31P NMR Kinetic Studies on Fenitrothion Hydrolysis in the Presence of Ag+ and Hg2+ Ions

Full text of DOI:10.1002/bkcs.10099

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DOI: 10.1002/bkcs.10099
BULLETIN OF THE
H. Choi et al.
KOREAN CHEMICAL SOCIETY
31P NMR Kinetic Studies on Fenitrothion Hydrolysis in the Presence
of Ag+ and Hg2+ Ions
*
Hojune Choi, Kiyull Yang, and In Sun Koo
Department of Chemistry Education and Research Institute of Natural Science, Gyeongsang
National University, Jinju 660-701, Korea. *E-mail: iskoo@gnu.ac.kr
Received September 28, 2014, Accepted November 12, 2014, Published online January 15, 2015
Keywords: 31P NMR, Fenitrothion, Organophosphorus pesticides, Hydrolysis, Degradation
Organophosphorus (OP) pesticides are widely used today for
meeting the world food supply (i.e., in modern agriculture
and in small fields) as well as in forest industries.1–3 The
bad effects these pesticides have on many organisms
strongly depend on their chemical activities in the aquatic
environment.3,4 Because of their high toxicity to humans
and their long-term environmental effects, there have been
numerous investigations on the degradation of OP pesti-
cides.1–4 Even so, it is important to learn more about their
chemistry in order to better determine their persistence in
the environment and to find ways to destroy them rapidly
and safely.3
Recent research has highlighted the role of metal ions in
the abiotic degradation processes of OP-ester and thioester
pesticides.3a,5,6 The increase in hydrolysis rates was in the
range of 20–3000 times in metal ion solutions.7 However,
no direct experimental evidence was provided to prove the
interaction of the OP pesticides with metal ions. It is postu-
lated that the interaction of metal ions with atoms within
O P or S P pesticides may enhance their hydrolytic degra-
dation. The SN2(P) degradation mechanism (Scheme 1)
for the hydrolytic decomposition of a P S pesticide (feni-
trothion or FN), is known to proceed by the following
process. Attack of the phosphorus atom by OH− ions results
in cleavage of the P–O aryl bond to yield O,O-
dimethylphosphorothioate anion (PA−) and 3-methyl-4-
nitrophenol (PNP).3,4,8b The hydrolysis products (PA− and
PNP) can readily be identified using UV or visible light,
or 31P NMR spectra.
31P NMR peak area for FN (δ = 64.0 ppm) decreased gradu-
ally during the FN hydrolysis reaction.
Also, during the hydrolysis of FN, the 31P NMR peak area
for PA (δ = 41.40–41.68 ppm for Ag+ metal ion and δ =
37.60–38.20 ppm for Hg2+ metal ion) increased gradually
(see Table 1). The FN peak was almost constant over time,
but the PA peak shifted slightly (Δδ = 0.28 ppm for Ag+ and
Δδ = 0.60 ppm) as a function of time. The changes in the inte-
gration areas under these peaks at δ = 64.0 ppm for the 31P
atom in the FN–Ag+ and FN–Hg2+ systems were followed
for 950 and 3142 min, respectively.
The plot of integration area near 41.50 ppm versus time is
given in Figures 1(a) and 2(a) for the hydrolysis of FN in
the presence of silver and mercury ions in a solution of water
and acetone (25%:75%) at 20.5 ꢀC. The plot of log (Ainf − At)
versus time is shown in Figures 1(b) and 2(b), where the Ainf is
the integration area of PA at infinite time. The rate constant is
obtained from the slope of the plot of log (Ainf − At) ver-
sus time.
ThereactionstudiedwasaffectedbytheadditionofAg+ and
Hg2+ metal ions. In order to determine the importance of cat-
alyzed transition-metal ions in the observed effect, the rate
constants were determined in the presence of Ag+ and Hg2+
metal ions. The rate constants (kobs/s) for the hydrolysis of
FN with Ag+ and Hg2+ metal ions at 20.5 ꢀC are reported in
Table 2. The rate constant is 9.58 × 10−8 s−1 for the hydrolysis
of FN in buffered distilled water at 23 ꢀC.7 As shown in
Table 2, the rate constants for the hydrolysis of FN with
Ag+ and Hg2+ metal ions at 20.5 ꢀC are hundreds of times
greater than those for the hydrolysis of FN in buffered distilled
water at 23 ꢀC.7
This result indicates that the reaction rate is remarkably
affected by the Ag+ and Hg2+ metal ions in their ground and
transition states. Koo et al. and others3,6 have found that the
31P chemical shift of FN undergoes upfield shifts in the pres-
ence of Ag+ and Hg2+ metal ions. This result indicates that the
chemical shift of the phosphorus atom bonded to the sulfur
atom is expected to be sensitive to the coordination of Ag+
and Hg2+ metal ions with sulfur atoms in the ground state
(Scheme 2).3,6 The Ag+ and Hg2+ metal ions have a strong
affinity for the S atom11 in the compounds, based on Pearson's
hard and soft acid–base (HSAB) theory.3a,12 The proposed
When used as a tool for structural identification in the
field of OP chemistry, 31P NMR is strongly influenced by
its sensitivity to chemical shifts in the molecular struc-
ture.3b,9,10 In turn, 31P NMR chemical shifts provide valuable
information about the nearest neighbors of P atoms within
the molecules.3b,9,10
In order to find direct experimental evidence of the interac-
31
tion of OP pesticides with soft metal ions, we performed
P NMR kinetic studies of FN hydrolysis in the presence of
Ag+ and Hg2+ ions.
The progress of FN hydrolysis in the presence of Ag+ and
Hg2+ metal ions at 20.5 ꢀC, in a solution of 25% water and
75% acetone was observed over time using 31P NMR. The
Bull. Korean Chem. Soc. 2015, Vol. 36, 707–710
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(a)
3.0
2.5
2.0
1.5
1.0
0.5
0.0
Scheme 1. SN2(P) pathway for the degradation of FN.
Table 1. 31P NMR peak area (δ = 41.40–41.68 ppm) versus time of
appearance of PA in the presence of Ag+ and the 31P NMR peak area
(δ = 37.60–38.20 ppm) versus time (in min) of appearance of PA in
the presence of Hg2+ in a solution of 25% water and 75% acetone at
20.5 ꢀC.
FN with Ag+
Intensity of PA
FN with Hg2+
Min Intensity of PA
0
500 1000 1500 2000 2500 3000 3500
Min
Time (min)
0
5
30
55
265
465
715
950
—
0.0000
0.0627
0.1240
0.4486
0.6402
0.6402
0.6689
0.7462
—
0
0.0000
0.2550
0.4010
0.5880
0.7610
1.174
2.003
2.444
2.672
2.852
2.998
3.078
0.0
-0.5
-1.0
-1.5
-2.0
-2.5
10
52
99
(b)
167
260
749
1294
1663
2437
3142
Ainf
—
—
—
—
Ainf
0.7500
0
500
1000 150 0 200 0 250 0 30 00 35 00
Time (min)
(a)
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
Figure 2. Plot of the peak area (δ = 37.60–38.20 ppm) versus time (in
min) of appearance of PA in the presence of Hg2+ in a solution of 25%
water and 75% acetone at 20.5 ꢀC. (b) Plot to determine the pseudo-
first-order rate constant (kobs) of the representative kinetic run using
the correct Ainf value of 3.078.
Table 2. Rate constant (kobs/s) for the hydrolysis of FN with Ag+ and
Hg2+ at 20.5 ꢀC.
kobs (s−1
)
0
200
400
Time (min)
600
800
1000
FN with Ag+a
FN with Hg2+b
2.37 × 10−5 (r = 0.984)
1.85 × 10−5(r = 0.994)
0.0
-0.2
-0.4
-0.6
-0.8
-1.0
-1.2
(b)
a [FN] = 1.05 × 10−2 M, [Ag+] = 5.25 × 10−2 M, pH = 3.3.
b [FN] = 3.50 × 10−3 M, [Hg2+] = 3.50 × 10−2 M, pH = 3.5.
FN–Ag+/Hg2+ complexes were consistent with the theoretical
structure (Scheme 3) and the electrospray ionization-mass
spectrometry (ESI-MS) studies of Ag+/Hg2+ metal-ion bind-
ing to FN, reported in a previous work.3
We conclude that the hydrolysis rate of FN in the presence
of Ag+ or Hg2+ ions is greater than that of the reaction of the
free substrate itself because positive charge is developed at the
phosphate group (Scheme 2) by the metal complexation, so
that the nucleophilic attack is feasible. Another possibility
of providing enhanced reactivity with metal ions would be a
transition structure. One could propose a four-center SN2(P)
cyclic transition-state structure (Scheme 4). The reaction via
-100
0
100
200
300
400
500
600
700
800
Time (min)
Figure 1. (a) Plot of the peak area (δ = 41.40–41.68 ppm) versus
minute of appearance of PA in the presence of Ag+ in a solution of
25% water and 75% acetone at 20.5 ꢀC. (b) Plot to determine the
pseudo-first-order rate constant (kobs) of the representative kinetic
run using the correct Ainf value of 0.750.
Bull. Korean Chem. Soc. 2015, Vol. 36, 707–710
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this transition state structure could befaster thanthereaction in
the metal-ion-free condition, because the metal ion complex-
ation canstabilize thetransitionstate. We confirmed suchtran-
sition structures using density functional theory (DFT)
computations showing the transition movements (depicted
by arrows in Scheme 4). Based on the 31P NMR kinetic studies
ofFNhydrolysisinthepresenceofAg+ andHg2+metalions, as
well as thetheoretical andexperimental 31P NMR and ESI-MS
results showing Ag+ and Hg2+ metal ions binding to FN,3 we
conclude that the reaction proceeds via this four-center cyclic-
transition-state mechanism.
Experimental
The reaction kinetics were determined using 31P NMR spec-
trometry (Bruker 400 MHz NMR spectrometer, Bruker, Sil-
berstreifen, Rheinstetten, Germany) at 20.5 ꢀC. Reactions
werecarried outunder pseudo-first-orderconditions. FNstock
solutions were prepared in acetone-d6, and the Ag+ and Hg2+
metal ion stock solutions were prepared in D2O. Metal ion
Scheme 2. The FN–Ag+/Hg2+ complex in the ground state.
Scheme 3. B3LYP/6-31G(d) structures of FN–Ag and FN–Hg complexes. All values are bond lengths (Å). (a) FN–Ag complex and (b) FN–Hg
complex (source: Ref. 3).
Scheme 4. Four-center SN2(P) cyclic-transition structures of (a) FN–Ag+ and (b) FN–Hg2+ complexes optimized byB3LYP/6-31G(d) basis sets.
All values are bond lengths (Å).
Bull. Korean Chem. Soc. 2015, Vol. 36, 707–710
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Bull. Korean Chem. Soc. 2015, Vol. 36, 707–710
© 2015 Korean Chemical Society, Seoul & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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