Kinetics of chemical degradation of isoxaflutole: Influence of the nature of aqueous buffers (alkanoic acid/sodium salt vs phosphate)

Article abstract of DOI:10.1002/ps.300

A kinetic study of the chemical degradation of isoxaflutole (5-cyclopropyl-1,2-oxazol-4-ylααα-trifluoro-2-mesyl-p-tolyl ketone) into its diketonitrile derivative (DKN), which is its active herbicide principle, in organic buffers at different pH values was carried out using a HPLC/UV detection method. The values of the pseudo-first-order rate constants K_obs for the reaction were calculated and compared with those previously obtained in inorganic buffers. In both cases, K_obs was found to be dependent on pH and temperature, but at pH 5.2 the degradation of isoxaflutole in CH₃COOH/CH₃COONa buffers was considerably faster than in KH₂PO₄/Na₂HPO₄ buffers, indicating that the compound was sensitive to the nature of the reagents used to prepare buffered solutions. The influence of phosphate and acetate concentrations and the influence of the R-substituent in RCOOH/RCOONa buffers were investigated. For the HA/A^- buffers studied, the values of K_obs were linearly dependent on HA and A^- concentrations, which meant that the degradation of isoxaflutole was subject to general catalysis. The values of K_obs were also found to be dependent on the number and the position of the CH₃ groups of the R-substituent. The known degradation product of DKN (a benzoic acid derivative) was not detected throughout this study.

Full text of DOI:10.1002/ps.300

Pest Management Science
Pest Manag Sci 57:366±371 ꢀ2001)
Kinetics of chemical degradation of isoxaflutole:
influence of the nature of aqueous buffers
(alkanoic acid/sodium salt vs phosphate)†
1
1
1
2
´ `
Estelle Beltran, Helene Fenet, Jean-Franc¸ois Cooper * and Camille-Michel Coste
1
´
Laboratoire de Chimie Analytique, Faculte de Pharmacie, 15 av Ch Flahault, F-34060 Montpellier Cedex 2, France
2Centre de Phytopharmacie, UMR CNRS 5054, 52 av de Villeneuve, F-66860 Perpignan, France
Abstract: A kinetic study of the chemical degradation of isoxa¯utole ꢀ5-cyclopropyl-1,2-oxazol-4-yl
aaa-tri¯uoro-2-mesyl-p-tolyl ketone) into its diketonitrile derivative ꢀDKN), which is its active
herbicide principle, in organic buffers at different pH values was carried out using a HPLC/UV
detection method. The values of the pseudo-®rst-order rate constants Kobs for the reaction were
calculated and compared with those previously obtained in inorganic buffers. In both cases, Kobs was
found to be dependent on pH and temperature, but at pH 5.2 the degradation of isoxa¯utole in
CH3COOH/CH3COONa buffers was considerably faster than in KH2PO4/Na2HPO4 buffers, indicating
that the compound was sensitive to the nature of the reagents used to prepare buffered solutions. The
in¯uence of phosphate and acetate concentrations and the in¯uence of the R-substituent in RCOOH/
RCOONa buffers were investigated. For the HA/AÀ buffers studied, the values of Kobs were linearly
dependent on HA and AÀ concentrations, which meant that the degradation of isoxa¯utole was subject
to general catalysis. The values of Kobs were also found to be dependent on the number and the position
of the CH3 groups of the R-substituent. The known degradation product of DKN ꢀa benzoic acid
derivative) was not detected throughout this study.
# 2001 Society of Chemical Industry
Keywords: isoxa¯utole; diketonitrile derivative; chemical degradation; basic catalysis; general catalysis
1
INTRODUCTION
tion of isoxa¯utole in various matrices ꢀwater, soil,
vegetation) leads to a diketonitrile derivative ꢀ3-cyclo-
propyl-1-ꢀaaa-tri¯uoro-2-mesyl-p-tolyl)-1,3-dioxo-
propane-2-nitrile; DKN, Fig 1) by an opening of the
isoxazole ring. This diketonitrile derivative is the active
herbicidal principle and acts by inhibiting a speci®c
enzyme of carotenoid synthesis ꢀ4-hydroxyphenyl-
pyruvate dioxygenase, HPPD). This inhibition leads
to bleaching in susceptible weed species, followed by
Isoxa¯utole ꢀ5-cyclopropyl-1,2-oxazol-4-yl aaa-tri-
¯uoro-2-mesyl-p-tolyl ketone; Fig 1) is a new herbi-
cide recently developed by Aventis and used in corn
and sugarcane. It provides effective control of both
grass and broad-leaf weeds after pre-emergence
application at a relatively low rate ꢀc 75ghaÀ1).
Ã
Preliminary studies ꢀReference 1 and Rhone±
Poulenc Agro, pers comm) have shown that degrada-
Figure 1. Concerted one-stage E2-type mechanism of the degradation of isoxaflutole (IFT) into the diketonitrile (DKN).
´
* Correspondence to: Jean-Franc¸ois Cooper, Laboratoire de Chimie Analytique, Faculte de Pharmacie, 15 av Ch Flahault, F-34060
Montpellier Cedex 2, France
E-mail: jcooper@balard.pharma.univ-montp1.fr
†
Based on a poster presented at the symposium ‘Degradation of Pesticides in Subsoil and Groundwater’ organised by Dr Margaret Mills on
behalf of Zeneca Agrochemicals, Monsanto and Aventis CropScience and held at the Third SETAC World Congress at Brighton, UK, 21–25
May, 2000
(Received 18 July 2000; revised version received 11 November 2000; accepted 6 December 2000)
# 2001 Society of Chemical Industry. Pest Manag Sci 1526±498X/2001/$30.00
366

Kinetics of chemical degradation of isoxa¯utole
growth suppression and necrosis. In plants, the
diketonitrile is rapidly converted into the inactive acid
derivative aaa-tri¯uoro-2-mesyl-p-toluic acid ꢀBA);
such a metabolic pathway seems to occur also in soil.
and analysed by HPLC/UV without further proces-
sing.
2.3 Analytical procedure
Ã
ꢀRhone±Poulenc Agro, pers comm)
Samples were analysed by HPLC/UV, using a
Shimadzu LC-10A pump with SPD 10AVP auto-
injector ꢀinjected volume: 20ml), the compounds were
detected by their UV absorbance at 267nm ꢀisoxa-
¯utole and BA) and 290nm ꢀDKN) using a Shimadzu
SPD-10AVP UV detector. The column used was
5-mm C18 Hypersil ODS stationary phase 250mmÂ
4.6mm, from Supelco, the mobile phase was aceto-
nitrile ‡ water ‡ tri¯uoroacetic acid ꢀ48 ‡ 52 ‡ 0.5 by
volume) delivered at 1ml minÀ1. Under these condi-
tions, the retention times of isoxa¯utole, DKN and BA
were 14.6, 13.7 and 4.9min, respectively. Identi®ca-
tion and analysis were performed by injection of
analytical standards for comparison. Data were col-
lected using Star software ꢀVarian).
Previous work2 has shown that the degradation of
isoxa¯utole in aqueous solutions followed pseudo-
®rst-order kinetics, was catalysed by hydroxide ions
and activated by temperature; log Kobs was linearly
dependent on pH, but the discontinuities observed
showed that the nature of the buffer constituents had
an effect on the rate of the degradation. In a critique of
the OECD and EPA methods recommended for the
determination of the rates of hydrolysis of chemicals in
the environment, Grayson3 pointed out the impor-
tance of experimental conditions such as temperature,
pH, buffer type and concentration, since these can
lead to variations in the values observed for rate
constants.
The objective of the present study was to extend
understanding of isoxa¯utole behaviour in aqueous
media by determining the way in which organic acids
and/or their conjugate bases catalyse its degradation in
water; various ionic strengths and concentrations of
the buffer constituents ꢀphosphate and acetate) and
several R-substituents ꢀfor buffers RCOOH/
RCOONa) were studied. Experiments were carried
out at 30°C and at a number of pH values between 4
and 7.
2.4 Quality criteria of the method
At 267nm, the limit of detection ꢀLOD) was 0.05 and
0.12mg litreÀ1 for isoxa¯utole and BA, respectively,
and that for DKN at 290nm was 0.07mg litreÀ1. The
limit of quanti®cation ꢀLOQ) was de®ned as the
sample concentration required to give a signal-to-noise
ratio of 6:1 and corresponded to twice the LOD for
each compound. Linearity of the detector response
was evaluated at the two wavelengths used in this study
by injecting solutions containing the three analytical
standards at ®ve different concentrations for each
compound varying from 1±100 times the LOQ. Each
injection was duplicated and the entire procedure
repeated the following day. The response of the
detector was found to be linear, with coef®cients of
determination ꢀr2) greater than 0.9993 for the three
compounds. We veri®ed that no compound interfered
with isoxa¯utole and its derivatives during chromato-
graphic analysis by injecting blank ꢀanalyte-free)
buffered solutions.
2
MATERIALS AND METHODS
2.1 Chemicals
Isoxa¯utole, DKN and BA of analytical standard
purity were provided by Aventis CropScience ꢀOngar,
UK). The solvents ꢀmethyl alcohol and acetonitrile for
HPLC) were supplied by Carlo Erba. Buffered
aqueous solutions were prepared with reagents of
purity ꢀ99% ꢀpotassium chloride and potassium
dihydrogen phosphate were supplied by Prolabo,
succinic acid by Fluka, isobutyric acid, trimethylacetic
acid, propionic acid and n-butyric acid by Aldrich,
sodium hydroxide by Carlo Erba, acetic acid and di-
sodium hydrogen phosphate anhydrous by Merck)
and with sterile and pyrogen-free water ꢀB Braun
Medical SA). Because of the low solubility of isoxa-
¯utole in water ꢀabout 6mg litreÀ1 at 20°C), solutions
containing 10mg litreÀ1 ꢀ2.78 10À5 M) of this com-
pound were prepared from methanolic solutions
ꢀ1000mg litreÀ1), giving a methanol content of 10ml
litreÀ1 in the ®nal aqueous solutions.
3
RESULTS AND DISCUSSION
3.1 Influence of ionic strength
There are signi®cant electrostatic interaction forces
between the ions in buffered solution. The ionic
strength m is given by the expression
ˆ 1 2Æꢁci Á z2i †
where ci is the concentration of each ion ꢀM), and zi, is
its charge. The in¯uence of ionic strength on Kobs has
been studied for phenylbutazone and cefotaxime4 and
found to be negligible at pH 2.2 and pH 6.0, but
signi®cant at higher pH values. In the present work,
the effect of m was studied in ®ve phosphate buffers
ꢀ[KH2PO4]=[Na2HPO4]=0.05M) at pH 6.7, with an
initial isoxa¯utole concentration of 10mg litreÀ1 and
at the same temperature ꢀ30°C), m being varied from
0.2 to 0.4M by adding increasing quantities of
2.2 Degradation in aqueous solutions
In order to evaluate the in¯uence of the nature of
buffer constituents on the hydrolysis of isoxa¯utole,
the above solutions were kept in the dark at a constant
temperature of 30°C under sterile conditions, and no
pH variation was observed during any of the experi-
ments. Samples were removed at appropriate times
Pest Manag Sci 57:366±371 ꢀ2001)
367

E Beltran et al
potassium chloride. No difference in Kobs values was
observed and, in these conditions, the average half-life
of isoxa¯utole was 34ꢀÆ3) h.
forms of these herbicides with respect to water
molecules.6 These discontinuities also corresponded
to changes in the buffer reagents used, indicating that
the nature of these reagents could have an important
effect on the reactivity of the sulfonylureas, as also
seemed to be the case for isoxa¯utole in the present
study.
3.2 Differences in the rate of the degradation with
respect to the buffers’ reagents
In a previous study,2 the degradation of isoxa¯utole
was studied in aqueous solutions prepared with in-
organic buffers over a wide range of pH values ꢀ1.8±
10.1) and at three temperatures ꢀ22, 30 and 50°C),
and shown to follow pseudo-®rst-order kinetics:5
To extend understanding of the mechanism of
isoxa¯utole degradation in water, additional studies
were conducted in aqueous buffered solutions pre-
pared with organic acids and their conjugate bases. As
buffers prepared with CH3COOH/CH3COONa are
widely used in kinetic experiments, the degradation of
isoxa¯utole in such solutions was studied at two pH
values and at 22, 30 and 50°C. As expected, the
degradation of isoxa¯utole followed pseudo-®rst-order
kinetics, but the values of the rate constant Kobs were
much higher than those obtained with a KH2PO4/
Na2HPO4 buffer for the same pH 5.2 ꢀTable 1).
The similar values of Ea indicated that the mechan-
ism of the reaction was a concerted one-stage E2 type
ꢀFig 1) in both the organic and inorganic buffers,
corresponding to the usual reaction undergone by
isoxazoles unsubstituted in the 3-position.7,8 The
difference observed in S≠ values showed that, in the
presence of RCOOH and RCOOÀ, the enolate ion
formed during the reaction was as constrained as
isoxa¯utole at such pH values ꢀ S≠ close to zero),
whereas in phosphate buffer the enolate ion was
apparently more constrained ꢀhighly negative value
of S≠).
C ˆ C0 exp ꢁÀKobs Á t†
ꢁ1†
where C is the concentration of isoxa¯utole ꢀM) at time
t, C0 its initial concentration ꢀM), Kobs the observed
rate constant of the reaction ꢀhÀ1) and t the incubation
time ꢀh). The effect of temperature on the rate of the
degradation of isoxa¯utole in aqueous solutions was
determined by using Arrhenius' law:
Kobs ˆ A exp ‰ÀEa ꢁRT†Š
ꢁ2†
where A is a speci®c constant of the reaction ꢀhÀ1), Ea
the activation energy in ꢀJ molÀ1), R the universal gas
constant ꢀ8.314J molÀ1
K
À1), and T the absolute
temperature ꢀK). The values of Ea were calculated
from the slope of ln Kobs against ꢀ1/T) and the
corresponding activation entropies S≠ were calcu-
lated from the y-intercept of the plot of ln ꢀKobs/T)
against ꢀ1/T).5 Thus, it was found that the degradation
of isoxa¯utole in aqueous solutions with inorganic
buffers was catalysed by the concentration of hy-
droxide ions and activated by temperature. Opening of
the isoxazole ring led to the diketonitrile derivative
DKN, and the acid derivative, BA, was not detected
during any of the experiments. The in¯uence of pH on
the observed rate constant was more dif®cult to
determine because of discontinuities in the plots of
log Kobs vs pH. Thus, for each of the three
temperatures, the slope of log Kobs against pH varied
with respect to the buffers used ꢀKH2PO4/Na2HPO4
for 5.2 ꢂ pH ꢂ 8.0 and H3BO3/NaOH for 8.3 ꢂ pH
ꢂ10.1). Such discontinuities have been observed in
the hydrolysis of sulfonylureas and attributed to the
different reactivities of dissociated and undissociated
Under the above conditions, the increase of Kobs
value with pH could be quanti®ed by plotting log Kobs
vs pH, which gave the equation
log Kobs ˆ 0 19 Á pH À 2 5
for acetate buffers. At 30°C, for similar pH values in
inorganic buffers,2 the equation was
log Kobs ˆ 2 8  10À4 Á pH ‡ 0 001
The small value of the slope in the inorganic buffers
proved the absence of catalysis due to excessively low
pH, and Kobs could therefore be considered a constant.
‡
For acetate buffers, since pH=Àlog [H3O ] and
Table 1. Comparison of Kobs, Ea and S≠ values obtained with acetate and phosphate buffers at 22, 30 and 50°C
Composition
ꢀby volume)
Temperature
Kobs
ꢀhÀ1Â103)
Ea
ꢀkJ molÀ1
S≠
Buffer
pH
ꢀ°C)
)
ꢀJ molÀ1 KÀ1
)
CH3COOH‡CH3COONa
61‡39
21‡79
98‡2
4.4
22
30
50
7
85.0
85.7
67.8
À7
À2
16.7
141.4
CH3COOH‡CH3COONa
KH2PO4 ‡Na2HPO4
5.2
5.2
22
30
50
8.3
22.4
172.1
22
30
50
1.6
2.6
À69
20.0
368
Pest Manag Sci 57:366±371 ꢀ2001)

Kinetics of chemical degradation of isoxa¯utole
‡
[OHÀ][H3O ]=kE =10À13.83mol2 litreÀ2 at 30°C,
Kobs could be expressed as a function of hydroxide
ion concentration:
Table 2. Influence of phosphate concentration on degradation of isoxaflutole
in aqueous buffers at 30°C and at pH 6.0 and 6.5
Concentration ꢀM)
a
KH2PO4
Na2HPO4
Kobs ꢀhÀ1Â103)
t1/2 a ꢀh)
if log Kobs ˆ a Á pH ‡ b then
0.0585
0.1754
0.02
0.0082b
0.0246b
0.02c
0.05c
0.1c
5.9
11.9
10.0
20 ꢀÆ1)
37
116.6
58.2
a
Kobs ˆ 10ꢁbÀaÁlog kE† ‰OHÀŠ
ꢁ3†
72.0
which gave Kobs =1.34Á[OHÀ]0.19 ꢀfor a =0.19 and
b =À2.5). In none of the buffers studied was the
reaction found to be ®rst order with respect to [OHÀ].
For any given temperature, the rate of the degrada-
tion of isoxa¯utole depended on the nature of the
buffer constituents and the pH ꢀbetween pH 5.2 and
pH 8.0 in phosphate buffer we found log Kobs =0.51Á
pH À5.2). In general, the rate constant Kobs of the
reaction can be written as follows:
0.05
35 ꢀÆ2)
18.7
0.1
a
Mean of three replicate experiments.
r =[HA]/[AÀ]=7.13 ꢀpH 6.0).
r =[HA]/[AÀ]=1 ꢀpH 6.5).
b
c
0.0046 and 0.3331 litre molÀ1
h
À1, respectively, and
indicated that, in such a solution, the catalytic action
of the acid H2PO4À was negligible in comparison with
HPO42À. The increase of Kobs with increasing concen-
tration showed that the hydrolysis of isoxa¯utole
underwent general catalysis in the solutions investi-
gated and the overall equation was:
‡
‡
Kobs ˆ kH O Á ‰H2OŠ ‡ k
Á ‰H3O Š
H3O
2
a
‡ kOH Á ‰OHÀŠ ‡ kHA Á ‰HAŠ ‡ kA Á ‰AÀŠ ꢁ4†
À
À
where the constituents of the buffer solution are HA
and AÀ, ie the acid and its conjugate base, respec-
Kobs ˆ 0 0046 ‰H2POÀ4 Š ‡ 0 3331‰HPO42ÀŠ ‡ 0 003
tively.9 To simplify this equation, kH O Á[H O ] can
‡
‡
3
3
be expressed as a function of [OHÀ] and, because in
dilute aqueous solutions the quantity of water can be
considered as invariable, [H2O] can be included in the
3.4 Influence of CH3COOÀ concentration
The results given in Table 1 reveal a signi®cant
difference between rates of hydrolysis in phosphate
and acetate buffers: at 30°C and pH 5.2, the half-lives
for phosphate and acetate were 267 and 31h,
respectively. Because it is known that under acidic
conditions isoxazoles unsubstituted in the 3-position
do not tend to open to give the corresponding b-ketone
derivatives,7,8 it was supposed that the greater rate of
degradation in acetate buffer compared with phos-
phate buffer was due to the strong catalytic activity of
the CH3COOÀ ion, the promoting action of which in
reactions of nucleophilic elimination is already
known.10 In order to establish a relationship between
Kobs and the CH3COOÀ concentration, the degrada-
tion of isoxa¯utole was followed in phosphate buffers
ꢀ[KH2PO4]=[Na2HPO4]=0.02M) to which different
quantities of acetate anion were added, and then
repeated with phosphate buffers containing KH2PO4
and Na2HPO4 at 0.05M.
constant rate of the `spontaneous' reaction: kH O
Á
2
[H2O]=k0. The values of the catalytic constants kHA
and kAÀ can be calculated by ®xing the ratio r =[HA]/
[AÀ] ꢀat constant pH) and eqn ꢀ4) becomes
À
Kobs ˆ kr ‡ ꢁkHA ‡ kA r†‰HAŠ
For a given ratio r, Kobs against [HA] is a straight line
and the separate values kHA and kAÀ can be determined
by comparing the slopes of two lines obtained for two
different values of r. Thus, to determine whether the
abiotic degradation of isoxa¯utole into DKN was
subject to general catalysis, and to calculate the values
of the separate catalytic constants kHA and kAÀ, the
in¯uences of phosphate and acetate concentrations
were studied.
3.3 Influences of KH2PO4 and Na2HPO4
concentrations
The initial isoxa¯utole concentration was 10mg
litreÀ1, the temperature of the experiments was 30°C,
m was 0.2M, and pH was 6.6 ꢀÆ0.2). Results are given
in Table 3.
The effect of increasing the concentration of phos-
phate anions was examined in aqueous solutions
having initial isoxa¯utole concentration of 10mg
litreÀ1 ꢀat the same temperature; 30°C). The degrada-
tion of isoxa¯utole was followed in two solutions at pH
6.0 and in ®ve solutions at pH 6.5. Results are given in
Table 2.
Table 3. Catalytic action of acetate anion on the degradation of isoxaflutole in
two aqueous phosphate buffers at 30°C and at pH 6.6 (Æ0.2)
At pH 6.0, r =[HA]/[AÀ]=7.13 and the equation
was:
K
obs ꢀhÀ1 Â103)
t1/2 ꢀh)
Sodium acetate
Concentration ꢀM) Buffer 1 a Buffer 2 b Buffer 1 a Buffer 2 b
Kobs ˆ 0 051 ‰H2POÀ4 Š ‡ 0 003
and at pH 6.5, r was 1 and the equation was:
Kobs ˆ 0 338 ‰H2POÀ4 Š ‡ 0 003
Thus, the values of k for H2PO4À and HPO42À were
0.16
0.1
74
58
21
63
48
10
9.4
12.0
32.7
10.9
14.5
72.0
0.0
a
[KH2PO4]=[Na2HPO4]=0.05 M.
[KH2PO4]=[Na2HPO4]=0.02 M.
b
Pest Manag Sci 57:366±371 ꢀ2001)
369

E Beltran et al
As for the phosphate buffers, the constant rate Kobs
was found to be linearly dependent on the CH3COOÀ
concentration ꢀeqns ꢀ5a) and ꢀ5b)):
solutions due to the catalytic action of the CH3COOÀ
anion, it can be supposed that the degradation of
isoxa¯utole in water is subject to general catalysis in all
types of buffer.
for ‰H2POÀ4 Š ˆ ‰HPO24ÀŠ ˆ 0 02M
Kobs ˆ 0 3362‰CH3COOÀŠ ‡ 0 0112
ꢁ5a†
ꢁ5b†
3.5 Influence of R-substituent in RCOOH/RCOONa
buffers
for ‰H2POÀ4 Š ˆ ‰HPO24ÀŠ ˆ 0 05M
Because of the obvious action of CH3COONa in the
degradation of isoxa¯utole in water, it seemed
important to establish whether the same properties
would be observed for other organic buffers. Since
alkyl groups are electron-repelling, increasing their
number in the a-position should lower the strength of
the acid, and consequently increase the strength of its
conjugate base. Dissociation of saturated dicarboxylic
acids in water occurs in two steps. In the ®rst
ionisation, the acid loses a proton from two positions,
but the corresponding ion formed can add a proton to
only one position. This negatively charged carboxylate
ion has an electrostatic effect which is assumed to
require additional energy to separate the proton from
the carboxyl group ꢀ®eld effect through the solvent).
The carboxylate anion exerts an inductive effect ꢀ‡I)
with resultant weakening of the acid. In the second
ionisation, the ion can lose a proton from only one
position, but the di-anion can add a proton to two
positions.12 In order to evaluate the in¯uence of the
R-substituent in R-COOH/R-COONa buffers, the
degradation of IFT was investigated at 30°C in buffers
consisting of the following acids and their sodium
salts: propionic ꢀR =C2H5), n-butyric ꢀR =C3H7),
isobutyric ꢀR = ꢀCH3)2CH) and trimethylacetic
ꢀR =ꢀCH3)3C), and the dicarboxylic acid, succinic
acid, and its conjugate base ꢀHOOCCH2CH2COOH/
NaOOCH2CH2COONa).
Kobs ˆ 0 3352‰CH3COOÀŠ ‡ 0 0219
Taking the difference between the slopes to be due to
experimental variation, an averaged slope of 0.3357
ꢀÆ5Â10À4) litre molÀ1 hÀ1 has been calculated. This
represents the catalytic constant kCH COOÀ of
3
CH3COOÀ for isoxa¯utole under the conditions
previously described. The difference observed be-
tween the y-intercept and the value calculated from
measurements in phosphate buffers without acetate
anion ꢀpreceding paragraph) may be attributed to
experimental errors and pH variation from one solu-
tion to another ꢀfrom pH 6.4to pH 6.8). As for the
phosphate buffers ꢀTable 2), the CH3COOH and
CH3COOÀ concentrations were increased in order to
quantify the catalytic constant kCH COOH. The values
3
of Kobs obtained for pH 5.2 ꢀr =[CH3COOH]/
[CH3COOÀ]=0.26) and at 30°C are given in Table 4.
The Kobs values obtained lead to a kCH COOH value
3
of 0.3085 litre molÀ1 hÀ1, which is similar to the value
for kCH COOÀ and allowed us to conclude that the
3
catalytic actions of CH3COOH and CH3COOÀ were
equivalent. The overall equation was
Kobs ˆ 0 3085 Á ‰CH3COOHŠ
‡ 0 3357 Á ‰CH3COOÀŠ À 0 01
Since the constant rate of the degradation of isoxa-
¯utole into DKN in a [HA]/[AÀ] buffer was a function
of [HA] and [AÀ], the velocity of the reaction could be
supposed to be related to the capacity of the catalytic
entity for approaching the isoxa¯utole molecule.
Because of the high number of water molecules that
can hydrate HPO42À ꢀup to 12 water molecules),11 the
formation of the enolate ion could be hindered by the
volume of the phosphate ion and this could explain the
lower value of S≠ ꢀTable 1) obtained in phosphate
buffer.
The similar pKa values of the above acid/base pairs
allowed a constant pH value with the same proportion
of acid and base for all the solutions ꢀno in¯uence of
the anion concentration). To have pH=pKa, buffers
were made with equal amounts of acid and base ꢀby
volume); the ®nal concentrations of the acid and its
conjugate base in solution were 0.05M and the pH was
4.7 ꢀÆ0.1). Results are given in Table 5.
The results obtained con®rmed that methyl groups
in the a-position increased the strength of the con-
jugate base RCOOÀ, since we found KobsꢀCH3)
Comparison of eqn ꢀ4) with eqns ꢀ5a) and ꢀ5b)
indicates that the rate constant varied positively with
increasing concentration for both organic and in-
organic buffers studied, which meant that, regardless
of the increased rate of degradation in aqueous
Table 5. Influence of the nature of the R-substituent
in alkanoic acid/sodium salt buffers at 30°C and pH
4.7 (Æ0.1)
R-substituent
Kobs ꢀhÀ1Â103)
t1/2 ꢀh)
CH3
C2H5
30
44
28
39
49
22
23.1
16.0
24.6
17.8
14.1
31.6
Table 4. Influence of concentration of components on Kobs
values in acetate buffers at pH 5.2 and at 30°C
C3H7
Concentration ꢀM)
ꢀCH3)2CH
ꢀCH3)3C
Succinic acida
Acetic acid Sodium salt Kobs ꢀhÀ1 Â103) t1/2 ꢀh)
0.021
0.042
0.079
0.158
22.4
55.4
30.9
12.5
a Data for an analogous succinic acid buffer included
for comparison.
370
Pest Manag Sci 57:366±371 ꢀ2001)

Kinetics of chemical degradation of isoxa¯utole
K
obsꢀꢀCH3)2CH)
K
obsꢀꢀCH3)3C). We noticed that
groundwater before predicting the fate of xenobiotics
in the natural environment. Since BA was not detected
throughout this study, it can be supposed that the
degradation of the diketonitrile DKN occurs in bio-
logically active media only. Studies currently taking
place with sterilised and unsterilised soils should
con®rm this point.
lengthening the carbon chain reduced the ‡I effect of
the methyl group and slowed the reaction ꢀthe half-life
obtained for R=C3H7 was higher than that obtained
for R=C2H5). The value of the rate constant
determined in a succinic acid/sodium succinate buffer
allowed us to con®rm that the absence of electron-
repelling groups decreased the velocity of the reaction.
By following the degradation of isoxa¯utole in a
succinic acid/sodium succinate buffer containing
ACKNOWLEDGEMENTS
The authors wish to thank Aventis CropScience,
Ongar ꢀUK) for their assistance throughout this study.
0.033M of each of these compounds ꢀKobs =0.016hÀ1
)
it could be veri®ed that the degradation of isoxa¯utole
in aqueous solutions is subject to general catalysis.
Above all, the relevant point is that Kobs values of the
same order of magnitude were obtained for the
chemical degradation of isoxa¯utole in buffers pre-
pared with various organic acids and their conjugate
bases. In all cases the Kobs values were at least ten times
greater than those observed in phosphate buffer
systems.
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371