Nucleophilic Dephosphorylation of p-Nitrophenyl Diphenyl Phosphate

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Nucleophilic Dephosphorylation of p-Nitrophenyl Diphenyl Phosphate in Cationic Micellar Media Kallol K. Ghosh,*,† Daliya Sinha,† Manmohan L. Satnami,† D. K. Dubey,‡ P. Rodriguez-Dafonte,§ and G. L. Mundhara† School of Studies in Chemistry, Pt. Ravishankar Shukla University, Raipur, 492010 India, Defence Research & Development Establishment, Gwalior, India, and Departamento de Quimica Fisica, Facultad de Quimica, Universidad de Santiago de Compostela, Spain Received May 8, 2005. In Final Form: June 30, 2005 The kinetics of nucleophilic dephosphorylation of p-nitrophenyl diphenyl phosphate by hydroxamate ions (R′(CdO)N(RO-)) have been investigated in aqueous cationic micellar media at pH 9.12 and 27 °C. The pseudo-first-order rate constant-surfactant profiles show micelle-assisted bimolecular reactions involving interfacial ion exchange between bulk aqueous media and micellar pseudophase. N-Substituted hydroxamate ion shows higher reactivity over the unsubstituted hydroxamate ions in cationic micellar media. The kinetic data are discussed in terms of the pseudophase ion exchange model.

Introduction The hydrolysis of phosphotriesters are classical reactions of fundamental importance in chemistry and biology. Many organophosphorus compounds, which are known toxic substances, are used as pesticides, insecticides, and chemical warfare agents (nerve gases). These compounds are extremely potent inhibitors of acetylcholinesterase, the enzyme responsible for regulating the concentration of the neurotransmitter acetylcholine at cholinergic synapses. These are also inhibitors of butyrylcholinesterase and neuropathy target esterase.l There is an immediate need to develop safe, effective, convenient, and economically feasible methods for detoxification of these neurotoxic compounds.2-12 Due to licensing and safety problem direct target G-series nerve agents, i.e., sarin (O-isopropyl methyl phosphonofluoridate) and soman (Opinacolyl methylphosphonofluoridate), cannot be used. Therefore, instead of the actual target compounds, the most popular and widely used simulant p-nitrophenyl diphenyl phosphate (I) is used. * To whom correspondence should be addressed. E-mail: [email protected]. † Pt. Ravishankar Shukla University. ‡ Defence Research & Development Establishment. § Universidad de Vigo. (1) Schafer, L. M.; Volker, T.; Barites, C. F.; Thompson, C. M.; Lockridge, O. Chem. Res. Toxicol. 2005, 18, 747. (2) Tsang, J. S.; Neverov, A. A.; Brown, R. S. J. Am. Chem. Soc. 2003, 125, 7602. (3) Dubey, D. K.; Gupta, A. K.; Sharma, M.; Prabha S.; Vaidyanathaswami, R. Langmuir 2002, 18, 10489. (4) (a) Yang, Y.-C.; Baker, J. A.; Ward, J. R. Chem. Rev. 1992, 92, 1729. (b) Yang, Y.-C. Acc. Chem. Res. 1999, 32, 109. (5) Ouarti, O.; Blagoeva, I. B.; El Seoud, O. A.; Ruasse, M. F. J. Phys. Chem. 2001, 14, 823. (6) Balakrishnan, V. K.; Han, X.; VanLoon, G. W.; Dust, J. M.; Toullec, J.; Buncel, E. Langmuir 2004, 20, 6586. (7) Wanger, G. W.; O’Connor, R. J.; Edward, J. L.; Brevett, C. A. S. Langmuir 2004, 20, 7146. (8) Couderc, S.; Toullec, J. Langmuir 2001, 17, 3819. (9) Bunton, C. A., Foroudian, H. J. Langmuir 1993, 9, 2832. (10) Buncel, E.; Albright, K. G.; Onyido, I. Org. Biomol. Chem. 2004, 2, 601. (11) Bunton, C. A.; Gillitt, N. D.; Foroudian, H. J. Langmuir 1998, 14, 4415. (12) Moss, R. A.; Rojas Morales H.; Vijayaraghavan, S.; Tian, J. V. J. Am. Chem. Soc. 2004, 126, 10923.

Over the past several years, a number of theoretical and experimental approaches have been used to address the problem of hydrolysis/dephosphorylation of anthropogenic organophosphorus esters.13-24 These include highly reactive R-nucleophiles as oximates, hydroxamates, hydrogen peroxide anion, o-iodosylbenzoate and its derivatives, hydrazine, hydroxylamine, hydroxybenzotriazoles or the use of metal ions as lewis acid catalyst.13-24 Besides these, metallomicelles,25 enzymes, and antibodies26,27 have also been used. In view of the recognized ability (13) Terrier, F.; Guevel, E. L.; Chatrousse, A. P.; Moutiers, G.; Buncel, E. Chem. Commun. 2003, 600. (14) Fountain, K. R.; Felkerson, C. J.; Driskell, J. D.; Lamp, B. D. J. Org. Chem. 2003, 68, 1810. (15) Shirin, S.; Buncel, E.; vanLoon, G. W. Can. J. Chem. 2002, 81, 42. (16) Costas-Costas, U.; Bravo- Diaz, C.; Chaimovich, H.; Cuccovia, I. M. Colloids Surf. A 2004, 250, 385. (17) Ghosh, K. K.; Satnami, M. L.; Sinha, D. Tetrahedron Lett. 2004, 45, 9103. (18) Simanenko, Y. S.; Prokop,eva, T. M.; Popov, A. F.; Bunton, C. A.; Karpichev, E. A.; Savelova, V. A.; Ghosh, K. K. Russ. J. Org. Chem. 2004, 40, 1337. (19) Brown, R. S.; Neverov, A. A.; Tsang, J. S. W.; Gibson, T. T.; Montoya-Pelaez, P. J. Can. J. Chem. 2004, 82, 1791. (20) (a) Rojas, Morales-H.; Moss, R. A. Chem. Rev. 2002, 102, 2497. (b) Moss, R. A.; Gong, P. K. Langmuir 2000, 16, 8551. (21) Kirby, A. J.; Lima, M. F.; Silva, D.; Nome, F. J. Am. Chem. Soc. 2004, 126, 1350. (22) Domingos, J. B.; Longhinotti, E.; Brandaos, T. A. S.; Eberlin, M. N.; Bunton, C. A.; Nome, F. J. Org. Chem. 2004, 69, 7898. (23) Nome, F.; Domingos, J. B.; Longhinotti, E.; Bunton, C. A. J. Org. Chem. 2003, 68, 7051. (24) (a) Bhattacharya, S.; Praveen Kumar, V. Langmuir 2005, 21, 71. (b) Praveen Kumar, V.; Ganguly B.; Bhattacharya, S. J. Org. Chem. 2004, 69, 8634. (c) Bhattacharya, S.; Snehalatha, K.; Praveen Kumar, V. J. Org. Chem. 2003, 68, 2741. (25) Xie, J. Q.; Jiang, B. Y.; Kou, X. M.; Hu, C. W.; Li, Y. T. Transition Met. Chem. 2003, 28, 787. (26) Vayron, P.; Renard, P. Y.; Taran, F.; Creminon, C.; Frobert, Y.; Grassi, J.; Mioskowski, C. Proceed. Natl. Acad. Sci. U.S.A. 2000, 97, 7058.

10.1021/la051223b CCC: $30.25 © 2005 American Chemical Society Published on Web 08/18/2005

Nucleophilic Dephosphorylation of PNPDPP

of cationic micelles to accelerate the cleavage of carboxylic esters, micelle catalyzed hydrolyses of phosphate esters have been studied. The present paper reports the cleavage of PNPDPP(I) by hydroxamate ions (II) using cationic surfactants. The hydroxamate ions are believed to act as R-nucleophiles and are good deacylating and dephosphorylating agents. The R-nucleophiles have a lone pair of electrons on an atom adjacent to the nucleophilic center and show remarkable reactivity compared with normal nucleophile of comparable (kR-nuc./knormal nuc.) pKa.

R ) H, R′ ) CH3 (acetohydroxamate ion, AHA) R ) H, R′ ) C6H5 (benzohydroxamate ion, BHA) R ) H, R′ ) 2-OH‚C6H4 (salicylhydroxamate ion, SHA) R ) C6H5 R′ ) C6H5 (N-phenylbenzohydroxamate ion, PBHA) R ) C6H5, R′ ) 4-FC6H4 (4-fluoro-N-phenylbenzohydroxamate ion, p F-PBHA) The extent of micellar catalysis is expected to be dependent on the relative amount of substrate incorporated in micelles. Therefore, for comparison, diethyl p-nitrophenyl phosphate (paraoxon) was also used (III).

Langmuir, Vol. 21, No. 19, 2005 8665 Scheme 1

Table 1. pH-Dependent Pseudo-First Order Rate Constants for the Nucleophilic Substitution Reaction of p-Nitrophenyl Diphenyl Phosphate with N-Phenylbenzohydroxamate (PBHA-) Ion in Micellar Solution at 27 °Ca pH

kobsd × 103/s-1

6.7 7.3 8.0 8.5 10.0 11.0

0.26 0.92 1.81 4.00 9.00 10.51

a µ ) 0.1 M KCl, [PNPDPP] ) 0.5 × 10-4 M, [PBHA] ) 0.5 × 10-3 M, [CTAB] ) 1.8 mM, Medium 4% MeCN.

Table 2. Nucleophile Concentration Dependent First-Order Rate Constants for the Reaction of PNPDPP with N-Phenylbenzohydroxamate Ion in Micellar Mediaa [PBHA]/mM

3 -1 kHA obsd × 10 /s

0.0 0.25 0.50 0.75 1.00

0.19 4.30 7.75 11.2 13.8

a µ ) 0.1 M KCl, Temp. 27 °C. [PNPDPP] 0.05 mM, pH ) 9.12, [CTAB] ) 1.80 mM.

Table 3. Kinetic Rate Data for the Nucleophilic Substitution Reactions of p-Nitrophenyl Diphenyl Phosphate with N-Phenylbenzohydroxamate Ion in Cationic Micellar Solutionsa

The PNPDPP is highly lipophilic, whereas paraoxon is much more hydrophilic and much less incorporated into the micelles. Due to the biological importance and their environmental significance the detoxification reaction is essential. This article reports a systematic study of the effect of different cationic surfactants (IV) varying, head, tail and counterions on the dephosphorylation reaction.

kobsd × 103/s-1 [surfactant]/ mM CPC CPB CTACl CTAB CDEAB TTAB DTAB 0.0 0.36 0.90 1.80 3.60 5.40 7.00 8.00 9.00

0.13 8.50 8.90 9.33 9.20 8.40 7.61 7.00 6.40

0.13 6.98 8.50 9.13 8.85 8.10 7.30 6.70 6.10

0.13 6.15 7.00 7.68 7.40 6.90 6.10 5.70 5.22

0.13 4.09 7.46 7.73 7.22 5.98 4.85 4.10 3.32

0.13 5.70 6.60 7.24 6.90 6.10 5.30 4.80 4.31

0.13 3.70 4.70 5.22 5.16 4.75 4.45 4.25 4.13

0.40 0.90 2.12

µ ) 0.1 M KCl, medium ) 4.0% (v/v), MeCN. [PNPDPP] ) 0.05, mM, pH ) 9.12, [PBHA] ) 0.5 mM. a

Biomimetic models such as micelles, reverse micelles, microemulsions, cyclodextrins, liposomes, and vesicles have been used to accelerate reactions of phosphate esters with nucleophiles. The application of the pseudophase model28 that does promise a satisfactory quantitative explanation of reactivity in micelles has been used. Results and Discussion Pseudo-first-order rate constants for the reaction of p-nitrophenyl diphenyl phosphate with hydroxamate ions (27) Zhang, J.; Lan, W.; Qiao, C.; Jiang, H.; Mulchandani, A.; Chen, W. Biotechnol. Prog. 2004, 20, 1567. (28) (a) Bunton, C. A.; Savelli, G. Adv. Phys. Org. Chem. 1986, 22, 213. (b) Bunton, C. A. J. Mol. Liq. 1997, 72, 231.

(Scheme 1) have been determined at 27 °C in 4% (v/v) MeCN aqueous media 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 rate of reaction shows drastic change at the pH where the 50% hydroxamic acid deprotonated, i.e., pKa, of hydroxamic acid (Table 1). The pKa of all of the hydroxamic acids were determined in the presence and absence of CTAB (Table 4). The effect of cationic surfactants on the pKa is not significant. The pKa value, thus, determined under micellar conditions agrees with the value determined pH-meterically in 10% (v/v) MeCN medium. The rate surfactant profile for the reaction of PNPDPP with N-phenylbenzohydroxamate ion in cationic micellar solution is typical of a pH-dependent nucleophilic reaction. Hydroxamic acids have been suggested to behave either as NH or OH acids depending on solvents.29-31 Numerous studies indicate that hydroxamic acids are OH, rather than NH, acids in H2O.29 It is known that the anion of hydroxamic acid (N-O-) acts as a reactive species in the

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Table 4. Kinetic Rate Data for the Reaction of p-Nitrophenyl Diphenyl Phosphate with Hydroxamate Ions in Cationic Micellar Solutions of Cetylpyridinium Bromide at 27 °Ca kobsd × 103/s-1 [CPB]/mM

p-F-PBHA (8.7)b

PBHA (8.9)b

SHA (7.2)b

BHA (8.6)b

AHA (9.2)b

0.0 0.36 0.90 1.80 3.60 5.40 7.00 8.00 9.00

0.10 7.25 8.71 9.60 9.30 8.30 7.10 6.30 5.44

0.13 6.98 8.50 9.13 8.85 8.10 7.30 6.70 6.10

0.19 0.72 1.20 1.59 1.50 1.41 1.10 0.91 0.73

0.15 0.52 0.81 1.28 1.19 1.00 0.80 0.75 0.71

0.12 0.50 0.70 0.65 0.63

a µ ) 0.1 M KCl. [PNPDPP] ) 0.5 mM, pH ) 9.12, [HA] ) 0.5 mM. b The pKa of nucleophiles in micelles are given in parentheses.

Figure 1. Plots of first-order rate constants vs pH (A) and log kψ vs pH (B) for the reaction of PNPDPP with N-phenylbenzohydroxamate ion in CTAB.

hydrolysis of esters. Consequently, the pKa for the conversion of the N-OH to N-O- form plays an important role for the cleavage of phosphate esters. A pH-rate constant profile for the nucleophilic cleavage of 0.05 mM PNPDPP by 0.5 mM hydroxamate ion in CTAB micellar media (1.8 mM) gave the apparent pKa values for each of the hydroxamic acids. Typically, the pseudo-first-order rate constants for the reaction of PNPDPP were determined at different pH values between 6.7 and 11.0. In Figure 1, we present a representative pH-rate constant profile for the cleavage of 0.05 mM PNPDPP by 0.5 mM of N-phenylbenzohydroxamic acid in micellar CTAB (1.8 mM) at 27° C. The plot of log kψ vs pH (Figure 1) gave a break at pH 8.9 which was taken as a systematic pKa for the PBHA under CTAB micellar conditions. To investigate the nucleophilic catalysis of hydroxamate ions for the decomposition of organophosphate, we have studied the reaction of PNPDPP in the presence and absence of hydroxamate ions. By comparing the observed pseudo-first-order rate constant in the presence of hydroxamic acids (kobs) and in buffer alone (k0), it is apparent that the addition of hydroxamic acids under these conditions increases the rate of nucleophilic reaction of PNPDPP significantly. The nucleophile concentration dependent first-order rate constant was determined for the reaction of PNPDPP with hydroxamic acids in excess. Table 2 summarizes the data for the reaction of PNPDPP with different concen(29) Exner, O.; Hradil, M.; Mollin, J. J. Collect. Czech. Chem. Commun. 1990, 55, 3980. (30) Bagno, A.; Commuzzi, C.; Scorrano, G. J. Am. Chem. Soc. 1994, 116, 916. (31) Um, I. H.; Yoon, H. W.; Lee, J. S.; Moon, H. J. Kwon, D. S. J. Org. Chem. 1997, 62, 5939.

3 -1 Figure 2. Kinetic plot of kHA vs concentration of obs × 10 /s PBHA in the micellar media.

tration of N-phenylbenzohydroxamate ion at pH 9.12. Kinetic data show additional support for the hypothesis that hydroxamic acid is acting as a nucleophilic catalyst for the reaction of PNPDPP. Equation 1 describes the reaction of PNPDPP with nucleophiles, and k0 defined in eq 2 corresponds to the intercept in the kobs vs [Nu] plot

kobs ) k0 + kNu[Nu]

(1)

k0 ) kH2O + kOH-[OH-]

(2)

The kH2O term may assume some significance for very weak nucleophiles and at very low OH- concentrations. At high pH, the intercept is dominated by the kOH- term. Plotting kobs vs [Nu-] gave a straight line (Figure 2) with intercept k0. This indicates that competition with other nucleophiles, i.e., OH- and H2O, is not expected and hydroxamate ions are very strong nucleophiles for the nucleophilic attack at the P center of PNPDPP and kobs is simply given by kobs ) kNu[Nu]. Under the identical condition the reaction of paraoxon with hydroxamate ion is slow. Kinetic Studies in Micelles. The rate constant data for the nucleophilic substitution reaction of PNPDPP with N-phenylbenzohydroxamate ion at 9.12 pH are given in Table 3. The results follow the typical biphasic pattern. Cationic micelle catalyzed the reaction and kψ passed through maxima with increasing surfactant concentration. The rate maxima are independent of the type of surfactants but the magnitude of the rate constant depends on the type of surfactants. The rate-surfactant concentration profiles obtained with various surfactants/catalysts are characteristic of the micelle catalyzed reaction.32 The variation of rate constants below the critical micelle concentration(cmc) is difficult to quantify due to reactant induced micellization and interaction with nonmicellized surfactants. The reaction was slightly faster when the counterion was chloride than bromide ion (Table 3). The fractional ionic dissociation, R, of micelle is often little affected by the nature or the concentration of the counterion. In other words, the micellar surface appears to be saturated with counterions, and the fractional coverage β ) 1 - R is constant. If β is constant, the rate of reaction should increases as substrate is taken up by the micelles, but once the substrate is fully bound, the rate should be independent of added surfactant or counterion. CTAB is more reactive than TTAB. The effect of DTAB is insignificant. The kψ values increase with increasing alkyl chain lengths of the surfactants, i.e., with increasing aggregation number of micelle. This increase in the order is mainly due to the increase in the electrical surface potential of (32) Romsted, L. S.; Bunton, C. A.; Yao, J. Curr. Opin. Colloid Interface Sci. 1997, 2, 622.

Nucleophilic Dephosphorylation of PNPDPP Scheme 2

the micelle and partially due to an increase of hydrophobicity of the palisade layer of micelle. The hydroxamate ion concentration in the vicinity of the micellar surface is expected to increase with increase in the aggregation number. According to Buncel et al.6 the electrostatic attraction of the cationic headgroups of the surfactants at the micelle surface to the nucleophilic anion counterions leads to augmentation of the local concentration of the nucleophile, whereas incorporation of the substrate in the micelle leads to a higher local concentration of the substrate. This enhanced concentration of the reactants accounts for the higher rate of reaction. Implicit in this explanation is the requirement that the reactive site of the PNPDPP be situated in close proximity to the nucleophile, that is, at the micelle-water interface, the Stern layer. The subsequent addition of the cationic surfactant after cmc caused a decrease in the reaction rate possibly due to the decrease in the catalyst/reagent concentration in the micellar pseudophase. The excess of unreactive counterions (X-) compete with hydroxamate ions for available sites in the Stern layer. The rate of the nucleophilic reaction of the anionic nucleophile depends on the binding of the substrate molecule hydrophobically and electrostatically attraction of anionic nucleophlies in to the micelle. Kinetic rate data for the reaction of PNPDPP with various hydroxamate ions in micellar solution of cetylpyridinium bromide shows that the rate of reaction increases with increasing hydrophobicity of the nucleophiles. By comparing the reaction rate of N-substituted and unsubstituted hydroxamate ions in aqueous media (Table 4), it can be concluded that the micellar system shows differential reactivity than bulk aqueous media. Table 4 shows the rate data for the reaction of PNPDPP with hydroxamate ions of different hydrophobicity. In aqueous media, reactivities of p-FPBHA and PBHA are comparable and slightly lower than the SHA, BHA, and AHA. In aqueous micellar media, p-F-PBHA and PBHA show higher reactivity than SHA, BHA, and AHA. The solubility of PBHA and PNPDPP in pure buffered aqueous solution was quite low. However, they are readily soluble in CTAB micelles. Since these hydrophobic substrates also partition into the micellar pseudophase, increased localization of catalysts and substrates lead to rate acceleration in the cleavage of phosphate esters. Similar observation has also been made for the reaction of paraoxon with hydroxamate ions. Paraoxon is less hydrophobic than PNPDPP; therefore, the reactivity of hydroxamate ions in micellar solution is not significant (data not shown). The hydrolysis reaction for PNPDPP proceeding via the steps outlined in Scheme 2. A point of interest was whether the catalyst was consumed as the reaction proceeded or was regenerated and indeed a true catalyst. When the concentration of PNPDPP was in large excess, the release of p-nitrophenoxide ion also followed pseudo-first-order

Langmuir, Vol. 21, No. 19, 2005 8667 Scheme 3

kinetics and yielded a consistent kobs value. With such a small proportion of the catalyst, the kinetics could only be first order if the catalyst was not consumed during the course of reaction. There is no direct experimental evidence for the complete regeneration of hydroxamic acids. Other R-nucleophiles such as O-iodosylcarboxylates,17 oximate,8 hydroperoxide,9 and hydroxybenzotriazoles24 rapidly cleaved phosphate esters with turnover in micelle. Pseudophase Model. Rate effects on bimolecular reactions in association colloids are rationalized by the pseudophase model,28 in which the aggregates and bulk solvent, typically water, are regarded as distinct reaction regions. The overall reaction rate is the sum of the rate in each pseudophase and depends on the rate constants and reactant concentrations in each pseudophase. A crucial requirement of this model is that component distributes them much faster than the reaction. The influence of cationic micelles on the reaction rate can be quantitatively analyzed according to the model of the micelle pseudophase.33-34 We assume that the presence of cationic surfactant does not change the pKa of the hydroxamic acids in water. Under these experimental conditions, Scheme 3 can be proposed for applying the pseudophase model. In Scheme 3, subscripts w and m indicate aqueous and micellar pseudophases, respectively, and Dn represents the micellized surfactant, that is, [Dn] ) [DT] - cmc, where [DT] is the stoichiometric 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 3 considers the distribution of PNPDPP between . This the aqueous and micellar pseudophases, KPNPDPP m association constant of PNPDPP has been previously obtained8 from studies of hydrolysis reactions of PNDPP ) 7000 M-1. in micellar media with a value of KPNPDPP m The distribution of the hydroxamate ion, HA, between both pseudophases is considered through the distribution constant KHA m . The different reactivities in the aqueous and micellar pseudophases have been taken into account through the corresponding second-order rate constants: m w kw 2 and k2 . The values of k2 have been obtained by studying the reaction in the absence of the surfactant. The hydroxamate concentration in the micellar pseudophase has been defined as the local, molar concentration h, within the micelle pseudo phase: [HA]T ) [HA]m/DnV where V h is the molar volume in dm3 mol-1 of the reaction region and [Dn]V h 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 (33) Garcia-Rio, L.; Herves, P.; Mejuto, J. C.; Perez-Juste, J.; Rodriguez-Dafonte, P. New J. Chem. 2003, 27, 372. (34) Bunton, C. A.; Carrasco, N.; Huang, S. K.; Paik, C. H.; Romsted, L. S. J. Am. Chem. Soc. 1978, 100, 5420.

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Table 5. Kinetic Parameters Obtained by Applying Pseudophase Model for the Nucleophilic Reaction of PNPDPP with N-Phenylbenzohydroxamate Ions in the Presence of Cationic Micelles

PBHA (CPC) PBHA (CPB) PBHA (CTACl) PBHA (CTAB) PBHA (CDEAB) PBHA (TTAB)

-1 s-1) kw 2 /(M

/(M-1) KPNDPP m

-1 KHA m /(M )

-1 -1 km 2 /(M ‚s )

0.13 × 10-3 0.13 × 10-3 0.13 × 10-3 0.13 × 10-3 0.13 × 10-3 0.13 × 10-3

7000 7000 7000 7000 7000 7000

86.2 73.7 73.4 118.6 96.8 43.9

(3.93 ( 0.15) × 10-2 (4.20 ( 0.11) × 10-2 (3.56 ( 0.10) × 10-2 (2.16 ( 0.22) × 10-2 (2.59 ( 0.09) × 10-2 (3.79 ( 0.07) × 10-2

Table 6. Kinetic Parameters Obtained by Applying Pseudophase Model for the Nucleophilic Reaction of PNPDPP with Hydroxamate Ions in the Presence of CPB (Cetylpyridinium Bromide) Micelles -1 s-1) kw 2 /(M

/(M-1) KPNDPP m

-1 KHA m /(M )

-1 s-1) km 2 /(M

7000 7000 7000 7000 7000

94.1 73.7 67.5 50.5 5.2

(3.51 ( 0.17) × 10-2 (4.20 ( 0.11) × 10-2 (6.78 ( 0.92) × 10-3 (6.62 ( 0.93) × 10-3 (3.67 ( 0.47) × 10-2

10-3

0.10 × 0.13 × 10-3 0.19 × 10-3 0.15 × 10-3 0.12 × 10-3

p-F-PBHA PBHA SHA BHA AHA

the micellar pseudophase, determined by Bunton34 as 0.14 dm3 mol-1. Micellar binding of both substrate, PNPDPP and hydroxamate ions HAs, is governed by hydrophobic and interactions and the equilibrium constants KPNPDPP m KHA are expressed by referring these concentrations to m the total volume of the observed rate constants, kobs, based on Scheme 3 and on the above considerations, is given by the following equation:

km 2 PNPDPP HA K Km [Dn] V h m [HA]T kobs ) (1 + KPNPDPP [Dn])(1 + KHA m m [Dn]) kw 2 +

(3)

earlier, N-substituted hydroxamic acids are more hydrophobic, associate with CPB micelle through hydrophobic interactions (Kp-FPBHA ) 86.2 M-1; KPBHA ) 73.7 M-1). m m The N-OH groups are considerably ionized as N-O- at pH 9.1 and therefore also bind to the quarternary ammonium headgroup through electrostatic attractions. N-Substituted hydroxamate ions, p-FPBHA and PBHA, w show 351- and 323-fold micellar catalysis (km 2 /k2 ) toward the reaction of PNPDPP, whereas SHA, BHA, and AHA show around 36, 44, and 31-fold catalysis, respectively. The satisfactory fit obtained for these experiments supports the validity of the model employed. Conclusions

Second-order rate constants at the micellar interface and association constants of the hydroxamate ions to the cationic micelles were obtained by fitting eq 3 to the experimental data and listed in Tables 5 and 6. In Figures 3 and 4, the kψ calculated values with this treatment are shown by solid lines. The results presented in Table 5 allow us to study the influence of the nature of the micelle for the reaction of PNPDPP with N-phenylbenzohydroxamate ion. From the -3 fitting of eq 3, we obtained kw M-1 s-1 in 2 ) 0.13 × 10 aqueous medium. Likewise we obtained value of km 2 ) 4.20 × 10-2 M-1 s-1, for the highly reactive CPB-PBHA combination. The CPB-PBHA system shows 323-fold w micellar catalysis (km 2 /k2 ) for the cleavage of PNPDPP. A very important aspect to take into account when dealing with nucleophilic reactions in micelles is the incorporation of different nucleophiles into the micelle. Table 6 lists the substrate and nucleophile distribution constants in cetylpyridinium bromide micelle. As stated

The study was undertaken with a view to develop a hydroxamate function based “hydrolyzing nucleophile” in micellar medium to detoxify the toxic phosphorus esters. For this purpose, PNPDPP was selected as simulant of nerve agents as huge kinetic data are available in open literature on this substrate with other nucleophiles, which could be compared with the data obtained with hydroxamic acids (HAs). To achieve this target, the HAs and surfactants with varying structures were selected for kinetic study and the best combination of “HA-surfactants” was formulated as potential esterolytically detoxifying system against nerve agents. According to Brown et al.,19 many effective methods for destruction of organophosphorus materials are available, but none is applicable to all situations or classes of compounds. Currently work is underway in our laboratory to access the nucleophilic reactivity of different unsubstituted and N-substituted hydroxamic acids in the presence of novel surfactants for

Figure 3. Simulated rate-surfactant profile for the reaction of p-nitrophenyl diphenyl phosphate with N-phenylbenzohydroxamate ion (lines are predicted values with model).

Figure 4. Simulated rate-surfactant profile for the reaction of PNPDPP with hydroxamate ions in cetylpyridinium bromide micellar solutions (lines are predicted values with model).

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Langmuir, Vol. 21, No. 19, 2005 8669

the cleavage of carboxylic and phosphate esters and ultimately target compounds. Experimental Section N-Phenylbenzohydroxamic acid, p-fluorobenzohydroxamic acid, and benzohydroxamic were prepared by the literature method.35 Salicylhydroxamic acid and acetohydroxamic acid, cetyltrimethylammonium bromide, tetradecyltrimethylammonium bromide, dodecyltrimethylammonium bromide, cetylpyridinium bromide and cetylpyridinium chloride, and CDEAB were obtained from Sigma/Aldrich. A sample of PNDPP was obtained from Professor C. A. Bunton, California. Kinetics. All of the reactions were followed at 27 °C ( 0.2 °C with a UV 2-300 Unicam spectrophotometer equipped with Techne circulator (C-85A) thermostated cell holder. The rate of nucleophilic reaction with PNPDPP 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). Borate buffer was employed. All reactions were conducted under pseudo-first-order conditions. For all of the kinetic runs, the absorbance/time result fit very well to the first-order rate equation

ln(A∞ - At) ) ln(A∞ - A0) - kt

(4)

The pseudo-first-order rate constants can be determined by leastsquares fits. A progressive reaction profile is shown in Figure 5. Each experiment was repeated at least twice, and the observed rate constant was found to be reproducible within a precision of about 3% or better. The spectrum exhibits an increase in absorbance at 400 nm with the formation of p-nitrophenoxide ion during the course of reaction. The pKa values of hydroxamic (35) (a) Priyadarshini, U.; Tandon, S. G. J. Chem. Eng. Data 1967, 12, 143. (b) Rajput, S. K. Ph D. Thesis, Pt. Ravishankar Shukla University Raipur, 1984.

Figure 5. Repeat scans (1-15) every minute showing the increase in absorbance at 400 nm. acids were determined pH metrically using Systronics (type335) pH meter.

Acknowledgment. The financial support from Defence Research Development Organization, Government of India, New Delhi (Grant No. ERIP/ER/0303406/M/01) is gratefully acknowledged. Authors are grateful to Professor C. A. Bunton, University of California, Santa Barbara, CA for the gift of PNPDDP and Professor J. R. Leis, University of Santiago de Compostela, Spain for helping us in applying pseudophase model. The constructive criticisms and suggestions from the reviewers/editor are acknowledged with appreciation. Authors are thankful to Prof. G. L. Mundhara, Head, SOS in Chemistry, Pt. Ravishanker Shukla University for providing laboratory facilities. LA051223B