Kinetic Investigation of Micellar Promoted Pyridine based Oximate and Hydroxamate Catalysis on Phosphotriester Pesticides

Article abstract of DOI:10.1007/s10562-016-1912-5

Abstract: The nucleophilic hydrolysis of paraoxon, parathion and fenitrothion by pyridine oximate (PyOx^?) and hydroxamate (PyHA^?) has been studied in aqueous and cationic micellar media. A noticeable kinetic changes has been observed at pH > pK_adue to effective nucleophilicity of oximate (–CH=NO^?) and hydroxamate (–CONHO^?) functions. The Ox^?nucleophile shows large reactivity than corresponding HA^?. The reactivity of nucleophiles toward the electrophilic center of P=O and P=S bond of phosphate ester shows prominent effect in the presence of cetyltrimethylammonium bromide (CTAB) and tetradecyltrimethylammonium bromide (TTAB) micelles than aqueous media. The adjacent lone pair of electron in nitrogen atom on the nucleophile moiety of 2-PyOx and 2-PyHA helps to augment the reactivity compared to presence in remote of 4-PyOx and 4-PyHA nucleophile. The application of the pseudophase model of micellar catalysis showed that the ratio k₂^m/k₂^wis higher for the reaction of esters. Graphical Abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-016-1912-5

Catal Lett
DOI 10.1007/s10562-016-1912-5
Kinetic Investigation of Micellar Promoted Pyridine based
Oximate and Hydroxamate Catalysis on Phosphotriester
Pesticides
1
Hitesh K. Dewangan · Rekha Nagwanshi · Kallol K. Ghosh ·
2
1
1
Manmohan L. Satnami
Received: 27 August 2016 / Accepted: 6 November 2016
Springer Science+Business Media New York 2016
©
Abstract The nucleophilic hydrolysis oꢀ paraoxon, para-
−
thion and ꢀenitrothion by pyridine oximate (PyOx ) and
ester shows prominent efect in the presence oꢀ cetyltri-
methylammonium bromide (CTAB) and tetradecyltri-
methylammonium bromide (TTAB) micelles than aqueous
media. The adjacent lone pair oꢀ electron in nitrogen atom
on the nucleophile moiety oꢀ 2-PyOx and 2-PyHA helps to
augment the reactivity compared to presence in remote oꢀ
4-PyOx and 4-PyHA nucleophile. The application oꢀ the
pseudophase model oꢀ micellar catalysis showed that the
−
hydroxamate (PyHA ) has been studied in aqueous and
cationic micellar media. A noticeable kinetic changes has
been observed at pH>pK due to efective nucleophilic-
a
−
−
ity oꢀ oximate (–CH=NO ) and hydroxamate (–CONHO )
−
unctions. The Ox nucleophile shows large reactivity than
ꢀ
−
corresponding HA . The reactivity oꢀ nucleophiles toward
the electrophilic center oꢀ P=O and P=S bond oꢀ phosphate
m
w
ratio k /k is higher ꢀor the reaction oꢀ esters.
2
2
Electronic Supplementary Material The online version
oꢀ this article (doi:10.1007/s10562-016-1912-5) contains
supplementary material, which is available to authorized users.
*
Manmohan L. Satnami
manmohanchem@gmail.com
1
2
School oꢀ Studies in Chemistry, Pt. Ravishankar Shukla
University, Raipur, Chhattisgarh 492010, India
Department oꢀ Chemistry, Govt. Madhav Science P. G.
College, Ujjain, Madhya Pradesh 456010, India
1
3

H. K. Dewangan et al.
Graphical Abstract
Keywords Pesticides · Oximate · Hydroxamate ions ·
pesticides, insecticides as well as a chemical warꢀare agent
[16, 17]. These compounds are extremely potent inhibitors
oꢀ acetylcholinesterase enzymes [18]. There is an imme-
diate required to develop efective, convenient and eco-
nomic method to detoxiꢀy them. A considerable amount oꢀ
α-nucleophiles was reportedly investigated ꢀor hydrolysis
oꢀ OP compounds, pesticides and chemical warꢀare agents
[19–25]. Nucleophiles such as peroxides [19], hypochlo-
rites [20], oximates [21, 22],o-iodosylcarboxylate [23] and
hydroxamates [24] have been used alone or in the presence
oꢀ surꢀactants. The study oꢀ nucleophilic reactivities oꢀ the
Micellar catalysis
1
Introduction
The tuning oꢀ oxime and hydroxamic acids has received
renewed interest due to their high aꢁnity to bind the metal
ions to ꢀorm metal complexes [1, 2]. Hydroxylamine is
the parent compound ꢀor the three typical classes oꢀ the
oxygen-containing amidoxime, oxime and hydroxamic
acid. Their deprotonated –NOH ꢀunctional group such
−
α-efect Ox ꢀunction led to the development oꢀ antidotes
−
−
as amidoximate (–C(NH )=NO ), oximate (–CH=NO )
2
ꢀor the reactivation oꢀ organophosphate-inhibited cholinest-
erase enzymes [26, 27]. Nucleophilic reactivity oꢀ other
nucleophiles such as monoperoxyphthalates [28], 4-N,N-
dialkylaminopyridines [29], hydroxybenzotriazoles [30],
5-alkyl-1H-tetrazoles [31] and have been studied in physi-
ological conditions.
−
and hydroxamate (–CONHO ) plays a very essential role
towards the scavenging oꢀ Fe(III) ꢀrom the environment
[
3, 4]. The ꢀunctional group oꢀ hydroxamic acids provide
a better platꢀorm in numerous applications such as bind-
ing oꢀ metal ions [5], inhibitors oꢀ enzyme [6] and prom-
ising drugs ꢀor the treatment oꢀ a variety oꢀ diseases. An
account oꢀ their antimalarial [7, 8], antiꢀungal [9] and
anticancer activities has been extensively studied [10–
−
−
Oximate (Ox ) and hydroxamate (HA ) ions are
known to be very efective α nucleophiles and their reac-
−
−
tivity are inꢂuenced by the basicity oꢀ the Ox and HA
1
2]. Recently, Danial et al. have used salinomycine based
ꢀunctions, at least in the pKa range 7–10 [32]. The use oꢀ
relatively acidic hydroxamic acids (HA) at pH close to
natural pH ensures a large ꢀraction oꢀ their highly reac-
tive conjugate bases. The reactivities oꢀ amphiphilic [33]
and aromatic N-substituted hydroxamate [34] ions are
increased by comicellization with surꢀactant in water.
Efective hydrolytic reactions oꢀ bis (2,4-dinitrophenyl)
hydroxamic acids as an efective anticancer agent [13]. The
coordination chemistry oꢀ acids with metal has also been
−
studied widely [14, 15]. The oximate (Ox ) and hydroxa-
−
mate (HA ) ions play a ꢀundamental role as deacylating
and dephosphorylating agent towards the organophosphate
esters. Organophosphate (OP) esters are widely used as
1
3

Kinetic Investigation of Micellar Promoted Pyridine based Oximate and Hydroxamate Catalysis…
phosphate (BDNPP) were also achieved with a long-
chain hydroxamic acid, i.e. hydroxamate polymers [35]
and desꢀeroxamine [36]. Surprisingly, nucleophilic
cleavage oꢀ OP compound has largely been studied using
2 Experimental
2.1 Materials and Method
−
Ox ions. However, the nucleophilic reactivity oꢀ pyri-
−
dine based HA ions towards dephosphorylation reaction
Syn-2-pyridinealdoxime (2-PyOx), 4-pyridinealdoxime
(4-PyOx), cetyltrimethylammonium bromide (CTAB), tetra-
decyltrimethylammonium bromide (TTAB), paraoxon, para-
thion and ꢀenitrothion were procured ꢀrom Sigma-Aldrich
Chemicals Pvt. Ltd. Bangalore (India). Pyridinehydroxamic
acids such as 2-pyridinehydroxamic acid (2-PyHA), 3-pyri-
dinehydroxamic acid (3-PyHA) and 4-pyridinehydroxamic
acids (4-PyHA) were prepared by reaction oꢀ esters with
hydroxylamine in alkaline condition [39]. Salicylaldoxime
(SOx) was prepared by the reaction oꢀ methanolic solution oꢀ
salicylaldehyde and hydroxylamine hydrochloride [40]. Other
chemicals used were oꢀ AR/high purity (99.0%) grade. All
the solutions were prepared in triple-distilled water.
oꢀ pesticides have not been claimed in previous studies.
−
−
We have documented efective catalysis oꢀ HA and Ox
ions towards the dephosphorylation oꢀ paraoxon, para-
+
−
thion and ꢀenitrothion in CTA X cationic micelles [37]
and microemulsions [38].
In the present exploration, we have examined the
reaction oꢀ model OP esters such as paraoxon, parathion
−
and ꢀenitrothion with pyridine based Ox and HA ions
−
−
Scheme 1). The reactivity oꢀ oximate (Ox ) and hydrox-
(
−
amate (HA ) ions is described by a common Brѳnsted
equation and believed that the main ꢀactor determining
−
the reactivity oꢀ Ox and HA ion is their nucleophi-
−
licity. The acid-base ꢀunctional groups present on some
−
HA ions could exert a nucleophilic efect on reaction
2.2 Determination of pK of Hydroxamic Acids
a
with P=O and P=S centers. Thereꢀore, we have also
−
studied the reactivity oꢀ HA ions, especially oꢀ those
The acid dissociation constant (pK ) values oꢀ all synthesized
a
oximes and hydroxamic acid were determined by potentio-
metric titration at 27°C (Fig. S1, a–e). Typically, 0.001 M
solutions oꢀ oxime and hydroxamic acids were titrated with
0.01 M NaOH using Orion Star A-211 pH-meter. The ionic
strength was maintained constant at 0.1 M by adding KCl.
−
possessing acidic-basic groups (PyHA ) towards phos-
phate and thiophosphate esters. To evaluate the ꢀactors
governing the nucleophilic reactivities and strength oꢀ
micelle interaction, the reactivity oꢀ structurally simi-
−
−
−
lar Ox ions (PyHA , PyOx ) were also investigated in
−
Under the experimental condition, the pK values were
a
+
cationic alkyltrimethylammonium bromide (RTA Br ;
R = C H and C H ) micelles.
obtained ꢀrom the measured pH values to the solution by
means oꢀ Eq. 1.
1
6
33
14 29
−
A ]
[
pK = pH − log
a
(1)
[
HA]
Scheme 1 Nucleophilic reaction oꢀ pyridine oximate and hydroxamate with phosphate esters
1
3

H. K. Dewangan et al.
2
.3 Kinetic Measurements
adding KCl as required. The pK values ꢀor the ionization
a
−
−
oꢀ Ox and HA group oꢀ oximes and hydroxamic acid
were independently determined in similar ways (Table 1).
Pyridiniumcarbaldehyde oximes constitute the most impor-
tant class oꢀ potential AChE reactivators [41]. The most
All reactions were perꢀormed at 27 °C with a Thermoꢃsher
Evolution 300 UV–Visible spectrophotometer equipped
ꢀor temperature control (Peltier). All the kinetic runs were
−
ꢀollowed under pseudo-ꢃrst-order conditions in which the
eꢁcient Ox reactivators are 2-PAM, TMB-4 and HI-6.
−
These compounds have both Ox ꢀunctionality and a posi-
−3
concentration oꢀ nucleophiles (1.0×10 M) was at least
1
0 times larger than the initial concentration oꢀ substrates
tively charged moiety, which is a disadvantage in terms oꢀ
penetration oꢀ biological membrane. Pyridine substituted
hydroxamate α-nucleophiles may meet these requirements
−4
(
1.0×10 M). The additions oꢀ nucleophile and substrate
were done under thermal control at 27°C. The cuvettes
were allowed to equilibrate thermally (27± 0.1°C) in the
cell holder ꢀor 30 min. Aꢀter temperature equilibrium, the
stock solutions oꢀ each substrate were added to cuvettes,
and the kinetic runs were started. The observed rate con-
stants (kobs) ꢀor the reaction oꢀ nucleophiles with ester
were determined by ꢀollowing the increase in absorption oꢀ
to be a ꢀunction as hydrolytic catalyst and AChE reactiva-
−
tors. The pK oꢀ 2-PyHA is ꢀound to be 8.73, which are
a
≈0.3 pK units greater than the pK values oꢀ correspond-
a
a
−
−
ing Ox ion (i.e. pK oꢀ 2-PyOx =8.41). Most importantly,
a
−
4-PyHA shows pK value oꢀ 7.2, which is ≈2.2 pK units
a
a
−
less than corresponding Ox ion, 4-PyOx (pK =9.39).
−
a
−
The pK value oꢀ 4-PyHA is even less than the well-known
4
-nitrophenoxide ion at 400 nm and 3-methyl-4-nitrophe-
a
noxide ion at 398 nm (Fig. S2). All the kinetic experiments
were perꢀormed at an ionic strength oꢀ 0.1 M KCl. Borate
bufer was employed. For all the kinetic runs, the absorb-
ance/time result ꢃt the ꢃrst-order rate equation very well.
reactivators, 2-PAM (pK =7.75) and 4-PAM (pK =8.27).
a
a
Thereꢀore, it is advantageous to investigate the catalytic
−
efect oꢀ the PyHA ions ꢀor the cleavage oꢀ environmen-
tal persistent OP pesticides, viz. paraoxon, parathion and
ꢀenitrothion.
ꢀ
ꢁ
ꢀ
0ꢁ − kt
ln A − A = ln A − A
∞
(2)
t
∞
The values oꢀ pseudo-ꢃrst-order rate constants (kobs
)
were obtained by least squares ꢃts. The second-order rate
3.2 Eꢀect of pH
constants (k ) in the aqueous medium were determined
2
ꢀrom linear plots oꢀ kobs versus nucleophile concentra-
Studying pH efect allows us to estimate the pK oꢀ nucleo-
a
tions. The intercept were negligible. Generally, kobs val-
ues were measured at ꢃve diferent concentrations oꢀ each
nucleophile.
philes and thereꢀore, provide situation to control the rate oꢀ
reaction at preꢀerable reaction conditions. There is a notice-
able efect oꢀ pH on the reaction rates. At pH>pK suꢁ-
a
cient rate enhancement may be observed due to oxime and
hydroxamic acid ꢀunction completely dissociated into the
−
−
3
Results and Discussion
oximate (–CH=NO ) and hydroxamate (–CONHO ) ions.
The relatively high acidity oꢀ hydroxamic acids (a) is one oꢀ
3
.1 Hydrolytic Reaction in Aqueous Media
the striking properties. A number oꢀ pK values ꢀor RCON-
a
HOH are reported in the literature [42, 43]. The structure oꢀ
the anion is still subject to controversy. At this point, it is
diꢁcult to generalize beyond one hydroxamic acid, and the
solvent systems used ꢀor the particular study. Another diꢀ-
ꢃculty lies in the detection oꢀ minute ꢀraction; this problem
also hindered the tautomerism. Widespread literatures have
been reported that hydroxamate anion represents a unique
type oꢀ reactive α-nucleophile since it behaves as a one-cut
selꢀ-destructive molecular scissor, losing its nucleophilic
ability aꢀter destroying the phosphate ester (Scheme 2c)
The acidity constant (pK ) oꢀ the oxime and hydroxamic
a
acids were measured at 27°C by standard potentiometric
titration methods. The ionic strength was kept at 0.1 M by
Table 1 Second-order rate constants ꢀor the reaction oꢀ ester
−
Nu ]
5
10 k (M
−1 −1
s
[
pKa
)
2
Paraoxon
Parathion
Fenitrothion
−
−
−
a
a
a
2
3
4
2
4
-PyHA
-PyHA
-PyHA
8.73 (8.2)
9.10 (8.8)
7.20 (7.1)
8.41
1.64± 0.12
1.08± 0.11
0.16± 0.12
4.67± 0.11
3.10± 0.10
4.81± 0.11
0.17± 0.13
0.11± 0.11
0.08± 0.11
3.25± 0.12
2.96± 0.12
3.50± 0.13
0.032± 0.11
0.023± 0.10
0.020± 0.11
0.043± 0.10
0.031± 0.10
0.060± 0.11
[
44]. It was observed that the rate oꢀ reaction increased
with the rise in pH as represented in Fig. 1a, b. A pH versus
rate constant proꢃle ꢀor the nucleophilic cleavage oꢀ par-
aoxon and parathion by oximate and hydroxamate ion gave
−
-PyOx
−
-PyOx
−
9.38
the apparent pK value (Table 1). The higher rate constant
a
SOx
9.20
was observed ꢀrom these experiments were at pH>pKa,
a
−3
Determined under the micellar condition: [CTAB]=5×10 M,
KCl]=0.1 M, pH=10, Temp.=27 °C
due to the appearance oꢀ dissociated oxime and hydroxamic
−
acid ꢀunctionality (–CH=NO and C(O)NHO ).
−
[
1
3

Kinetic Investigation of Micellar Promoted Pyridine based Oximate and Hydroxamate Catalysis…
Scheme 2 Ionization oꢀ
hydroxamic acid and nucleo-
−
philic attack oꢀ HA Ion
−4
Fig. 1 pH efect oꢀ oximate and hydroxamate ions with a Paraoxon and b Parathion. Reaction condition: [Ester]=1×10
[
M,
− −3
Nu ]=1×10 M, Temp.=27 °C
Equation (3) describes the reaction oꢀ phosphate esters
(Fig. 2). Second-order rate constants (k ) were calculated
2
−
ꢀrom the slope oꢀ plots oꢀ kobs versus [Nu ] (Tables S1–S3);
with nucleophiles; k deꢃned in Eq. (4). It corresponds to
o
−
the intercept in the kobs versus [Nu ] plot.
results ꢀor the various pyridine based nucleophiles are pre-
−
−
sented in Table 1. It can be seen that 2-PyHA , 2-PyOx
−
kobs = k + k [Nu ]
o
2
(3)
(4)
−
and SOx ions had more eꢁciency to attack the electro-
philic center oꢀ phosphate ester than other nucleophile. We
have applied two types oꢀ electrophilic model substrate, i.e.
P=O (paraoxon) and P=S (parathion and ꢀenitrothion); the
more electronegativity oꢀ oxygen has more tendencies to
attract the nucleophile as compared to the sulꢀur substituted
electrophiles. Fenitrothion has extra methyl group, i.e. elec-
tron donor group which response to repulsion takes place
between nucleophile and ꢀenitrothion. All nucleophile
shows more reactivity towards the P=O center than P=S
center oꢀ ester. The order oꢀ reactivity ꢀor all the substrate
−
+ kHO[HO ]
ko = k
H O
2
3
.3 Eꢀect of Nucleophiles
The plots oꢀ kobs versus nucleophile concentration ꢀor the
−
reactions oꢀ phosphate ester with HA and Ox anions
−
were linear with positive intercepts k . This indicates that
o
−
the contribution oꢀ other nucleophiles i.e. H O/OH is
,
2
negligible and hydroxamate and oximate ions are very
strong nucleophiles ꢀor the hydrolysis oꢀ phosphate esters
1
3

H. K. Dewangan et al.
note that salicylaldoximate ions may act as a biꢀunctional
catalyst (Scheme 3) at operational pH (pH=10) in which
both oꢀ the oximate and phenolate ions attack at the P=X
(O, S) center [32].
3
.4 Hydrolytic Reaction in Micellar Media
The kobs values ꢀor the reactions in micellar system are
evaluated through kobs versus surꢀactant concentration pro-
ꢃles. At ꢃxed concentration oꢀ ester and nucleophile, under
pseudo-ꢃrst order conditions, with increasing the concen-
tration oꢀ surꢀactant gives kobs versus [surꢀactant] proꢃles
which go through a maximum. This maximum corresponds
to the balance point between a rate increase caused by the
transꢀer oꢀ the nucleophile into the micellar pseudophase
and a rate decrease due to dilution oꢀ nucleophile as the
surꢀactant concentration increases. The extent oꢀ micel-
lar catalysis is expected to be depending on the relative
amount oꢀ substrate incorporated in micelles. Thereꢀore, we
describe here an examination oꢀ micellar mediated nucleo-
philic reactivity oꢀ model substrate, ꢀollowed by their appli-
cation to decomposition oꢀ OP pesticides i.e. paraoxon,
parathion and ꢀenitrothion.
−
Fig. 2 Plots oꢀ kobs versus [SOx ] ꢀor the reaction oꢀ paraoxon, par-
−4
athion and ꢀenitrothion. Reaction condition: [Ester]=1×10 M,
pH=10, Temp.=27 °C
−
4
The kobs values ꢀor the reaction oꢀ 1.0×10 M phos-
−
phate esters with 1.0×10 M nucleophiles at pH 10 were
3
−
determined under micellar conditions. All the HA and
−
Ox ions showed signiꢃcant catalysis ꢀor hydrolysis oꢀ
esters. The results ꢀollow the typical biphasic pattern. Cati-
onic micelles catalyzed the reaction and kobs passed through
maxima at the value closed to the critical micelle concen-
tration (cmc) oꢀ the surꢀactants. The positions oꢀ rate max-
ima are independent oꢀ the type oꢀ surꢀactants but the mag-
nitude oꢀ the observed rate constants depends on the types
oꢀ surꢀactant. The variation oꢀ rate constants below the
cmc is diꢁcult to quantiꢀy due to reactant induced micel-
lization and interaction with nonmicellized surꢀactants.
Scheme 3 Biꢀucntional attack oꢀ salicyladoximate ion on P=O and
P=S center oꢀ ester
−
The reactions are ꢀaster with 2-PyHA in comparison to
other nucleophiles which may be due to strong interaction
−
with the micelles. SOx showed signiꢃcant rate accelera-
can be predicted as paraoxon>parathion>ꢀenitrothion.
We also compared the nucleophilicity oꢀ all pyridine based
nucleophile towards the hydrolysis oꢀ phosphate ester. We
tion ꢀor the reaction oꢀ phosphate ester due to its intrinsic
nucleophilicity and efective micellar interaction at micel-
lar stern layer. The structure oꢀ surꢀactant head group plays
an important role towards the molecular packing oꢀ organ-
ized selꢀ-assemblies [45]. An increase in the head group
size leads to an increase in the interꢀace area, and the space
between two head group is also enhanced [46]. The space
between two head groups allows the α-nucleophiles to solu-
bilize itselꢀ smoothly at the interꢀacial region. Because oꢀ
the enhanced interꢀacial region, the concentration oꢀ nucle-
ophile and substrate increased and showed higher reactivity
(Scheme 4). The micellar-mediated nucleophilic reactivity
oꢀ nucleophiles are strongly depends on the structure and
charge oꢀ the surꢀactant head group. Analysis oꢀ kinetic
−
−
observed that the 2-PyHA and 2-PyOx had more abili-
ties to cleave the ester because oꢀ the presence oꢀ lone pair
electron oꢀ nitrogen in ortho position to the hydroxamate
and oximate ꢀunction oꢀ 2-PyHA and 2-PyOx thereꢀore, it
increased the nucleophilicity compared to presence in para
position oꢀ 4-PyHA and 4-PyOx; hence, it behaves as an
excellent nucleophile than other pyridine based nucleo-
−
philes. Comparison oꢀ the reactivity oꢀ Ox ions with those
−
oꢀ HA ions ꢀor the reaction oꢀ ester provided a reliable
−
evidence ꢀor the large α-efect oꢀ Ox ions. In general, Ox
−
−
ions are more reactive than HA ions. It is interesting to
1
3

Kinetic Investigation of Micellar Promoted Pyridine based Oximate and Hydroxamate Catalysis…
oꢀ phosphate esters. The overall reaction rate is the sum oꢀ
the rate in each pseudophase and depends on the rate con-
stants and reactant concentrations in each pseudophase
[27]. The diferent reactivities in the aqueous and micel-
lar pseudophases have been taken into account through the
w
corresponding second-order rate constants: k and k .
m
2
2
w
The values oꢀ k2 have been obtained by studying the reac-
−
−
tion in absence oꢀ the surꢀactant. The Ox and HA con-
centration in the micellar pseudophase has been deꢃned as
the local, molar concentration within the micellar pseudo
−
T
−1
−
̄
phase: [Nu ] = [Nu ] ∕DV, where V is the molar volume
̄
m
3
̄
in dm mol oꢀ the reaction region and [D]V denotes the
micellar ꢀractional volume in which the reaction occurs.
̄
We assume V equal to the partial molar volume oꢀ the
interꢀacial reaction region in the micellar pseudophase,
3
determined by Bunton as 0.14 dm mol [49]. Micellar
−1
−
binding oꢀ substrate and Nu , is governed by hydropho-
substrate
bic interactions and the equilibrium constants Km
and
Nu
Km are expressed by reꢀerring these concentrations to the
total volume oꢀ the observed rate constants, kobs, based on
Scheme 5 and on the above considerations, is given by the
ꢀollowing equation:
−
Scheme 4 Enhance the nucleophilicity oꢀ HA and Ox in cationic
−
m
k2
k + Km
−
Km [D]
w
substrate Nu
micellar media
2
V̄
−
kobs = ꢀ1 + K
ꢀ
ꢁꢁ[Nu ]
(5)
−
[D] 1 + K [D]
T
substrate
m
Nu
m
data indicates that CTAB showed higher reactivity than
TTAB ꢀor the reaction oꢀ paraoxon, parathion and ꢀeni-
trothion. The increase oꢀ kobs values with increasing chain
length oꢀ the surꢀactant, that is, with an increasing aggrega-
tion number oꢀ micelles, is mainly due to the increase in the
electrical surꢀace potential and partially due to an increase
in hydrophobicity oꢀ the palisade layer oꢀ the micelle.
m
Second-order rate constants (k ) at the micellar inter-
2
−
−
ꢀace and association constants oꢀ the Ox and HA ions to
the cationic micelles were ꢃtted by Eq. 5 to the experimen-
tal data. Figures 3, 4 and 5 shows the rate-surꢀactant pro-
ꢃles ꢀor the reaction oꢀ paraoxon, parathion and ꢀenitrothion
−
−
−
−
−
with SOx , 2-PyOx , 4-PyOx , 2-PyHA , 3-PyHA and
−
-PyHA in CTAB (A) and TTAB (B) cationic micel-
4
3
.5 Application of Pseudophase Model [PPM]
lar media. It is possible to evaluate the association con-
Nu
stant ꢀor micelle-nucleophile interaction (K ) ꢀrom Eq. 5.
SOx
The micellar association constant oꢀ SOx, Km =478.9
m
For the quantitative treatment oꢀ surꢀactants on the rate
oꢀ bimolecular reactions are rationalized by pseudophase
model [47, 48]. In this model the eꢁcacy oꢀ nucleophiles in
both pseudophase have determined towards the hydrolysis
and 453.36 in CTAB and TTAB micelles respectively. It
Ox
shows the association constants oꢀ oximate, K , is rela-
HA
tively higher than hydroxamate Km in both the cationic
m
micelles (Tables 2, 3, 4). Larger micellar interaction oꢀ
−
SOx results greater micellar catalysis (k / k ) oꢀ the
m
w
2
2
ꢀactor ≈118 and 37 in CTAB and TTAB micelles respec-
tively with paraoxon. However, signiꢃcant acceleration oꢀ
−
kobs values have been observed ꢀor the reaction oꢀ SOx
in CTAB and TTAB micelles, which may be due to the
−
cooperative intrinsic nucleophilicity oꢀ SOx with aggre-
gate micelle. The N–OH groups are considerably ionized
−
as N–O at operational pH and thereꢀore, also bind to the
quaternary ammonium through electrostatic interactions.
−
-PyHA showed 143- and 117-ꢀold micellar catalysis in
2
CTAB and TTAB respectively. The satisꢀactory ꢃt obtained
ꢀrom these experiments supports the validity oꢀ the model
Scheme 5 Pseudophase model
1
3

H. K. Dewangan et al.
Fig. 3 Simulated rate-surꢀactant proꢃle ꢀor the reaction oꢀ paraoxon with hydroxamate and oximate ions in the presence oꢀ CTAB (a) and TTAB
(
−
4
−
b). Reaction condition: [Paraoxon]=1.0×10 M, [Nu ]=1.0×10 M, pH=10, Temp.=27 °C
−3
Fig. 4 Simulated rate-surꢀactant proꢃle ꢀor the reaction oꢀ parathion with hydroxamate and oximate ions in the presence oꢀ CTAB (a) and
−
4
−
−3
TTAB (b). Reaction condition: [Parathion]=1.0×10 M, [Nu ]=1.0×10 M, pH=10, Temp.=27 °C
Fig. 5 Simulated rate-surꢀactant proꢃle ꢀor the reaction oꢀ ꢀenitrothion with hydroxamate and oximate ions in the presence oꢀ CTAB (a) and
−
4
−
TTAB (b). Reaction condition: [Fenitrothion]=1.0×10 M, [Nu ]=1.0×10 M, pH=10, Temp.=27 °C
−3
1
3

Kinetic Investigation of Micellar Promoted Pyridine based Oximate and Hydroxamate Catalysis…
Table 2 Kinetic parameters obtained by applying pseudophase
model ꢀor the reaction oꢀ paraoxon in the presence oꢀ cationic
micelles
oꢀ rate enhancement oꢀ nucleophiles is CTAB>TTAB. All
nucleophile shows higher rate constant with paraoxon than
parathion and ꢀenitrothion. This may be due to the incor-
poration oꢀ paraoxon and parathion into the micelle core
and would be inert to hydroxide ion and nucleophile cata-
lyzed hydrolysis. It can be seen that the alliance oꢀ oxime
group oꢀ pyridinealdoxime with surꢀactants are more ꢀacile
than hydroxamic group oꢀ pyridinehydroxamic acid, there-
−
Nu ]
5
10 k (M
w
2
−1 −1
s
Nu
Km (M
−1
3
m
10 k (M
−1 −1
s )
[
)
)
2
CTAB TTAB CTAB
TTAB
SOx
4.81± 0.02
478.59 453.36 5.70± 0.1 3.80± 0.1
467.35 434.53 3.20± 0.1 2.10± 0.1
415.63 447.36 2.70± 0.1 1.95± 0.1
389.96 381.54 2.35± 0.1 1.92± 0.1
385.63 361.44 1.40± 0.1 1.53± 0.1
314.10 368.91 0.42± 0.1 1.05± 0.1
2-PyOx 4.67± 0.01
4-PyOx 3.10± 0.01
2-PyHA 1.64± 0.01
3-PyHA 1.08± 0.01
4-PyHA 0.16± 0.01
ꢀore the surꢀactants carrying reactants in close proximity
by binding the substrates via hydrophobically binding and
coulombically attracting the negatively charged nucleophile
oꢀ oxime and hydroxamic acid.
Micellar association constant oꢀ paraoxon in CTAB and TTAB
paraoxon
−1
(Km
)=200 M
4
Conclusions
Table 3 Kinetic parameters obtained by applying pseudophase
model ꢀor the reaction oꢀ parathion in the presence oꢀ cationic
micelles
Kinetic studies relevant to pesticide detoxiꢃcation are
reported ꢀor the hydrolysis oꢀ model phosphate esters in
aqueous bufer and alkyltrimethylammonium bromide with
−
Nu ]
5
10 k (M
w
2
−1 −1
s
Nu
Km (M
−1
3
10 k (M
m
−1 −1
s
[
)
)
)
−
−
−
2
Ox and HA . Comparison oꢀ the reactivity oꢀ Ox ions
−
with those oꢀ corresponding HA ions ꢀor the reaction oꢀ
CTAB TTAB CTAB
TTAB
triester provides a reliable evidence ꢀor the large α-efect
−
oꢀ Ox ions. The pK value oꢀ PyHA and PyOx decreased
SOx
3.50± 0.13
394.64 461.96 3.10± 0.1 2.10± 0.1
482.38 414.70 1.70± 0.1 2.10± 0.1
328.95 431.69 1.40± 0.1 2.40± 0.1
345.61 365.34 1.70± 0.1 0.50± 0.1
345.76 299.25 1.20± 0.1 0.50± 0.1
325.62 298.45 1.10± 0.1 0.50± 0.1
a
2-PyOx 3.25± 0.12
4-PyOx 2.96± 0.12
2-PyHA 0.17± 0.13
3-PyHA 0.11± 0.10
4-PyHA 0.08± 0.12
in the cationic micelle due to an enhanced local pH at the
cationic surꢀace oꢀ the aggregate, and this is not expected
−
to modiꢀy the nucleophilicity oꢀ the micellar HA s. The
micellar-mediated hydrolysis oꢀ paraoxon, parathion and
−
enitrothion with SOx /CTAB in oximate and 2-PyHA /
−
ꢀ
Micellar association constant oꢀ parathion in CTAB and TTAB
parathion
CTAB in hydroxamate are most efective system ꢀor the
−1
(Km
)=366 M
m
cleavage oꢀ OP pesticides. The ratio k /k ꢀor the hydrol-
w
2
2
ysis oꢀ paraoxon is much larger relative to parathion and
enitrothion.
ꢀ
Table 4 Kinetic parameters obtained by applying pseudophase
model ꢀor the reaction oꢀ ꢀenitrothion in the presence oꢀ cationic
micelles
Acknowledgements Financial support ꢀrom Department oꢀ Science
and Technology (DST), New Delhi (Grant No. SR/FT/CS-39/2011,
dated 01.08.2012) is grateꢀully acknowledged.
−
Nu ]
5
10 k (M
w
2
−1 −1
s
Nu
Km (M
−1
3
m
10 k (M
−1 −1
s )
[
)
)
2
CTAB TTAB CTAB
TTAB
Supporting Information Determination oꢀ acid dissociation con-
stant (pK ) oꢀ oxime and hydroxamic acid, Repeat scan spectra oꢀ
a
SOx
0.060± 0.11
374.02 103.70 0.20± 0.1 0.20± 0.1
307.65 96.61 0.08± 0.1 0.20± 0.1
235.36 92.20 0.08± 0.1 0.10± 0.1
349.07 148.64 0.04± 0.1 0.06± 0.1
255.24 90.07 0.05± 0.1 0.08± 0.1
339.43 88.83 0.03± 0.1 0.07± 0.1
production oꢀ 3-methyl-4-nitrophenoxide ion (λmax =398 nm) ꢀor the
reaction oꢀ ꢀenitrothion and nucleophilic efect data are available in
supporting inꢀormation.
2-PyOx 0.043± 0.10
4-PyOx 0.031± 0.10
2-PyHA 0.032± 0.11
3-PyHA 0.023± 0.11
4-PyHA 0.020± 0.11
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