. 26, No. 9 (2014)
Oxidation of Organic Sulfides by Imidazolium Fluorochromate 2599
TABLE–2
DEPENDENCE OF THE REACTION RATE ON HYDROGEN-ION CONCENTRATION
−
−
3
[IFC] 0.001 (mol dm )
3
[Sulphide] 0.10 (mol dm )
Temprature 298 K
−
3
0.10
6.15
0.20
7.18
0.40
8.91
0.60
10.5
0.80
12.6
1.00
14.6
[TsOH] (mol dm )
104 kobs (s-1)
TABLE – 3
RATE CONSTANTS FOR THE DECOMPOSITION OF IFC−SULFIDES COMPLEXES AND ACTIVATION PARAMETERS
104 k2 (dm3 mol-1 s-1)
∆H*
− ∆S*
(mol-1 K-1)
∆G*
Subst.
(kJ mol-1)
70.3 0.2
67.9 0.3
65.5 0.4
72.0 0.5
73.6 0.3
83.0 0.4
78.8 0.2
77.1 0.3
73.7 0.4
68.3 0.3
60.7 0.4
67.3 0.5
65.5 0.2
73.7 0.3
73.8 0.2
73.1 0.3
83.1 0.3
76.7 0.3
76.1 0.2
71.4 0.1
88.9 0.2
84.6 0.4
81.5 0.4
83.4 0.3
85.6 0.2
66.0 0.3
(kJ mol-1)
89.1 0.2
87.4 0.2
85.5 0.3
88.9 0.3
89.9 0.3
95.3 0.3
93.3 0.2
92.5 0.2
89.9 0.3
87.1 0.2
82.7 0.3
87.7 0.4
87.4 0.2
91.4 0.2
91.5 0.2
91.1 0.2
96.5 0.2
93.0 0.3
91.0 0.2
88.7 0.1
97.9 0.2
95.9 0.3
94.1 0.3
94.8 0.2
95.4 0.2
86.0 0.3
288 K
298 K
308 K
318 K
H
5.49
11.3
24.9
5.81
3.84
0.37
0.88
1.26
3.77
12.6
82.8
9.88
11.7
2.04
2.01
2.37
0.23
1.04
2.33
6.39
0.12
0.29
0.63
0.46
0.35
20.7
15.2
30.1
64.6
16.2
11.0
1.20
2.70
3.78
10.8
33.8
198
40.6
77.4
162
98.1
180
62
66
68
57
55
42
50
52
55
64
74
69
73
60
60
61
46
55
51
59
31
38
43
39
34
67
1
1
1
2
1
1
1
1
1
1
1
2
1
1
1
1
1
1
1
1
1
1
1
1
1
1
p-Me
p-OMe
p-F
360
45.0
30.6
3.78
8.01
11.0
30.1
87.3
468
108
p-Cl
76.6
10.7
21.4
28.8
75.5
204
p-NO2
p-COMe
p-COOMe
p-Br
p-NHAc
p-NH2
m-Me
m-OMe
m-Cl
990
26.1
30.6
5.88
5.79
6.75
0.75
3.12
6.93
18.0
0.43
0.95
20.2
1.52
1.18
54.0
67.5
75.6
16.3
16.1
18.6
2.36
9.08
19.8
48.1
1.44
30.6
6.26
4.81
3.81
135
153
171
40.8
40.3
46.3
6.66
23.4
51.3
117
m-Br
m-I
m-NO2
m-CO2Me
o-Me
o-OMe
o-NO2
o-COOMe
o-Cl
4.38
8.91
17.1
13.5
11.2
306
o-Br
o-I
o-NH2
(ii) Alkyl phenyl sulfides
Et
9.18
5.85
7.74
2.14
25.2
17.1
22.5
6.58
66.6
48.1
64.4
19.8
161
122
165
58.5
70.3 0.2
74.7 0.2
75.4 0.4
81.4 0.8
59
48
43
33
1
1
1
3
87.8 0.2
88.8 0.2
88.1 0.3
91.1 0.6
Pr
i-Pr
t-Bu
(iii) Other sulfides
Me2S
Pr2S
11.7
18.0
2.16
28.8
43.2
6.12
73.8
112
180
255
48.6
67.0 0.9
65.3 0.9
76.8 0.9
69
71
49
3
3
3
87.4 0.8
86.4 0.7
91.2 0.7
Ph2S
18.0
4.5
with the solvent chosen. The kinetics were similar in all the
solvents. The values of k2 and K are recorded in Table-5.
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
There is a linear correlation between the activation enthal-
pies and entropies of the oxidation of the thirty-four sulfides
(r = 0.9432), indicating the operation of a compensation effect11.
A correlation between the calculated values of enthalpies and
entropies is often vitiated by the experimental errors associated
with them. The reaction, however, exhibited an excellent iso-
kinetic relationship, as determined by Exner's method12. An
Exner's plot between log k2 at 288 K and at 318 K was linear
(r2 = 0.9964, slope = 0.8302 0.0090) (Fig. 3). The value of
isokinetic temperature evaluated from the Exner's plot is 651
43 K. The linear isokinetic correlation implies that all the
sulfides are oxidized by the same mechanism and the change
in the rate of oxidation is governed by changes in both the
enthalpy and entropy of the activation.
0
1
2
3
4
log k318
Fig. 3. Exner's isokinetic relationship in the oxidation of sulphides by IFC

2600 Mathur et al.
Asian J. Chem.
TABLE – 4
FORMATION CONSTANTS FOR THE DECOMPOSITION OF IFC-SULFIDE COMPLEXES AND THERMODYNAMIC PARAMETERS
K (dm3 mol–1)
–∆H*
(kJ mol-1)
–∆S
(mol-1 K-1)
–∆G*
(kJ mol-1)
Diols
288
298
308
318 K
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
m-Br
m-I
m-NO2
m-CO2Me
o-Me
o-OMe
o-NO2
o-COOMe
o-Cl
o-Br
o-I
6.15
6.22
5.89
5.56
5.44
5.81
5.46
5.90
6.18
5.85
5.32
6.02
6.13
5.99
5.45
5.73
5.66
6.20
5.27
5.56
5.31
6.03
5.94
5.49
6.05
5.98
5.66
5.40
5.09
4.76
4.63
5.02
4.66
5.10
5.36
5.05
4.50
5.22
5.31
5.20
4.68
4.90
4.86
5.40
4.45
4.77
4.52
5.25
5.15
4.65
5.26
5.20
4.80
4.62
4.23
3.96
3.80
4.20
3.84
4.32
4.59
4.24
3.72
4.41
4.53
4.39
3.81
4.12
4.05
4.55
3.64
3.96
3.70
4.40
4.33
3.90
4.42
4.33
4.05
3.78
3.46
3.15
3.03
3.39
3.06
3.48
3.75
3.42
2.88
3.59
3.71
3.57
3.05
3.33
3.24
3.73
2.88
3.15
2.93
3.63
3.53
3.05
3.66
3.58
13.2 0.9
15.0 0.7
16.0 0.6
16.8 0.8
17.3 0.7
16.1 0.7
17.1 0.7
15.8 0.8
15.0 0.7
16.0 0.7
17.9 0.9
15.5 0.7
15.1 0.6
15.6 0.7
17.3 0.8
16.2 0.6
16.6 0.8
15.4 0.7
17.8 0.7
16.8 0.8
17.6 0.9
15.4 0.6
15.7 0.7
17.2 0.9
15.3 0.6
15.6 0.6
22 3
29 2
33 2
36 3
38 2
33 2
37 2
32 3
29 2
33 2
40 3
31 2
29 2
31 2
37 3
33 2
35 2
30 2
40 2
36 3
39 3
30 3
31 2
37 3
30 2
31 2
6.70 0.7
6.64 0.5
6.47 0.5
6.31 0.6
6.23 0.6
6.44 0.6
6.25 0.6
6.49 0.6
6.62 0.5
6.46 0.6
6.17 0.7
6.54 0.5
6.59 0.5
6.53 0.6
6.25 0.6
6.39 0.5
6.36 0.6
6.62 0.5
6.14 0.6
6.31 0.6
6.17 0.6
6.55 0.5
6.51 0.5
6.26 0.7
6.56 0.5
6.52 0.5
o-NH2
(ii) Alkyl phenyl sulfides
Et
Pr
i-Pr
t-Bu
6.18
5.91
5.45
5.86
5.41
5.12
4.68
5.04
4.53
4.32
3.86
4.23
3.72
3.45
3.06
3.45
15.4 0.7
16.0 0.8
17.1 0.8
15.9 0.6
30
33
37
32
2
3
3
2
6.62 0.6
6.49 0.7
6.26 0.7
6.46 0.5
(iii) Other sulfides
Me2S
Pr2S
Ph2S
5.61
5.83
5.79
4.82
5.04
4.95
4.01
4.25
4.20
3.15
3.42
3.33
17.0 0.9
15.9 0.8
16.3 0.8
373
322
343
6.330.7
6.450.6
6.420.6
Solvent effect: The rate constants for oxidation, k2, in
eighteen solvents (CS2 was not considered, as the complete
range of solvent parameters was not available) were correlated
in terms of the linear solvation energy relationship (eqn. 6) of
Kamlet et al.13.
log k2 = -2.69 + 0.54 ( 0.38) β
r2 = 0.1153; sd = 0.45; n = 18; y = 0.97
(10)
Kamlet's13 triparametric equation explains 87 % of the
effect of solvent on the oxidation. However, by Exner's criterion
the correlation is not even satisfactory (cf. eqn. 7). The major
contribution is of solvent polarity. It alone accounted for 81 %
of the data. Both β and α play relatively minor roles.
The data on the solvent effect were analyzed in terms of
Swain's equation15 of cation- and anion-solvating concept of
the solvents as well.
log k2 = A0 + pπ* + bβ + aα
(6)
In this equation, π* represents the solvent polarity, β the
hydrogen bond acceptor basicities and α is the hydrogen bond
donor acidity. A0 is the intercept term. It may be mentioned
here that out of the 18 solvents, 13 have a value of zero for α.
The results of correlation analyses in terms of eqn. 6, a bipara-
metric equation involving π* and β and separately with π* and
β are given below in eqn. 7-10. We have used the standard
deviation (sd), the coefficient of multiple determination (R2)
and Exner's14 parameter, ψ, as the measures of goodness of fit.
Here n is the number of data points.
log k2 = aA + bB + C
(11)
HereA 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 analysed in terms of eqn. 11,
separately with A and B and with (A + B).
log k2 = 1.36 ( 0.04)A + 1.68 ( 0.03) B -3.99 (12)
R2 = 0.9956; sd = 0.03; n = 19; y = 0.07
log k2 = -4.25 + 1.81 ( 0.20) π* + 0.13
( 0.16) β + 0.34 ( 0.16) α
R2 = 0.8735; sd = 0.18; n = 18; ψ = 0.39
log k2 = -4.33 + 1.69 ( 0.21) π* + 0.24 ( 0.17) β (8)
R2 = 0.8316; sd = 0.20; n = 18; ψ = 0.44
log k2 = -4.38 + 1.75 ( 0.21) π*
r2 = 0.8101; sd = 0.21; n = 18; ψ = 0.45
(7)
log k2 = 1.12 ( 0.55) A-2.83
r2 = 0.1946; sd = 0.45; n =19; y = 0.92
(13)
(14)
(15)
log k2 = 1.58 ( 0.24) B - 3.56
r2 = 0.7126; sd = 0.27; n = 19; y = 0.55
log k2 =1.57 0.05 (A + B) - 3.94
r2 = 0.9846; sd = 0.02; n = 19; y = 0.13
(9)

Vol. 26, No. 9 (2014)
Solvents
Oxidation of Organic Sulfides by Imidazolium Fluorochromate 2601
TABLE – 5
SOLVENT EFFECT ON THE OXIDATION OF MESPH BY IFC AT 318 K
K (dm-3 mol-1)
104 k2 (s-1)
Solvents
K (dm-3 mol-1)
104 k2 (s-1)
Chloroform
1,2-Dichloroethane
Dichloromethane
DMSO
5.67
4.86
4.54
4.59
5.76
4.70
5.94
4.32
6.20
5.62
70.8
66.1
57.5
209
Toluene
5.31
6.21
4.75
4.38
4.27
5.40
4.95
5.90
5.40
13.2
67.6
25.7
27.5
26.3
12.6
5.62
32.4
18.2
Acetophenone
THF
t-butylalcohol
1,4-Dioxane
1,2-Dimethoxyethane
CS2
Acetone
47.9
98.1
39.8
63.1
17.4
1.45
DMF
Butanone
Nitrobenzene
Benzene
Acetic acid
Ethyl acetate
Cyclohexane
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. How-
ever, the correlations individually with A and B were poor. In
view of the fact that solvent polarity is able to account for 99
% of the data, an attempt was made to correlate the rate with
the relative permittivity of the solvent.
log k2 = L σl + D σd + R σe + S υ + h
(18)
where υ is the well-known Charton's steric parameter based
on van der Waals radii20.
The rates of oxidation of ortho-, meta- and para- substi-
tuted sulfides show excellent correlations in terms of the LDR/
LDRS equations (Table-4). The values of the independent vari-
ables, σl, σd, σe and υ, were obtained from the work of Charton
and Charton19. Though the number of data-points is less than
the optimum number, the correlations are excellent as per
Exner's14 criterion also. Exner's ψ parameter takes into account
the number of data-point also.
However, a plot of log k2 against the inverse of the relative
permittivity is not linear (r2 = 0.4766; sd = 0.36; ψ = 0.74).
Correlation analysis of reactivity: The oxidation of diffe-
rent sulfides follows the order of their nucleophilicity: Pr2S >
Me2S > MeSPh > Ph2S.
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 eqn. 18, reflecting the electron-
donating power of the substituent and its capacity to stablize a
cationic species.
(i) Aryl methyl sulfides: The correlation of the effect of
substituents on the reactivity has been widely attempted in
terms of the Hammett eqn.16 or with dual substituent-parameter
eqn.17,18. Charton19 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 deloca-
lization. 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, there-
fore, begun a study of structural effects on reactivity by means
of the LDR equation. In this work, we have applied the LDR
eqn. 16 to the rate constants, k2.
The negative value of S indicates that the reaction is subjec-
ted to steric hindrance by the ortho-substituent. 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 regre-
ssion 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 [eqn. 19-21] showed that all the four substituent
constants are significant.
log k2 = -2.01 ( 0.47)σl - 1.33 ( 0.39)
σd - 1.98 ( 2.74)σe - 3.17
(19)
(20)
(21)
log k2 = L σl + D σd + R σe + h
(16)
R2 = 0.8941, sd = 0.29, n = 10, ψ = 0.40
log k2 = -1.89 ( 0.44)σd + 0.54 ( 0.18)
σe - 1.84 ( 0.55) υ - 2.94
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 eqn. 17.
R2 = 0.8506, sd = 0.34, n = 10, ψ = 0.47
log k2 = -2.13 ( 0.88)σl - 5.37 ( 4.39)
σe - 0.70 ( 0.88) υ - 2.72
σD =ησe + σd
(17)
Here η represents the electronic demand of the reaction site
and represents the ratio of regression coefficient of the sensi-
tivity parameter, σe and that of resonance parameter, σd i.e. η
= R/D. σD represents the delocalized electrical parameter of
the diparametric LD equation.
R2 = 0.6837, sd = 0.50, n = 10, ψ = 0.69
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 collinearity between the various substi-
tuent constants for the three series.
For ortho-substituted compounds, it is necessary to account
for the possibility of steric effects and Charton19 therefore,
modified the LDR equation to generate the LDRS eq. 18.

2602 Mathur et al.
Asian J. Chem.
TABLE – 6
CORRELATION OF RATE OF OXIDATION OF ALKYL PHENYL SULFIDES WITH PAVELICH-TAFT EQUATIONA
ρ*
δ
Temperature K
R2
sd
288
0.9999
0.9993
0.9957
0.9832
0.01
0.03
0.08
0.11
−2.79 0.02
−2.81 0.10
−2.87 0.23
−2.85 0.25
0.80 0.01
0.78 0.02
0.76 0.04
0.69 0.05
298
308
318
a No. of data points = 5
TABLE – 7
TEMPERATURE DEPENDENCE FOR THE REACTION CONSTANTS FOR THE OXIDATION OF ORGANIC SULFIDES BY IFC
T/K
–S
R2
sd
PD
PS
−L
−D
−R
η
ψ
para-substituted
288
298
308
318
1.44
1.35
1.26
1.17
1.80
1.70
1.62
1.52
1.53
1.44
1.34
1.28
-
-
-
-
0.85
0.9999
0.9998
0.9989
0.9999
0.001
0.002
0.001
0.003
0.01
0.02
0.04
0.01
37.7
37.9
38.4
38.3
-
-
-
-
0.85
0.83
0.84
meta-substituted
288
298
308
318
1.80
1.71
1.62
1.51
1.44
1.36
1.26
1.18
1.19
1.09
0.97
0.82
-
-
-
-
0.83
0.80
0.9999
0.9998
0.9999
0.9998
0.001
0.002
0.001
0.005
0.01
0.02
0.01
0.02
32.5
32.7
32.7
33.6
-
-
-
-
0.77
0.69
ortho-substituted
0.83
288
298
308
318
1.54
1.48
1.35
1.27
1.62
1.53
1.44
1.34
1.34
1.25
1.15
1.14
1.26
1.17
1.08
0.99
0.9999
0.9989
0.9999
0.9998
0.005
0.002
0.001
0.005
0.01
0.04
0.01
0.02
36.0
36.3
36.5
35.7
21.9
21.7
21.5
21.0
0.82
0.80
0.85
The percent contribution20 of the delocalized effect, PD is
given by the following eqn. 22.
log k2 = ρ* σ* + δ Es + log k0
(24)
The correlations are excellent (Table-6). Though the number
of compounds is small (five) for any analysis by a DSP equa-
tion, 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.
PD = (|D| × 100)/(|L| + |D| + |R|)
(22)
Similarly, the per cent contribution of the steric para-
meter20 to the total effect of the substituent, PS, was determined
by using eqn. 23.
PS = (|S| × 100 )/(|L| + |D| + |S| + |R|)
(23)
The values of PD and PS are also recorded in Table-7. The
value of PD for the oxidation of para-substituted sulfides is
52 % whereas the corresponding values for the meta- and
ortho-sobstituted aldehydes are 39 and 49 % respectively. The
less pronounced resonance effect from the ortho- position than
from the para-position may be due to the twisting away of
the methyl-thio 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)21, periodate
(-1.40)22, permanganate (-1.52)23 and peroxydisulfate (-0.56)24.
Large negative reaction constants were exhibited by oxidations
involving formation of halogeno-sulfonium cations e.g. by
chloramine-T (-4.25)25, bromine (-3.2)26 and N-bromoacetamide
(-3.75)27.
Mechanism
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 (25).
O2CrClOImH + TsOH
[OCr(OH)ClOImH]+ [TsO]– (25)
In view of the absence of any effect of radical scavenger,
acrylonitrile, on the reaction rate, it is unlikely that a one elec-
tron reaction giving rise to free radicals, is operative in this
oxidation. The observed Michaelis-Menten kinetics with respect
to sulfides led us to suggest the formation of a 1:1 complex of
IFC and sulfides in a rapid pre-equilibrium. With present set
of data, it is difficult to state the definite nature of the inter-
mediate complex. Theoretical calculations [30] have shown
that there is a substantial charge transfer from the metal to the
oxygen in dichromate. The most logical mode of interaction
between sulfides and IFC would, therefore be nucleophilic
attack at the metal. Donation of a unshared pair of electrons to
an empty d-orbital on the metal would result in the formation
of a coordinate covalent bond. The initial formed intermediate
is likely to undergo a further rapid reaction in which the inci-
pient oxide and sulfonium ions bond to form a highly structured
In the oxidation by N-chloroacetamide (NCA)28 the values
of field (ρI) and resonance (ρ+ ), at 298 K are -1.3 and -1.7,
respectively.
R
(ii) Alkyl phenyl sulfides: The rates of oxidation of alkyl
phenyl sulfides did not yield any significant correlation sepa-
rately with Taft's σ* or Es values. The rates were therefore
analysed in terms of Pavelich-Taft's29 dual substituent-parameter
(DSP) eqn. 24.

Vol. 26, No. 9 (2014)
Oxidation of Organic Sulfides by Imidazolium Fluorochromate 2603
intermediate, that would rearrange to give a sulfoxide
(Scheme-I).
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••
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S
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PRODUCTS
••
OR
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O
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Scheme-I
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intermediate as has been suggested in many reactions of
Cr(VI). The cyclic transition state will be highly strained in
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will be higher as compared to those of reaction (Scheme-I)
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.
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ACKNOWLEDGEMENTS
Thanks are due to University Grants Commission, New
Delhi for financial support in the form of BSR-One Time Grant,
No. F. 4-10/2010 (BSR) dated 07.03.2012.