Kinetics and mechanism of the reaction of α-phenoxypropanoic acids with sodium salt of N-chlorobenzene-sulphonamide: EDTA catalysis

Article abstract of DOI:10.1002/kin.10016

EDTA smoothly catalyses the oxidation cum chlorination of some 17 α-phenoxypropanoic acids with sodium salt of N-chlorobenzenesulphonamide in acidic solution. A ternary intermediate can be envisaged for describing the enhanced reactivity. Imperfections are observed in the linear Hammett relationship in the case of-NO₂ substituents, irrespective of the position. The susceptibility constant, p(≈ + 1) indicates the development of an electron-rich transition state.

Full text of DOI:10.1002/kin.10016

Kinetics and Mechanism
of the Reaction of
-Phenoxypropanoic
Acids with Sodium Salt
of N-Chlorobenzene-
sulphonamide: EDTA
Catalysis
SUBBIAH MEENAKSHISUNDARAM, M. SELVARAJU
Department of Chemistry, Annamalai University, Annamalainagar 608002, India
Received 29 December 2000; accepted 2 August 2001
ABSTRACT: EDTA smoothly catalyses the oxidation cum chlorination of some 17
-
phenoxypropanoic acids with sodium salt of N-chlorobenzenesulphonamide in acidic
solution. A ternary intermediate can be envisaged for describing the enhanced reactivity. Im-
perfections are observed in the linear Hammett relationship in the case of NO2 substituents,
irrespective of the position. The susceptibility constant,
(
+1) indicates the development
C
of an electron-rich transition state. 2001 John Wiley & Sons, Inc. Int J Chem Kinet 34: 27–33,
2002
these aromatic insulated acids have had widespread
application as herbicides and pesticides in recent years,
but this has resulted in environmental contamination
of soil and ground water. Hence, degradation stud-
ies are very much essential. A survey of literature
shows that some systematic studies were carried out
on the reactions of phenoxyacetic acids by various
oxidants such as Mn(VII) [2], Ce(IV) [3], chromic
acid [4], bipyridinium chromate [4], phenyliodoso ac-
etate [5], lead tetraacetate [6], PFC [7], pyridinium-
hydrobromide perbromide [8], periodate complex of
Cu(II) [9], NCP [10], PDC [11], picolinic acid cat-
alyzed Cr(VI) [12], and QFC [13]. The kinetics and
INTRODUCTION
Perron et al. [1] have synthesized numerous new
penicillins by condensing various phenoxypropanoic
acids with 6-aminopenicillanic acid (6-APA) and the
penicillins obtained are analytically pure. Phenoxy-
acetic and ␣-phenoxypropanoic acids (␣-PPA) pos-
sess potential biological properties. Substituents of
Correspondence to: SP. Meenakshisundaram; e-mail:meenas19-
57@sify.com.
c
2001 John Wiley & Sons, Inc.
DOI 10.1002/kin.10016

28
MEENAKSHISUNDARAM AND SELVARAJU
related studies of the reaction of ␣-PPA have not been
reported so far. In this paper, we present results of
the EDTA catalyzed reaction of several ␣-PPA by
N-chlorobenzenesulphonamide (CAB) and the sub-
stituent effects. The structure–reactivity relationships
are ascertained.
Stoichiometry and Product Analysis
The stoichiometric runs carried out in the presence of
excess of CAB at 40 C reveal that 2 mol of the oxidant
are consumed by 1 mol of ␣-PPA. The reaction mix-
ture was extracted with ether from an actual kinetic
run after about 70% completion of the reaction. The
solvent was removed by evaporation and the product
was methylated with methyl sulphate and alkali. The
product was made acidic and extracted with ether. It
was found to be chlorophenol as evidenced by its IR
spectra.
EXPERIMENTAL
Pure chlorine gas bubbled through a solution of ben-
zenesulphonamide in 4 M sodium hydroxide over a
period of 1 h at 70 C. CAB obtained was dried and re-
crystallized from water. Sodium salt of EDTA (Merck)
was used as such. Acetic acid was purified by a method
similar to that of Weissberger [14]. All the ␣-PPA were
prepared by literature methods [1]. The pure acids were
obtainedbyrecrystallizationfromsuitablesolvents. All
the other chemicals used were of AnalaR grade.
The reactions were carried out in 60% (v/v) aque-
ous acetic acid. Perchloric acid was used as the proton
source. The studies were carried out in the tempera-
ture range of 303–323 K. All the solutions were kept in
a thermostat at constant temperature, which was con-
trolled using Gallenkemp thermostat to an accuracy
of 0.1 . The required volumes of these solutions for
each run were mixed and 2 ml aliquots of the reaction
mixture were pipetted out at convenient time intervals
and quenched in 10 ml 2% KI solution and the liber-
ated iodine was titrated against standard thiosulphate
to a starch end point. The pseudo-first-order rate con-
stants were evaluated from log titre vs. time plots. All
the rate constants reported are average of two or more
determinations.
RESULTS AND DISCUSSION
First-order dependence on [CAB] was evidenced by
the linear semilogarithmic plots of titre vs. time. A
perfect linearity is observed (r > 0.996; s < 0.024) in
the first-order plots and the pseudo-first-order rate co-
efficients are almost constant. The order dependence
on the concentration of substrate is <1. Tendency for
curvature is noticed in kobs vs. [␣-PPA] plot as shown in
Fig. 1 (r = 0.982; s = 0.222). The inverse–inverse plot
(Fig. 2) reaches a limiting value. The catalytic activ-
ity of EDTA is depicted in Fig. 3. EDTA enhances the
conversion of the insulated acids and the system can be
characterized as,
kobs = a + b [EDTA]
where a and b are rate constants for catalyzed and un-
catalyzed reactions respectively. The rate of conver-
sion is proportional to the acidity. No leveling off rate
Figure 1 Plot of kobs vs. [ -PPA] in the EDTA catalyzed reaction of [ -PPA] with CAB.

REACTION OF ␣-PHENOXYPROPANOIC ACIDS
29
Figure 2 Michaelis–Menten plot in the EDTA catalyzed reaction of -PPA with CAB.
constants takes place at higher acidities. The kinetic
data are summarized in Table I.
employing 17 meta-, para-, and ortho-substituents at
three different temperatures and the results are listed
in Table IV. It appears that the substituent effects are
small in this reactivity as expected since the order de-
pendence on the concentration of substrate is very low.
A scan of Table IV reveals quite interesting results.
Exceptional behavior is observed in the case of NO2
substituents, irrespective of the position. Although the
Hammett plot of log k vs. shows a very poor corre-
lation (r = 0.275; s = 0.525) when considering all the
substituents, when the NO2 groups and the other sub-
stituents are evaluated separately, good correlations are
The catalyzed reaction is accelerated in the pres-
ence of Cl . The reactivity of the insulated acids in-
creases as the amount of acetic acid in the solvent is in-
creased as shown in Table II. The insensitivity on rates
of added acrylonitrile rules out a free radical process.
Ionic strength has a significant effect on the reactivity
(Table III).
The Hammett correlations are primarily used for
reactions occurring in homogeneous solutions. Effect
of substituents on the reactivity has been examined by
3
Figure 3 Catalytic activity of EDTA on the reaction rate. (a) 102 [EDTA] = 1.00 mol dm 3, 103 [CAB] = 1.20 mol dm
;
(b) 102 [S] = 2.00 mol dm 3, 103 [CAB] = 1.20 mol dm 3; (c) 102[S] = 2.00 mol dm 3, 102 [EDTA] = 1.00 mol dm 3, 103
[CAB] = 1.20 mol dm 3; (d) 102[S] = 2.00 mol dm 3, 102 [EDTA] = 4.00 mol dm 3, 103 [CAB] = 1.20 mol dm
.
3

30
MEENAKSHISUNDARAM AND SELVARAJU
Table I Effect of Reactants on the Reaction Rate in the
Table III Dependence of the Reaction Rate at 313 K
EDTA Catalyzed Reaction of -PPA by CAB at 313 K
on Ionic Strength and Acrylonitrile
103[CAB]
(mol dm
102[ -PPA]
10[HClO4]
104kobs
102[NaClO4]
103[Acry]
(mol dm
104kobs
3
3
3
1
3
3
1
)
(mol dm
)
(mol dm
)
(s
)
(mol dm
)
)
(s
)
0.80
1.20
1.80
2.30
1.20
1.20
1.20
1.20
1.20
1.20
1.20
1.20
1.20
1.20
1.20
2.00
2.00
2.00
2.00
1.00
2.00
4.00
6.00
10.00
14.00
2.00
2.00
2.00
2.00
2.00
6.00
6.00
6.00
6.00
6.00
6.00
6.00
6.00
6.00
6.00
3.00
4.50
6.00
7.50
9.00
5.03
4.99
4.98
4.87
4.38
4.99
5.43
5.99
6.51
7.30
2.25
3.53
4.99
6.53
7.89
–
1.00
2.00
3.00
4.00
–
–
–
–
–
–
–
–
–
–
4.99
6.09
7.91
9.85
11.59
4.99
4.93
5.06
4.89
5.10
–
1.00
2.00
4.00
7.00
[CAB] = 1.20 10 mol dm 3; [EDTA] = 1.00 10 mol
3
2
3
2
1
dm [␣-PPA] = 2.00 10 mol dm 3; [H+] = 6.00 10 mol
3
dm
.
[EDTA] = 1.00 10 mol dm 3; AcOH : H2O = 60 : 40 (v/v).
2
exceptionally well with a correlation coefficient,
r = 0.985 and standard deviation, s = 0.075. The sus-
ceptibility constant ( ) values are listed in Table V.
The behavior of NO2 substituents is puzzling. The
general nonadherence to the Hammett equation sug-
gests that the nitro and other substituents exhibit dif-
ferent reaction mechanisms. Similar type of imperfec-
tions are observed in the Os(VIII)- catalyzed oxidation
of sulfides [15] by CAB. Also, it has been reported [16]
that for an electrically charged group Hammett’s equa-
tion would be in error because of the direct effect in
addition to inductive and resonance effects.
observed (Fig. 4) (r = 0.947; s = 0.111). Few ortho-
substituents are not included in the correlation since
the corresponding
ature. The Brown–Okamoto
values are+not available in liter-
0
correlations are in-
values (corre-
ferior (r = 0.889; s = 0.158). The
lation for p-substituents only) describe the reactivity
Table II Dependence of [EDTA], Solvent Composition,
The susceptibility constant, is a measure of the re-
action to polar substitution. A positive value ( +1)
in this system means that the rate is increased by
electron-attracting substituents. Low values indicate
that there is only a moderate degree of charge sepa-
ration in the transition state. A transition state similar
to the one shown below can be envisaged which can
explain the positive values.
and [NaCl] on the Reaction Rate at 313 K
102[EDTA]
104[NaCl]
(mol dm
104kobs
3
3
1
(mol dm
)
Da
)
(s
)
0.50
1.00
2.00
3.00
4.00
6.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
35.14
35.14
35.14
35.14
35.14
35.14
49.60
42.37
35.14
27.90
20.67
35.14
35.14
35.14
35.14
35.14
–
–
–
–
–
–
–
–
–
3.38
4.99
7.96
9.34
12.23
16.05
2.51
3.22
4.99
7.56
17.98
7.15
–
–
Charge separation in the transition state is supported by
the marked increase in the rate of conversion with de-
crease in dielectric constant of the medium (Table III).
2.00
3.50
5.00
7.50
10.00
8.89
10.40
12.22
13.75
Mechanism
The probable oxidizing species in acid medium [17] are
CAB, RNHCl, RNCl2, and HOCl. It seems that the ex-
perimental observations can be explained by assuming
CAB as the effective electrophile. It has been reported
3
2
[CAB] = 1.20 10 mol dm 3; [ -PPA] = 2.00 10 mol
1
3
dm 3; [H+] = 6.00 10 mol dm
.
a Dielectric constant values are calculated from the values of pure
solvents.

REACTION OF ␣-PHENOXYPROPANOIC ACIDS
31
Table IV Rate Constants and Activation Parameters for the EDTA Catalyzed Reaction of -PPA with CAB
1
103k0a/(s
)
1H#
(kJ mol
1S#
(J K mol
1G#
(kJ mol
1
1
1
1
No.
Substrate
30 C
40 C
50 C
)
)
)
r
s
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
H
p-CH3
p-OCH3
p-Cl
4.99
1.92
1.60
5.34
4.84
0.61
18.64
5.90
5.64
0.30
1.87
9.12
11.69
9.49
1.83
1.73
1.42
10.50
4.59
3.54
14.16
10.90
1.16
38.56
13.49
12.61
0.57
21.15
7.96
6.20
31.11
32.45
2.75
92.07
25.73
27.72
1.18
64.5
55.6
53.1
69.4
74.9
58.6
62.3
57.6
62.4
53.4
60.4
63.7
57.0
67.7
63.6
58.7
47.6
96.9
132.6
142.5
67.7
62.0
132.8
92.0
116.9
101.6
155.8
117.3
93.1
113.4
79.8
107.1
123.4
162.2
94.8
97.1
97.7
90.6
94.3
100.2
91.1
94.1
94.2
102.2
97.1
92.8
92.5
92.6
97.1
97.4
98.4
0.997
0.993
0.996
0.999
0.994
0.995
0.997
0.998
0.999
0.999
0.999
0.999
0.997
0.999
0.999
0.999
0.991
0.087
0.114
0.082
0.053
0.136
0.099
0.076
0.061
0.010
0.044
0.027
0.057
0.074
0.020
0.027
0.015
0.108
p-Br
p-NO2
p-COOH
m-OCH3
m-Cl
m-NO2
o-OCH3
o-Cl
4.28
8.79
22.51
22.88
22.64
4.09
3.79
2.34
46.25
50.70
53.53
9.29
7.74
4.92
o-Br
o-CHO
o-COOH
o-COOCH3
o-NO2
3
3
1
3
[S] = 2.00 10 2 mol dm 3; [CAB] = 1.20 10 mol dm [H+] = 6.00 10 mol dm AcOH : H2O = 60 : 40 (v/v); [EDTA] =
2
3
1.00 10 mol dm
.
ak0 = kobs/[S]n where n is the order with respect to substrate.
[18] that an electrophilic attack by the positive halogen
of the oxidant at the neutral nitrogen of EDTA gives
the intermediate C1. Structure of C1 is given as,
The observed 1:2 stoichiometry between the sub-
strate and oxidant very much supports the proposed
mechanism.
Rate Law
CH2COOH
¨
RNCl + CH2
N
d[CAB]
dt
k3 K1 K2[S][H+][EDTA][OX]t
CH2COOH
=
1 + K1[EDTA] + K1 K2[S][EDTA]
.....
Cl
NR
(5)
.
.
˙
.
K1
CH2 N CH2 COOH
k3 K1 K2[S][H+][EDTA]
1 + K1[EDTA] + K1 K2[S][EDTA]
kobs
=
(6)
CH2 COOH
(C1)
(1)
(2)
+
,
Table V The Reaction Constant Values for
and Correlations
,
K2
C1 + PhOCHCOOH
CH3
C2
+
Correlation
Temp ( C)
r
s
n
30
40
50
30
40
50
30
40
50
1.19a
1.16a
1.34a
0.73a
0.72a
0.85a
0.98b
1.02b
1.13b
0.918
0.919
0.938
0.872
0.889
0.902
0.984
0.972
0.970
0.140
0.136
0.138
0.172
0.158
0.171
0.075
0.096
0.118
8
+
k3
+
C2 + H+
→
OH + other products (3)
Slow
8
6
+
OH + RNHCl
fast
am-NO2 and p-NO2 excluded in the correlation.
bCorrelation for para-substituents only.
→ Cl OH + RNH2
(4)

32
MEENAKSHISUNDARAM AND SELVARAJU
Figure 4 The Hammett plot in the EDTA catalyzed reaction of phenoxypropanoic acids with CAB (numbered as in Table IV).
This equation predicts a linear relationship between
1/kobs and 1/[S] at constant [H+] and [EDTA] as per
Eq. (7).
region of the substrate. The enhanced catalytic activity
is explained by envisaging a ternary complex (C2). This
complex containing EDTA, oxidant, and the substrate
offers the reaction a more favorable pathway. EDTA
forms an intermediate complex with CAB, which act
as a more powerful electrophile than the uncomplexed
CAB. This is supported by the observation that a plot
of kobs vs. EDTA is a straight line (Fig. 5) (r = 0.995;
s = 0.504) with a positive slope and intercept. The
1
1
1
1
=
+ 1 +
kobs k3 K2[S][H+] K1[EDTA]
k3[H+]
(7)
1
1
Figure 2 shows that there is a linearity at high [ -PPA].
It appears that the linearity is limited to a concentration
graph of kobs vs. [ -PPA] (Fig. 2) shows a limit-
ing value which further supports the formation of a
Figure 5 Plot of kobs vs. [EDTA] in the EDTA catalyzed reaction of -PPA with CAB.

REACTION OF ␣-PHENOXYPROPANOIC ACIDS
33
Figure 6 The Hammett plot on the EDTA catalyzed reaction of phenoxypropanoic acids with CAB (numbered as in Table IV).
ternary intermediate. Enhanced reactivity in the EDTA
catalyzedhydrazine-Cr(VI)reaction[19]andisopropyl
alcohol oxidation by Cr(VI) in the presence of oxalic
acid [20] are explained by envisaging the formation of
a intermolecular complex. Highly negative activation
entropy suggests that there is a highly ordered transi-
tion state.
Enhanced reactivity in the presence of Cl can be
explainedbytheformationofmolecularchlorine, anef-
fective electrophile under the experimental conditions
maintained. kobs is a composite quantity. Attempts have
been made to evaluate the rate coefficients of the slow
step (k3) separately using the intercept of the double
reciprocal plot of 1/kobs vs. 1/[S] at constant [H+] and
[EDTA] (Eq. (7)). Deviation of NO2 substituents in
the established Hammett line using k3 values deter-
mined at high substrate concentrations, is again quite
clear in Fig. 6 (r = 0.530; s = 0.441). The treatment
by the Yukawa–Tsuno equation [21], which can give
an idea regarding the extent of involvement of cross
conjucation in the transition state also fails to explain
the reactivity (100 R 73, where R is the correlation
coefficient).
2. Gurumurthy, R.; Gopalakrishnan, M. Ind J Chem
Sect A 1986, 25, 476.
3. Gurumurthy, R.; Gopalakrishnan, M.; Prabhakaran, A.
Asian J Chem 1989, 1, 7.
4. Gurumurthy,
R.;
Ananthabaskaran,
T.;
J
Sathiyanarayanan, K.; Karunakaran, K. Asian
Chem 1994, 6, 449.
5. Gurumurthy, R.; Karunakaran, K.; Sathiyanarayanan, K.
Oxid Commun 1993, 16(3-4), 351.
6. Trillas, M.; Peral, J.; Domench, X. Appl Catal, B: Envi-
ron 1993, 3(1), 45.
7. Nagarajan, S.; Gopalakrishnan, M. Oxid Commun 1995,
18(2), 162.
8. Karunakaran, K.; Elango, K. P. Oxid Commun 1994,
17(3-4), 274.
9. Elango, K. P.; Karunakaran, K. Oxid Commun 1996,
19(1), 44.
10. Kabilan, S.; Sankar, P.; Krishnasamy, K. Oxid Commun
1994, 17(1-2), 115.
11. Kabilan, S.; Uma, M.; Krishnasamy K.; Sankar, P. J Ind
Council Chem 1994, X(1), 21.
12. Chellamani, A.; Rajeswari, V. Afinidad 1999, 56(483),
322.
13. Karunakaran, K. Pol J Chem 1998, 72, 916.
14. Weissberger, A.; Prabankar, E. S. Organic Solvents
Physical Properties and Methods of Purification, 2nd ed.;
Interscience Publishers: London, 1955; p. 390.
15. Meenakshisundaram, S. P.; Sockalingam, R. M. J Mol
Catal A: Chemical 2000, 160, 269.
16. Norman; Hayes, V.; Gerald; Branch, E. K. J Am Chem
Soc 1943, 65, 1555.
17. Thimme Gowda B.; Mahadevappa, D. S. J Chem Soc
Perkin Trans 1983, 2, 323.
Except COOH substituent, in all the other cases
the ratio of rate constants of ortho- and para-substituted
-PPA is >1 and <2. It suggests that in CAB oxida-
tion of these ortho-derivatives the steric inhibition is
slightly overcompensated by an effective anchimeric
assistance.
18. Mohan, N.; Ananda, S.; Mahadevappa, D. S. Ind J Chem
Sect A 1986, 25, 666.
BIBLIOGRAPHY
19. Beck, M. T.; Durham, D. A. J Inorg Nucl Chem 1971,
33, 461.
20. Hasan, F.; Rocek, J. J Am Chem Soc 1972, 94, 3181.
21. Yukawa, Y.; Tsuno, Y. Bull Chem Soc Jpn 1959, 32, 971.
1. Perron, Y. G.; Minor, W. F.; Holdrege, C. T.; Gottstein,
W. J.; Godfrey, J. C.; Crast, L. B.; Babel, R. B.; Cheney,
L. C. J Am Chem Soc 1960, 82, 3934.