ALKALINE HYDROLYSIS OF ETHIOFENCARB: KINETIC STUDY AND MECHANISM DEGRADATION
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performed using HPLC with ultraviolet or fluorescence
detection and gas chromatography (GC) in combina-
tion with nitrogen phosphorus detection after a deriva-
tization step.
The mobile phase was acetonitrile–water (45:55
v/v). It was set at a flow rate of 2 mL min−1. The
measured wavelength was 200 nm.
Standards and Reagents
The degradation of carbamate derivatives is pos-
sible through chemical, microbiological, and photo-
chemical processes. A detailed study of voltammetric
behavior of ethiofencarb was reported using a glassy
carbon electrode and a hanging mercury drop electrode
[16]. The photodegradation of ethiofencarb was inves-
tigated in aqueous media. Half-lives were measured,
and photoproducts were assessed by GC with mass
spectrometry as a detector, allowing the establishment
of a cleavage mechanism [6].
A kinetic study of the hydrolysis of ethiofencarb
in pure water and in aqueous solutions at different pH
values and different temperatures has been carried out
using a gas chromatographic nitrogen phosphorus de-
tection method. The values of the first-order rate con-
stants for the degradation reaction were found to be
dependent on pH and temperature [17]. Zamy et al.
studied the hydrolysis of four organophosphorus pesti-
cides and two N-methylcarbamate derivatives (oxamyl
and ethiofencarb). At pH 8, ethiofencarb was found sta-
ble (t1/2 < 1 month). A degradation product was iden-
tified by LC-MS [18]. The corresponding mechanism
involves either an addition of OH− onto the carbonyl
with a further elimination of the RO− leaving group or
an abstraction of the hydrogen of the methyl carbamate
moiety again evolving with the elimination of the RO−
group.
Ethiofencarb and 2-ethylthiomethylphenol were
obtained from Supelco (Saint-Quentin Fallavier,
France). Standard stock solutions were prepared as
1000 μg mL−1 in methanol at 4◦C. Working solutions
were prepared by sequential dilution at 20 μg mL−1 in
various buffer and in sodium hydroxide solutions rang-
ing from pH 9.80 to 11.71. Ionic strength (I) made up
1 with KCl at 25◦C.
These aqueous solutions were prepared with deion-
ized water, which was distilled over permanganate and
sodium hydroxide. Nitrogen was bubbled through the
distilled water used for the preparation of sodium hy-
droxide solutions.
Acetonitrile and methanol were of HPLC grade.
Borax (Na2B4O7·10 H2O), NaHCO3, KH2PO4, HCl,
NaOH, and KCl were purchased from Fluka (Saint-
Quentin Fallavier, France).
Kinetics Measurements
Spectrophotometric Method. The changes in concen-
tration of ethiofencarb were followed spectrophoto-
metrically by recording changes in the absorbance cor-
responding to the appearance of a hydrolysis product
(λ = 295 nm).
All reactions exhibited first-order kinetics with re-
spect to the substrate. The absorbance versus time
plots (Fig. 1) gave the pseudo–first-order rate constants
graphically using the experimental infinity value. The
observed rate constants kobs were obtained by plotting
Since no systematic studies on the mechanistic as-
pect of the hydrolysis of ethiofencarb in aqueous solu-
tion had been reported in the literature, the objective of
this study was to determine the mechanism of the hy-
drolysis of ethiofencarb employing spectrophotometric
UV and liquid chromatographic methods.
log (A∞ − At ) versus time, where A , and At are the
∞
absorbance at infinity and at time t, respectively:
kobs
log (A∞ − At ) = −
t + log (A∞ − A0) (1)
2, 3
EXPERIMENTAL
Materials
Liquid Chromatographic Method. We followed by
reversed-phase liquid chromatography the evolution of
2-ethylthiomethylphenol chromatogram (peak 2) ver-
sus time during the hydrolysis of ethiofencarb (peak 1)
in alkaline solution (Fig. 2).
A Beckman DU 640B spectrophotometer fitted with a
thermostated multiple cell compartment is used for all
spectroscopic measurements.
The observed rate constants kobs were obtained by
The liquid chromatography diode array detection
(LC-DAD) system consists of a gradient model pump
from Varian G 1600 A, a rehodyne six-port injection
valve model 7125 with a 20-μL loop. A model Pro
Star 330 photodiode array detector was connected to a
computer station. Separations were performed using a
Symmetry C18, 150 × 4.6 mm i.d., 3-μm particulate
size analytical column.
plotting log (A∞ − At ) versus time, where A , and At
∞
are the peak area at infinity and at time t, respectively
(Eq. (1)).
Entropy Activation
The logarithms of the observed rate constants were
ꢀ
plotted versus 1 T , a straight line was observed, and
International Journal of Chemical Kinetics DOI 10.1002/kin.20748