Comparative nucleophilic reactivities in carboxylate, phosphinate, and thiophosphate esters cleavage

Article abstract of DOI:10.1002/poc.1370

Nucleophilic substitution reaction of p-nitrophenyl acetate (PNPA),p-nitrophenyldiphenyl phosphinate, and pesticide parathion with different a-nucleophiles [I] have been studied at 27°C in different pH in the presence of a novel cationic surfactant. [image omitted] The kinetic study was performed spectrophotometrically under pseudo-first order conditions with the α-nucleophile in excess. The pK_a of nucleophiles have also been determined by kinetic method. In the presence of surfactant, the rate constant increased with increasing surfactant concentration up to a limiting value. This behavior has been analyzed in quantitative terms on the basis of pseudo-phase model of micellar catalysis. Finally the nucleophilic reactivity of hydroxamate ions has been compared with other α-nucleophiles, like oxime, hydroxybenzotriazole, and 2-iodosobenzoic acid (IBA). The order of cleavage of electrophilic centers, that is, C=O, P=O, and P=S have also been discussed. Copyright

Full text of DOI:10.1002/poc.1370

Research Article
Received: 25 October 2007,
Revised: 1 March 2008,
Accepted: 4 March 2008,
Published online in Wiley InterScience: 2008
(www.interscience.wiley.com) DOI 10.1002/poc.1370
Comparative nucleophilic reactivities in
carboxylate, phosphinate, and thiophosphate
esters cleavagey
a
a
a
a
*
Kallol K. Ghosh , Sunita Bal , Sancheeta Kolay and Ashish Shrivastava
Nucleophilic substitution reaction of p-nitrophenyl acetate (PNPA), p-nitrophenyldiphenyl phosphinate, and pesticide
parathion with different a-nucleophiles [I] have been studied at 27 -C in different pH in the presence of a novel cationic
surfactant.
The kinetic study was performed spectrophotometrically under pseudo-first order conditions with the a-nucleophile
in excess. The pKa of nucleophiles have also been determined by kinetic method. In the presence of surfactant, the rate
constant increased with increasing surfactant concentration up to a limiting value. This behavior has been analyzed in
quantitative terms on the basis of pseudo-phase model of micellar catalysis. Finally the nucleophilic reactivity of
hydroxamate ions has been compared with other a-nucleophiles, like oxime, hydroxybenzotriazole, and
—
—
—
2-iodosobenzoic acid (IBA). The order of cleavage of electrophilic centers, that is, C O, P O, and P S have also
been discussed. Copyright ß 2008 John Wiley & Sons, Ltd.
—
—
—
Keywords: a-nucleophile; pKa; pseudo-phase model
enhance the rate of hydrolysis of such compounds via micellar
catalysis.[11–16] Moss and others[17–21] have studied extensively
the catalytic cleavage of carboxylate and phosphate esters by a
series of o-iodosobenzoic acid. Similarly Bhattacharya et al.
introduced tetrazole,[22] hydroxybenzotriazole,[23] and their
suitably designed derivatives as powerful ester cleaving reagents.
Recently Buncel et al. studied the reactivities of fenitrothion with
a series of oximate a-nucleophiles with pKa values ranging from
7.7 to 11.8.[12] Recently, biomimetic nanocatalyst, magnetic
nanoparticles, and functionalized polymer nanofibre membrane
have also been used for detoxification.[26–28]
INTRODUCTION
There is a growing need for fast detoxification and technological
advancements to combat chemical and biological warfare
agents. The a-effect has been reported in many different types
of reactions in solutions.[1–4] Classical physical organic chemistry
studies over the last 20 years have yielded some important clues
about the nature of a- effect.[5–10] Although many attempts have
been made to rationalize a-effects in terms of physicochemical
factors (e.g., polarizability, hydrogen bonding, single electron
transfer character, orbital splitting, and other), mechanistic details
have not been sufficiently clear. a-nucleophiles are also
characterized by anomalously high nucleophilic reactivity with
respect to electron deficient centers of various origins, for
example, carbon, sulfur, and phosphorus. This fact attracts
interest from the viewpoints of utilization of organophosphorus
ecotoxicants and search for effective and novel detoxicants.
Various strategies[11–28] have been studied to enhance the
hydrolysis of phosphoester and carboxylate esters, and these
include the employment of metal ions which act as Lewis acid
catalysts, metallomicelles, enzymes, nanoparticles/biomimetic
nanocatalyst, and reactive a-effect nucleophiles such as
oximates, hydroximates, hydrazines, and hydroxylamine to name
a few. It has also been recognized that cationic micelles serve to
Changing the electrophilic center from a carbonyl to a sulfonyl
or phosphonyl group would exert significant effect on their
*
School of Studies in Chemistry, Pt. Ravishankar Shukla University, Raipur
492010, Chhattisgarh, India.
E-mail: kallolkghosh@yahoo.com
a K. K. Ghosh, S. Bal, S. Kolay, A. Shrivastava
School of Studies in Chemistry, Pt. Ravishankar Shukla University, Raipur
492010, Chhattisgarh, India
y
Presented in part at 12th Asian Chemical Congress, Kuala Lumpur, Malaysia,
23–25 August 2007.
J. Phys. Org. Chem. 2008, 21 492–497
Copyright ß 2008 John Wiley & Sons, Ltd.

COMPARATIVE NUCLEOPHILIC REACTIVITIES
electrophilicity. However systematic studies on changing
such electrophilic centers have been lacking. Only scattered
information on the reactivity of carbonyl, sulfonyl, and
phosphonyl esters of similar structures is available.[29–31]
RESULTS AND DISCUSSION
pH-dependent reaction
Pseudo-first order rate constants for the reaction of PNPDP with
a series of a-nucleophiles, that is, IBA, 2,3-Butanedione mono-
xime (oxime), and PBHA have been determined over a pH range
6.5–11.1 at 27 8C. The apparent pKa of these a-nucleophiles were
determined from the rate constant versus pH plots for the
cleavage reactions.
The rate data (data not shown) indicate that the rate of
reaction increases with increasing pH values. Plot of log kobs
versus pH (Fig. 1) gave a discontinuity at definite pH value for IBA,
oxime, and PBHA. These break points were taken as
apparent pKa values for these nucleophiles (IBA ¼ 7.45,
oxime ¼ 9.0, PBHA ¼ 8.9). These values are in close agreement
with literature values.[30–35]
In the past few years, our laboratory has aimed at demonstrat-
ing the role of hydroxamate ions (a-nucleophiles) for the
degradation of carboxylic and neurotoxic phosphate esters.[32–35]
In the present work comparative nucleophilic reactivities in
carboxylate, phosphate and thiophosphate esters cleavage have
been studied. The nucleophilic substitution reaction of
p-nitrophenyl acetate (PNPA), p-nitrophenyl diphenyl phosphinate
(PNPDP) and p-nitrophenyl diethyl phosphorothioate (parathion)
with different a-effect nucleophiles (I), that is, N-phenylben-
zohydroxamic acid (PBHA), oximate (butane-2,3-dione mono-
xime), 1-hydroxybenzotriazole, and 2-iodosobenzoic acid (IBA)
have been investigated in the absence and presence of cationic
surfactant cetyltriphenylphosphonium bromide (C16PPh3Br)(II).
Effect of cationic surfactant
Cationic surfactants are known to accelerate the hydrolysis of
carboxylic and phosphate esters. The ability of micellized
surfactants to control rates of moderately slower reactions is
well established.[30–36] Acceleration of organic reactions in
micelle solution is determined mainly by two factors that is
concentration of the reactants in the micelle pseudophase and
considerable increase in the rate of reaction. Cationic micelles
bring reactants closer by hydrophobically binding of substrate
and coulombically attracting the negatively charged nucleophile.
The effects of C16PPh3Br on the hydrolysis of PNPA, PNPDP, and
Scheme 1.
J. Phys. Org. Chem. 2008, 21 492–497
Copyright ß 2008 John Wiley & Sons, Ltd.
www.interscience.wiley.com/journal/poc

K. K. GHOSH ET AL.
Figure 1. Plots of observed rate constant versus pH and log of observed
rate constant versus pH for the hydrolysis of p-nitrophenyl diphenyl
phosphinate by IBA (A), oxime (B), and PBHA (C) at 27 8C. This figure is
available in colour online at www.interscience.wiley.com/journal/poc
parathion are shown in Table 1. It has been shown that observed
first order rate constants increase sharply with increase in the
concentrations of the surfactants. The rate-surfactant concen-
tration profiles obtained with various surfactants/catalysts
are characteristic of micelle catalyzed reaction.[36] The reactivity
of these a-nucleophiles, that is, IBA, oxime, PBHA, and HOBT
have been observed to be more significant for the hydrolysis of
PNPA. Figure 2 shows that under comparable conditions, the kobs
values for hydrolysis of PNPA by PBHA was found to be greater
than IBA which in turn was more reactive than oxime and HOBT.
It is illustrated in Table 1 that the observed first order rate
constant reactivity for the hydrolysis of PNPA, PNPDP, and
parathion increases with surfactant concentration. The nucleo-
philic reactivity of micelle depends upon the binding of substrate
and interaction with anionic nucleophiles. In case of phosphinate,
on comparison with other a-nucleophiles, IBA was found to be
the most reactive and HOBT the least reactive. In case of
parathion, irrespective of concentration of surfactants, IBA shows
the maximum rate as compared to oxime, PBHA, and HOBT. PBHA
and oxime exhibited comparable reactivity. The reactivity order
for all the nucleophiles was IBA > PBHA > oxime > HOBT. In case
www.interscience.wiley.com/journal/poc
Copyright ß 2008 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2008, 21 492–497

COMPARATIVE NUCLEOPHILIC REACTIVITIES
Figure 2. Rate surfactant plots of kobs (rate constant) versus [surfactant]
for the nucleophilic reaction of PNPA with different a-nucleophiles [I]. This
figure is available in colour online at www.interscience.wiley.com/journal/
poc
Figure 3. Rate surfactant plots of kobs (rate constant) versus [surfactant]
for the reaction of PNPA, PNPDP, and parathion using PBHA. This figure is
available in colour online at www.interscience.wiley.com/journal/poc
of all the substrates studied, IBA showed the highest reactivity
among all the nucleophiles. PBHA was the next most reactive
relative to other nucleophiles.
An attempt has been made to compare the reactivity of
different a-nucleophiles in carboxylate, phosphate, and thiopho-
sphate esters in the presence of C16PPh3Br. As shown in Table 1
and Fig. 3, the reactivity of PNPA, PNPDP, and parathion
—
—
toward these a-nucleophiles is PNPA (C O) > PNPDP (P
—
—
—
O) > parathion (P S). The nucleophilic reactivity of these
—
—
—
a-nucleophiles toward P S center is less than P O center
—
—
Scheme 2.
J. Phys. Org. Chem. 2008, 21 492–497
Copyright ß 2008 John Wiley & Sons, Ltd.
www.interscience.wiley.com/journal/poc

K. K. GHOSH ET AL.
—
—
—
due to strong pp–dp interaction in P O than in P S center.
distribution of the nucleophiles, Nu, between both pseudophases
is considered through the distribution constant KmNu. The different
reactivities in the aqueous and micellar pseudophases have been
taken into account through the corresponding-second order rate
constants: k2w and k2m. The values of k2w have been obtained by
studying the reaction in the absence of the surfactant.
—
—
—
Electrophilicity of central atom in P O and P S esters reduce in
—
—
same order due to pp–dp bonding, which hinders the attack of
a-nucleophile in the rate determining step. On the contrary, the
non-existence of pp–dp bonding manifested to highest
—
reactivity of C O ester.
—
The concentration of nucleophile in the micellar pseudophase
has been defined as the local, molar concentration within the
micelle pseudo phase. V is the molar volume in dm3 molꢀ1 of
the reaction region and [Dn] denotes the micellar fractional
volume in which the reaction occurs. We assume V equal to the
partial molar volume of the interfacial reaction region in the
micellar pseudophase, determined by Bunton[42] as 0.14 dm3
molꢀ1. Micellar binding of substrates, PNPA, PNPDPP, and
parathion and nucleophile is governed by hydrophobic inter-
actions and the equilibrium constants KmPNPA, KmPNPDP, and KmNu are
expressed by referring these concentrations to the total volume of
the micelle. The observed rate constant, kobs, based on Scheme 2
and on the above considerations, is given by the following:
Quantitative treatment of rate data: pseudophase model
The pseudophase model treats water and micelles as distinct
reaction regions and rationalizes
a great deal of data
quantitatively.[37–41] The interfacial ion exchange and the binding
constant of the substrate are the key factors at the origin of
micellar catalysis since it is now recognized that accelerations in
surfactant solutions arise not because of an increase in the
micellar rate constants, km, as compared to those in water, kw, but
because of large reagent concentrations in the small interfacial
volume in which the reaction occurs. A quantitative interpret-
ation of the experimental behavior observed can be carried out
by means of formalism of the micellar pseudophase. The
influence of cationic micelles on the kobs values for the
nucleophilic bimolecular reactions of PNPA, PNPDP, and para-
thion with a-nucleophiles can be described as illustrated in
Scheme 2.
k2m
k2w
þ
KmNuKmSubstrate½Dnꢂ
V
À
ÁÀ
Á
kobs
¼
½Nuꢂ
(1)
1 þ KmNu½Dnꢂ 1 þ KmSubstrate½Dnꢂ
Second order rate constants at the micellar interface and
association constants of the hydroxamate, oximate, hydroxy-
benzoate, and 2-iodosobenzoate ions to the cationic micelles
were obtained by fitting Eqn 1 to the experimental data that are
listed in Table 2. Figure 4 shows the simulated rate-surfactant
profiles for the reaction of PNPA, PNPDP, and parathion with
different a-nucleophiles [I] in the presence of C16PPh3Br micelles.
In Scheme 2, subscripts w and m indicate aqueous and micellar
pseudophases, respectively, and Dn represents the micellized
surfactant, that is, [Dn] ¼ [DT]-cmc, where [DT] is the stoichio-
metric surfactant concentration and cmc the critical micellar
concentration, obtained under the experimental conditions as
the minimum surfactant concentration required to observe any
kinetic effect.
Scheme 2 considers the distribution of PNPA, PNPDP, and
parathion between the aqueous and micellar pseudophases,
KmPNPA, KmPNPDP, and KmParathion. The association constants of PNPA,
PNPDP, and parathion have been obtained from fitting the
EXPERIMENTAL
Materials
reaction data with the values of KmPNPA ¼ 300 Mꢀ1, KmPNPDP
¼
PNPA was purchased from s.d.fine and was used as received.
PNPDP, parathion, and PBHA were prepared by literature method
900 Mꢀ1, and KmParathion ¼ 500 Mꢀ1 in C16PPh3Br micelles. The
Figure 4. Simulated rate-surfactant profiles for the reaction of PNPA (A), PNPDP (B), and parathion (C) with different a-nucleophiles [I] in the presence of
cetyltriphenylphosphonium bromide micelles (lines are predicted values with model)
www.interscience.wiley.com/journal/poc
Copyright ß 2008 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2008, 21 492–497

COMPARATIVE NUCLEOPHILIC REACTIVITIES
at the Vertox laboratory of Defence Research Development
Establishment, Gwalior. C16PPh3Br surfactant was obtained from
Prof. R. M. Palepu, St. Francis Xavier University, Antigonish,
Canada. Oxime, IBA, and HOBT were purchased from Sigma.
[3] I. H. Um, Y. K. Park, E. Buncel, Chem. Commun. 2000, 1917.
[4] E. Buncel, I. H. Um, Tetrahedron Lett. 2004, 60, 7801.
[5] F. M. Menger, H. Bark, Recl. Trav. Chim. Pays Bas. 1994, 113, 176.
[6] A. P. Grekov, Y. Vesclor, Usp. Khim. 1978, 47, 1200.
[7] K. H. Ibone-Rassa, J. O. Edward, J. Am. Chem. Soc. 1962, 84, 763.
[8] D. J. Palling, W. P. Jencks, J. Am. Chem. Soc. 1984, 106, 4869.
[9] J. E. Dixon, T. C. Bruice, J. Am. Chem. Soc. 1971, 93, 3248.
[10] J. E. Dixon, T. C. Bruice, J. Am. Chem. Soc. 1971, 93, 6592.
[11] Y. C. Yang, L. L. Szafraniec, W. T. Beaudry, J. Org. Chem. 1993, 58, 6964.
[12] X. Han, V. K. Balakrishnan, G. W. vanLoon, E. Buncel, Langmuir 2006,
22, 9009.
[13] S. Bhattacharya, P. V. Kumar, Langmuir 2005, 21, 71.
[14] K. R. Fountain, C. J. Felkerson, J. D. Driskell, B. D. Lamp, J. Org. Chem.
2003, 68, 1810.
[15] K. R. Fountain, D. B. Tad-y, T. W. Paul, M. V. Golynskiy, J. Org. Chem.
1999, 64, 6547.
Methods
All of the reactions were followed at 27 ꢃ 0.2 8C with a Varian
Cary-50 spectrophotometer and Systronics (Type-104) spectro-
photometer. Substrate stock solution of 0.0015 M was prepared
with triply distilled water. The rate of nucleophilic reaction was
determined by following the increase in absorption of
p-nitrophenoxide anion (400 nm). All of the kinetic experiments
were performed at an ionic strength of 0.1 M (with KCl).
Phosphate buffer was employed to control the pH of the media.
The pH of the reaction medium was measured using Systronics
(Type-335) pH-meter. All reactions were conducted under
pseudo-first order conditions.
[16] A. J. Kirby, J. E. Davies, T. A. S. Brandao, P. F. da Silva, W. R. Rocha, F.
Nome, J. Am. Chem. Soc. 2006, 101, 1253.
[17] R. A. Moss, P. K. Gong, Langmuir 2000, 16, 8551.
[18] R. A. Moss, B. A. McKernan, Tetrahedron Lett. 2002, 43, 4179.
[19] R. A. Moss, Y. Ihara, J. Org. Chem. 1983, 48, 588.
[20] H. M. Rojas, R. A. Moss, Chem. Rev. 2002, 102, 2497.
[21] R. A. Moss, H. Morales-Rojas, Langmuir 2000, 16, 6485.
[22] S. Bhattacharya, P. V. Kumar, J. Org. Chem. 2005, 70, 9677.
[23] P. V. Kumar, B. Ganguly, S. Bhattacharya, J. Org. Chem. 2004, 69,
8634.
CONCLUSIONS
The hydrolysis was studied with PNPDP with IBA, oxime, and
PBHA to obtain the pKa values, which was found to be
approximately 7.45, 9.0, and 8.9, respectively. The effects
of C16PPh3Br on the hydrolysis of PNPA, PNPDP, and parathion
have been studied and the observed first order rate constants
increases sharply with increase in the concentrations of the
surfactant upto a limiting value. PBHA was found to be the most
reactive, and HOBT the least reactive for the hydrolysis of PNPA.
The order of cleavage by a-nucleophiles of different electrophilic
[24] F. Terrier, P. R. Dafonte, E. L. Guevel, G. Moutiers, Org. Biomol. Chem.
2006, 4, 4352.
[25] Y. Zhang, C. Duanmu, Y. Gao, Org. Lett. 2006, 8, 3215.
[26] S. J. Lee, C. H. Huang, S. Shian, K. H. Sandhage, J. Am. Ceram. Soc.
2007, 90, 1632.
[27] L. Bromberg, T. Alan. Hatton, Ind. Eng. Chem. Res. 2005, 44, 7991.
[28] R. Ramaseshan, S. Sundarrajan, Y. Liu, R. S. Barhate, N. L. Lala, S.
Ramakrishna, Nanotechnology 2006, 17, 2947.
[29] C. A. Bunton, in: Kinetics and Catalysis in Microheterogeneous Systems
(Eds: M. Gratzel, K. Kalyanasundaram ) Marcel Dekker, New York,
1991, Chapter 2.
—
—
—
center is in the order of C O > P O > P S.
—
—
—
[30] A. T. Kotchevar, R. A. Moss, P. Scrimin, P. Tecilla, H. Zhang, Tetrahedron
Lett. 1994, 35, 4927.
[31] S. Couderc, J. Toullec, Langmuir 2001, 17, 3819.
[32] K. K. Ghosh, D. Sinha, M. L. Satnami, D. K. Dubey, P. R. Dafonte, G. L.
Mundhara, Langmuir 2005, 21, 8664.
Acknowledgements
[33] K. K. Ghosh, J. Vaidya, M. L. Satnami, Int. J. Chem. Kinet. 2005, 38, 26.
[34] K. K. Ghosh, D. Sinha, M. L. Satnami, J. Surf. Sci. Technol. 2003, 19, 159.
[35] K. K. Ghosh, Y. Simanenko, M. L. Satnami, S. K. Sar, Indian J. Chem.
2004, 43B, 1990.
[36] J. H. Fendler, Membrane Mimetic Chemistry, Wiley Interscience, New
York, 1982.
[37] S. Tascioglu, Tetrahedron 1996, 52, 11113.
The authors are grateful for the financial support from Defence
Research Development Organization, Government of India, New
Delhi (Grant No. ERIP/ER/0303406/M/01). The authors are also
grateful to Dr P. R. Dafonte, University of Santiago de Compostela,
Spain for applying Pseudo-phase model.
[38] L. S. Romsted, C. A. Bunton, J. Yao, Curr. Opin. Colloid Interface Sci.
1997, 2, 622.
[39] C. A. Bunton, F. Nome, F. H. Quina, L. S. Romsted, Acc. Chem. Res. 1991,
24, 354.
[40] R. A. Mackay, Adv. Colloid Interface Sci. 1981, 15, 131.
[41] L. Garcia-Rio, P. Herves, J. C. Mejuto, J. Perez-Juste, P. R. Dafonte, New J.
Chem. 2003, 27, 372.
REFERENCES
[1] E. Buncel, H. Wilson, C. Chuaqui, J. Am. Chem. Soc. 1982, 104, 4896.
[2] K. R. Fountain, K. Hutchinson, C. C. Mulhearn, Y. B. Xu, J. Org. Chem.
1993, 58, 7883.
[42] C. A. Bunton, N. Carrasco, S. K. Huang, C. H. Paik, L. S. Romsted, J. Am.
Chem. Soc. 1978, 100, 5420.
J. Phys. Org. Chem. 2008, 21 492–497
Copyright ß 2008 John Wiley & Sons, Ltd.
www.interscience.wiley.com/journal/poc