Kinetics and mechanism of oxidation of dimethyl sulphoxide by mono- and di-substituted N,N-dichlorobenzenesulphonamides in aqueous acetic acid

Article abstract of DOI:10.1515/znb-2003-0811

In an effort to introduce N,N-dichloroarylsulphonamides of different oxidising strengths, four mono- and five di-substituted N,N-dichlorobenzenesulphonamides are prepared, characterised and employed as oxidants for studying the kinetics of oxidation of dimethyl sulphoxide (DMSO) in 50% aqueous acetic acid. The reactions show first order kinetics in [oxidant], fractional to first order in [DMSO] and nearly zero order in [H ⁺]. Increase in ionic strength of the medium slightly increases the rates, while decrease in dielectric constant of the medium decreases the rates. The results along with those of the oxidation of DMSO by N,N-dichlorobenzenesulphonamide and N,N-dichloro-4-methylbenzenesulphonamide have been analysed. Effective oxidising species of the oxidants employed in the present oxidations is Cl⁺ in different forms, released from the oxidants. Therefore the introduction of different substituent groups into the benzene ring of the oxidant is expected to affect the ability of the reagent to release Cl⁺ and hence its capacity to oxidise the substrate. Significant changes in the kinetic and thermodynamic data are observed in the present investigations with change of substituent in the benzene ring. The electron releasing groups such as CH₃ inhibit the ease with which Cl⁺ is released from the oxidant, while electron-withdrawing groups such as Cl enhance this ability. The Hammett equation, log k_obs = -3.19 + 1.05 σ, is found to be valid for oxidations by all the p-substituted N,N-dichlorobenzenesulphonamides. The substituent effect on the energy of activation, E_a and log A for the oxidations is also analysed. The enthalpies and free energies of activation correlate with an isokinetic temperature of 320 K.

Full text of DOI:10.1515/znb-2003-0811

Kinetics and Mechanism of Oxidation of Dimethyl Sulphoxide
by Mono- and Di-Substituted N,N-Dichlorobenzenesulphonamides
in Aqueous Acetic Acid
B. Thimme Gowda, K. L. Jayalakshmi, and K. Jyothi
Department of Post-Graduate Studies and Research in Chemistry, Mangalore University,
Mangalagangothri-574 199, Mangalore, India
Reprint requests to Prof. B. Thimme Gowda. Fax: 91 824 2287 367. E-mail: gowdabt@yahoo.com
Z. Naturforsch. 58b, 787 – 794 (2003); received February 6, 2003
In an effort to introduce N,N-dichloroarylsulphonamides of different oxidising strengths, four
mono- and five di-substituted N,N-dichlorobenzenesulphonamides are prepared, characterised and
employed as oxidants for studying the kinetics of oxidation of dimethyl sulphoxide (DMSO) in 50%
aqueous acetic acid. The reactions show first order kinetics in [oxidant], fractional to first order in
[DMSO] and nearly zero order in [H+]. Increase in ionic strength of the medium slightly increases
the rates, while decrease in dielectric constant of the medium decreases the rates. The results along
with those of the oxidation of DMSO by N,N-dichlorobenzenesulphonamide and N,N-dichloro-4-
methylbenzenesulphonamide have been analysed. Effective oxidising species of the oxidants em-
ployed in the present oxidations is Cl+ in different forms, released from the oxidants. Therefore the
introduction of different substituent groups into the benzene ring of the oxidant is expected to affect
the ability of the reagent to release Cl+ and hence its capacity to oxidise the substrate. Significant
changes in the kinetic and thermodynamic data are observed in the present investigations with change
of substituent in the benzene ring. The electron releasing groups such as CH3 inhibit the ease with
which Cl+ is released from the oxidant, while electron-withdrawing groups such as Cl enhance this
ability. The Hammett equation, log kobs = −3.19 + 1.05 σ, is found to be valid for oxidations by all
the p-substituted N,N-dichlorobenzenesulphonamides. The substituent effect on the energy of activa-
tion, Ea and log A for the oxidations is also analysed. The enthalpies and free energies of activation
correlate with an isokinetic temperature of 320 K.
Key words: Kinetics, Oxidation, Dimethyl Sulphoxide, N,N-Dichloroarylsulphonamides
Introduction
reported the spectroscopic studies on a number of aryl-
sulphonamides [12], the sodium salts of N-chloro com-
pounds [13] and N,N-dichloroarylsulphonamides [14].
Dimethyl sulphoxide is a versatile solvent in indus-
Many sulphonamides and their N-halo compounds
are of fundamental chemical interest as they show
distinct physical, chemical and biological properties. try and chemical laboratories as medium for carrying
They also exhibit pharmacological activity. Further, out reactions. It is also a popular medicinal compound
many of these compounds exhibit fungicidal and her- because of its low toxicity and high skin permeability.
bicidal activities, due to their oxidising action in aque- Hence the reactions of dimethyl sulphoxide are of in-
ous, partial aqueous and non-aqueous media [1 – 6]. terest. The kinetics of oxidation of DMSO by a number
This has stimulated recent interest in their chemistry. of oxidants have been reported [15– 20].
Therefore an understanding of the formation, proper-
ties and reactions of arylsulphonamides, their N-chloro
and N,N-dichloro compounds is of interest in medic-
inal and redox chemistry, and a great deal of work
on the spectroscopic, structural and kinetic aspects of
these compounds needs to be done for correlating their
reactivities with the chemical bond parameters. We are
interested in the chemistry of these compounds in par-
ticular and amides in general [6 – 14] and have recently
We report herein the kinetics of oxidation of
dimethyl sulphoxide by four mono- and five di-
substituted N,N-dichlorobenzenesulphonamides and
analyse the results along with those by N,N-
dichlorobenzenesulphonamide and N,N-dichloro-4-
methylbenzenesulphonamide. The oxidants or N,N-di-
chloro compounds employed were of the configuration
ArSO2NCl2, where Ar = C6H5 (1); 4-CH3-C6H4 (2);
4-C2H5-C6H4 (3); 4-F-C6H4 (4); 4-Cl-C6H4 (5); 4-Br-
c
0932–0776 / 03 / 0800–0787 $ 06.00 ꢀ 2003 Verlag der Zeitschrift fu¨r Naturforschung, Tu¨bingen · http://znaturforsch.com
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B. Thimme Gowda et al. · Kinetics and Mechanism of Oxidation of Dimethyl Sulphoxide
103[DCSB]o 10[DMSO]o
10[HClO4]
104kobs (s−1) for i-X-C6H4SO2NCl2, i-X =
Table 1. Pseudo-first order rate
constants (kobs) for the ox-
idation of dimethyl sulphox-
ide (DMSO) by monosubsti-
tuted N,N-dichlorobenzenesul-
phonamides (DCSB) in 1:1
(v/v) water-acetic acid medium
at 303 K (I = 0.30 mol dm−3).
(mol dm−3
)
(mol dm−3
)
(mol dm−3
)
4-H
4-CH3
4-C2H5
4-F
4-Cl
4-Br
Effect of varying [DCSB]o
0.5
1.0
2.0
4.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
11.9
12.1
12.3
13.0
9.0
9.4
9.9
11.6
11.8
12.2
12.9
14.8 20.0
15.2 20.7
15.3 21.1
15.8 21.6
20.4
21.8
22.3
22.5
10.5
Effect of varying [DMSO]o
1.0
1.0
1.0
1.0
0.1
0.5
1.0
2.0
1.0
1.0
1.0
1.0
1.9
7.2
12.1
26.6
1.4
5.4
9.4
2.0
5.8
11.8
21.5
2.0
2.8
2.9
11.2
21.8
43.7
8.4 10.5
15.2 20.6
29.1 40.9
18.4
Effect of varying [HClO4]
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
0.1
0.5
1.0
2.0
12.1
12.1
12.2
12.9
8.6
9.1
9.4
9.9
10.5
10.5
11.8
11.9
15.3 20.0
15.3 20.3
15.3 20.6
15.4 20.6
21.3
21.3
21.7
21.7
103[DCSB]o 10[DMSO]o 10[HClO4]
(mol dm−3 (mol dm−3 (mol dm−3) 2,3-(CH3)2 2,4-(CH3)2 2-CH3-4-Cl 2,4-Cl2 3,4-Cl2
Effect of varying [DCSB]o
104kobs (s−1) for i-X-j-Y-C6H3SO2NCl2, i-X-j-Y =
Table 2. Pseudo-first order
rate constants (kobs) for the
oxidation of dimethyl sulphox-
ide (DMSO) by disubstituted
N,N-dichlorobenzenesulphon-
amides (DCSB) in 1:1 (v/v)
water-acetic acid medium at
303 K (I = 0.30 mol dm−3).
)
)
0.5
1.0
2.0
4.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
8.9
9.5
9.7
20.2
21.0
22.0
22.2
24.2
24.9
25.5
26.2
118.1
119.6
119.8
120.2
41.0
41.9
43.4
44.0
10.2
Effect of varying [DMSO]o
1.0
1.0
1.0
1.0
0.1
0.5
1.0
2.0
1.0
1.0
1.0
1.0
1.5
5.2
9.5
4.2
14.0
21.0
42.2
4.4
15.7
24.9
50.3
12.4
60.0
119.6
−
4.8
22.6
41.9
73.1
17.5
Effect of varying [HClO4]
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
0.1
0.5
1.0
2.0
9.6
9.6
9.7
9.9
20.8
20.9
21.0
21.0
24.7
24.7
24.9
25.1
119.6
119.6
119.7
120.4
41.7
41.7
41.8
41.8
Table 3. Effect of variation in ionic
strength (I) and solvent composition of
the reaction medium on the rate of ox-
idation of dimethyl sulphoxide (DMSO)
by the mono- and di-substituted N,N-
dichlorobenzenesulphonamides (DCSB)
in 1:1 (v/v) water-acetic acid medium (ex-
cept during its variations) at 303 K.
10[I]
104kobs (s−1) for
i-X-j -Y-C6H3SO2NCl2, i-X-j-Y =
2,4-(CH3)2 2-CH3−4-Cl 2,4-Cl2
107.2
(mol dm−3
)
i-X-C6H4SO2NCl2, i-X =
4-H 4-C2H5
4-Cl
1.0
2.0
3.0
5.0
11.8
12.0
12.1
13.3
9.9
9.9
11.8
11.9
19.4
19.8
20.7
21.1
20.7
21.0
21.0
21.2
22.5
23.8
24.9
25.4
107.2
119.6
126.9
% AcOH
104 kobs (s−1) for
103 [DCSB]0 = 10 [DMSO]0 = 10[HClO4] =
(v/v)
i-X-C6H4SO2NCl2, i-X =
i-X- j-Y-C6H3SO2NCl2, i-X-j-Y =
1.0 mol dm−3
.
4-H 4-C2H5
4-Cl
2,4-(CH3)2 2-CH3-4-Cl
2,4-Cl2
20
30
50
70
22.5
18.7
12.1
7.4
19.6
16.1
11.8
7.3
44.6
33.6
20.7
11.2
52.8
41.4
21.0
12.9
60.3
45.7
24.9
13.5
–
156.9
119.6
70.1
C6H4 (6); 2,3-(CH3)2-C6H3 (7); 2,4-(CH3)2-C6H3 (8); first order conditions with [substrate] ꢀ [oxidant]
2-CH3-4-Cl-C6H3 (9); 2,4-Cl2-C6H3 (10) and 3,4-Cl2- (5 to 50 times). The reactions were initiated by the
C6H3 (11).
rapid addition of requisite amounts of oxidant solu-
tion (0.0005– 0.004 mol dm−3), pre-equilibrated at a
desired temperature, to solution mixtures containing
known amounts of dimethyl sulphoxide (0.01– 0.2 mol
dm−3), perchloric acid (0.01– 0.2 mol dm−3), acetic
Results and Discussion
Kinetic measurements: The kinetic runs were made
in glass stoppered pyrex boiling tubes under pseudo-
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B. Thimme Gowda et al. · Kinetics and Mechanism of Oxidation of Dimethyl Sulphoxide
789
Table 4. Kinetic data for the oxidation of dimethyl sulphox-
ide (DMSO) by the mono- and di-substituted N,N-di-
chlorobenzenesulphonamides (DCSB) in 1:1(v/v) water-
acetic acid medium at 303 K.
Orders
in
[DCSB]
[DMSO]
[HClO4]
i-X-C6H4SO2NCl2, i-X =
4-H
1.0
0.9
0
4-CH3
1.0
4-C2H5
1.0
4-F
1.0
1.0
0
4-Cl
1.0
1.0
0
4-Br
1.0
1.0
0
0.8
0
0.7
0
i-X-j-Y-C6H3SO2NCl2, i-X-j-Y =
2,3-(CH3) 2,4-(CH3)2 2-CH3-4-Cl 2,4-Cl2 3,4-Cl2
[DCSB]
[DMSO]
[HClO4]
1.0
0.8
0
1.0
0.7
0
1.0
1.0
0
1.0
1.0
0
1.0
1.0
0
Fig. 2. Plot of kobs vs. [DMSO]0; 103 [DCSB]0
=
10 [HClO4] = 3.33 I = 1.0 mol dm−3; Temp. 303 K.
(DCSB: i-X-j-YC6H3SO2NCl2; i-X-j-Y = 2,3-(CH3)2; 2,4-
(CH3)2).
Fig. 1. Plot of kobs vs. [DMSO]0; 103 [DCSB]0
=
10 [HClO4] = 3.33 I = 1.0 mol dm−3; Temp. 303 K. (DCSB:
i-XC6H4SO2NCl2; i-X = 4-CH3 or 4-C2H5).
acid and water, thermostated at the same temperature.
The progress of the reactions was monitored for two
half-lives by the iodometric estimation of unreacted
oxidant at regular intervals of time. The pseudo-first
order rate constants were computed by the graphical
methods and the values were reproducible within 3%
error.
The kinetic data on the oxidation of dimethyl
sulphoxide by the substituted N,N-dichlorobenzene-
sulphonamides (DCSB) 3 – 11, under varying [DCSB],
Fig. 3. Plot of kobs vs. % acetic acid; 103 [DCSB]0
=
10 [HClO4] = 3.33 I = 1.0 mol dm−3; Temp. 303 K. (DCSB:
i-XC6H4SO2NCl2; i-X: 4-H, 4-C2H5 or 4-Cl and i-X-j-
YC6H3SO2NCl2 = 2,4-(CH3)2; 2-CH3-4-Cl; 2,4-Cl2).
[DMSO], [HClO4], ionic strength and solvent com- and Figures 1 – 4. The kinetics of oxidation of DMSO
position of the medium, are shown in Tables 1 – 4 by the oxidants 1 and 2 have also been carried out
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B. Thimme Gowda et al. · Kinetics and Mechanism of Oxidation of Dimethyl Sulphoxide
Activation
parameters
Ea (kJ mol−1
log A
∆H= (kJ mol−1
i-X-C6H4SO2NCl2, i-X =
Table 5. Activation parameters for the
oxidation of dimethyl sulphoxide by
mono-substituted N,N-dichlorobenzenesul-
phonamides in 1:1 (v/v) water-acetic acid in
presence of perchloric acid.
4-H
4-CH3
4-C2H5
4-F
4-Cl
4-Br
)
65.5
8.4
64.0
58.3
7.0
56.3
59.0
7.2
67.3
8.8
65.1
8.5
62.4
62.2
8.1
58.6
)
55.2
60.2
∆S= (JK−1 mol−1
∆G= (kJ mol−1
Optimised values with reference to log A of the parent oxidant
)
−89.8 −117.3
−119.0 −100.2 −90.4 −102.7
)
91.2 91.8
91.2
90.6
89.8
89.7
Ea (kJ mol−1
∆H= (kJ mol−1
)
65.7
64.0
66.3
64.6
65.7
64.0
65.1
63.4
64.3
62.7
64.2
62.5
)
Optimised values with reference to E of the parent oxidant
a
log A
∆S= (JK−1 mol−1
8.4
−89.7
8.3
−91.8
8.4
−89.9
8.5
8.6
8.7
−84.8
)
−87.8 −85.3
Table 6. Activation parameters for the oxidation
of dimethyl sulphoxide by di-substituted N,N-
dichlorobenzenesulphonamides in 1:1 (v/v) water-acetic
acid in presence of perchloric acid.
Activation
parameters
i-X-j-Y-C6H3SO2NCl2, i-X-j-Y =
2,3-(CH3)2 2,4-(CH3)2 2-CH3-4-Cl 2,4-Cl2 3,4-Cl2
Ea (kJ mol−1
)
61.6
7.6
59.7
65.7
8.6
58.7
58.4 53.5 58.0
7.5 7.3 7.6
55.6 52.0 56.5
−111.4 −110.3 −104.1
89.4 85.4 88.0
log A
∆H= (kJ mol−1
)
∆S= (JK−1 mol−1) −106.0
−102.5
∆G= (kJ mol−1
)
91.8
89.8
Optimised values with reference to log A of the parent oxidant
Ea (kJ mol−1
∆H= (kJ mol−1
)
66.2
64.6
64.3
62.6
63.8 59.9 62.5
62.2 58.2 60.9
)
Optimised values with reference to E of the parent oxidant
a
log A
∆S= (JK−1 mol−1
8.3
−91.7
8.6
−85.1
8.7
9.4
8.9
)
−83.7 −70.7 −79.4
[DMSO] with fractional to first order dependence of
the rate on [DMSO] (Tables 1, 2, and 4). The kobs ver-
sus [DMSO] plots were linear passing through the ori-
gin with the oxidants 4 – 6 and 9 – 11. The plots were
also linear with the oxidants 1 – 3, 7, 8 but with inter-
cepts on the ordinates (Fig. 1 and 2).
Fig. 4. Plot of (i) log kobs vs. σρ and (ii) ∆H= vs. ∆S=;
103 [DCSB]0 = 10 [HClO4] = 3.33 I = 1.0 mol dm−3
Temp. 303 K. (DCSB: i-XC6H4SO2NCl2; i-X: 4-H, 4-CH3,
4-C2H5, 4-F, 4-Cl or 4-Br and i-X-j-Y-C6H3SO2NCl2;
i-X-j-Y=2,3-(CH3)2; 2,4-(CH3)2; 2-CH3-4-Cl; 2,4-Cl2 or
3,4-Cl2).
;
Effect of varying [HClO4]: At constant [DCSB]0
and [DMSO]0, the rates remained almost unaffected by
the changes in [HClO4], indicating nearly zero order
dependence of rates on [H+] (Tables 1 and 2).
under identical conditions and included for complete-
ness.
Effect of varying other parameters: The rates
slightly increased with increase in ionic strength of the
medium, while decrease in dielectric constant of the
medium by increasing the acetic acid composition of
the solvent decreased the rates (Table 3 and Fig. 3).
The rates were measured at different temperatures and
the activation parameters have been computed from the
Arrhenius and Eyring plots (Tables 5 and 6).
Effect of varying [DCSB]0 : At constant [DMSO]
(5 – 50 fold excess over [DCSB]) and [H+], first order
plots of log[DCSB] versus time were linear up to at
least 75% completion of the reaction. The pseudo-first
order rate constants (kobs) computed from the plots re-
mained unaffected by the changes in [DCSB], estab-
lishing first order dependence of the rate on [DCSB],
in all the cases (Tables 3 and 4).
N,N-dichloroarylsulphonamides furnish the follow-
Effect of varying [DMSO]0: At constant [DCSB]0 ing reactive species in its aqueous acetic acid solu-
and [HClO4], the rates increased with increase in tions [2, 4, 6]: ArSO2NCl2, ArSO2NHCl and HOCl
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B. Thimme Gowda et al. · Kinetics and Mechanism of Oxidation of Dimethyl Sulphoxide
791
Table 7. The calculated rate constants k1 and k2ꢂ
at different temperatures for the oxidation of di-
methyl sulphoxide by mono and di-substituted N,N-
dichlorobenzenesulphonamides in 1:1 (v/v) water-acetic
acid medium in the presence of perchloric acid.
Table 8. Activation parameters for path 1 of the mecha-
nism for the oxidation of dimethyl sulphoxide by mono-
and di-substituted N,N-dichlorobenzenesulphonamides in
1:1 (v/v) water-acetic acid medium in the presence of per-
chloric acid.
Activation
parameters
i-X-C6H4SO2NCl2, i-X-j-Y-C6H3SO2NCl2,
i-X = i-X-j-Y =
4-H 4-CH3 4-C2H5 2,3-(CH3)2 2,4-(CH3)2
Temp.
(K)
103k1(dm3mol−1s−1
)
i-X-C6H4SO2NCl2,
i-X =
i-X-j-Y-C6H3SO2NCl2, =
i-X-j-Y =
Ea (kJ mol−1
log A
)
55.4 64.4 68.9
7.7 9.1 9.9
54.2 61.3 66.4
65.6
9.3
62.7
43.8
5.8
45.1
4-H 4-CH3
4-C2H5
2,4-Cl2
3,4-Cl2
∆H= (kJ mol−1
)
293
298
303
308
6.2
8.9
4.0
5.5
3.9
5.3
3.2
5.0
12.8
16.6
19.6
30.6
∆S= (JK−1 mol−1) −102.2 −82.0 −63.9
−77.5
−129.1
∆G= (kJ mol−1
)
85.2 86.1 85.7
86.1
84.2
12.9
16.0
9.0
13.8
10.5
14.1
8.9
10.7
Optimised values with reference to log A of the parent oxidant
Ea (kJ mol−1
∆H= (kJ mol−1
)
55.6 56.5 56.2
54.2 55.2 54.8
56.6
55.2
54.6
53.2
105 kꢂ2 (s−1
)
)
293
298
303
308
2.0
4.0
6.0
8.0
2.5
5.0
7.5
7.5
7.5
10.0
10.0
17.5
4.0
6.0
6.0
8.0
0
Optimised values with reference to E of the parent oxidant
a
10.0
30.0
30.0
log A
7.7
7.6
7.6
7.6
7.9
∆S= (JK−1 mol−1) −102.2 −105.2 −104.0
−105.3
−98.8
Table 9. Activation parameters for path 2 of the mecha-
nism for the oxidation of dimethyl sulphoxide by mono
and di-substituted N,N-dichlorobenzenesulphonamides in
1:1 (v/v) water-acetic acid medium in the presence of per-
chloric acid.
(where Ar = C6H5, 4-CH3-C6H4, 4-C2H5-C6H4, 4-F-
C6H4, 4-Cl-C6H4, 4-Br-C6H4, 2,3-(CH3)2-C6H3, 2,4-
(CH3)2-C6H3, 2-CH3-4-Cl-C6H3, 2,4-Cl2-C6H3 and
3,4-Cl2-C6H3).
Mechanism of oxidation: The kinetics of first
order each in [DCSB] and [DMSO], and nearly
zero order in [H+] observed for the oxidation of
dimethyl sulphoxide by 4-fluoro, 4-chloro, 4-bromo, 2-
methyl-4-chloro, 2,4-dichloro and 3,4-dichloro substi-
tuted N,N-dichlorobenzene-sulphonamides in aqueous
acetic acid medium may be explained by the simple
scheme
Activation
parameters
i-X-C6H4SO2NCl2, i-X =
2,3-(CH3)2C6
H3SO2NCl2
4-H 4-CH3
4-C2H5
Ea (kJ mol−1
log A
)
82.5 68.9
38.3
2.6
39.9
34.6
1.8
32.4
10.0
7.8
∆H= (kJ mol−1
)
82.1 79.8
∆S= (JK−1 mol−1) −55.1 −60.8
−190.0
−219.0
∆G= (kJ mol−1
)
98.7 98.2
97.5
98.7
Optimised values with reference to log A of the parent oxidant
Ea (kJ mol−1
∆H= (kJ mol−1
)
82.5 81.9
82.1 81.5
81.2
80.8
82.5
82.1
)
Optimised values with reference to E of the parent oxidant
k
1
a
ArSO2NCl2 +DMSO −→ Products (slow) (1)
The related rate law is
log A
10.0 10.1
10.2
−50.8
10.0
−55.1
∆S= (JK−1 mol−1) −55.1 −53.2
The combined rate law based on equation (1) and
Scheme 1 is given by
−d[oxidant]/dt
or kobs
=
=
k1[ArSO2NCl2]
k1[DMSO]
(2)
(3)
−d[oxidant]/dt
=
k1[oxidant][DMSO]+
k2[oxidant][H2O]
The plots of kobs versus [DMSO] gave straight lines
passing through the origin in conformity with the
above rate law (eq. (3)).
(4)
or kobs = k1[DMSO]+k2[H2O] =
k1[DMSO]+kꢂ , where kꢂ = k2[H2O]
The kinetic behaviour for the oxidation of DMSO
by the parent, 4-methyl, 4-ethyl, 2,3-dimethyl and
2,4-dimethyl substituted N,N-dichlorobenzenesul-
phonamides was slightly different. Oxidations showed
first order kinetics each in [DCSB], fractional order in
[DMSO] and zero order in [H+]. This is also reflected
in the magnitudes of the rate constants. These results
may be explained either by a two pathway mechanism
(eq. (1) and Scheme 1) or a Michaelis-Menten type
mechanism (Scheme 2).
(5)
2
2
The plots of kobs versus [DMSO] were linear with
finite intercepts on the ordinates in accordance with the
rate law (eq. (5)) (Fig. 1 and 2). The constants k1 and
k1
ArSO2NCl2 + H2O −→ HOCl + ArSO2NHCl
(slow)
(fast)
HOCl + DMSO
Scheme 1.
→
Products
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B. Thimme Gowda et al. · Kinetics and Mechanism of Oxidation of Dimethyl Sulphoxide
K3
ArSO2NCl2 + DMSO
Intermediate
Products
log A values were optimised with reference to the Ea
of the parent oxidant through the equation log A =
logkobs +Ea/2.303RT. Log A values were little higher
for oxidants with the electron withdrawing groups,
ꢀ
k4
X
−→
Scheme 2.
k2ꢂ were calculated from the slopes and intercepts of while the effect of electron releasing groups on log A
the plots. The substrate concentrations were vaꢂried at is negligible. The free energies of activation remain al-
different temperatures. The constants k1 and k2 were most the same in both the optimisations indicating the
calculated at each temperature ( Table 7). Thus two sets operation of similar mechanisms in all the cases.
of activation parameters have been computed from the
Conclusions
plots of log k versus 1/T and log (k/T) versus 1/T
(Tables 8 and 9).
Based on Scheme 2, the rate laws ( eq. (6 – 8) ) have
been obtained,
Effective oxidising species of the oxidants em-
ployed in the present oxidations is Cl+ in differ-
ent forms, released from the oxidants. The oxida-
tive strengths of N,N-dichloroaryl-sulphonamides de-
pend on the ease with which Cl+ is released from the
reagents. Therefore the introduction of different sub-
stituent groups into the benzene ring of the oxidant is
expected to affect the ability of the reagent to release
Cl+ and hence its capacity to oxidise the substrate.
Significant changes in kinetic and the thermodynamic
data were observed in the present investigations with
change of substituent in the benzene ring of the oxi-
dant. The electron releasing groups such as CH3 inhibit
the ease with which Cl+ is released from the oxidant,
while electron-withdrawing groups such as Cl enhance
this ability.
−d[oxidant]/dt = K3 k4[oxidant]
[DMSO]/1+K3[DMSO]
(6)
kobs = K3 k4[DMSO]/1+K3[DMSO]
1/kobs = 1/K3 k4[DMSO]/1+k4
(7)
(8)
The plots of 1/kobs versus 1/[DMSO] were also lin-
ear in conformity with the rate law in eq. (8). But the
former direct plots had better correlations than the lat-
ter double reciprocal plots, favouring the two pathway
mechanism over the Michaelis-Menten type mecha-
nism.
The following form of the Hammett equation is
found to be valid for the oxidation of dimethyl
sulphoxide by p-substituted-N,N-dichlorobenzenesul-
phonamides (Fig. 4).
Experimental Section
Materials and methods
The parent, mono- and di-substituted N,N-dichloro-
benzenesulphonamides employed in the present studies
were prepared as follows in three steps [12 – 14, 21].
logkobs = −3.19+1.05σ
(9) In the first step, the corresponding arylsulphonamides of
the configuration ArSO2NH2 were obtained by the re-
The enthalpies and the free energies of activation for action of chloroosulphonic acid on the respective sub-
stituted benzenes dissolved in chloroform at 0 0C and
subsequent treatment with liquid ammonia. Then the
corresponding N-chloroarylsulphonamides with constitu-
reactions of all the N,N-dichloro-arylsulphonamides
have been correlated. The plot of ∆H = versus ∆S=
(Fig. 4) was reasonably linear with an isokinetic tem-
perature of 320 K. Further, to see the effect of sub-
stituent in benzene ring of oxidant on the energy of ac-
tivation, Ea values of N,N-dichloroaryl-sulphonamides
were optimised with reference to the log A value
of the parent oxidant, N,N-dichlorobenzenesulphon-
tion ArSO2NaNCl·xH2O were prepared by bubbling pure
chlorine gas through clear aqueous solutions of aryl-
sulphonamides in 4 M NaOH at 70 ◦C for about 1 h. The pre-
cipitated sodium salts of N-chloroarylsulphonamides were
purified and dissolved in water and further chlorinated by
passing pure chlorine gas through the solution.
The precipitated N,N-dichloroarylsulphonamides were
filtered, washed, dried and recrystallised from chloroform.
The purity of all the reagents was checked by determining
amide, through the equation, Ea = 2.303RT(logA −
logkobs). The energies of activation for the oxidants
with electron withdrawing groups in the benzene ring
are slightly lower than those of the parent oxidant,
N,N-dichlorobenzenesulphonamide, while the effect
of electron releasing groups on Ea is negligible. En-
their melting points and by estimating iodometrically the
amounts of active chlorine present in them. All the aryl-
sulphonamides, N-chloroarylsulphonamides and their N,N-
thalpies of activations have the same trend. Similarly, dichloro compounds were further characterised by recording
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B. Thimme Gowda et al. · Kinetics and Mechanism of Oxidation of Dimethyl Sulphoxide
793
Table 10. Melting points
(◦C) of arylsulphonamides
and their N-chloro and N,N-
dichloro compounds.
Sodium salts of N-chloro-
arylsulphonamides
(ArSO2NaNCl·xH2O)
obsd. [12 – 14]
N,N-Dichloro-
arylsulphonamides
(ArSO2NCl2)
obsd.
Aryl group (Ar)
Arylsulphonamides
(ArSO2NH2)
obsd. [12 – 14]
4-C2H5-C6H4
4-F-C6H4
4-Cl-C6H4
99 – 101
194
198
191 (190)
179 (178)
167
154
172
210
192
58
55 – 56
81
102
58
63
70
67
55
125 (124 – 125)
143 (142 – 143)
162 (161.5)
138 – 140
140 – 142
180 – 182 (184-185)
178 – 180 (179-180)
141 – 143 (134-135)
4-Br-C6H4
2,3-(CH3)2-C6H3
2,4-(CH3)2-C6H3
2-CH3-4-Cl-C6H3
2,4-Cl2-C6H3
3,4-Cl2-C6H3
their infrared spectra [12 – 14]. The melting points of the pre- paper chromatography using benzyl alcohol saturated with
pared compounds are given in Table 10.
water as the solvent with 5% vanillin in 1% HCl solu-
tion in ethanol as spraying reagent (Rf = 0.95). 2,4-Di-
methyl-benzenesulphonamide among the reaction products
in N,N-dichloro-2,4-dimethylbenzene-sulphonamide oxida-
tion was detected by TLC [22] using petroleum ether-
chloroform-butanol (2:2:1 v/v) as the solvent system and
iodine as spray reagent. The Rf value of the reduced 2,4-
dimethylbenzenesulphonamide was 0.97 (compared with the
value of 0.96 for the pure compound). The presence of
dimethyl sulphone among the reaction products was con-
firmed by TLC using petroleum ether-chloroform-n-butanol
(2:2:1, v/v) as the solvent and iodine as the detecting reagent.
The observed stoichiometry may be represented by
The stock solutions (0.1 mol dm−3) of the N,N-
dichloroarylsulphonamides were prepared in distilled acetic
acid and stored in amber coloured bottles. The solutions
were standardised by the iodometric method. A stock solu-
tion (1.0 mol dm−3) of analytical grade dimethyl sulphox-
ide (E. Merck, Germany) in doubly distilled water was pre-
pared as and when required. Ionic strength of the medium
was maintained at (0.30 mol dm−3) by employing concen-
trated aqueous solution of sodium nitrate. All other reagents
used were of analytical grades of purity.
Stoichiometry and product analysis
Reaction mixtures containing excess [oxidant] over
[DMSO] were kept at 27 2 ◦C in the presence of perchlo-
ric acid for 24 h. In a typical experiment, 4-ethylbenzene-
sulphonamide among the reaction products in N,N-dichloro-
4-ethylbenzenesulphonamide oxidation was detected by
ArSO2NCl2 +2(CH3)2SO+2H2O
→ ArSO2NH2 +2(CH3)2SO2 +2H+ +2Cl− (10)
As = Arylgroups 1-11.
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B. Thimme Gowda et al. · Kinetics and Mechanism of Oxidation of Dimethyl Sulphoxide
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