Kinetic study on acid-base catalyzed hydrolysis of azimsulfuron, a sulfonylurea herbicide

Article abstract of DOI:10.1002/(SICI)1097-4601(1999)31:4<253::AID-KIN2>3.0.CO;2-X

Pseudo-first-order rate constants (k_obs) for hydrolysis of a sulfonylurea herbicide, azimsulfuron, AZIM, {N-[[(4,6-dimethoxy-2-pyrimidinyl)amino]carbony]-1-methyl-4- (2-methyl-2H-tetrazol-5-yl)-1H-pyrazole-5-sulfonamide} (AZS) follow an empirical relationship: k_obs = α₁+α₂[^-OH]+α₃ [^-OH]² within the [NaOH] range of 0.1-2.0 M at different temperatures ranging from 40 to 55 °C. The contribution of α₃[^-OH]² term is small compared with α₂[^-OH] term and this turns out to be zero at 60 °C. Pseudo-first-order rate constants (k_obs) for hydrolysis of AZS within the [H⁺] range from 2.5×10^-6 to 1.4 M follow the relationship: k_obs = (α₁K_a+β₁[H*]+β₂ [H⁺]²)/([H⁺]+K_a) where pK_a = 4.37 at 50 °C. The value of β₁ is nearly 25 times larger than that of α₁. The rate of alkaline hydrolysis of AZIM is weakly sensitive to ionic strength.

Full text of DOI:10.1002/(SICI)1097-4601(1999)31:4<253::AID-KIN2>3.0.CO;2-X

Kinetic Study on Acid-Base
Catalyzed Hydrolysis of
Azimsulfuron, a
Sulfonylurea Herbicide
M. NIYAZ KHAN,1 BAKI B. BAKAR,2 FENNY W. N. YIN2
1Department of Chemistry, Universiti Malaya, 50603 Kuala Lumpur, Malaysia
2Institute of Biological Sciences, Universiti Malaya, 50603 Kuala Lumpur, Malaysia
Received 19 February 1998; accepted 5 November 1998
ABSTRACT: Pseudo-first-order rate constants (kobs) for hydrolysis of a sulfonylurea herbicide,
azimsulfuron, AZIM௡, {N-[[(4,6-dimethoxy-2-pyrimidinyl)amino]carbony]-1-methyl-4-(2-methyl-
2H-tetrazol-5-yl)-1H-pyrazole-5-sulfonamide} (AZS) follow an empirical relationship: kobs
ϭ
Ϫ
Ϫ
␣1 ϩ ␣2[ OH] ϩ ␣3[ OH]2 within the [NaOH] range of 0.1–2.0 M at different temperatures
Ϫ
Ϫ
ranging from 40 to 55ЊC. The contribution of ␣3[ OH]2 term is small compared with ␣2[ OH]
term and this turns out to be zero at 60ЊC. Pseudo-first-order rate constants (kobs) for hydrolysis
of AZS within the [Hϩ] range from 2.5 ϫ 10Ϫ6 to 1.4 M follow the relationship: kobs ϭ (␣1Ka ϩ
␤1[Hϩ] ϩ ␤2[Hϩ]2)/([Hϩ] ϩ Ka) where pKa ϭ 4.37 at 50ЊC. The value of ␤1 is nearly 25 times
larger than that of ␣1. The rate of alkaline hydrolysis of AZIM is weakly sensitive to ionic
strength. ᭧ 1999 John Wiley & Sons, Inc., Int J Chem Kinet 31: 253–260, 1999
INTRODUCTION
Azimsulfuron (AZS) {N-[[(4,6-dimethoxy-2-pyrimi-
midinyl)amino]carbonyl]-1-methyl-4-(2-methyl-2H-
wet rice paddies. Sulfonylurea herbicides have a com-
mon functional group (9SO2NHCONH9) in which
the carbonyl carbon is the most likely electrophilic site
for nucleophilic attack by water and other nucleophiles
in the aqueous degradation of these herbicides.
tetra-zol-5-yl)-1H-pyrazole-5-sulfonamide},
origi-
nally known as DPX-A8947, AZS is one of the most
effective sulfonylurea herbicides for weed control in
Correspondence to: M. Niyaz Khan (niyaz@kimia.um.edu.my)
Contract grant sponsor: National Scientific Research and De-
velopment Council of Malaysia (IRPA)
Contract grant number: 01-02-03-0040
᭧ 1999 John Wiley & Sons, Inc.
CCC 0538-8066/99/040253-08

254
KHAN, BAKAR, AND YIN
The acid-base catalyzed hydrolysis of urea has been
the subject of several reports [1–4] partly because of
its relevance as a model system for urease-catalyzed
hydrolysis reactions. The sulfonylurea herbicides are
subject to both chemical hydrolysis and microbial deg-
radation in soils [5,6]. Although AZS is N,NЈ-disub-
stituted urea, its aqueous stability is expected to be low
compared with urea due to chemical modification on
the leaving ability of the leaving group. There is a
paucity of kinetic information on the acid-base cata-
lyzed hydrolysis of AZS. Such an in-vitro study on
AZS may be useful in the rationalization of de-
gradation data obtained in the presence of soils of dif-
ferent pH and physico-chemical characteristics. The
aim of the present study was to investigate the mech-
anistic details of hydrolytic cleavage of AZS under
acidic and alkaline pH. The experimental results and
their probable explanations are described in this
article.
pared frequently in acetonitrile. The values of water
solubility of azimsulfuron at 25ЊC are 72 mg/L at pH
5 and 1050 mg/L at pH 7 [7].
Kinetic Measurements
Apparently, the base-catalyzed hydrolysis products of
AZS absorbed much more strongly than AZS at 285
nm in alkaline medium. However, in acidic medium,
AZS showed a stronger absorption than the hydrolysis
products at 240 nm. The reaction rates of aqueous
cleavage of AZS under alkaline and acidic pH were
studied spectrophotometrically by monitoring the ap-
pearance of products at 285 nm and disappearance of
reactant (AZS) at 240 nm, respectively. The ionic
strength of the reaction medium was kept constant at
2.0 M with sodium chloride. The acetate buffer solu-
tions of desired pH were prepared just before the start
of the kinetic runs.
In a typical kinetic run, all the reaction ingredients
(except AZS) required for an appropriate experimental
condition were put in a 25 mL reaction vessel, which
was then placed in a thermostatted water bath. A total
volume of 9.9 mL of reaction solution was allowed to
equilibrate for about 15 min at the desired temperature.
The reaction was then initiated by adding 0.1 mL (us-
ing a 100 ␮l Hamilton syringe) of 0.015 M AZS. An
aliquot of nearly 2.5 mL was quickly withdrawn from
the reaction mixture and transferred to a 3 mL quartz
cuvette. The cuvette was then quickly placed into the
thermostatted cell compartment of a Shimadzu model
UV-2101/3101PC UV-VIS-NIR scanning spectropho-
EXPERIMENTAL
Materials
Reagent and commercial grades of AZS {N-[[(4,6-di-
methoxy-2-pyrimidinyl)amino]carbonyl]-1-methyl-4-
(2-methyl-2H-tetrazol-5-yl)-1H-pyrazole-5-sulfon-
amide} were obtained as a generous gift from Du Pont
Far East Inc. 2-Amino-4,6-dimethoxypyrimidine was
obtained from Maybridge Chem. Co. Ltd., UK. All
other chemicals used were commercial products of re-
agent grade. Glass-distilled water was used throughout
the kinetic study. Stock solutions of AZS were pre-
tometer. The absorbance of the reaction mixture (Aobs
)

ACID-BASE CATALYZED HYDROLYSIS OF AZIMSULFURON
255
was recorded at a fixed wavelength of either 285 nm
or 240 nm and the reaction mixture (i.e., aliquot) was
transferred back quickly into the thermostatted bulk
reaction mixture. There was always a maximum lapse
of nearly 30 s on the average between the time of
removal of the aliquots and the time when their ab-
sorbance was recorded. Reactions were generally fol-
lowed up to Ͼ80% completion with at least 13 data
points as well as a reading after 24–240 h. The pH
values of the reaction mixtures containing acetate buf-
fers were determined with WITEG W500 Digital pH/
mV/Temp meter.
Entire kinetic runs were carried out under pseudo-
first-order kinetic conditions. Pseudo-first-order rate
constants (kobs) were calculated from eq. (1) and eq.
(2) for the reactions where disappearance of reactant
and appearance of product(s), respectively, were mon-
itored as a function of reaction time (t). In eqs. (1) and
(2), Aobs, Aϱ, and A0 represent absorbance at any time,
t, at t ϭ ϱ, and t ϭ 0, respectively, Eapp is the apparent
molar absorption coefficient, and [X]0 is the initial con-
centration of AZS.
A
obs ϭ Eapp[X]0exp(Ϫkobst) ϩ Aϱ
(1)
(2)
Aobs ϭ Eapp[X]0[1 Ϫ exp(Ϫkobst)] ϩ A0
The nonlinear least-squares technique was used to
calculate three unknown parameters, kobs, Eapp, and Aϱ
or A0 from eqs. (1) or (2). The observed data were
found to fit reasonably well with eq. (1) or eq. (2) for
almost all the kinetic runs. The percentage deviations
between the observed and calculated absorbance val-
ues were less than 2% for up to 6–8 halflives for most
of the kinetic runs.
It is difficult to ascertain whether 9SO2NH2 or
9NH2 is better leaving group because the pKa of
C6H5SO2NH2, C6H5NH2, and 3 at 50ЊC are 10.11 [9],
27 [10], and 8.4 [11], respectively.
Product Characterization
The most probable electrophilic sites for oxy nucleo-
philic attack are the carbon of 9CO9 or sulfur of
9SO2NH9 groups in AZS. But the electrophilicity
of carbonyl carbon is much larger than the sulfur of
sulfonamide for nucleophilic attack by nucleophiles
containing oxygen and nitrogen as nucleophilic sites
[8]. Thus, in the hydrolysis of AZS, the attack by ϪOH
or H2O should occur at the carbonyl carbon of the
herbicide. The expected immediate stable products
should be 1 and 2, whether 9SO2NH2 or 9NH2
moiety leaves as a result of nucleophilic attack at car-
bonyl carbon.
Recently, it has been reported that the main
products of hydrolysis of sulfonylurea herbicides,
ArSO2NHCONHHet, are ArSO2NH2, H2NHet, and
CO2
and
anionic
sulfonylurea
herbicides,
ArSO2N-CONHHetMϩ, are inert toward hydrolysis

256
KHAN, BAKAR, AND YIN
[6,12]. However, rimsulfuron rapidly hydrolyzed in
both acidic and alkaline conditions through contrac-
tion of the sulfonylurea bridge [5].
check its ultraviolet spectra under the reaction condi-
tions of the kinetic runs. However, the value of
4
1
kobs (ϭ 1.44 ϫ 10Ϫ minϪ at 50ЊC ) for the reaction
of monoanionic AZS with H2O obtained by monitor-
ing the appearance of products at 285 nm is not sig-
The values of pseudo-first-order rate constants for
hydrolysis of a few sulfonylurea herbicides are shown
in Table I. It is evident from Table I that thifensulfuron
methyl and thifensulfuron hydrolyzed 4–5 times and
10–300 times slower than rimsulfuron at pH 5 and 9,
respectively. The initial steps in the acidic and alkaline
hydrolyses of thifensulfuron methyl and thifensulfu-
ron involved the cleavage of the sulfonylurea bridge,
whereas rimsulfuron hydrolyzed through contraction
of the sulfonyl bridge. The rate of hydrolysis of rim-
sulfuron increased 3–6 times with each 10ЊC increase
in temperature [5]. Thus, the allowance of temperature
difference made kobs larger for rimsulfuron than that
for AZS by more than 6-fold and 190-times at pH 5
and 9, respectively. But the values of kobs for AZS
differ from corresponding kobs for thifensulfuron
methyl by the factors of 2–3 and 3–4 at pH 5 and 9,
respectively (Table I). These results suggest that the
acidic and alkaline hydrolysis of AZS involve the
cleavage of the sulfonylurea bridge.
4
nificantly different from the value of kobs (1.90 ϫ 10Ϫ
minϪ1 at 50ЊC) obtained by monitoring the disappear-
ance of AZS at 240 nm and pH 5.6. These results
suggest that the rate of hydrolysis of AZS monitored
at both 285 and 240 nm under mild pH corresponds
to the same slow reaction step. Furthermore, if 1 and
2 were not the immediate products in the hydrolytic
cleavage of AZS, then it was highly unlikely for re-
action kinetics to obey a simple first-order rate law
under both acidic and alkaline conditions.
RESULTS AND DISCUSSION
Effect of [NaOH] on Rate of Hydrolysis of
AZS
A series of kinetic runs was carried out within the
[NaOH] range of 0.1–2.0 M at 40ЊC and 2.0 M ionic
strength. Pseudo-first-order rate constants kobs, as
shown in Figure 1, revealed an increase with the in-
crease in [NaOH]. Similar observations were obtained
at different temperatures ranging from 40 to 60ЊC. The
observed data were treated with the following empir-
ical equation.
In a typical kinetic run, the products of the reaction
4
mixture containing 1.5 ϫ 10Ϫ M AZS and 0.6 M
NaOH were analyzed using a mass spectrometric tech-
nique. The mass spectra of the reaction products ob-
tained at different time intervals (2 min; 3, 12, and 25
days) showed a strong peak at m/z 156 which may be
due to 2. Although the mass spectrum of the reaction
mixture at reaction time ϳ2 min did not show a peak
at m/z equivalent to the molecular ion peak for 1, the
presence of a peak for 2 suggested that the immediate
products of the aqueous cleavage of AZS were 1 and
2. The uv spectra of an authentic sample of 2 showed
k
obs ϭ ␣1 ϩ ␣2[ϪOH] ϩ ␣3[ϪOH]2
(3)
The empirical parameters, ␣1, ␣2, and ␣3, were cal-
culated using the least-squares technique, and these
results are summarized in Table II. The fitting of the
observed data to eq. (3) is evident from the plots of
Figure 1 where solid lines are drawn through the least-
squares calculated points. Most of the ␣1 values are
1
a weak absorption at 285 nm (␧ ϭ 270 Ϯ 60 MϪ1 cmϪ
where ␧ represents molar extinction coefficient) within
the [NaOH] range of 0.1–2.0 M at 2 M ionic strength.
An authentic sample of 1 could not be obtained to
Table I Values of Pseudo-First-Order Rate Constants (kobs) for Hydrolysis of Sulfonylurea Herbicides at Different pH
Ϫ1
105kobs/min
Herbicide
Temp/ЊC
pH ϭ
4
5
9
Thifensulfuron methyla
Derivative of thifensulfuron methyla
Thifensulfurona
28
28
28
28
25
50
23.3
14.0
11.6
25.8
–
2.17
3.67
3.17
5.83
10.2
35.0
Ͻ11.8
–
0.35
Ͻ0.53
120
Derivative of thifensulfurona
Rimsulfuronb
Azimsulfuronc
353
ϳ14.4
a Ref. 6
b Ref. 5
c This study

ACID-BASE CATALYZED HYDROLYSIS OF AZIMSULFURON
257
be neglected in eq. (3), and the linear least-squares
calculated values of ␣1 and ␣2 are (1.44 Ϯ 0.02) ϫ
4
1
3
1
1
10Ϫ minϪ and (7.32 Ϯ 0.08 ) ϫ 10Ϫ MϪ minϪ ,
respectively.
It may be worth mentioning that the contribution
of the ␣3[ϪOH]2 term compared with other terms of
eq. (3) is Ͻ40% under the experimental conditions of
entire kinetic runs. The standard deviations associated
with ␣3 values are rather high and because of the low
contribution of the ␣3[ϪOH]2 term, the values of ␣3
(Table II) may be considered to be less reliable.
It is interesting to note that the value of
1
␣2(ϭ 3.55 ϫ 10Ϫ4 MϪ1 sϪ ) is slightly more than 800-
fold larger than the corresponding value of 4.28 ϫ
7
1
1
10Ϫ MϪ sϪ obtained in the alkaline hydrolysis of
urea at 60ЊC [4]. However, the pseudo-first-order rate
4
1
constant (kobs ϭ 3.77 ϫ 10Ϫ sϪ ) for hydrolysis of
AZS at 1.0 M NaOH and 60ЊC is nearly 370-fold
6
1
larger than kobs (ϭ 1.02 ϫ 10Ϫ sϪ ) obtained for hy-
drolysis of urea under similar experimental conditions
[4]. Significantly larger rate of alkaline hydrolysis of
AZS than that of urea may be attributed to increased
leaving ability of the leaving group in AZS compared
to that in urea.
Ϫ
Figure 1 Effects of [ OH] on pseudo-first-order rate con-
stants (kobs) for the cleavage of AZS at 40 (᭺), 45 (᭝), 50
(□), 55 (᭞), and 60ЊC (᭛).
Effect of Acidic pH on Rate of Hydrolysis of
AZS
The rate of hydrolysis of AZS was also studied under
the [HCl] range of 0.001–1.4 M at a constant ionic
strength of 2.0 M (by NaCl) and under the pH range
3.6–5.6 at 50ЊC. The observed pseudo-first-order rate
constants, kobs, as summarized in Table III, were found
to fit to the following empirical equation:
associated with high standard deviations which make
them rather unreliable. The maximum contribution of
␣1 is Ͻ50% compared to other terms of eq. (3) under
the experimental conditions imposed. A relatively
more reliable value of ␣1 was determined from kobs
obtained within the [NaOH] range of 0.001–0.050 M
at 50ЊC. Under such conditions, ␣3 [ϪOH]2 term may
k
obs ϭ (␣1Ka ϩ ␤1[Hϩ]
ϩ ␤2[Hϩ]2)/([Hϩ] ϩ Ka)
(4)
Table II Values of Empirical Parameters, ␣1, ␣2, and ␣3, Calculated from Eq. (3)a
Temp
ЊC
104 ␣1
min
103 ␣2
104 ␣3
[NaOH] range
M
Ϫ
1
Ϫ
Ϫ1
Ϫ
Ϫ1
M
1 min
M
2 min
40
45
50
2.00 Ϯ 2.51b
4.12 Ϯ 1.18
7.11 Ϯ 2.65
2.52 Ϯ 1.57
5.00 Ϯ 3.01
14.3 Ϯ 7.5
11.0 Ϯ 11.4
13.8 Ϯ 7.4
2.37 Ϯ 0.56b
3.86 Ϯ 0.42
5.81 Ϯ 0.70
7.86 Ϯ 0.46
7.38 Ϯ 0.70
11.2 Ϯ 1.7
22.1 Ϯ 2.5
21.3 Ϯ 0.6
7.28 Ϯ 2.53b
12.1 Ϯ 2.0
24.2 Ϯ 2.9
11.5 Ϯ 2.4
13.4 Ϯ 3.3
14.8 Ϯ 8.2
Ϫ0.4 Ϯ 1.1
0
0.1 Ϫ 2.0
0.1 Ϫ 2.0
0.1 Ϫ 2.0
0.001 Ϫ 2.0c
0.1 Ϫ 2.0c
0.1 Ϫ 2.0
0.1 Ϫ 2.0
0.1 Ϫ 2.0
55
60
a Conditions: 1.5 ϫ 10Ϫ4 M AZS, ␭ ϭ 285 nm, 2.0 M ionic strength, 1% (v/v) acetonitrile in the aqueous solvent, unless otherwise stated,
reagent grade AZS was used.
b Error limits are standard deviations.
c In these kinetic runs, the commercial grade AZS was used.

258
KHAN, BAKAR, AND YIN
Table III Pseudo-First-Order Rate Constants (kobs) for
2.0 M at a constant temperature, no regular trend was
observed in the variation of ⌬H* and ⌬S* values with
the change in [NaOH] (Table IV).
Hydrolysis of AZS Under Acidic pHa
Ϫ1
Ϫ1
[HCl] (pH)/M
104 kobs/min
104 kcalcdb/min
(5.6)
(5.0)
(4.6)
(4.0)
(3.6)
0.001
0.005
0.01
0.05
0.1
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.90 Ϯ 0.40c
3.50 Ϯ 0.40
12.3 Ϯ 0.9
35.3 Ϯ 4.0
30.5 Ϯ 2.0
34.8 Ϯ 4.3
36.4 Ϯ 2.6
43.6 Ϯ 2.3
41.4 Ϯ 2.4
43.0 Ϯ 1.0
42.9 Ϯ 0.9
45.8 Ϯ 1.0
46.0 Ϯ 1.1
52.8 Ϯ 1.2
55.7 Ϯ 1.4
58.7 Ϯ 1.8
57.3 Ϯ 1.9
3.57
8.74
15.6
28.4
34.3
38.3
39.6
39.9
40.6
41.3
42.7
45.6
48.4
51.3
54.2
57.0
59.9
Effect of Ionic Strength on Alkaline
Hydrolysis of AZS
The effect of ionic strength on rate of alkaline hy-
drolysis of AZS was studied by carrying out the kinetic
runs within the [NaCl] range of 0.0–3.0 M at 0.6 M
NaOH and 50ЊC. Pseudo-first-order rate constants,
kobs, showed an increase of nearly 22% with the in-
crease in ionic strength from 0.6–3.6 M (Table V).
An effect of ionic strength was not observed in the
alkaline hydrolysis of urea [4]. Although the ionic
strength effect appears to be small, it is certainly real.
The pKa of the most acidic hydrogen in AZS should
be ca. 4 [13]. Thus, at 0.6 M NaOH, the reactants in
the hydrolysis of herbicide should be ionized AZS
(SϪ) and ϪOH; hence the rate of hydrolysis should be
sensitive to ionic strength. The alkaline hydrolysis of
saccharin (at 1.0 M NaOH) containing a similar re-
active functional group as AZS, was found to be more
sensitive to ionic strength [14]. But the value of ␣3/␣2
a Conditions: 1.5 ϫ 10Ϫ4 M AZS, ␭ ϭ 240 nm, unless other-
wise noted ionic strength is 2.0 M, 1% (v/v) acetonitrile in the aque-
ous solvent, 50ЊC.
b Calculated from eq. (4) using the values of ␤1, ␤2, and Ka as
mentioned in the text.
c Parenthesized values are pH (maintained by the use of acetate
buffer, 0.2 M total buffer concentration, where ionic strength was
not adjusted by using any inert salt).
1
(ϭ 1.8 MϪ at 60ЊC) for saccharin was significantly
larger (Ͼ6 times) than that for AZS. It should be noted
that the negative charge in SϪ is not localized as in
ϪOH and that is why the rate of reaction between SϪ
and ϪOH showed a weak ionic strength effect.
where ␣1 is calculated from eq. (3), ␤1 and ␤2 are
empirical parameters, and Ka is the ionization constant
of acidic group in AZS. The nonlinear least-squares
calculated values of ␤1, ␤2, and Ka are (3.99 Ϯ 0.13)
Mechanistic Speculations
The rate constant [ϪOH] profiles (Figure 1) indicate
that there is a change in the rate-determining steps with
the change in [ϪOH]. A similar conclusion was first
advanced by Biechler and Taft in the interpretation of
1
1
ϫ 10Ϫ3 minϪ , (1.43 Ϯ 0.19) ϫ 10Ϫ3 MϪ1 minϪ , and
5
(4.29 Ϯ 1.12) ϫ 10Ϫ M, respectively, with ␣1 ϭ
1
1.44 ϫ 10Ϫ4 minϪ . The fitting of the observed data to
eq. (4) appeared to be good, as is evident from the
calculated values of rate constants shown in Table III.
Table IV Values of Activation Parameters at Different
[NaOH] Calculated from Eq. (5) Using kobs
Effect of Temperature on Rate of Alkaline
Hydrolysis of AZS
Ϫ1
Ϫ1
Ϫ1
[NaOH]/M
⌬H*/kcal mol
⌬S*/cal K mol
Pseudo-first-order rate constants, kobs, obtained at a
constant [NaOH] and within the temperature range of
40–60ЊC, were found to obey the Eyring equation {eq.
(5)}, where all the symbols have their usual meanings.
The nonlinear least-squares technique was used to cal-
culate ⌬H* and ⌬S* from eq. (5)
0.1
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
22.61 Ϯ 1.26a
19.45 Ϯ 1.27
19.20 Ϯ 1.78
18.79 Ϯ 0.70
21.25 Ϯ 1.61
19.32 Ϯ 0.72
17.33 Ϯ 0.60
19.58 Ϯ 0.81
17.54 Ϯ 0.98
16.86 Ϯ 0.82
16.37 Ϯ 0.42
9.8 Ϯ 3.8a
18.8 Ϯ 3.8
18.4 Ϯ 5.4
19.0 Ϯ 2.1
10.9 Ϯ 4.9
16.4 Ϯ 2.2
22.0 Ϯ 1.8
14.8 Ϯ 2.4
20.8 Ϯ 3.0
22.7 Ϯ 2.5
23.9 Ϯ 1.3
k
obs ϭ (kT/h) exp(⌬S*/R) exp(Ϫ⌬H*/RT) (5)
and these calculated values of ⌬H* and ⌬S* are sum-
marized in Table IV. Although kobs increased by nearly
10- to 15-fold with the increase in [NaOH] from 0.1–
a Error limits are standard deviations.

ACID-BASE CATALYZED HYDROLYSIS OF AZIMSULFURON
259
Table V Effect of Ionic Strength on Alkaline
are 35 [22], 27 [10], 15.4 [19], and 9.5 [23], respec-
tively. In view of these results it seems that the pKa of
9CONH9 group of AZIM should be Ͻ 4.
The measured pKa value of 4Hϩ is 4.37 at 39ЊC
[24]; hence the kinetically determined pKa (ϭ 4.37)
may be assigned for pyrimidinium ion of AZS.
Hydrolysis of AZSa
Ϫ1
Ionic Strength/M
104 kobs/min
0.6
0.6
1.1
1.6
2.6
3.1
3.6
47.6 Ϯ 1.4b
44.7 Ϯ 0.4
48.2 Ϯ 0.6
50.6 Ϯ 0.9
52.6 Ϯ 0.6
52.7 Ϯ 0.7
56.3 Ϯ 1.5
a Conditions: 1.5 ϫ 10Ϫ4 M AZS, ␭ ϭ 285 nm, 1% (v/v) ace-
tonitrile in the aqueous solvent, 0.6 M NaOH, 50ЊC, ionic strength
was maintained by the use of NaCl.
b Error limits are standard deviations.
kinetic data on the alkaline hydrolysis of trifluoroac-
etanilide and N-methylanilides [15]. These authors
proposed, for the first time, the existence of highly
unstable oxydianionic tetrahedral (ODT) intermedi-
ates in the hydrolysis of anilides under highly alkaline
medium. The existence of ODT intermediates have
been proposed in many related studies [16]. In an ear-
lier study on alkaline hydrolysis of formamide it was
concluded that the existence of an ODT intermediate
is not an implicit function of either alkalinity of the
reaction medium or the structural features of the sub-
strate but rather is a composite function of both effects.
The existence of an ODT intermediate was also pro-
posed in the alkaline hydrolysis of urea [4]. The ex-
istence and non-existence of trianionic tetrahedral in-
termediates in the alkaline hydrolysis of saccharin [14]
and phthalimide [17], respectively, were attributed to
the stronger electron withdrawing ability of ͘SO2
group compared to the ͘CO group [18]. The stronger
electron withdrawing nature of the ͘SO2 group com-
pared to that of the ͘CO group could also be seen in
the difference between the pKa of saccharin (1.31) [8a]
and that of phthalimide (8.3) [19], (9.5) [20].
The previous discussion reveals that the pKa value
of 4.37 cannot be assigned with certainty to either
9SO2NHCO9, 9CONH9, or pyrimidinium ion
of AZS. However, nearly 25-fold larger value of ␤1
than that of ␣1 creates doubt about pyrimidinium ion
being an acidic group of pKa 4.37, because the acidic
site is quite far from the expected reaction site.
The authors thank the National Scientific Research and De-
velopment Council of Malaysia under the IRPA program for
financial support (Grant No. 01-02-03-0040) and Mr. Sa-
budin Ali from Du Pont Far East Inc. for the samples of
Azimsulfuron.
The approximate value of pKa of 9SO2NH9
group may be estimated as follows. The pKa of sac-
charin is 1.3 [8a] and pKa values of phthalimide are
8.3 [19] and 9.5 [20a]. Thus, the replacement of CO
by SO2 causes the reduction in pKa by nearly 7–8 pK
units. The reported value of pKa of diacetylamine
(CH3CONHCOCH3) is 12.9 [21]; hence, the expected
pKa value for CH3SO2NHCOCH3 is 5–6. The kineti-
cally determined value of pKa (ϭ 4.37) therefore ap-
pears to be for the 9SO2NHCO9 group of AZS.
The values of pKa of protonated triazine (3Hϩ) and
the side chain amino group of 3 are 1.5 and 8.4, re-
spectively [11]. Thus, the pKa of the primary amino
group of 2 should be nearly 8–9. The values of pKa
of NH3, C6H5NH2, CH3CONH2, and CF3CONHC6H5
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