Degradation of the Pesticide Fenitrothion as Mediated by Cationic

Sep 19, 2006 - The reaction of fenitrothion with a series of R-nucleophile oximates having pKa values in the range of 7.7-11.8 was studied both in the...
0 downloads 0 Views 150KB Size
Download PDF
Langmuir 2006, 22, 9009-9017

9009

Degradation of the Pesticide Fenitrothion as Mediated by Cationic Surfactants and r-Nucleophilic Reagents Xiumei Han,† Vimal K. Balakrishnan,‡ Gary W. vanLoon,*,† and Erwin Buncel*,† Department of Chemistry, Queen’s UniVersity, Kingston, ON, Canada K7L 3N6, and National Water Research Institute, EnVironment Canada, Burlington, ON, L7R 4A6 ReceiVed March 8, 2006. In Final Form: June 29, 2006 The reaction of fenitrothion with a series of R-nucleophile oximates having pKa values in the range of 7.7-11.8 was studied both in the absence and presence of cetyltrimethylammonium (CTA+) surfactants. Reaction with CTAoximates was found to proceed through two pathways: SN2(P) and SN2(C); an SNAr pathway was not observed. Accordingly, the observed rate constants were dissected into the two corresponding SN2(P) and SN2(C) pathways. Use of the pseudophase ion exchange (PPIE) model for micellar catalysis in the CTA+ system allowed evaluation of micellar second-order rate constant (k2m) parameters and binding constants, (KS). KS values for CTA-oximates were found to vary with the counterion, and the rate enhancement depended on a combination of KS and k2m values. k2m/k2w values ranged from 0.0025 to 0.64, suggesting that a concentration effect is mainly responsible for the rate enhancement. In the absence of surfactant, an R-effect (i.e., kR/knormal) varying from 8 to 450 was observed for the oximate reaction, decreasing with increasing pKa. It is proposed that differential solvation (transition-state imbalance) is a cause of the R-effect in this system.

Introduction Organophosphorus (OP) pesticides are widely used in agriculture for field crop and fruit tree protection against a variety of insects. The acute lethality of OP compounds against pests may be attributed to the inhibition of acetylcholinesterase, and this also leads to toxicity with respect to humans and other mammals. Because of concerns related to their broad-ranging toxicity, numerous studies have been carried out to examine their long-term behavior in the plant-soil-water environment.1-6 These have included studies of microbial and chemical degradation processes, both those that occur naturally and those that can be enhanced or inhibited by human intervention. There are situations, such as pesticide spills, where accelerating the rate of pesticide degradation is of critical importance. Among chemical methods to bring about rate enhancements, the use of so-called R-nucleophiles has been recommended for this purpose.7 An R-nucleophile is one that bears a lone pair of electrons at the R position next to the nucleophilic site, for example, hydroxylamine, hydrazine, the anions of hypochlorite, hydroxamic acids, oximes, and hydroperoxides.8 The R-effect has been defined as * Corresponding author. † Queen’s University. ‡ Environment Canada. (1) Greenhalgh, R.; Dhawan, K. L.; Weinberger, P. J. Agric. Food. Chem. 1980, 28, 102-105. (2) Wan, H. B.; Wong, M. K.; Mok, C. Y. Pestic. Sci. 1994, 42, 93-99. (3) Ohshiro, K.; Kakuta, T.; Sakai, T.; Hirota, H.; Hoshino, T.; Uchiyama, T. J. Ferment. Bioeng. 1996, 82, 299-305. (4) Lartiges, S. B.; Garrigues, P. P. EnViron. Sci. Technol. 1995, 29, 12461254. (5) Lacorte, S.; Barcelo´, D. EnViron. Sci. Technol. 1994, 28, 1159-1163. (6) Durand, G.; Mansour, M.; Barcelo´, D. Anal. Chim. Acta 1992, 262, 167178. (7) (a) Buncel, E.; Um, I.-H. Tetrahedron 2004, 60, 7801-7825. (b) Balakrishnan, V. K.; Buncel, E.; vanLoon, G. W. EnViron. Sci. Technol. 2005, 39 (15), 5824-5830. (8) (a) Wilson, I. B.; Ginsburg, S. Arch. Biochem. Biophys. 1995, 54, 569571. (b) Tarkka, R. M.; Buncel, E. J. Am. Chem. Soc. 1995, 117, 1503-1507. (c) Um, I.-H.; Buncel, E. J. Org. Chem. 2000, 65 (2), 577-582. (d) Hoz, S.; Buncel, E. Isr. J. Chem. 1985, 26, 313-319. (e) Ouarti, N.; Marques, A.; Blagoeva, I.; Ruasse, M. F. Langmuir 2000, 16, 2157-2163. (f) Ghosh, K. K.; Sinha, D.; Satnami, M. L.; Dubey, D. K.; Dafonte, P. R.; Mundhara, G. L. Langmuir 2005, 21, 8664-8669. (g) Morales-Rojas, H.; Moss, R. A. Chem. ReV. 2002, 102, 2497-2521.

“a positive deviation exhibited by an R-nucleophile from a Brφnsted-type nucleophilicity plot”.8d Oximate ions exhibit the characteristic rate enhancement of R-nucleophiles in their reaction with phosphate triesters; however, recent studies have described a “leveling-off” effect in reactivity (i.e., log k vs pKa plot) as pKa increased beyond ∼8.5.9 Also pertinent to the present study are the findings by Bunton,10 Toullec,11 and their co-workers that the rate of degradation of phosphoesters in the presence of oximate ions was greatly enhanced by cationic surfactants.10,11 Recent studies of the degradation of fenitrothion (1), [O,Odimethyl O-(3-methyl-4-nitrophenyl) phosphorothioate], a broadspectrum insecticide, with nucleophiles have revealed that different pathways may operate, as shown in Scheme 1.1,12-14 Balakrishnan et al.14 found that ethanolysis with alkali metal alkoxides (M+ -OEt; M+ ) Li+, K+, and Na+) in anhydrous ethanol proceeds by nucleophilic attack at both P (SN2(P)) and aliphatic carbon (SN2 (C)) centers; a minor SNAr route (e7%) was also detected. Meanwhile, Omakor et al.12 found that, in comparison to normal oxygen nucleophiles such as HO- and CF3CH2O-, enhanced reactivity was observed for the reaction of fenitrothion with the R-nucleophiles 2,3-butanedione monoximate and hydrogen peroxide anion. Rate acceleration was also observed in the presence of the cationic surfactants cetyltrimethylammonium (CTA+) with the counterions OH- and R-nucleophilic antipyruvaldehyde 1-oximate (MINA-, where MINA- is CH3-COCHdN-O-).15 (9) (a) Terrier, F.; MacCormack, P.; Kizilian, E.; Halle´, J.-C.; Demerseman, P.; Guir, F.; Lion, C. J. Chem. Soc., Perkin Trans. 2 1991, 153-158. (b) Degorre, F.; Kiffer, D.; Terrier, F. J. Med. Chem. 1988, 31, 757-763. (10) Bunton, C. A.; Ihara, Y. J. Org. Chem. 1977, 42, 2865-2869. (11) Couderc, S.; Toullec, J. Langmuir 2001, 17, 3819-3828. (12) Omakor, E. J.; Onyido, I.; vanLoon, G. W.; Buncel, E. J. Chem. Soc., Perkin Trans. 2 2001, 324-330. (13) Onyido, I.; Omakor, J. E.; vanLoon, G. W.; Buncel, E. ARKIVOC 2001, 134-142. (14) Balakrishnan, V. K.; Dust, J. M.; vanLoon, G. W.; Buncel, E. Can. J. Chem. 2001, 79, 157-173. (15) (a) Balakrishnan, V. K. Ph.D. Thesis, Queen’s University, 2002. (b) Balakrishnan, V. K.; Han, X.; vanLoon, G. W.; Dust, J. M.; Toullec, J.; Buncel, E. Langmuir 2004, 20 (16), 6586-6593.

10.1021/la060641t CCC: $33.50 © 2006 American Chemical Society Published on Web 09/19/2006

9010 Langmuir, Vol. 22, No. 21, 2006

Han et al.

Scheme 1. Possible Pathways for the Reaction of Fenitrothion with Nucleophiles Including 31P NMR Chemical Shifts of Degradation Products 2 and 3

In this report, we present the results of a systematic study of the effect of R-nucleophilic reactive counterion surfactants on the degradation of fenitrothion. Oximate ions (Figure 1) with pKa values in the range of 7.7-11.8 were investigated both in the absence and presence of surfactants. Rate comparisons are made with the normal nucleophiles shown in Figure 2. Dual degradation pathways (SN2(P) and SN2(C)) were found to operate in the presence of surfactants, where the rate data obtained were treated according to the pseudophase ion exchange (PPIE) model of micellar catalysis.16

Results The reactions of fenitrothion with the oximate R-nucleophiles were carried out spectrophotometrically under pseudo-first-order conditions at 25.0 °C, by monitoring the formation of the product, 3-methyl-4-nitrophenoxide (see Experimental Section). Pseudofirst-order rate constants, kobs, were obtained from plots of

log(Ainf - At) versus t, which were linear for ∼90% of the reaction; second-order rate constants (k2w) in the absence of surfactants were obtained from linear plots (with negligible intercepts) of kobs versus [Nu-]. In the presence of surfactants, it was found that the observed infinity absorbance values at 398 nm (due to the formation of 3-methyl-4-nitrophenoxide) were below the Ainf value obtained in the absence of surfactant (Table 1S, Supplementary Information). Pseudo-first-order rate constants, kobs, for the reaction with CTA-oximates are shown in Figure 3. Ainf values were used for the dissection of kobs into the constituent rate constants for attack at phosphorus (kPobs) and at the aliphatic carbon (kCobs) according to eqs 1 and 2.17 The concentration of 3-methyl-4-nitrophenoxide was determined from the absorbance values at infinite time (Ainf), and the concentration of demethylfenitrothion, which does not absorb at 398 nm, was then obtained by subtracting the

Figure 1. Structures and pKa values of the R-nucleophiles used in the present study.

Degradation of the Pesticide Fenitrothion

Langmuir, Vol. 22, No. 21, 2006 9011

Table 1. The Distribution Coefficients of the Oximate Nucleophiles Used (log D), the Ainf Values at 398 nm (Observed and Theoretical), and the Percentage of SN2(P) and SN2(C) Pathways for the Reaction of Fenitrothion with CTA-Oximatesa CTA-oximates

log Db

average Ainf (observed)

Ainf (theoretical)c

percent SN2(P)d

percent SN2(C)d

9-CTA 8-CTA 7-CTA 6-CTA 10-CTA 12-CTA 11-CTA

-0.08 0.90 0.47 -0.06 1.35 0.98 0.96

1.14 ( 0.03 1.11 ( 0.03 1.13 ( 0.03 1.12 ( 0.06 0.89 ( 0.06 0.98 ( 0.09 0.57 ( 0.16

1.22 1.19 1.24 1.20 1.10 1.26 1.13

91.2 ( 3.2 92.7 ( 2.3 90 ( 2 89 ( 6 79 ( 5 77 ( 7 45.5 ( 5.5

8.8 ( 3.2 7.3 ( 2.3 10 ( 2 11 ( 6 21 ( 5 23 ( 7 54.5 ( 5.5

a Dissections are performed as described in eqs 1 and 2. b Log D values calculated at pH ) pKa using ChemAxon Software: Marvin 4.0. c Theoretical Ainf values assuming 100% reaction at phosphorus center. d Average values given are based on the variation in Ainf values as a function of [CTA-Ox-].

Figure 3. Pseudo-first-order rate plots for the reaction of fenitrothion with CTA-oximates at 25.0 °C. Table 2.

Figure 2. Structures and pKa values of normal nucleophiles from ref 12.

concentration of 3-methyl-4-nitrophenoxide from the initial concentration of fenitrothion. The dissected rate constants are provided in Tables 2S-8S (Supporting Information).

kobs ) kPobs + kCobs kPobs kCobs

)

% SN2(C) )

reaction system 5-CTA 6-CTA 7-CTA 8-CTA 9-CTA 10-CTA 11-CTA 12-CTA

(2) a

The Ainf values were also used for the calculation of percent SN2(P) and SN2(C) pathways for the reaction of fenitrothion with CTA-oximates according to eqs 3a and 3b. The results are presented in Table 1.

% SN2(P) )

above CMC

(1)

[3-methyl-4-nitrophenoxide] [demethylfenitrothion]

Ainf(experimental) Ainf(theoretical)

× 100%

A inf(theoretical) - Ainf(experimental) Ainf(theoretical)

(3a)

× 100% (3b)

31P

Product formation was followed via NMR, in the absence and presence of surfactant. In the absence of surfactants, one product peak at 58.9 ppm was observed for the reaction with (16) (a) Menger, F. M.; Portnoy, C. E. J. Am. Chem. Soc. 1967, 89, 4698. (b) Mittal, K. L., Lindman, B., Eds. Surfactants in Solution; Plenum Press: New York, 1982; Vol. 2. (17) Frost, A. A.; Pearson, R. G. Kinetics and Mechanism, 2nd ed.; John Wiley and Sons: New York, 1962.

31 P Chemical Shifts (ppm) Above and Below the CMC for the Reaction Products of Fenitrothion with CTA-r-Nucleophilic Systems at 25.0 °Ca

103

CMC (M) 1.15 1.21 0.95 0.89 0.52 0.94 1.50

below CMC

3

2

3

2

59.4 59.3 59.5 59.4 59.4 59.2 60.0 59.6

54.1 53.9 54.1 54.0 54.0 53.8 53.8 54.1

58.9 59.0 59.0 59.1 59.0 59.0 59.0

53.8 53.9 54.0 54.0 54.0 54.0 54.0

NaOH 59.07 (3); 2-PAMox 59.00 (3).

NaOH and with 2-pyridinealdoximate methochloride, while two product peaks (53.8-54.2 ppm, 58.2-59.3 ppm) were observed for the reaction with CTA-oximates. The product appearing at 58.2-59.3 ppm corresponds to O,O-dimethylphosphorothioate ((MeO)2P(S)O-), while the product appearing at 53.8-54.2 ppm corresponds to demethylfenitrothion.15 31P NMR chemical shifts corresponding to the above reaction products 2 and 3 are presented in Table 2.

Discussion Concurrent Degradation Pathways. We were alerted that competing reaction pathways were operating in the reactions of fenitrothion with CTA-oximates through the observation that the Ainf values of the product (3-methyl-4-nitrophenoxide) were not consistent with the values obtained in the absence of micelles for 100% of the reaction at the phosphorus center (Table 1). Conclusive identification of the dual reaction pathways was obtained by 31P NMR analyses (Table 2) above and below the critical micelle concentration (CMC). These studies revealed

9012 Langmuir, Vol. 22, No. 21, 2006

Han et al.

Scheme 2. Pathways for the Reaction of Fenitrothion with CTA-Oximates at 25.0 °C, with Surfactant Concentrations Both Below and Above the CMC

Scheme 3. Depiction of Fenitrothion Orientation within a Micelle of CTA-Oximatea,b

a

(a) SN2(C) pathway; (b) SN2(P) pathway. bAdapted from ref 15b.

two reaction products: O,O-dimethylphosphorothioate ((MeO)2P(S)O-) (3) at 59.0-60.0 ppm, and demethylfenitrothion (2) (53.8-54.2 ppm). Assignment of these two peaks has been discussed previously.15 Formation of demethylfenitrothion is in accord with attack by oximates at the aliphatic methoxy carbon center in an SN2(C) process. On the other hand, formation of (MeO)2P(S)O- accords with attack at the phosphorus center by the SN2(P) pathway. In the case of 2-PAMox (5), only one product was observed, corresponding to (MeO)2P(S)O- at 59.0 ppm, produced upon hydrolysis of the phosphorylated oxime, which was formed in the primary SN2(P) process. In our previous studies, we adduced evidence that the SNAr pathway was very unlikely,15b and, consequently, Scheme 2 with the SN2(P) and SN2(C) operative pathways shown is in accordance with our experimental observations.

Orientation of Fenitrothion in the Micelles. Orientation of fenitrothion in CTA-oximate micelles can be described by the model presented by Balakrishnan et al.,15 which invokes the binding of fenitrothion within the Stern layer, as shown in Scheme 3. Depending on the extent to which the nucleophile penetrates the micelle, the given orientation rationalizes the occurrence of two concurrent pathways of fenitrothion degradation. The polarity of the interfacial region is known to approximate that of ethanol,18 and, as such, “solvent” effects can be anticipated. For example, in aqueous solution, O-nucleophiles are known to react with fenitrothion only via attack at phosphorus.12 However, in ethanol, the ethoxide anion was shown to react with fenitrothion via multiple reaction pathways.14 In aqueous micellar solutions, these (18) Fendler, J. H.; Fendler, E. J.; Infante, G. A.; Shih, P.-S.; Petterson, L. K.; J. Am. Chem. Soc. 1975, 97, 89-95.

Degradation of the Pesticide Fenitrothion

Langmuir, Vol. 22, No. 21, 2006 9013

Table 3. Summary of Kinetic Parameters Derived from the PPIE Model (k2m and KS) for the SN2(P) Pathway in the Degradation of Fenitrothion in CTA-Oximates at 25.0 °C with k2w Data Included for Comparison CTA-oximate

pKa

103k2wa (M-1 s-1)

Ks (M-1)

103k2m (M-1 s-1)

k2m/k2w

rate enhancementb

5-CTAc 6-CTA 7-CTA 8-CTA 9-CTA 10-CTA 11-CTA 12-CTA

7.75 8.34 9.42 10.09 10.35 10.86 11.48 11.82

2.50 ( 0.15 1.15 ( 0.03 1.84 ( 0.17 6.47 ( 0.17 6.86 ( 0.16 6.33 ( 0.16 1.52 ( 0.08 8.89 ( 0.12

1250 ( 98 1920 ( 115 1750 ( 175 870 ( 40 1600 ( 154 540 ( 174 250 ( 51

0.66 ( 0.02 1.18 ( 0.06 1.65 ( 0.04 1.69 ( 0.07 0.46 ( 0.02 0.66 ( 0.01 0.92 ( 0.04

0.57 0.64 0.26 0.25 0.07 0.43 0.10

0.3 33 60 92 72 29 16 44

a Quoted uncertainty in kobs is taken as the difference between the average of two experimental kobs values and either experimental value. b Rate enhancement ) kobs (CTA-Ox)/kobs(OH-), with [OH-] ) 0.01 M and [micelle] ) C - CMC ) 0.01 M. c KS and k2m values could not be determined for 5-CTA (see text).

Scheme 4. Representation of the PPIE Model of Micellar Catalysis

Table 4. Summary of KS and k2m Parameters Derived from the PPIE Model for SN2(C) Pathway in the Degradation of Fenitrothion with CTA-Oximates at 25.0 °C CTA-oximate

pKa

Ks (M-1)

104k2m (M-1 s-1)

5-CTAa 6-CTA 7-CTA 8-CTA 9-CTA 10-CTA 11-CTA 12-CTA

7.75 8.34 9.42 10.1 10.4 10.9 11.5 11.8

900 ( 600 850 ( 123 330 ( 91 1070 ( 226 875 ( 56 1450 ( 276 70 ( 10

0.29 ( 0.10 1.13 ( 0.1 1.73 ( 0.12 0.96 ( 0.16 1.14 ( 0.05 1.49 ( 0.06 6.64 ( 0.17

a

same multiple pathways are again observed with the various O-nucleophiles used in this study. Partition coefficients provide a measure for the distribution of a species between a polar liquid and a nonpolar solvent. Partition coefficients are generally defined for the octanol-water system: P is expressed as the ratio of the concentration of a given compound in octanol to that found in water. However, with ionizable species (such as oximates), it is preferable to estimate a distribution coefficient, D, that accounts for both the ionic and neutral species present in solution (i.e., D ) ΣP0...n, for n charged states).19 For oximates, the log D values (Table 1) provide a basis for evaluating the partitioning of oximates from the aqueous phase into the micelle, where they can participate in nucleophilic attack at the methoxy carbon of fenitrothion. It appears that there is a relationship between the lipophilic character of the oximate as expressed by log D and the extent of the SN2(C) pathway (Table 1), with syn-Box (10) and Apox (11) exhibiting the highest percent SN2(C). Note, however, that this relationship is not uniform since the extent of attack at carbon is influenced by a number of factors apart from lipophilicity, including the size of the nucleophile and the aggregate size of the micelle (which might change with each nucleophile). Detailed studies of the factors which produce the observed regioselectivity are still needed. Kinetic Parameters Determined for the Degradation of Fenitrothion in CTA-Oximates. The reaction of fenitrothion with various nucleophiles under the PPIE model of micellar catalysis is depicted in Scheme 4. KS represents the binding Nu constant with the micellar phase; kNu 2M and k2W are the secondorder rate constants for the reaction of a given nucleophile, Nu, with fenitrothion in the micellar and aqueous phases, respectively; θNu denotes the fractional association of the nucleophile with the micelle. A full derivation of the PPIE equation is provided in the Appendix. (19) Alacorn, C. J.; Simpson, R. J.; Leahy, D. E.; Peters, T. J. Biochem. Pharmacol. 1994, 47, 1105-1106.

See footnotes in Table 3.

The reaction of fenitrothion with OH- in CTA micelles proceeds via only the SN2(P) pathway, hence kOH 2m for the SN2(C) Ox pathway is zero. kOH 2w and k2w for attack at carbon are also zero since, in the absence of micelles, the SN2(C) pathway for the reaction of OH- and Ox- was not observed. The binding constants, Ks, and the second-order rate constants, k2m, for the SN2(P) and SN2(C) pathways are presented in Tables 3 and 4, respectively. The second-order rate constants (k2w) for the SN2(P) reaction with oximates in the absence of micelles and the k2m/k2w ratio for the SN2(P) pathway is also given in Table 3. The results are discussed below. The derived k2m values based on the PPIE model (see Appendix) for the two competitive pathways in the reaction of fenitrothion with CTA-oximates are presented in Tables 3 and 4. It is seen that the ratio k2m/k2w is less than 1, suggesting that the rate enhancement is not a true catalytic effect involving reduced activation energy; rather, it must be due to the increased concentration of the reactants within the micelles.20 The data in Table 3 for the SN2(P) pathway in the reaction of fenitrothion with CTA-oximates show that the rate enhancement depends on the combination effect of the KS and k2m values. A large rate enhancement is associated with surfactants that have relatively large Ks and relatively large k2m values, such as CTA2-Pox (8-CTA), CTA-3-Pox (9-CTA) and CTA-Buox (7CTA). The rate enhancement for the oximates with higher pKa values is smaller, due to relatively small Ks and k2m values. It is also noted that CTA-2-PAMox (5-CTA) does not display a rate enhancement; on the other hand, an inhibition effect is observed. One possible reason is that the positive charge on the pyridinium nitrogen diminishes binding to the Stern layer of the cationic CTA micelles, while fenitrothion is solubilized into the micelles. Therefore, the nucleophiles would be electrostatically repelled from the micelle containing the fenitrothion, thus (20) (a) Couderc, S. Ph.D. Thesis. Universite´ de Versailles-St. Quentin en Yvelines, 1999. (b) Bunton, C. A.; Gillitt, N. D.; Kumar, A. J. Phys. Org. Chem. 1997, 10, 221-228.

9014 Langmuir, Vol. 22, No. 21, 2006

Han et al.

Table 5. Magnitude of r-Effect in SN2(P) Reaction of Fenitrothion with Nucleophiles in the Absence of Surfactant nucleophile

pKa

105 k2w (M-1s-1)

(R) 2-PAMox (5) (normal) 4-CNPhO- (15) (R) 2,3-Buox (7) (normal) 4-ClPhO- (16) (R) 2-Pox (8) (normal) PhO- (17) (R) Aox (12) (normal) CF3CH2O- (18)

7.75 7.72 9.44 9.38 10.1 9.99 11.8 12.4

250 0.560 184 4.88 647 79.8 889 44.7

R-effect ) kR-Nu/knormal 450 38.0 8.00 20.0

inhibiting reaction with 2-PAMox (5). We also note that 12CTA (acetaldoxime) shows a large rate enhancement factor (44fold) despite having a relatively small KS value; however, the rate enhancement was calculated on the basis of observed rate constants, and did not distinguish between reaction in the aqueous or micellar phases. Given that 12 had a high k2W value (a result of the significant R-effect shown in Table 5), coupled with the weak affinity of fenitrothion for 12-CTA, we conclude that the majority of the observed rate enhancements from 12-CTA are due to reactions occurring in the aqueous phase. As shown in Tables 3 and 4, the binding constants (Ks) obtained for fenitrothion with CTA-oximates are variable. This indicates different solubilization of fenitrothion in micelles composed of identical surfactants but different counterions. A change in counterion leads to changes in the aggregation number, CMC, and fractional charge of a micelle.21 In a study concerning the partition coefficients of different molecules to the micelles, Armstrong22 suggested that two additional factors are responsible for the binding of compounds in micelles: (a) the nature and size of the hydrophobic core of the micelle, and (b) Stern-layer electrostatic effects. Bunton et al.23 also suggested that the binding of an organic substrate to micelles is governed by both Coulombic and hydrophobic interactions. Therefore, we conclude that, with respect to the CTA-oximates studied here, the presence of different counterions affects both the aggregation behavior and the interfacial electrostatic effects, thereby affecting fenitrothion binding to the micelle. Brφnsted-type Plot for Reaction of Fenitrothion with Oximates in the Absence of Surfactant. The second-order rate constants for the reaction of fenitrothion with oximates by the SN2(P) pathway in the absence of surfactant are given in Table 3, and the corresponding Brφnsted plot, log k2w versus pKa, is shown in Figure 4, which includes for comparison the data for the reaction of fenitrothion with normal oxygen nucleophiles in aqueous solution.12 Considering first the normal nucleophile plot in Figure 4, the points up to pKa 10 (PhO-) lie on a linear plot (R2 ) 0.9889); however, at higher pKa, CF3CH2O-(12.4) and OH-(15.7), there is a clear downward deviation. This type of negative deviation has been well documented by Jencks and is ascribed to abnormal solvation of the strongly basic nucleophiles. Proceeding to the oximate R-nucleophiles and disregarding for the moment the point for Apox (11), after initial scatter, the plot also exhibits a plateau at higher pKa. Jencks24 and Bernasconi25 have ascribed the relative plateauing in Brφnsted-type plots at low pKa (8-8.5) to the lack of (21) Fendler, J. H. Membrane Mimetic Chemistry; Wiley-Interscience: New York, 1982. (22) Armstrong, D. W.; Stine, G. Y. J. Am. Chem. Soc. 1983, 105, 29622964. (23) Bunton, C. A.; Nome, F.; Quina, F. H.; Romsted, L. S. Acc. Chem. Res. 1991, 24, 12, 357-364. (24) Jencks, W. P.; Brant, S. R.; Gandler, J. R.; Fendrich, G.; Nakamura, C. J. Am. Chem. Soc. 1982, 104, 7045-7051. (25) Bernasconi, C. F. AdV. Phys. Org. Chem. 1992, 27, 119-238.

Figure 4. Brønsted-type plot for the reaction of fenitrothion with R-nucleophiles (Ox-) and normal oxygen nucleophiles (ArO-) by the SN2(P) pathway in the absence of micelles. Data for normal nucleophiles are obtained from ref 12, with their structures shown in Figure 2.

Figure 5. Brønsted-type plot of log k2m vs pKa (k2m data derived using the PPIE model) for the reaction of fenitrothion with CTAOx-: (A) high percent SN2(P) pathway, 88-95%; (B) intermediate percent SN2(P) pathway, 40-84%; (C) reaction only via the SN2(C) pathway.

synchronization between partial desolvation of the nucleophile and bond formation in the transition state, referred to as an “imbalance”. Terrier et al.9a also observed a leveling off at pKa 8-8.5 in the reactivity of oximates, that is, imbalance. Interestingly, the linearity of the Brφnsted plots for the PNPA-oximate system was restored in Me2SO-rich media, which suggests that the imbalance between desolvation and bond formation is no longer significant in those media.26 As seen from Figure 4, Apox (11) with a high pKa (11.48), shows the largest negative deviation in k2w. This can be ascribed to steric hindrance associated with the methyl group attached to the carbon at the β position. In fact, MINA (6) (pKa ) 8.43) and Buox (7) (pKa ) 9.42), which also possess a methyl group at the carbon in the β position, also exhibit somewhat smaller negative deviations from the plot. Brφnsted-type Plot in the Presence of Surfactant. Figure 5 presents the Brφnsted-type plot, log k2m versus pKa, for the reaction of CTA-oximates with fenitrothion. Recall that two parallel processes were observed for the reaction of the oximate R-nucleophiles with fenitrothion in the micellar system: reaction at the phosphorus center, SN2(P), and reaction at the aliphatic (26) Buncel, E.; Cannes, C.; Chatrousse, A. P.; Terrier, F. J. Am. Chem. Soc. 2002, 124, 8766-8767.

Degradation of the Pesticide Fenitrothion

carbon center, SN2(C). The dissection of the respective pseudofirst-order rate constants for each process was accomplished as described earlier. As seen from Figure 5, curve C, representing oximate attack at the carbon center shows definite curvature, which sets in at pKa ∼ 9, that is, at lower pKa than the aforementioned onset of curvature generally observed for normal nucleophiles.12 Interestingly, the dissected rate data for oximate attack at P fall on two lines: (A) high percent (88-95%) SN2(P) pathway, and (B) intermediate percent (40-84%) SN2(P) pathway. While a definitive rationale for this dichotomy cannot be given at present, it may be suggested that the two lines represent two concurrent mechanisms of displacement at P: (A) concerted and (B) twostage, addition-elimination, (SN2(Pi)). Given the greater degree of bond formation required in the SN2(Pi) mechanism, it is reasonable that the addition-elimination mechanism (B) would have a higher Brφnsted slope than the concerted mechanism (A), as observed. Caution is needed in further discussion in view of the fact the oximates used in the plots comprise different structural/ hydrophobic effects in addition to possessing changing basicity. R-Effect. According to the Brφnsted-type plot for the reaction of fenitrothion with R-nucleophiles (oximates) and normal nucleophiles in the absence of micelle, (Figure 4), all oximate R-nucleophiles used in this study exhibit a sizable R-effect. The magnitude of the R-effect, defined as R-effect ) kR/knormal, was estimated by comparison with normal nucleophiles of comparable pKa (Table 5). The data in Table 5 show a generally decreasing R-effect with increasing pKa of the R-nucleophile. This is consistent with leveling in the reactivity of oximates having high pKa values, shown in Figure 4, and also with the idea that solvational imbalance is one of the causes for the R-effect in the reaction of fenitrothion with oximates, in common with other systems.24-26 Bunton and Foroudian27 showed that mixed CTA+ micellar systems containing both Cl- and HOO- as counterions were able to efficiently degrade p-nitrophenyl diphenyl phosphate. Meanwhile, Toullec and Moukawim demonstrated that CTAOOH promoted the rapid degradation of phosphorus compounds such as paraxon (a structural analogue of fenitrothion).28 In our own previous work, we found that, in the presence of CTAOOH micelles, fenitrothion itself was subject to a significant R-effect.7b Furthermore, given that CTAMINA has been demonstrated to be effective in accelerating the hydrolysis of a variety of phosphate esters, including paraxon,11 it is quite probable that the R-effect plays a significant role in the rate enhancements observed in the micellar systems employed throughout the current study.

Conclusions The reactions of fenitrothion with a series of oximate R-nucleophiles with pKa values ranging from 7.7 to 11.8 were studied at 25.0 °C, both in the absence and presence of CTA surfactant. Product analysis experiments were performed, while the kinetic data were treated using the PPIE model of micellar catalysis. We conclude the following: (1) On the basis of UV-visible (UV-vis) and 31P NMR data, the reaction of fenitrothion with CTA-oximates proceeded through two concurrent pathways: SN2(P) and SN2(C). The observed rate constants were dissected into the two corresponding pathways. (2) The fenitrothion CTA-oximate binding constants, KS, varied between 70 and 1500 M-1 as a result of the effect of (27) Bunton, C. A.; Foroudian, H. J. Langmuir 1993, 9, 2832-2835. (28) Toullec, J.; Moukawim, M. Chem. Commun. 1996, 221-222.

Langmuir, Vol. 22, No. 21, 2006 9015

different counterions on both the aggregation behavior of the micelle and the interfacial electrostatic effects. (3) The rate enhancement for the reaction of fenitrothion with CTA-oximates depends on the combined effect of KS and k2m values. The low k2m/k2w values obtained were consistent with a concentration effect being mainly responsible for the rate enhancement. (4) Of the R-nucleophilic reactive counterion surfactants studied, CTA-2-Pox (8-CTA) displays the largest, while CTAAPox (11-CTA) displays the smallest rate acceleration. (5) Brφnsted-type plots (log k2 vs pKa) were constructed for the reaction of fenitrothion with oximates in the absence and presence of surfactants. In the absence of surfactant, a leveling in reactivity at high pKa suggests an imbalance, or nonsynchronicity, in the partial desolvation of the nucleophile and bond formation with the electrophilic center. In the presence of surfactant, leveling in reactivity by oximate nucleophiles was also observed for the SN2(C) pathway, while the SN2(P) pathway fell on two lines, suggesting two concurrent mechanisms of displacement at P. (6) Finally, an R-effect, ranging from 8 to 450, was observed for the oximates used in this study. The leveling in reactivity of oximates with increasing pKa results in a decrease in the R-effect with the more strongly basic oximates. This has implications in detoxification studies, since higher basicity of oximate introduced for the purpose of remediation or detoxification would not be accompanied by increased reactivity. Experimental Section Materials. Reagents and solvents were commercial and used as received unless otherwise specified. 3-Pyridinealdehyde oxime was recrystallized from H2O.29 2,3-Butanedione monoxime was recrystallized from chloroform.30 syn-Benzaldoxime and acetophenone oxime were recrystallized from hexane. Reagent-grade 1,4-dioxane was refluxed over anhydrous stannous chloride under nitrogen followed by distillation to remove peroxides. The distillate was refluxed over sodium metal to remove water followed by distillation and was stored in the freezer under nitrogen.29 Distilled deionized water was boiled and degassed with nitrogen during cooling. Fenitrothion, [O,O-dimethyl-O-(3-methyl-4-nitrophenyl) phosphorothioate] was a gift from Sumitomo Chemical Company and was purified by column chromatography using 20% diethyl ether80% chloroform as eluent. The purity of the product was checked by 1H NMR and 31P NMR.31 Stock buffer solutions of R-nucleophiles (oximates) were prepared by mixing standard NaOH and oxime solutions (1:2) under nitrogen. Similarly, stock buffer solutions of hexadecyltrimethylammonium oximates (CTA-oximates) were prepared by the 1:2 mixing of CTAOH solution and oxime solution under nitrogen. In all cases, solutions were prepared such that pH ) oximate pKa (see Table 3). CTA-OH (10 wt %) from Aldrich was standardized against potassium hydrogen phthalate. Determination of Micellar Parameters. The CMC values were determined by electrical conductivity and are given in Table 2. The degree of counterion association with the micelles (β) for CTAsyn-Box (10-CTA) and CTA-APox (11-CTA) was determined using the conductivity method developed by Nome and co-workers.32 The results are provide in the Supporting Information in Tables 9S-15S. Kinetic Measurements. Kinetic experiments were performed by UV-vis spectrophotometry using a Varian Cary 3 double-beam spectrophotometer, a Hewlett-Packard 8452A diode array spectro(29) Perrin, D. D.; Armarego, W. L. F. Purification of Laboratory Chemicals, 3rd ed.; Pergamon Press: New York, 1988. (30) Omakor, E. J. M.Sc. Thesis, Queen’s University, 1997. (31) Han, X. M.Sc. Thesis, Queen’s University, 2002. (32) Neves, M. F. S.; Zanette, D.; Quina, F.; Moretti, M. T.; Nome, F. J. Phys. Chem. 1989, 93, 1502-1505.

9016 Langmuir, Vol. 22, No. 21, 2006

Han et al.

photometer, or a Perkin-Elmer Lambda-20 spectrophotometer, each being equipped for temperature control. All kinetic runs were followed under pseudo-first-order conditions in which the concentration of nucleophiles was at least 10 times the initial concentration of fenitrothion. The addition of solvent and nucleophile solution to the reaction cuvettes was done under nitrogen using a gastight syringe. The cuvettes were allowed to equilibrate thermally (25.0 ( 0.1 °C) in the cell holder for 40 min. After temperature equilibration, 20 µL of fenitrothion stock solution (9.54 × 10-3 M) was added to each cuvette, and the kinetic run was started. The initial concentration of fenitrothion in the cuvettes was generally 7.57 × 10-5 M. The product of the degradation of fenitrothion with the nucleophiles studied, 3-methyl-4-nitrophenoxide, shows maximum absorption at 398 nm (λmax), except in the case of 2-pyridinealdoximate methochloride (5). Aqueous solutions of 2-pyridinealdoximate methochloride have a high absorption near 400 nm, and therefore, in this instance, the formation of 3-methyl-4-nitrophenoxide was followed at 430 nm. Reaction was then monitored at fixed wavelength corresponding to λmax using the Cary 3 or Lambda-20 spectrophotometers. Generally, each reaction was followed for 10 half-lives, and the infinity absorbance value (Ainf) was taken after 10 half-lives. The pseudo-first-order rate constants (kobs) were obtained from linear plots of log(A∞ - At) versus time. The second-order rate constants (k2w) in the aqueous phase were determined from the linear plots of kobs versus nucleophile concentrations which showed negligible intercepts. Generally, kobs was measured at 5 different concentrations for each nucleophile. Product Analysis: Concurrent Pathways. (a) The theoretical Ainf values for the reaction of fenitrothion with CTA-oximates were measured by injecting 20 µL of 3-methyl-4-nitrophenol dioxane solution (9.54 × 10-3 M, equivalent to fenitrothion in the kinetic study) into the corresponding CTA-oximate solution. The observed Ainf values for the reaction with CTA-oximates were lower than the calculated Ainf both above and below the CMC. (b) The reaction products were examined by 31P NMR to determine the phosphorus-containing products formed. 31P NMR spectra were acquired using a 500 MHz Bruker NMR instrument with a sealed capillary tube containing DMSO-d6 placed into the NMR sample tube as a signal lock. Spectra were acquired for 13 h overnight. 31P NMR product analyses were performed for the reaction mixture of fenitrothion (7.57 × 10-5 M) with CTA-syn-Box (10-CTA), CTA-APox (11-CTA) and CTA-Aox (12-CTA), both above and below the CMC. Product analyses were also done for the reaction mixture of fenitrothion with CTA-Buox (7-CTA), CTA-2-Pox (8-CTA), and CTA-3-Pox (9-CTA), with a higher concentration of fenitrothion (2.24 × 10-4 M) being used both above and below the CMC. A 31P NMR spectrum was also obtained for the reaction mixture of fenitrothion (7.57 × 10-5 M) with CTA-2-PAMox (5CTA) (1.17 × 10-2 M) after 80 h (3 half-lives) of reaction. The reaction products of fenitrothion with hydroxide, 2-pyridinealdoximate methochloride (5) were also examined by 31P NMR for comparison with the results in the presence of surfactants.

Acknowledgment. Financial support of this research by the Natural Science and Engineering Research Council of Canada (NSERC) and from Queen’s University is gratefully acknowledged.

The relative association of the various anions with the micelle is defined as β (eq A-2), where θOH- and θOx- are the fractional association of OH- and Ox- with the micelle, respectively.

β ) θOH- + θOx-

Note that the concentration of nucleophile in the micellar pseudophase, [Nu]M, is then

[Nu]M )

Kb

Ox-(w) + H2Oy\z HOx + HO-

kobs )

OH kOH 2w [OH ]w + k2m [OH ]mKs(CT - CMC)

1 + Ks(CT - CMC)

(A-2b)

+

Ox kOx 2w[Ox ]w + k2m[Ox ]mKs(CT - CMC)

1 + Ks(CT - CMC)

(A-3)

where k2w and k2m represent the second-order rate constant of fenitrothion with the given nucleophile in the aqueous and micellar phases, respectively; KS is the binding constant for fenitrothion with the CTA-oximate micelle; CT is the total concentration of surfactant; and CMC is the critical micelle concentration. Rearranging eq A-3 yields the corrected rate constants in the micellar phase (eq A-4). Note that in the PPIE model of micellar catalysis, there are only two pseudo-phases: nucleophile in the water phase or nucleophile dissolved in the micellar phase. Accordingly, at all surfactant concentrations below the CMC, reactivity is treated as being equivalent to that observed with any nucleophile in water.11,14,15

kobs Ox OH kOH 2w [OH ]w + k2w[Ox ]w + k2m [OH ]mKs(CT - CMC)

1 + Ks(CT - CMC) kOx 2m[Ox ]mKs(CT - CMC)

1 + Ks(CT - CMC) kOH obs )

OH kOH 2w [OH ]w + k2m [OH ]mKs(CT - CMC)

1 + Ks(CT - CMC)

)

(A-4)

(A-5)

Defining kOH obs as the first-order rate constants associated with OH- in both the aqueous and micellar phases (eq A-5) and kOx w as the first-order rate constant associated with Ox- in the aqueous phase (eq A-6),

kOx w )

kOx 2w[Ox ]w

1 + Ks(CT - CMC)

(A-6)

Then, eq A-4 can be rewritten as eq A-7: Ox kcorr ) kobs - (kOH obs + kw ) )

(A-1)

β VM

where VM is the molar volume of the micelle (taken as 0.60 L mol-1).15 The equilibrium constant, Kb, in eq A-1 is equal to Kw/Ka, where Kw is the autoprotolysis constant of water (10-14), and Ka is the acid dissociation constant of the protonated oxime. According to the PPIE model, the observed pseudo-first-order constant is equal to the sum of the individual rate constants associated with the two counterions (eq A-3).

Appendix Application of PPIE Model for Data Treatment of Fenitrothion-CTA-Oximate System. The CTA-oximate kinetic studies were conducted in buffer solutions that contained two reactive counterionsshydroxide ions and oximate ionssas a result of hydrolysis (eq A-1).

(A-2a)

kOx 2m[Ox ]mKs(CT - CMC)

1 + Ks(CT - CMC)

(A-7)

Degradation of the Pesticide Fenitrothion

Langmuir, Vol. 22, No. 21, 2006 9017

The binding constants (Ks) of fenitrothion with micelles were obtained by fitting plots of kcorr versus (C - CMC) to eq A-7 of the PPIE model using a nonlinear least-squares curve fitting program (GraphPad Prism). Then, second-order rate constants (k2m) were obtained from the linear plot of kcorr versus FsθOxaccording to eqs A-8 and A-9.

kcorr )

k2m θ F Vm Ox- s

(A-8)

is discussed after kobs has been dissected into the two corresponding pathways as described above by eqs 1 and 2. Having obtained the first-order dissected rate constants for SN2(P) and SN2(C) pathways, the binding constants and secondorder rate constants were dissected for these pathways by fitting the reaction data to the appropriate PPIE equation. The dissected rate constant for the SN2(P) pathway is corrected using eq A-7, while, for the SN2(C) pathway, the dissected rate constant is corrected using eq A-10.

kCobs

where

Fs )

Ks(CT - CMC) 1 + Ks(CT - CMC)

(A-9)

The observed rate constants, kobs, obtained in the kinetic measurements are the total rate constants of the reaction (Figure 3). In the following, the fit of the reaction data to the PPIE model

)

kOx 2m[Ox ]mKs(CT - CMC)

1 + Ks(CT - CMC)

(A-10)

Supporting Information Available: Supplementary information describing the Ainf values (Table 1S), dissected kPobs and kCobs values (Tables 2S-8S), and β and θNu values (Tables 9S-15S) is provided. This material is available free of charge via the Internet at http://pubs.acs.org. LA060641T