Nucleophilic attack of salicylhydroxamate ion at C=O and P=O centers in cationic micellar media

Article abstract of DOI:10.1021/jp1066549

The reaction between the salicylhydroxamate anion (SHA^-) and p-nitrophenyl benzoate (PNPB), tris(3-nitrophenyl)phosphate (TRIS), and bis(2,4-dinitrophenyl)phosphate (BDNPP) have been examined kinetically. The α-nucleophile, SHA^-, incorporated into cetyltrimethylammonium bromide (CTAB) micelles accelerates dephosphorylation of tris(3-nitrophenyl) phosphate (TRIS) over the pH range 6.7-11.4. With a 1.0 mM of SHA in CTAB, the nucleophilicity of SHA followed the order of reactivity, PNPB (C=O, carboxylate ester) > TRIS (P=O, triester) > BDNPP (P=O, diester), and monoanionic SHA^- and dianionic SA^2- are the reactive species. The critical micelle concentration, cmc, of cetyltrimethylammonium bromide (CTAB) decreases and the fractional ionization constant, α, increases with increasing the concentration of SHA^-. Addition of 1 and 10 mM SHA under the reaction conditions (pH 9.2, borate buffer) led to saturation of the micellar surface and provided qualitative information for the micellar incorporation of hydroxamate ion. Plots of the pseudo-first-order rate constant, k_obs, log k_obs, fraction of hydroxamic acid ionized, α_SHA- and α_SA2-, vs pH showed bifunctional nucleophilicity of hydroxamic acid under micellar condition. Plotting k _obs vs [SHA]_T gave a straight line with intercept k ₀. This indicates that hydroxamate ions are very strong nucleophiles for nucleophilic attack at the C and P center. The pseudo-first-order rate constant-surfactant profiles show micelle-assisted bimolecular reactions involving interfacial ion exchange between bulk aqueous media and micellar pseudophase.

Full text of DOI:10.1021/jp1066549

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J. Phys. Chem. B 2010, 114, 16759–16765
16759
Nucleophilic Attack of Salicylhydroxamate Ion at CdO and PdO Centers in Cationic
Micellar Media
Manmohan Lal Satnami,*,† Sunita Dhritlahre, Rekha Nagwanshi,‡ Indrapal Karbhal,†
Kallol K. Ghosh,† and Faruk Nome§
School of Studies in Chemistry, Pandit RaVishankar Shukla UniVersity, Raipur (Chhatisgarh), India 492010,
Department of Chemistry, GoVernment College Barnagar (M.P.), India, 456010, and Departamento de
Quimica, UniVersidade Federal de Santa Catarina, 88040-900 Floriano’polis, Santa Catarina, Brazil
ReceiVed: July 17, 2010; ReVised Manuscript ReceiVed: October 31, 2010
The reaction between the salicylhydroxamate anion (SHA-) and p-nitrophenyl benzoate (PNPB), tris(3-
nitrophenyl)phosphate (TRIS), and bis(2,4-dinitrophenyl)phosphate (BDNPP) have been examined kinetically.
The R-nucleophile, SHA-, incorporated into cetyltrimethylammonium bromide (CTAB) micelles accelerates
dephosphorylation of tris(3-nitrophenyl)phosphate (TRIS) over the pH range 6.7-11.4. With a 1.0 mM of
SHA in CTAB, the nucleophilicity of SHA followed the order of reactivity, PNPB (CdO, carboxylate ester)
> TRIS (PdO, triester) > BDNPP (PdO, diester), and monoanionic SHA- and dianionic SA2- are the reactive
species. The critical micelle concentration, cmc, of cetyltrimethylammonium bromide (CTAB) decreases and
the fractional ionization constant, R, increases with increasing the concentration of SHA-. Addition of 1 and
10 mM SHA under the reaction conditions (pH 9.2, borate buffer) led to saturation of the micellar surface
and provided qualitative information for the micellar incorporation of hydroxamate ion. Plots of the pseudo-
-
2-
first-order rate constant, kobs, log kobs, fraction of hydroxamic acid ionized, RSHA and RSA , vs pH showed
bifunctional nucleophilicity of hydroxamic acid under micellar condition. Plotting kobs vs [SHA]T gave a
straight line with intercept k0. This indicates that hydroxamate ions are very strong nucleophiles for nucleophilic
attack at the C and P center. The pseudo-first-order rate constant-surfactant profiles show micelle-assisted
bimolecular reactions involving interfacial ion exchange between bulk aqueous media and micellar pseudophase.
Introduction
amine is the parent compound for the three typical classes of
the oxygen-containing R-nucleophiles: oximate and hydroxamic
Esterolytic reactions of carboxylate, phosphate, thiophosphate,
and sulfonate have recently received intensive attention due to
the importance in chemical and biological processes.1-5 The
phosphotriester-based Paraoxon and Parathion are used as
pesticides and are most frequently responsible for the poisoning
of agricultural field workers.6 Due to their potential toxicity,
considerable attention has been directed toward detection and
methods of facilitating the hydrolysis or decomposition of
organophosphates.7-10 Nucleophile-aided hydrolysis is the most
preferred reaction to detoxify them.11-15 In this regard, nucleo-
philes such as peroxides,11 hypochlorites,12 oximates,13 o-
iodosylcarboxylate,14 and hydroxamate15 have been investigated
alone or in concert with surfactants. Considerable research
efforts have been focused on the nucleophilic reagents such as
monoperoxyphthalates,16 4-N,N-dialkylaminopyridines,17 and
metallomicelles.18 Recently, ester cleavage properties and
significant turnover catalysis of synthetic hydroxybenzotriaz-
oles19 and 5-alkyl-1H-tetrazoles20 have been reported.
acid.22 Investigation of the R-effect of hydroxamic acids in
micellar media is of paramount importance, as compared to
those in homogeneous aqueous solvents, since it is currently
claimed that solvation/desolvation phenomena are the source
of the enhanced reactivity of the R-nucleophiles.23
Reactivities of hydroxamate ions are increased by incorpora-
tion in functional polymers or by comicellization with inert
surfactants in water.24 As expected on the basis of pseudophase
treatments of micellar rate effects,25 high local concentrations
of hydroxamate ions in the interfacial micellar region are largely
responsible for these rate increases.24b Considering that (i)
cationic micelles of cetyltrimethylammonium bromide, CTAB,
accelerate the spontaneous hydrolysis of BDNPP up to 30-fold
and high pH accelerates the reaction by concentrating nucleo-
philes,25b,26 e.g., OH-, and (ii) cationic micelles accelerate the
reaction of hydroxamates and phosphate triesters.27
We studied micellar effects on reactions of the p-nitrophenyl
benzoate (PNPB), tris(3-nitropheny)phosphate (TRIS), and
bis(2,4-dinitrophenyl)phosphate (BDNPP) with salicylhydrox-
amic acid (SHA) in cetyltrimethylammonium bromide micellar
media (Chart 1). In view of micellar incorporation of SHA-,
we reported the critical micelle concentration, cmc, and
fractional ionization constant, R, in the presence of hydroxamic
acid under buffered condition.
Hydroxamate ions are R-effect nucleophiles, i.e., their
reactivity is higher than that predicted by the Bronsted relation-
ship between nucleophilicity and basicity.21 They are effective
deacylating and dephosphorylating agents, and reactivities of
amphiphilic hydroxamate ions are increased by comicellization
with surfactant in water. It is interesting to note that hydroxyl-
* To whom correspondence should be addressed. Phone: +91-99071-
55681. E-mail: manmohanchem@gmail.com.
† Pandit Ravishankar Shukla University.
Experimental Section
‡ Government College Barnagar.
Materials. Salicylhydroxamic acid, p-nitrophenyl benzoate,
and cetyltrimethylammonium bromide were obtained from
§ Universidade Federal de Santa Catarina.
10.1021/jp1066549  2010 American Chemical Society
Published on Web 11/24/2010

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16760 J. Phys. Chem. B, Vol. 114, No. 50, 2010
Satnami et al.
CHART 1
Sigma/Aldrich. Samples of tris(3-nitrophenyl)phosphate (TRIS)
and bis(2,4-dinitrophenyl)phosphate (BDNPP) were synthesized
in the Department of Chemistry, Federal University of Santa
Catarina, Florianopolis-SC, Brazil.
Kinetics. All reactions were followed at 27 °C with a SL
159 Elico UV-visible spectrophotometer. The rate of nucleo-
philic reaction with esters was determined by following the
increase in the absorption of p-nitrophenoxide and 3-nitrophe-
noxide anion (400 nm). All kinetic experiments were performed
at an ionic strength of 0.1 M (with KCl). Borate buffer was
employed. All reactions were conducted under pseudo-first-order
conditions. For all of the kinetic runs, the absorbance/time results
fit very well to the first-order rate equation
Figure 2. Specific conductance as a function of [CTAB] in the presence
of [SHA] ) (9) 0.001 and (b) 0.01 M at pH 9.2. (Inset) Plots of specific
conductance versus [CTAB] in the absence (∆) and presence (O) of
buffer (pH 9.2).
TABLE 1: Critical Micelle Concentrations and r at
Different [SHA] in CTAB/HA Mixtures
[SHA] (mM)
R
103 cmc, M
0
0.28
0.49
0.40
0.47
1.10
0.90
0.97
0.87
ln(A∞ - At) ) ln(A∞ - A0) - kt
(1)
0 (pH 9.2)
1.0
10.0
The pseudo-first-order rate constants can be determined by least-
squares fits (Figure 1). Each experiment was repeated at least
twice, and the observed rate constant was found to be reproduc-
ible within a precision of about 3% or better. The spectrum
exhibits an increase in absorbance at 400 nm with the formation
of p-nitrophenoxide and 3-nitrophenoxide ions during the course
of reaction. The pKa values of hydroxamic acids were deter-
mined by pH metrically and spectrophotmetric titration using a
Systronics (Type-335) pH meter.
Conductance Measurements. Conductance measurements
were carried out using a Systronics (Type-306) conductimeter.
Values of the cmc were determined in the usual manner from
plots of specific conductance against surfactant concentration.
Solutions were buffered by borate buffer, pH 9.2.
Results and Discussion
Micelle Formation and Fractional Ionization. Critical
micelle concentrations, cmc, and fractional micellar ionizations,
R, for the micelles of CTAB in the presence of buffer and SHA
(buffered at pH 9.2) were estimated from plots of conductance
against [surfactant], Figure 2. Breaks in these plots give cmc
values that change little with increasing concentration of SHA,
but approximate values of R, from the ratio of slopes,28 increase
with increasing [SHA], indicating that addition of salicylhy-
droxamate ion effectively neutralizes the cationic micelles,
decreasing the micellar surface potential and affinity for
counteranions. This effect is very sensitive to [SHA] (Table 1).
The cmc of CTAB in water is 1.1 × 10-3 M, but for a mixture
of CTAB and SHA, the cmc decreases to 0.97 × 10-3 M in the
presence of 1.0 × 10-3 M SHA and 0.87 × 10-3 M in the
presence of 10.0 × 10-3 M SHA. The fractional micellar
ionizations, R, sharply increase from 0.28 to 0.47, i.e., SHA
significantly increases the fractional micellar charge of CTAB
depending on SHA concentration. The fractional ionization
constants in the presence of buffer and SHA are comparable,
indicating the competitive efficiency to bind cationic micellar
surface. However, the catalytic efficiency of SHA is much higher
than the HO- ion (buffer alone).
Figure 1. Kinetic plots of log(Ainf - A0)/(Ainf - At) vs time for
production of nitrophenolate from reaction of SHA with (A) TRIS,
[TRIS] ) 1.0 × 10-4 M, [SHA] ) 1.0 × 10-3, [CTAB] ) 2.0 × 10-3
M, pH ) 10.0, borate buffer, (B) PNPB, [PNPB] ) 1.0 × 10-4 M,
[SHA] ) 1.0 × 10-3 M, [CTAB] ) 3.7 × 10-3 M, pH ) 9.2, borate
buffer, and (C) BDNPP, [BDNPP] ) 1.0 × 10-4 M, [SHA] ) 1.0 ×
10-3 M, [CTAB] ) 5.4 × 10-3 M, pH ) 9.2, borate buffer.

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Nucleophilic Attack of Salicylhydroxamate Ion
J. Phys. Chem. B, Vol. 114, No. 50, 2010 16761
SCHEME 1: Nucleophilic Reaction of
Salicyhydroxamate Ions (Nu-) with Esters
Kinetic Studies in Cationic Micelles. Pseudo-first-order rate
constants for the reactions of PNPB, TRIS, and BDNPP with
salicylhydroxamate ion (Scheme 1) have been determined at
27 °C in 4% (v/v) MeCN micellar media (2.0 × 10-3 M CTAB)
with the nucleophiles in large excess over the substrate. The
pH-dependent rate constant increases with increasing pH in the
range pH 6.7-11.0. The kinetic curves plotted in Figure 3
illustrate variation of the absorption of 4-nitrophenoxide (PNPB)
and 3-nitrophenoxide ion (TRIS) with time at different pH for
the reaction of SHA with esters in the presence of 5.4 mM
CTAB. The curves correspond to accumulation of 4-nitrophe-
noxide and 3-nitrophenoxide ions in the system as a result of
substrate dissociation. The rate of reaction shows drastic change
at the pH where the 50% hydroxamic acid deprotonated, i.e.,
pKa, of salicylhydroxamic acid. The pKa1 of salicylhydroxamic
acid was determined pH metrically. The plot of log kobs vs pH
(Figure 4) gave a break at 9.45, which is taken as pKa2 for the
SHA. The rate-pH profile for the reaction of TRIS with
salicylhydroxamate ion in cationic micellar solution is typical
of a pH-dependent nucleophilic reaction. Hydroxamic acids have
been suggested to behave as either NH or OH acids depending
on solvents.29 Numerous studies indicate that hydroxamic acids
are OH, rather than NH, acids in H2O.29a It is known that the
anion of hydroxamic acid (N-O-) acts as a reactive species in
the hydrolysis of esters. Consequently, the pKa for conversion
of the N-OH to the N-O- form plays an important role for
cleavage of phosphate esters. A pH-rate constant profile for
the nucleophilic cleavage of 0.1 mM TRIS by 1.0 mM
Figure 4. Plot of first-order rate constants vs pH for the reaction of
TRIS with salicylhydroxamate ion in CTAB (2.0 × 10-3 M). (Inset
(B)): Plot of log kobs vs pH under identical condition.
TABLE 2: Nucleophile-Dependent Pseudo-First-Order Rate
Constant for the Reaction of BNPB and TRIS
kobs × 10-3, s-1
PNPB
[SHA], mM
TRIS
0.0
0.5
1.0
1.5
2.0
2.5
0.50
1.26
1.75
2.70
3.58
0.62
0.77
0.87
1.03
1.14
1.25
salicylhydroxamate ion in CTAB micellar media (2.0 mM) gave
the apparent pKa value for the salicylhydroxamic acid. Typically,
the pseudo-first-order rate constants for reaction of TRIS were
determined at different pH values between 6.7 and 11.0. In
Figure 4, we present a representative pH-rate constant profile
for cleavage of 0.1 mM TRIS by 1.0 mM salicylhydroxamic
acid in micellar CTAB (2.0 mM) at 27 °C.
The nucleophile concentration-dependent first-order rate
constant was determined for reaction of PNPB and TRIS with
salicylhydroxamic acids in excess. Table 2 summarizes the data
for reaction of PNPB and TRIS with different concentrations
of salicylhydroxamate ion at pH 9.12. Kinetic data show
additional support for the hypothesis that hydroxamic acid is
acting as a nucleophilic catalyst for reaction of PNPB and TRIS.
Equation 2 describes the reaction of PNPB and TRIS with
nucleophile, and k0 defined in eq 3 corresponds to the intercept
in the kobs vs [Nu-] plot (Figure 6)
kobs ) k0 + kNu-[Nu-]
(2)
(3)
k0 ) kH 0 + kOH-[OH-]
2
The kH O term may assume some significance for very weak
2
nucleophiles and at very low OH- concentrations. At high pH,
Figure 3. Time-dependent increase of absorbance at 400 nm upon
hydrolysis of PNPB (at pH 9.2) and TRIS (at pH 10) by salicylhy-
droxamate ion in the presence of CTAB. Conditions: 27 °C, [SHA] )
1.0 × 10-3 M, [CTAB] ) 5.4 × 10-3 M. Substrate concentrations:
[PNPB] ) 1.0 × 10-4 M, [TRIS] ) 1.0 × 10-4 M.
-
the intercept is dominated by the kOH term. Plotting kobs vs
[Nu-] gave a straight line (Figure 6) with intercept k0. This
indicates that competition with other nucleophiles, i.e., OH-
and H2O, is not expected, hydroxamate ions are very strong

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16762 J. Phys. Chem. B, Vol. 114, No. 50, 2010
Satnami et al.
SCHEME 2: pH-Dependent Ionization of
Salicylhydroxamic Acid
nucleophiles for the nucleophilic attack at the C and P centers
of PNPB and TRIS, and kobs is simply given by kobs ) kNu[Nu-].
It is assumed that catalysis by nucleophile is dependent upon
the ionization state of the hydroxamic acid. Equation 4 may be
written to describe the actual value of kobs. In this equation, kHA
is second-order rate constant, [SHA]T is the analytical concen-
tration of hydroxamic acid, and RHA- is the fraction of [SHA]T
ionized.
Figure 6. Plots of kobs vs [SHA-] (b) and [SA2-] (2) for the reaction
of TRIS at pH 9.2 (0.01 M borate buffer).
HA
obs
k
) k0obs + kHA-[HA]TRHA
(4)
-
can be explained by the hypothesis that a complex of type I
with intramolecular hydrogen bonding exists in micellar solution.
The salicylhydroxamic acid, containing two proton-active sites,
may give two types of reactive forms SHA- and SA2- (Scheme
2)
kobs ) k0obs + kSHA-[SHA]TRSHA + kSA2-[SHA]TRSA2-
-
(5)
Since RHA is equal to Ka1/Ka1 + [H+] and RSA is equal to
Ka2/Ka2 + [H+], where Ka1 and Ka2 are the kinetically apparent
dissociation constants of salicylhydroxamic acid, this equation
-
2-
The degree of proton transfer in this complex is not obvious,
and so either (a) the complex of type I may act as a true
nucleophilic reagent or (b) nucleophilic attack on the substrate
with participation of such a complex is subjected to general
base catalysis relative to the phenoxide ion. In the first case,
proton transfer in I is accomplished in the initial state of the
reaction. In the second case, in the initial state of the reaction
only partial transfer in I occurs, while the final transfer occurs
only in the transition state of the reaction. These two mecha-
nisms are indistinguishable kinetically. Additional support for
the fact that the dianionic form is the reactive species comes
from the linear dependence of kobs on [SHA-] and [SA2-], the
active concentration of hydroxamate mono- and dianionic
species (Figure 6). The second-order rate constant of the
is applicable to the condition of catalyst in excess. Plotting kHA
obs
versus [HA]T (Figure 6) should give a straight line with slope
) kHARHA and intercept ) k0
.
obs
Figure 5 represents the plots of kobs and fraction of SHA
-
2-
ionized (RSHA and RSA ) vs pH for reaction of TRIS with
salicylhydroxamic acid. It is evident from plots that monoan-
ionic, SHA-, is the reactive species below the pH 9.0, and
dianionic form becomes significant after pH 9.5. This behavior
monoanionic hydroxamate ion (SHA-) kSHA is 0.2427 M-1 s-1
-
-1
and dianionic species (SA2-), kSA , is 0.6795 M s-1 for the
2-
reaction of TRIS. This behavior supports the fact that (1) the
nucleophilicity of hydroxamate ions in the dianionic form (SA2-)
is intrinsically greater than that of the monoanionic hydroxamate
ion (SHA-) or (2) the phenolate anion in SA2- is directly
involved in the reaction besides the hydroxamate anion.
The micelle-bound salicylhydroxamate ion shows large
catalytic activity for the decarboxylation and dephosphorylation
reactions. The rate constant for reaction of BDNPP with
hydroxide ion at pH 9.15, in the absence of SHA-, is ca. 4 ×
10-7 s-1. The rate constant of spontaneous water reaction of
BDNPP is ca. 1.90 × 10-7 s-1 (reported).30 At the rate maxima
(Figure 7), kobs for the reaction of BDNPP with micellar-bound
salicylhydroxamate ion is ca. 3.2 × 10-4 s-1, that is, there is
up to 800-fold catalysis over the reaction of hydroxide ion and
1684-fold rate enhancement over the spontaneous water reaction.
In the previous paper, we studied the comparative catalytic
reactivity of acetohydroxamate ion (AHA-), benzohydroxamate
(BHA-), and salicylhydroxamate for cleavage of paraoxon in
2-
Figure 5. Plots of kobs, RHA- (A), and RSA vs pH for the reaction of
TRIS with salicylhydroxamic acid. Condition: [TRIS] ) 1.0 × 10-3
M, [SHA]T ) 1.0 × 10-3 M, [KCl] ) 0.1 M, [CTAB] ) 2.0 ×
10-3 M.