Reactivity studies of carbon, phosphorus and sulfur-based acyl sites with tertiary oximes in gemini surfactants

Article abstract of DOI:10.1002/poc.3142

Kinetic studies of the reactions of tertiary oximes (monoisonitroso acetone; MINA and butane 2,3 dione monooxime; BDMO) with some carboxylate (p-nitrophenyl acetate and p-nitrophenyl benzoate), phosphate (p-nitrophenyl diphenyl phosphate and bis (2,4-dini

Full text of DOI:10.1002/poc.3142

Research Article
Received: 7 March 2013,
Revised: 23 April 2013,
Accepted: 29 April 2013,
Published online in Wiley Online Library: 18 June 2013
(wileyonlinelibrary.com) DOI: 10.1002/poc.3142
Reactivity studies of carbon, phosphorus and
sulfur-based acyl sites with tertiary oximes in
gemini surfactants
Bhanushree Guptaa, Rahul Sharmaa, Namrata Singha, Yevgen Karpichevb,
Manmohan L. Satnamia and Kallol K. Ghosha*
Kinetic studies of the reactions of tertiary oximes (monoisonitroso acetone; MINA and butane 2,3 dione monooxime;
BDMO) with some carboxylate (p-nitrophenyl acetate and p-nitrophenyl benzoate), phosphate (p-nitrophenyl
diphenyl phosphate and bis (2,4-dinitrophenyl) phosphate) and sulfonate (p-nitrophenyl p-toluene sulphonate)
esters in gemini surfactants have been conducted. The observed first-order rate constant versus surfactant profiles
show micelle-assisted bimolecular reactions involving interfacial ion exchange between bulk aqueous media and
micellar pseudophase. Experimental results showed that MINA exhibited better nucleophilic activity towards ester
cleavage than BDMO. Pseudophase model has been applied in order to determine micellar second-order rate
constants and binding constants. Copyright © 2013 John Wiley & Sons, Ltd.
Supporting information may be found in the online version of this paper
Keywords: a-nucleophile; tertiary oxime; gemini surfactant; organophosphates; kinetics
bis (2,4-dinitrophenyl) phosphate (BDNPP) with benzohydroximate
INTRODUCTION
anion kinetically. Recently, Renard[35] et al. synthesized a series of
new a-nucleophiles (oximes and amidoximes) and examined their
ability to cleave P–S bond of organophosphorus nerve agents.
Oximes are potent a-effect nucleophiles, as they exhibit characteris-
tic reactivity than that predicted by Bronsted-type relationship of
nucleophilicity and basicity.[36] Tertiary oximes are uncharged ox-
imes and are proved to be better reactivating agents for inhibited
cholinesterase in the central nervous system (CNS).[37,38]
Monoisonitroso acetone (MINA) (pKa = 8.34) is highly lipid soluble
oxime, which can easily penetrate the blood–brain barrier to
reactivate inhibited AChE in CNS.[39]
It is an established fact that micelles and other self-organized
assemblies enhance the reactivity of nucleophiles.[40] Currently,
some novel surfactants have been reported,[41,42] which produce
different types of micellar aggregates upon solubilization in
water. Among all the surfactants, gemini[43,44] seems to be an at-
tractive host due to lower critical micelle concentration (CMC),
high surface activity, low Krafft point, higher viscoelasticity and
enhanced tendency to lower the oil–water interfacial tension in
comparison to other conventional surfactants.[45–47] Several
research groups[48,49] have been investigating the micellar-
The facile hydrolysis of organophosphate esters is ubiquitous
and has drawn great attention due to the importance in various
chemical and biological reactions.[1–5] These highly toxic,
lipophilic and life-threatening compounds are used as chemical
warfare agents (sarin, soman, tabun, etc.), pesticides and
insecticides[6] (parathion, paraoxon, diazinon, etc.). Due to
extreme toxicity, safety and other licensing reasons, few
molecules, known as simulants or surrogates, are used to find
mechanism of their hydrolysis.[7] Carboxylate, phosphate and
sulphonate esters are few existing simulants exhibiting limited
toxicity and are comparatively less expensive as well.
They mimic similar behavior as nerve agents and can be employed
as a safer substrate with less vigorous laboratory settings.[8] Hence,
different types of nucleophiles (oximates,[9] hydroxamates,[10] per-
oxides,[11,12] o-iodosocarboxylates,[13] hydroxybenzotriazoles,[14–16]
functionalized surfactants,[17] etc.) along with amphiphilic moie-
ties, metallosurfactants,[18] solid supports and latexes[19,20]have
been applied as hydrolytic micellar catalysts.[21–23] For many years,
efforts have been devoted to design and evaluate the efficiency
of a-nucleophiles as decontaminants by several research
groups.[24–29] The most systematic study of the effect of a series
of a-nucleophiles oximates with pKa values in the range of 7.7–
11.8 on the degradation of pesticide fenitrothion in cationic
miceller media, has been provided by Buncel et al.,[30] and further
kinetic data were interpreted in terms of pseudophase ion ex-
change model. Nucleophilic substitution of p-nitrophenyl diphenyl
phosphate (PNPDPP) with butanedione mono-oximate in aqueous
cetyltrimethylammonium bromide were performed by Ruasse and
workers.[31] The reaction of hydroxylamine with 2,4-dinitrophenyl
diethyl phosphate have been monitored by Nome research
group.[32] The group[33,34] further examined the hydrolysis of diester
* Correspondence to: K. K. Ghosh, School of Studies in Chemistry, Pt.
Ravishankar Shukla University, Raipur (C.G), 492010, India.
E-mail: kallolkghosh@yahoo.com
a B. Gupta, R. Sharma, N. Singh, M. L. Satnami, K. K. Ghosh
School of Studies in Chemistry, Pt. Ravishankar Shukla University, Raipur, C.G,
492010, India
b Y. Karpichev
L.M. Litvinenko Institute of Physical Organic and Coal Chemistry, Donetsk
83114, Ukraine
J. Phys. Org. Chem. 2013, 26 632–642
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REACTIVITY STUDIES OF CARBON, PHOSPHORUS AND SULFUR-BASED ACYL SITES
catalyzed hydrolysis of various toxic and non-toxic simulants
MATERIALS AND METHOD
using a-effect nucleophiles. Generally, the standard simulants
are persistent and hydrophobic in nature; therefore, aqueous
solutions of micellar aggregates are employed for their
hydrolysis and dephosphorylation reactions. Bhattacharya[50]
et al. have examined the esterolytic reaction of phosphotriesters
(p-nitrophenyl diphenyl phosphate and p-nitrophenyl hexanoate)
in cationic micellar media using 4,4’-(dialkylamino)pyridine-
based nucleophiles. Many research groups[51,52] have investi-
gated the hydrolysis of different esters using butane 2,3
dione monooxime (BDMO) as a cleaving agent. A compara-
tive study of catalytic hydrolysis of carboxylate esters using
conventional and cationic gemini surfactant has been
monitored by Zeng[53] and co-workers. Gemini surfactants
containing hydroxyl and ester groups in spacer have recently
been synthesized, and alkaline hydrolysis of paraoxon
(4-nitrophenyl diethyl phosphate) and arminum (4-nitrophenyl
diethyl phosphonate) in the presence of gemini micelles was
studied by Popov[54,55] and research group.
To the best of our knowledge very limited work has been done
in the detoxification of chemical simulants using tertiary oximes in
water-soluble gemini micellar media. Over the last few years, we
have examined the catalytic efficiencies of various a-nucleophiles
against ester hydrolysis in self-organized systems.[56–61] As a
continuation of these investigations herein, we have examined
the nucleophilic micellar-mediated hydrolytic reactions of
(shown in Chart 1 and Scheme 1) p-nitrophenyl acetate (PNPA),
p-nitrophenyl benzoate (PNPB), PNPDPP, BDNPP and p-nitrophenyl
p-toluene sulphonate (PNPTS) using MINA at pH 9.0 at 27 ꢀC. The
observed rate constants were compared with that of BDMO. Two
cationic water-soluble gemini surfactants (alkanediyl-a,o bis
(hydroxyethylmethylhexadecylammonium bromide), 16-s-16, MEA
2Brꢁ (s = 4, 6) have been used as micellar catalysts. The acid
dissociation constants (pKa) of both the nucleophiles have also been
determined kinetically and spectrophotometrically.
Materials
PNPA was procured from Fluka (USA), and PNPB was procured
from Lancaster (Lancashire, England). PNPDPP and PNPTS were
prepared at Defence Research Development Establishment,
Gwalior (India), and bis(2,4-dinitrophenyl) phosphate (BDNPP)
was synthesized in the Department of Chemistry, Federal Univer-
sity of Santa Catarina, Florianopolis-SC, Brazil. Dimeric gemini
surfactants were synthesized in the laboratory of P. Quagliotto,
Department of Chemistry, University of Torino, Torino, Italy.
MINA and BDMO were purchased from Sigma/Aldrich (St. Louis,
Missouri, U.S.A). All solutions of oximes were prepared in triple
distilled water.
Methods
Surface tension measurement
The surface tension of investigated gemini surfactants at pH 9.0
was determined with a surface tensiometer (Jencon, India) using
a platinum ring by the ring detachment technique. The tensiom-
eter was calibrated against distilled water. Platinum ring was
thoroughly cleaned and dried before each measurement. The
ring was hung to the balance, dipped into the solution and then
pulled out. The maximum force needed to pull the ring through
the interface was measured and correlated to the surface
tension. Each experiment was repeated several times until good
reproducibility was achieved. The results were accurate within
ꢂ0.1 mNmꢁ1
.
Determination of acid dissociation constant (pKa)
The acid dissociation constant (pKa) values of both the tertiary
oximes were determined in absence and presence of 0.25 mM
of 16-s-16 MEA 2Brꢁ spectrophotometrically by using the
method of Albert and Sergeant.[62] The method depends on
the direct determination of the ratio of molecular species
(protonated) to dissociated (deprotonated) species in buffer so-
lutions. An aliquot (3 ml) of a stock solution (5ꢃ 10ꢁ4 M) of ox-
ime in triple distilled water was diluted with 25 ml phosphate
buffer solution. Different pH values ranging from 7.12 to 10.5
were selected to determine the pKa values of oximes. The
pH of the solution was measured using Systronics (Type-362)
pH-meter, and the spectrum was recorded using buffer solution
as a blank. The absorption spectrum was recorded using Varian
Cary 50 UV–VIS spectrophotometer in the range of 200–400 nm
in the solutions of different acidity. The average values of ten
measurements were considered as the pKa of the compound
with respect to oximino functionality. The spectrophotometric
determination of pKa of MINA is reproduced in Fig. 4 at a
temperature 27 ꢀC. The pKa calculations were made around half
neutralization using following equation:
O
R
C
O
NO2
O
R= CH3, (PNPA)
R= C6H5, (PNPB)
O
P
O
NO2
O
PNPDPP
NO2
O
O
NO2
H3C
S
O
NO2
O2N
O P
O
O
-O
PNPTS
BDNPP
NO2
O
O
N
H3
C
OH
N
H3C
OH
CH3
MINA
BDMO
AbsΨ ꢁ AbsHox
pKa ¼ pHexp ꢁ log
(1)
2 Br
CH2CH2OHCH2CH2OH
(CH2)s CH3
C16H33 C16H33
(s = 4, 6)
Absox ꢁ AbsΨ
H3C
N
N
Where, AbsHOx is the absorbance of unionized form of com-
pound, Absc is the absorbance of partially ionized form of
compound and AbsOx is the absorbance of completely ionized
form of compound.
Gemini surfactant (C16-s-C16, MEA 2Br-).
Chart 1. Substrates, nucleophiles and surfactants used in the pres-
ent investigation
J. Phys. Org. Chem. 2013, 26 632–642
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B. GUPTA ET AL.
Gemini
Surfactant
O
C
O
C
-O
NO2
R
O
NO2
Nu-
R
Nu
R= CH3, p -nitrophenyl acetate (PNPA)
R= C6H5, p -nitrophenyl benzoate (PNPB)
NO2
NO2
O2N
-O
Gemini
Surfactant
O
O
NO2
O2N
O P
Nu
O2N
O
P
-O
NO2
Nu-
O
-O
NO2
bis (2,4-dinitrophenyl) phosphate (BDNPP)
Gemini
Surfactant
O
O
Nu-
-O
NO2
H3C
S
O
NO2
H3C
S
Nu
O
O
p-nitrophenyl p-toluene sulphonate (PNPTS)
Scheme 1. Nucleophilic attack of Nu- (tertiary oximate ions) at C = O, P = O and S = O centers
Kinetic measurements
PNPTS) concentration was kept same for all the reactions
(0.5ꢃ 10ꢁ4 M). The kinetic study was performed at various
concentrations of gemini surfactant to investigate the effect of
surfactant on the cleaving efficiency of oximes.
The reactions were studied spectrophotometrically with a Varian
Cary 50 spectrophotometer by monitoring the appearance of
the leaving p-nitrophenoxide (for PNPA, PNPB, PNPDPP and
PNPTS) and 2,4-dinitrophenoxide ion (for BDNPP) ion at 400 nm
at 27 ꢂ 0.2 ꢀC as represented in Fig. 1. Entire kinetic experiments
were performed at an ionic strength of 0.1M KCl. Each experiment
was repeated at least twice, and the observed rate constants were
found to be reproducible within a precision of about 3% or better.
Phosphate buffer (0.1M) was employed to control the pH of the
media. All the pH measurements were obtained using a Systronics
pH meter (type 362). The reactions were conducted under
pseudo-first-order conditions, i.e. large excess of oximate anions
over the substrate (1:10). For all the kinetic runs, the absor-
bance/time result fits very well with the first-order rate Eqn 2.
RESULTS AND DISCUSSION
We have studied the surface properties of investigated gemini
surfactants in water and buffer medium inclusive of their profi-
ciency as strong micellar catalyst for various esterolytic reactions.
The comparative nucleophilic efficacy of both the tertiary oximes
has also been examined by the means of pH effect, nucleophilic
effect, substrate effect and by applying the cationic gemini
micro-organized media as a catalyst. The pseudo-first-order rate
constants were determined for all the kinetic runs.
lnðA1 ꢁ AtÞ ¼ lnðA1 ꢁ A0Þ ꢁ kt
(2)
Surface properties of gemini surfactants in buffer medium
With the surface tension measurements, several parameters
referring to interfacial properties have been determined at
The pseudo-first-order rate constants (kobs) were determined
from the plots of absorbance versus time with A0, At and A
being the absorbance values at zero, time and infinite time,
1
pH 9.0: (i) Surface excess concentration of surfactant (Γmax
)
and (ii) minimum area per molecule at the air–water interface
(Amin) were determined from surface tension data using Eqns
(3) and (4)
respectively. The substrate (PNPA, PNPB, PNPDPP, BDNPP and
ꢀ
ꢁ
1
dg
Γmax ¼ ꢁ
(3)
(4)
2:303nRT d logC
T;
P
Amin ¼ 1=N Γmax
Where, R is the gas constant (8.314 Jmolꢁ1 Kꢁ1), T is the abso-
lute temperature, C is the surfactant concentration, (dg/dlogC) is
the slope of the g versus logC plot taken at the CMC and N is
Avogadro’s number. Here, constant (prefactor) n = 3 for investi-
gated cationic gemini surfactant system which is made up of a
divalent surfactant ion and two univalent counterions.[63,64] The
value of the surface pressure at the CMC (pCMC) was obtained
from Eqn (5),
Figure 1. UV spectra collected at different reaction times showing the
increase in absorbance of p-nitrophenoxide ion for the cleavage of PNPB
with BDMO in the presence of gemini surfactant (C16-6-C16, MEA 2Brꢁ).
Reaction conditions: [PNPB] = 0.5ꢃ 10 ꢁ 4 M, [BDMO] = 0.5ꢃ 10 ꢁ 3 M,
[KCl] = 0.1 M, [16-s-16] = 0.5ꢃ 10 ꢁ 3 M, pH = 9.0, Temp. = 27 ꢀC
pcmc ¼ gο ꢁ gCMC
(5)
where go indicates the surface tension of pure solvent, and gCMC
is the surface tension at the CMC. This parameter shows that the
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REACTIVITY STUDIES OF CARBON, PHOSPHORUS AND SULFUR-BASED ACYL SITES
maximum reduction of surface tension is due to the adsorption
of surfactant molecules and observed when surfactant begins
to micellize and not adsorb anymore at the surfaces.[65]
Therefore, it demonstrates the effectiveness of the surface
tension reduction. Hence, greater the pCMC values, higher the
effectiveness of the surfactants pCMC values are listed in
Table 1.
Gemini surfactants has extremely low CMC (as presented in
Fig. 2), and this unusual tendency of low CMC is due to confor-
mational changes and higher magnitude of energy of transfer
coming from both the tails.[66,67] The low CMC for 16-s-16 MEA
2Brꢁ are in general agreement with the reported values previ-
ously at 30 ꢀC.[61] It is confirmed from the Table 1 that CMC of
MEA-type surfactants reduce significantly in the buffer medium
as compared to the aqueous solution of 16-s-16 type cationic
gemini surfactants indicating that pH has a direct effect on
surface properties. This is clearly inferred from the observed
data that, there is very little increase in the values of pCMC and
pKa value (8.40) for the MINA as represented in Fig. 4 (a).
Typically, the pseudo-first-order rate constants for reaction of
PNPA were determined at different pH values between 6.7
and 11.0. The significant effect of pH was observed on
pseudo-first order rate constant rate of the nucleophilic reac-
tions. The most prominent advantage observed from these
experiments were at pH > pKa, due to the appearance of
completely dissociated oxime functionality (–CH = NOꢁ).
Figure 4 (a) represents the plots of kobs and fraction of MINA
ionized (degree of nucleophilic dissociation; a) versus pH for
the reaction of PNPA with MINA. Owing to the assumptions,
the ionization state of MINA (Nuꢁ) is responsible for the catalysis
in nucleophilic aided hydrolysis reaction. Hence, Eqn (6) may
give the actual value of kobs
.
HA
obs
k
¼ k0obs þ kHA ꢁ ½HAꢄ a
(6)
ꢁ
HA
T
Nucleophile MINA has only one proton active site which
releases the reactive species as shown in Eqn (7)
A
min as spacer chain length increases from 4 to 6 where as Гmax
decreases. The calculated surface properties indicate that
spacer chain length effect is not much pronounced in the inves-
tigated gemini surfactants.
kobs ¼ k0obs þ k
ꢁ ½MINAꢄT aMINHA
(7)
ꢁ
MINA
Since aꢁMINA is equal to Ka/Ka + [H+] and Ka represents kineti-
cally apparent dissociation constant of MINA, it is confirmed
that at pH 9.0, i.e. above the pKa of MINA, mono-anion is the re-
active species.[56–59]
Acid dissociation constant (pKa)
As oximate anion takes part in the hydrolysis reaction, it was
necessary to follow the pKa and pH effect on the deprotonation
of the oximes. The pKa of both the oximes were determined
spectrophotometrically and kinetically (Fig. 3). The observed
values are in good agreement with the literature values.[39,52]
To follow the effect of surfactants under the reaction medium
on the acid dissociation constants, pKa of nucleophiles have
been determined in the absence and presence of 0.25 mM
gemini surfactants. It can be concluded from the Table 2 that,
pKa of both the nucleophiles decrease slightly in the
presence of gemini surfactants illustrating increased ionization
of nucleophiles in miceller core. Hence, the observed results
favor the shift of acid base equilibria (Scheme 2) and accounts
for the higher rate constants. Observed pKa values support MINA
to be superior Nuꢁ than BDMO.
Effect of nucleophile [Nuꢁ]
The nucleophilic concentration-dependent first-order rate con-
stants were determined spectrophotometrically for the reaction
Effect of pH
Study of pH effect helps to evaluate the pKa of Nuꢁ and enables
to control the reaction rate at preferable conditions. In support
of the above mentioned facts, the effect of pH was studied on
cleavage of carboxylate ester with 0.5 ꢃ 10ꢁ3 M of MINA. It
was observed that the rate of reaction increased with the rise
in pH. A pH versus rate constant profile for the nucleophilic
cleavage of PNPA and PNPB by MINA ion gave the apparent
Figure 2. Plots of surface tension versus log conc. of C16-s-C16, MEA
2Brꢁ at pH 9.0
Table 1. Surface properties of gemini surfactants (C16-s-C16, MEA 2Brꢁ) in water and buffer medium (pH = 9.0)
Surfactants
Medium
CMC(mM)
ΠCMC mNmꢁ1
Гmax 106 mol mꢁ2
Amin 1020 m2
C16-4-C16, MEA 2Brꢁ
Watera
Buffer
Watera
Buffer
2.000
0.035
3.000
0.045
32.0
7.50
34.0
10.9
1.390
0.417
1.270
0.405
119.4
398.1
130.7
409.9
C16-6-C16, MEA 2Brꢁ
aRef 41,42
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B. GUPTA ET AL.
Figure 3. Representative absorption spectra of MINA, c = 5.3ꢃ 10–5 M; in water, Temp.=27 ꢀC. (a) in the absence of gemini surfactants at different pH
values. (b) in the presence of gemini surfactants at different pH values
Ka
O
NOH
O
NO-
-H+
Table 2. Acid dissociation constants (pKa) of MINA and
BDMO [0.2ꢃ10ꢁ4 M] in the absence and presence of 16-s
ꢁ16 MEA 2Brꢁ (2 5 ꢃ 10ꢁ4 M) by spectrophotometric method
at Temp. = 27 ꢀC
+H+
H3C
H
H3C
H
Scheme 2. pH-dependent ionization of monoisonitroso acetone
Gemini
pKa
surfactant
-
-
MINA
BDMO
kobs ¼ kNu ½Nu ꢄ
(10)
Nil
16-4-16
16-6-16
8.40
8.11
8.15
9.50
9.02
9.10
Table 4 provides apparent second-order rate constants for
both the nucleophiles against different substrates. Observed
values of k2,app support MINA to have higher rate constants over
BDMO for all the esters. Taking into account deprotonation de-
grees of both nucleophiles at pH 9.0, we can calculate their nu-
cleophilicities (see Table 4). Indeed, MINA falls on the Bronsted
plot for low basicity nucleophiles (pKa ≤ 9.0) characterized by
the slope aNu ca. 0.5[9,18] whereas BDMO falls on the leveling-
off Bronsted plot for the oximate ions with pKa ≥ 9.0 having neg-
ligible sensitivity to the oxime strucuture. (aNu ꢅ0). Thus, the two
tertiary oximes, MINA and BDMO, have a difference in acidity
constants of one order of magnitude with only two to three
times difference in the nucleophilicities.
of PNPA, PNPB and BDNPP with MINA in excess, at 27 ꢀC and
pH 9.0, as shown in Fig. 5. The data for reaction of carboxylate
and phosphate esters with different concentrations of MINA
and BDMO at pH 9.0 is summarized in Table 3.
The obtained kinetic data completely supports the hypothesis
that tertiary oximate anion act as nucleophilic catalyst for the
hydrolysis of both the esters. Here, Eqn (8) describes the reaction
of carboxylate and phosphate esters with MINA, and k0 corre-
sponds to the intercept of the kobs versus [Nuꢁ] plot in Fig. 5.
Effect of gemini surfactants on the hydrolysis of
different esters
-
-
kobs ¼ k0 þ kNu ½Nu ꢄ
(8)
(9)
The kinetic plots shown in Fig. 6 represent variation of the
absorption of 4-nitrophenoxide ion of (PNPA) and (PNPB) with
time at pH 9.0 for the reaction of MINA with esters in the
presence of 0.5 mM 16-4-16 MEA 2Brꢁ. The plots correspond
to formation of 4-nitrophenoxide ions in the system as a result
of substrate dissociation. Pseudo-first-order rate constants
(kobs) for the nucleophilic cleavage of five different esters
(PNPA, PNPB, PNPDPP, BDNPP and PNPTS) in the presence of
cationic gemini surfactants have been summarized in Table 5
and illustrated in Fig. 7. Among all the investigated esters,
cationic gemini surfactants displayed the highest catalytic
activity towards the hydrolysis of carboxylate ester; PNPA and
lowest activity for the sulphonate ester PNPTS.
-
-
k0 ¼ kH2O þ kOH ½OH ꢄ
The term kH2O in Eqn (9) represents the rate constant for
very weak nucleophile at very low OHꢁ concentration.[56–59]
-
OH
At higher pH, the intercept is governed by the term k
.
Figure 5 gave a straight line with intercept k0 indicating no
competition with other nucleophiles as OHꢁ, H2O for PNPB
and BDNPP. However, in the case of hydrolysis of PNPA, there
is a contributory effect of OHꢁ and H2O (followed Eqn 8).
However, the whole hydrolysis reaction for PNPA, PNPB
and BDNPP at large concentration of nucleophiles can be
represented as
The hydrolytic property of MINA and BDMO was executed
from the rate constant versus [gemini surfactant] profiles
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REACTIVITY STUDIES OF CARBON, PHOSPHORUS AND SULFUR-BASED ACYL SITES
Figure 4. (a) Plots of log kobs versus pH for PNPA and PNPB with MINA (Inset): Plot of first-order rate constants versus pH for the reaction of PNPA with MINA.
(b) Plot of kobs and a versus pH for the reaction of PNPA with MINA. Reaction conditions: [Substrate] = 0.5ꢃ 10–4 M, [MINA] = 0.5ꢃ 10–3 M, [KCl] = 0.1M, Temp.
= 27 ꢀC
Figure 5. Plots for the nucleophilic effect of MINA on the hydrolysis of PNPA and PNPB (a) MINA (b) BDMO
(Fig. 7 and Fig. S1.) for cleavage of different esters at C = O, P = O
and S = O centers. The observed rate constants were dependent
on both the type of nucleophile and gemini surfactant used. The
hydrolysis of carboxylate esters by MINA was facile (PNPA: 3.56 fold
PNPB: 2.90 fold) as compared to BDMO (PNPA: 2.95 PNPB: 1.98
fold) in cationic gemini micellar media. Similarly, the cleavage of
PNPDPP (2.10 fold), BDNPP (1.68 fold) and PNPTS (1.06 fold) was
easier in MINA as compared to BDMO (PNPDPP: 1.79 fold, BDNPP:
1.13 fold PNPTS: 1.02 fold).
Rate-surfactant profile shows that the rate of reaction in-
creases with increasing surfactant concentration up to a certain
concentration and then decreases at higher concentration of
surfactants. The trend supports a typical micellar-assisted
bimolecular reactions. The rate maxima are independent of
the type of surfactants, but the magnitude of rate constant
depends on type of surfactants. The observed rate maxima are
due to the dilution effect of enhanced concentration of gemini
surfactant.[65] Gemini surfactants, linked by different spacer
groups, offer di-cationic head group that is significantly higher
in electrostatic interactions. Hydrophobicity of substrate
and properties of surfactant undoubtedly play key role for
the hydrolytic reactions in micellar media.[66–68] Preferably,
increasing concentration of surfactant releases more cationic
gemini micelles and initially results in increased rate of reaction.
Due to the conformational changes, cationic gemini surfactants
bring reactants in close proximity by binding the substrates via
hydrophobically binding and coulombically attracting the
negatively charged nucleophile[11,12] As the number of micelles
increases, all the substrate molecules get associated in the
micellar phase. Hence, further increment of surfactant enhances
the number of micelles which take up the oximate anions
into the volume element of micellar-assisted reaction by
deactivating the substrate. As a consequence of this, substrate
bound to one micelle becomes unable to react with nucleophile
in another.[52,56–59] It is evident from the observed data (Table 5)
that 16-4-16 MEA 2Brꢁ gemini surfactants are slightly more
efficient catalyst for the hydrolytic reactions as compared to
16-6-16 MEA 2Brꢁ surfactants. It may be due to the low CMC
which favors the immediate micelles formation and provides
higher aggregation number in the reaction media at the same
total surfactant concentration.[60] Another important factor
which supports 16-4-16 to be a better catalyst is the spacer
chain length (s = 4) in comparison to 16-6-16 (s = 6). If s = 4,
elongated spherical or cylindrical micelles are likely forming
J. Phys. Org. Chem. 2013, 26 632–642
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B. GUPTA ET AL.
Table 3. Nucleophilic effect on the hydrolysis of carboxylate esters using 1 = MINA and 2 = BDMO
[Nuꢁ] mM
kobs ꢃ 103 (sꢁ1
)
PNPA
PNPB
BDNPP
2
1
2
1
2
1
Nil
0.5
1
1.5
2.0
2.5
3.0
0.476
13.2
16.0
26.8
32.0
-
0.189
8.74
12.1
20.4
25.8
26.4
28.7
0.578
2.32
4.88
7.01
8.03
0.131
1.91
3.37
6.87
7.34
0.058
0.141
0.179
0.205
0.251
0.298
0.345
0.031
0.079
0.109
0.137
0.169
0.205
0.241
12.2
16.5
11.5
14.8
35.7
Reaction conditions: [Substrate] = 0.5 ꢃ 10ꢁ4 M, [KCl] = 0.1 M, pH = 9.0, Temp. = 27 ꢀC
Table 4. Second-order rate constants for [Nuꢁ] against
different substrates
Substrate
Second-order rate constant k2
(Mꢁ1 sꢁ1
)
MINA
BDMO
PNPA
PNPB
PNPDPP
BDNPP
PNPTS
14.6
6.3
1.83
40.5
20.0
4.0
0.111
0.28
0.005a
0.015a
aEstimated from Bronsted plot Ref 16: 25 ꢀC
Figure 6. Time-dependent increase of absorbance at 400 nm upon
hydrolysis of PNPA and PNPB (at pH 9.0) by oximate ion in the
presence of gemini 16-4-16 MEA 2Brꢁ. Reaction conditions: [MINA] and
[16-4-16 ] = 0.5ꢃ 10–3 M. Substrate concentrations: [PNPA] and [PNPB] =
0.5ꢃ 10–4 M, Temp. = 27 ꢀC
with increased viscoelasticity, but in s = 6 prolate micelles are
formed with decreased aggregation properties. Hence, 16-4-16
MEA 2Brꢁ proved to be proficient[11,12] with better kinetic re-
sults of micellar catalysis. It is noteworthy that the two gemini
surfactants provide high observed reaction rates along with
low Krafft temperature caused by introducing OHꢁ groups into
head groups. The latter structure modifications allow us to use
gemini surfactant at considerably high concentrations without
a co-surfactant at room temperature. Another factor is the
change of the microenvironment due to the presence of OHꢁ
groups which makes micellar interface less water deficient and
therefore facilitates reactions involving charged species.[54,55]
oximes in water (see Table 2); hence, scheme 3 can be proposed.
In this scheme, (S), OHꢁ and Oxꢁ represent substrate, hydroxide
ions and oximate ions, respectively. Superscript w and m indicate
aqueous phase and micellar pseudophase in the reaction. Sub-
strate is mainly cleaved by oximates. As it can be seen from
Bronsted plots for oximate ions,[9,18] reactivities of MINA and
OHꢁ ions are comparable, but observed reaction rates in water
for oximes under chosen conditions are more than 10 times
higher. The role of hydroxide ions at pH 9.0 is minor; hence, we
can apply PPM[73–75] taking into account the main route
(oxymolysis) and alkaline hydrolysis enhanced by micelles of
gemini surfactants:
Application of pseudophase model (PPM)
The effect of micelles on the bimolecular reactions is quantita-
tively rationalized by the PPM (Berezin approach).[69–72] This
approach treats micelles and a solvent (water) both as distinct
reaction phases. The overall reaction rates depend on the
concentration of reactants including rate constants. The whole
reaction rates are depicted by the sum of the rate in each
distinct phase.
In the case of our reaction systems, there are two reactive
species: hydroxide ions from bulk aqueous phase and oximate
ions produced by tertiary oximes. We assume that the presence
of gemini surfactants very slightly change the pKa of tertiary
ꢂ
ꢃ
km2;Ox=VM ꢆKSꢆKOxꢆC þ kw2
Ka;app
kobsd
¼
ꢆ
½Oxꢄ0
þ
ð1 þ KSꢆCÞꢆð1 þ KOxꢆCÞ Ka;app þ аH
ꢂ
ꢃ
km2;OH=VM ꢆKSꢆKOHꢆC þ kwOH
ð1 þ KSꢆCÞꢆð1 þ KOHꢆCÞ
þ
½OHꢄ0
(11)
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J. Phys. Org. Chem. 2013, 26 632–642

REACTIVITY STUDIES OF CARBON, PHOSPHORUS AND SULFUR-BASED ACYL SITES
Where, km2 and kw2 (Mꢁ1 sꢁ1) are the second-order rate constants
indicating nucleophilicity of the oximate species, KS (Mꢁ1) is the
substrate binding equilibrium constant, KOx is the oxime binding
constant, VM (L/mol) is the partial mole volume of surfactant,
w-
C (M) is concentration of micellized surfactant, k and a-OH corre-
OH
spond to contribution of alkaline hydrolysis, Ka, app is apparent con-
stant for acid ionization of oxime in the presence of surfactant, a+H is
the proton activity and a is the oximate ion deprotanation degree.
We depicted the quantitatively rationalized kinetic data for the
cleavage of carboxylate (PNPA), phosphate (PNPDPP) and
sulphonate (PNPTS) esters by MINA and BDMO in the presence
of 16-4-16 MEA 2Brꢁ and 16-6-16 MEA 2Brꢁ in Table 6 and Fig. 8.
The kinetic profiles for three substrates are adequately described
in terms of PPM. PNPTS and PNPDPP are very hydrophobic
(KS ≥ 3000 Mꢁ1); their concentration profiles reach maximum
(optimal concentration, Copt) at Dtot lower 1 mM which corre-
sponds ca. 75% binding substrate. PNPA is considerably less
hydrophobic (low value of binding constant KS ca. 250 Mꢁ1) and
reactive substrate with sufficient alkaline hydrolysis route which
can be satisfactorily predicted using Eqn (11); see Fig. 8 c,d.
At 3 mM, about 40% of PNPA is incorporated in the micellar
pseudophase. This allows nucleophiles, both OHꢁ ion and
oximate ions, to cleave esters in micelle with the observed rates
of almost one order of magnitude higher than in water. The OHꢁ
ion binding is low and can be assumed to be 30 to 50 Mꢁ1[54,55]
that makes oxymolysis the main route in the micellar
pseudophase. As it can seen from the Table 6, binding constant
of oximate ions is lower for 16-6-16 MEA almost for all the
systems, irrespective of substrate, that makes 16-4-16 MEA more
efficient surfactant in the studied reactions.
The nucleophilicities of oximate ions (parameter km/VM in
Table 6) are somewhat smaller than in water, and correct estima-
tion of the intrinsic nucleophilicities requires the knowledge of
VM. For conventional surfactants, it was considered to be from
0.14 (estimated Stern volume) to 0.34[66,67] sometimes, a value
of 0.5 was used.[64] For gemini surfactants, this value might be
different since the gemini micelles have smaller aggregation
numbers and can elongate to form rod-like micelles at relatively
low concentrations. As it can be seen from Table 6, the effect of
substrate concentration caused by binding both substrate and
nucleophile in the micellar pseudophase is the main factor
determining rate enhancement in the presence of gemini
micelles under mild pH conditions.
Comparative efficacy of tertiary oximes
In order to compare the reactivity of both the tertiary oximes
MINA and BDMO, the rate of hydrolysis of PNPA, PNPB, PNPDPP,
BDNPP and PNPTS has been evaluated. According to data
presented in Table 3, MINA shows better nucleophilic efficiency
than butane 2,3-dione monoxime towards different electrophilic
centers (C = O, P = O and S = O centers). The reactivity difference
of both the nucleophiles can be explained on the basis of
their acid dissociation constant (pKa). Lower pKa of MINA
(pKa = 8.30)[39] as compared to BDMO (pKa = 9.50)[37] increases
the electron withdrawing effect of MINA; hence, it provides
sufficient concentration of oximate anions to the cleavage of
various substrate.
Changing the electrophilic centers from carbonyl to phosphonyl
or to sulphonyl group can visualize significant effect on their elec-
trophilicity. The order of reactivity for all the substrates can be
J. Phys. Org. Chem. 2013, 26 632–642
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B. GUPTA ET AL.
Figure 7. Observed rate constant for the hydrolysis of BDNPP and PNPTS by MINA as a function of [16-s-16], (s = 4, 6), pH 9.0 at Temp.=27 ꢀC in borate
buffer 0.01 M. (Inset). Observed rate constant for the hydrolysis of PNPA, PNPB and PNPDPP as function of [16-s-16], (s = 4, 6) pH = 9.0 at Temp.=27 ꢀC
in borate buffer 0.01 M
k2m, OH
k2m
(S)Micelle + OH-
PS
(S)water + OH-
(S)Micelle + Ox-
PS
(S)water + Ox-
Products
Products
Products
Products
kOwH
k w
Scheme 3. Schematic distribution of reactive spices in aqueous and micellar phase
Table 6. Kinetic parameters obtained by applying pseudophase model
Nuꢁ
Gemini surfactant
kw2 (Mꢁ1 ꢆ sꢁ1
)
a (at pH 9.0)
km/VM (sꢁ1
)
KS (Mꢁ1
)
Kox )
ꢁ
(Mꢁ1
PNPTS
MINA
16-4-16
16-6-16
16-4-16
16-6-16
0.005
0.005
0.015
0.015
0.89
0.88
0.49
0.44
0.006
0.011
0.012
0.02
3000
3000
3000
2000
230
190
200
100
BDMO
PNPA
MINA
16-4-16
16-6-16
16-4-16
16-6-16
14.6
14.6
40.5
40.5
0.89
0.88
0.49
0.44
2.0
1.2
2.4
1.5
250
250
250
250
580
350
750
410
BDMO
PNPDPP
MINA
16-4-16
16-6-16
16-4-16
16-6-16
1.64
1.64
4.0
0.89
0.88
0.49
0.44
0.4
0.2
0.3
0.4
5000
4000
5000
5000
200
350
280
160
BDMO
4.0
Reaction conditions: [Substrate] = 0.5 ꢃ 10ꢁ4 M, [MINA] = 0.5 ꢃ 10ꢁ3 M, KCl = 0.1 M, pH = 9.0, Temp. = 27 ꢀC
depicted as PNPA > PNPB > PNPDPP> BDNPP> PNPTS. The
nucleophilic efficacy of both the nucleophiles is quite less
towards the suphonyl ester when compared to phosphonyl and
carboxyl esters.[50,65] This difference may be due to least
electrophilicity of S = O center, than P = O and C = O centers. The
comparative reactivity of phosphate and carboxylate esters can
be accounted on the basis of reduced electrophilicity of P = O cen-
ter due to pp–dp interaction, which hinders the attack of
nucleophile in rate determining step. On the contrary, the absence
of pp–dp bonding at C = O center results in facile hydrolysis of car-
boxylate esters. Among the carboxylate esters (PNPA and PNPB),
PNPA was hydrolyzed extremely fast in the presence of only
oximes and micelle-assisted oximes. This can be accounted on
the basis of high stability of PNPB as compared to PNPA[52]
that leads to decreased reactivity in all the nucleophilic substitu-
tion reactions.
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J. Phys. Org. Chem. 2013, 26 632–642

REACTIVITY STUDIES OF CARBON, PHOSPHORUS AND SULFUR-BASED ACYL SITES
(a)
(b)
(c)
(d)
(e)
(f)
Figure 8. Plots for the effect of gemini surfactants for the cleavage of PNPTS by MINA, (a) 16-4-16, (b) 16-6- 16; PNPDPP by MINA, (c) 16-4-16 (d) 16-6-16;
PNPA by BDMO, (e) 16-4-16, (f) 16-6-16; Reaction conditions: [Substrate]= 0.5 ꢃ 10–4 M, [MINA]= [BDMO] = 0.5 ꢃ 10–3 M, KCl= 0.1M, pH= 9.0, Temp. = 27ꢀC
(lines predicted with model)
useful in developing an effective mechanism for the degrada-
tion of chemical and biological warfare agents under potential
CONCLUSIONS
The present investigation was performed to determine the
kinetic efficiencies of tertiary oxime-based reactivators against
various simulants of chemical warfare agents. With this motive,
different esters were selected as substrate. The nucleophile
MINA showed better nucleophilicity than BDMO for all the
hydrolysis reactions at pH 9.0, which makes it model candidate
for decontamination reactions. Gemini surfactant-assisted
hydrolysis showed tremendous enhancement in the rate of
reactions, and the effects were analyzed in terms of PPM. The
observed first-order rate constants were found to increase with
increasing surfactant concentration, which further remains
almost constant at a higher surfactant concentration, showing
the nature of micellar-catalyzed reactions and can be accounted
on the basis of surface properties and of pKa of nucleophiles in
the presence of surfactant. Results of this investigation are
therapy for the treatment of human exposure.
Acknowledgement
BG and NS are grateful to Pt. Ravishankar Shukla University, Rai-
pur for providing university fellowship.
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