Langmuir 2000, 16, 10101-10105
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Hydrolyses of Dinitroalkoxyphenyl Phosphates in Aqueous Cationic Micelles. Acceleration by Premicelles Lucia Brinchi,† Pietro Di Profio,† Raimondo Germani,† Gianfranco Savelli,*,† Monica Tugliani,† and Clifford A. Bunton‡ Dipartimento di Chimica, Universita` di Perugia, Via Elce di Sotto 8, I-06123 Perugia, Italy, and Department of Chemistry and Biochemistry, University of California, Santa Barbara, California 93106 Received June 6, 2000. In Final Form: September 26, 2000 Hydrolyses of dianionic 2,4-dinitrophenyl phosphate, substituted at position 5 by Me, OMe, n-OC14H29 (DNMePP2-, DNMeOPP2-, and DNTDOPP2- respectively) are accelerated by cationic micelles of cetyl trialkylammonium bromide (CTABr, CTPABr, and CTBABr, with alkyl ) Me, n-Pr, and n-Bu, respectively). There are well-defined rate maxima in dilute surfactant, below the critical micelle concentration, for reaction of DNTDOPP2- in the sequence CTBABr > CTPABr > CTABr, because of acceleration by premicelles which is not observed with the methoxy and unsubstituted derivatives. The rate maxima disappear as these species “dissolve” in micelles at higher surfactant concentrations.
Introduction
Scheme 1
Aqueous cationic and sulfobetaine micelles accelerate spontaneous hydrolyses of dinitrophenyl phosphate and acyl phosphate dianions.1 These reactions have been extensively investigated in various solvents; there is extensive P-O bond cleavage in the transition state,2 and substituent effects in the aryl phosphates follow the Hammett equation with F ) 2.6.2c However, free metaphosphate ion, PO3-, is not an intermediate in polar solvents, although it can be trapped in aprotic media,3 because there is inversion of configuration at phosphorus4 and some tertiary amines catalyze hydrolysis.5 In the present work, we examine substituent effects at position 5 of dinitrophenyl phosphates, especially as regards acceleration by cationic surfactants (Scheme 1). The substituents are methoxy and n-tetradecyloxy (R ) MeO, n-C14H29O), and a few experiments were made with the 5-methyl derivative. We designate the substrates as DNPP2-, DNMePP2-, DNMeOPP2-, and DNTDOPP2for the unsubstituted, methyl, methoxy, and tetradecyloxy derivatives, respectively. Spontaneous hydrolyses of DNPP2- and the p-nitrobenzoyl phosphate1e dianion are accelerated by cationic and, for the former, by sulfobetaine micelles1,a,d which favor reactions of anionic substrates in which charge is dispersed in the transition state. Micellar effects on reaction rates
and equilibria are generally described by a pseudophase model which treats aqueous and micellar pseudophases as distinct reaction media.6 The dependence of first-order rate constants, kobs, on [surfactant] for spontaneous reactions is given by eq 1, where k′W and k′M are first-order rate constants in the aqueous and micellar pseudophases, respectively, and KS is the association constant of the substrate, S, with micellized surfactant (detergent), Dn; that is, [Dn] ) [Dtotal] - cmc. The critical micelle concentration, cmc, in the reaction conditions is taken as the concentration of monomeric surfactant. Equation 1 was developed for micellar effects on reactions of nonionic substrates, but it fits the data for hydrolyses of 2,4- and 2,6-dinitrophenyl phosphate dianions in CTABr.1a
* To whom correspondence should be addressed. E-mail: Savelli@ unipg.it. Tel: +39-075-585-5538. Fax: +39-075-585-5560. † Universita ` di Perugia. ‡ University of California. (1) (a) Bunton, C. A.; Fendler, E. J.; Sepulveda, L.; Yang, K.-U. J. Am. Chem. Soc. 1968, 90, 5512. (b) Buist, G. J.; Bunton, C. A.; Robinson, L.; Sepulveda, L.; Stam, M. J. Am. Chem. Soc. 1970, 92, 4072. (c) Bunton, C. A.; Dorwin, E. L.; Savelli, G.; Si, V. C. Recl. Trav. Chim. Pays-Bas 1990, 109, 64. (d) Del Rosso, F.; Bartoletti, A.; Di Profio, P.; Germani, R.; Savelli, G.; Blasko`, A.; Bunton, C. A. J. Chem. Soc., Perkin Trans. 2 1995, 673. (e) Bunton, C. A.; McAneny, M. J. Org. Chem. 1977, 42, 475. (2) (a) Di Sabato, G.; Jencks, W. P. J. Am. Chem. Soc. 1961, 83, 4393. (b) Di Sabato, G.; Jencks, W. P.; J. Am. Chem. Soc. 1961 83, 4400. (c) Kirby, A. J.; Varvoglis, A. G. J. Am. Chem. Soc. 1967, 89, 415. (d) Bunton, C. A.; Fendler, E. J.; Fendler, J. H. J. Am. Chem. Soc. 1967, 89, 1221. (3) Rebek, J. J. Am. Chem. Soc. 1975, 97, 454. (4) Buchwald, S. L.; Friedman, J. M.; Knowles, J. R. J. Am. Chem. Soc. 1984, 106, 4911. (5) Kirby, A. J.; Varvoglis, A. G. J. Chem. Soc. B 1968, 135.
kobs )
k′W + k′MKS[Dn] 1 + KS[Dn]
(1)
This treatment, which predicts that values of kobs will increase monotonically and become constant with a fully bound substrate, fits extensive data for spontaneous reactions.6 However, in many reactions kobs increases below the cmc in water, possibly because of reactant-induced micellization or reaction in so-called premicelles which, in dilute surfactants, may coexist with monomers and micelles.6-8 The term premicelles is applied to these small (6) (a) Romsted, L. S. In Surfactants in Solution; Mittal, K. L., Lindman, B., Eds.; Plenum Press: New York, 1984; Vol. 2, p 1015. (b) Bunton, C. A.; Savelli, G. Adv. Phys. Org. Chem. 1986, 22, 213. (c) Bunton, C. A.; Nome, F.; Romsted, L. S.; Quina, F. H. Acc. Chem. Res. 1991, 24, 357. (d) Bunton, C. A. In Surfactants in Solution; Mittal, K. L., Shah, D. O., Eds.; Plenum Press: New York, 1991; Vol. 11, p 17. (e) Romsted, L. S.; Bunton, C. A.; Yao, J. Curr. Opin. Colloid Interface Sci. 1997, 2, 622. (f) Bunton, C. A. J. Mol. Liq. 1997, 72, 231. (7) Buckingham, S. A.; Garvey, C. J.; Warr, G. G. J. Phys. Chem. 1993, 97, 10236. (8) Niu, S.; Gopidas, K. R.; Turro, N. J.; Gabor, G. Langmuir 1992, 8, 1271.
10.1021/la000799s CCC: $19.00 © 2000 American Chemical Society Published on Web 12/01/2000
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surfactant assemblies, although the size distribution of micelles complicates the distinction between micelles and premicelles and the latter are usually invoked to rationalize results that do not fit the pseudophase rubric, for example, eq 1. Monotonic rate increases below the cmc are often seen with bimolecular reactions involving hydrophobic organic substrates and in reactions of polyvalent inorganic ions,9,10 both of which reduce the cmc and can induce micellization.6 Drennan et al. carefully analyzed this problem in discussing complexation with Ni2+ and showed that rate increases in very dilute sodium dodecyl sulfate can be ascribed to induced micellization by the metal cation rather than formation of premicelles.10,11 They note that polyvalent counterions markedly reduce the cmc of ionic surfactants. Evidence for the kinetic intervention of cationic premicelles comes from observation of rate maxima in unimolecular cyclizations of o-(ωhaloalkoxy)phenoxide ions,12 which mechanistically are equivalent to SN2 reactions.13 With ω ) 3, eq 1 describes variations of kobs with cationic [surfactant], but as the tether length is increased, for example, ω g 7, values of kobs increase sharply at [surfactant] < cmc in water and then decrease to plateau values in accordance with eq 1 as forming micelles “dissolve” premicelles and substrate. These rate extrema are seen in cetyl and dodecyl trialkylammonium bromides.12 Maxima in kobs at [surfactant] below or near the cmc12,14,15 followed by constant values of kobs show that there are spontaneous reactions in water, premicelles, and micelles, and the premicelles must be kinetically more effective than micelles. It is important to recognize that observation of monotonic rate changes in very dilute surfactant does not demonstrate intervention by premicelles,10 unless reactant-induced micellization is disproved by physical evidence. Consistently, some reactions are accelerated by nonmicellizing, hydrophobic ammonium ions,16,17 and in some cases the accelerations are described quantitatively in terms of association of these ions with reactants. For example, tri-n-octyl ethylammonium mesylate does not form micelles, but it accelerates the spontaneous hydrolysis of DNPP2- 17c because of its association with the hydrophobic ammonium ion. It seemed probable that dephosphorylation (Scheme 1) could be accelerated by a monomeric or premicellar surfactant. Micellar effects on this hydrolysis in aqueous quaternary ammonium ion or sulfobetaine surfactants fit eq 1,1a-d and values of kobs increase as micelles form at the cmc in the reaction medium, but it seemed probable that introduction of hydrophobic substituents would allow reaction in premicelles and generate rate maxima at or near the cmc. Decarboxylation of 6-nitrobenzisoxazole-3-carboxylate (NBIC) and its derivatives and hydrolysis of DNPP2- are (9) Graciani, M. del M.; Rodriguez, A.; Fernandez, G.; Moya, M. L. Langmuir 1997, 13, 4239. (10) Drennan, C. E.; Hughes, R. J.; Reinsborough, V. C.; Soriyan, O. O. Can. J. Chem. 1998, 76, 152. (11) Mukerjee, P.; Mysels, K. J. Critical Micelle Concentrations of Aqueous Surfactant Systems; National Bureau of Standards: Washington, DC, 1970. (12) Cerichelli, G.; Mancini, G.; Luchetti, L.; Savelli, G.; Bunton, C. A. Langmuir 1994, 10, 3982. (13) Mandolini, L. Adv. Phys. Org. Chem. 1986, 22, 1. (14) Germani, R.; Ponti, P. P.; Savelli, G.; Spreti, N.; Cipiciani, A.; Cerichelli, G.; Bunton, C. A. J. Chem. Soc., Perkin Trans. 2 1989, 1767. (15) Brinchi, L.; Di Profio, P.; Germani, R.; Giacomini, V.; Savelli, G.; Bunton, C. A. Langmuir 2000, 16, 222. (16) Kunitake, T.; Okahata, Y.; Ando, R.; Shinkai, S.; Hirakawa, S. J. Am. Chem. Soc. 1980, 102, 7877. (17) (a) Bunton, C. A.; Hong, Y. S.; Romsted, L. S.; Quan, C. J. Am. Chem. Soc. 1981, 103, 5788. (b) Biresaw, G.; Bunton, C. A.; Quan, C.; Yang, Z.-Y. J. Am. Chem. Soc. 1984, 106, 7178. (c) Bunton, C. A.; Quan, C. J. Org. Chem. 1985, 50, 3230.
Brinchi et al.
accelerated by a decrease in solvent polarity2c,18 and by incorporation into cationic or zwitterionic micelles.1a-d,19 We see rate enhancements by premicelles with the 5-tetradecyloxy derivative of NBIC15 but not with the less hydrophobic derivatives. On the basis of similar medium effects on decarboxylation and spontaneous dephosphorylation, we hoped to see similar premicellar rate effects with DNTDOPP2- (Scheme 1). Micellar rate enhancements are much larger for decarboxylation than for dephosphorylation,1,19 because of different hydration requirements, which might affect sensitivities to premicelles. The tetradecyloxy derivative of NBIC is too sparingly soluble for its reaction to be followed in the absence of surfactant,15 and we hoped to avoid this problem with the more soluble DNTOPP2-. It is easier to obtain kinetic evidence for the existence of premicelles in spontaneous than in bimolecular reactions involving a second reagent where rate constants go through maxima with increasing [surfactant], because of transfer equilibria of the two reactants.6 However, double rate maxima have been observed in reactions of nucleophilic anions with 2,4-dinitronaphthalene20 and 2-(4chlorophenoxy) quinoxaline21 in solutions of quaternary ammonium ion surfactants. They are ascribed to reactions in premicelles induced by interaction of substrate and ammonium ion and in some conditions disappear if the substrate is very dilute or micellization is induced by addition of electrolyte.20,21 The surfactants are cetyl trialkylammonium bromide, n-C16H33NR3Br, R ) Me, n-Pr, n-Bu, CTABr, CTPABr, CTBABr. Hydrolysis of DNPP2- had been examined in these surfactant solutions, and k′M increased with increasing bulk of the headgroup.1d Values of kobs in surfactants with the larger headgroups increased significantly at [surfactant] < cmc, but the increase was monotonic and kobs became constant with a fully bound substrate, as predicted by eq 1. These observations are understandable in terms of either substrate-induced micellization or reaction in premicelles but do not distinguish between them. We hoped that introduction of an apolar, hydrophobic group, as in DNTDOPP2-, would provide rate maxima, demonstrating the role of premicelles.15 In early work, synthesis of the alkoxy derivatives, especially that of DNTDOPP2-, presented some difficulties and required careful selection of reaction conditions. Results and Discussion Reactions in Water. Methoxy and tetradecyloxy groups should have similar electronic effects. Substituent effects of MeO and Me on kobs in water (Table 1) are as expected from the σm values (0.12 and -0.02 for OMe and Me, respectively).2c,22 However, DNTDOPP2- is a little less reactive than DNMeOPP2-, but a bulky substituent can reduce electronic withdrawal by NO2 by restricting its coplanarity with the phenyl group.22 This effect is apparently unimportant with the Me and MeO derivatives. (18) Paul, D. S.; Kemp, K. G. J. Am. Chem. Soc. 1975, 97, 7305. (19) (a) Bunton, C. A.; Minch, M. J.; Hidalgo, J.; Sepulveda, L. J. Am. Chem. Soc. 1973, 95, 3262. (b) Germani, R.; Ponti, P. P.; Savelli, G.; Spreti, N.; Cipiciani, A.; Cerichelli, G.; Bunton, C. A.; Si, V. J. Colloid Interface Sci. 1990, 138, 44. (c) Di Profio, P.; Germani, R.; Savelli, G.; Cerichelli, G.; Spreti, N.; Bunton, C. A. J. Chem. Soc., Perkin Trans. 2 1995, 1505. (20) (a) Bunton, C. A.; Bacaloglu, R. J. Colloid Interface Sci. 1987, 115, 288. (b) Bacaloglu, R.; Bunton, C. A. J. Colloid Interface Sci. 1992, 153, 140. (21) Cuenca, A. Langmuir 2000, 16, 72. (22) Hammett, L. P. Physical Organic Chemistry, 2nd ed.; McGrawHill: New York, 1970.
Hydrolyses of Dinitroalkoxyphenyl Phosphates
Langmuir, Vol. 16, No. 26, 2000 10103
Table 1. Rate Constants in Water and in Micellesa surfactant
DNPP2- b
DNMePP2- c
DNMeOPP2-
DNTDOPP2-
CTABr CTPABr CTBABr
2.20 (26) 3.30 (38) 4.11 (48)
0.68 (17) 0.90 (23) 1.09 (28)
1.72 (14) 2.47 (21) 2.64 (22)
1.09 (11) 1.39 (14) 1.56 (16)
Values of 104k′M (s-1) for micellar-bound substrates at 25.0 °C with 2 × 10-5 M substrate and in 3.3 × 10-4 M NaOH unless otherwise specified. In water, 104k′W ) 0.086, 0.039, 0.12, and 0.097 s-1 for DNPP2-, DNMePP2-, DNMeOPP2-, and DNTDOPP2respectively. Values in parentheses are k′M/k′W. b Reference 1d, in 10-4 M NaOH. c Mean values in 0.03 and 0.04 M surfactant. a
Dephosphorylation but not decarboxylation of the more hydrophobic derivatives can be followed in water without surfactant. Reactions in Fully Formed Micelles. Micellar rate effects are qualitatively similar to those observed earlier for hydrolyses in cationic micelles1a-d at 25.0 °C, and values of k′M increase with increasing bulk of the surfactant headgroup. (The hydrolysis of DNMePP2- was examined only over a limited range of [surfactant], where the substrate was fully micellar-bound). These hydrolyses are sensitive to solvent composition, because hydrogenbonding to the phosphoryl group inhibits reaction,2 and bulky alkyl groups reduce the availability of water in the interfacial region. Polarities of these regions, estimated by using spectral probes, are similar to those of polar organic solvents or their mixtures with water.23 Methyl and alkoxy substituents at position 5 decrease k′M/k′W, and the increase in hydrophobicity in going from the methoxy to the tetradecyloxy derivative slightly decreases k′M (Table 1), as in decarboxylations.15 These experiments were made with [surfactant] such that substrates were fully micellar-bound. Increases in substrate hydrophobicity often accelerate micellar-mediated reactions, by influencing the transfer equilibria between water and micelles rather than the reactivity in the latter.6 Micellar rate effects depend on interactions with both the initial and transition states, relative to those in water, and insofar as initial state stabilization inhibits reaction, the different behaviors of DNPP2- and its derivatives may be related to strong interactions between substrate and micelle which stabilize the initial state, especially with DNTDOPP2-. In the transition state for dephosphorylation, charge in the dianionic initial state is formally distributed into an aryloxide ion and the PO3- moiety, which is interacting with nucleophilic water,4 whereas in anionic decarboxylation, for example, a single negative charge is distributed into the organic residue.18 The generalization that micelles tend to favor reactions in which charge in the initial state is dispersed over an extended region applies not only to unimolecular reactions, such as dephosphorylation,1 decarboxylation,14,15,19,24 and cyclization,12 but also to some bimolecular reactions. For example, cationic and sulfobetaine micelles favor E2 over SN2 reactions, provided that reactant transfer equilibria from water into micelles are taken into account.25 (23) (a) Ramachandran, C.; Pyter, R. A.; Mukerjee, P. J. Phys. Chem. 1982, 86, 3198. (b) Quina, F. H.; Alonso, E. O.; Farah, J. P. S. J. Phys. Chem. 1995, 99, 11708. (c) Abraham, M. H.; Chadha, H. S.; Dixon, J. P.; Rafols, C.; Treiner, C. J. Chem. Soc., Perkin Trans. 2 1995, 887. (d) Novaki, L. P.; El Seoud, O. A. Phys. Chem. Chem. Phys. 1999, 1, 1957. (e) Novaki, L. P.; El Seoud, O. A. Langmuir 2000, 16, 35. (24) (a) Germani, R.; Ponti, P. P.; Romeo, T.; Savelli, G.; Spreti, N.; Cerichelli, G.; Luchetti, L.; Mancini, G.; Bunton, C. A. J. Phys. Org. Chem. 1989, 2, 553. (b) Di Profio, P.; Germani, R.; Savelli, G.; Cerichelli, G.; Spreti, N.; Bunton, C. A. J. Colloid Interface Sci. 1996, 182, 301. (25) (a) Wilk, K. A. J. Phys. Chem. 1991, 95, 3405; 1992, 96, 901. (b) Brinchi, L.; Di Profio, P.; Germani, R.; Savelli, G.; Bunton, C. A. Langmuir 1997, 13, 4583. (c) Brinchi, L.; Germani, R.; Savelli, G.; Bunton, C. A. J. Phys. Org. Chem. 1999, 12, 890.
Table 2. Activation Parameters in Water and in Micellesa medium
DNPP2- b DNMePP2- DNMeOPP2- DNTDOPP2-
H2O 30 (19) CTABr 24 (7) CTBABr 25 (10)
24 (5)
30 (18) 24 (5) 23 (4)
28 (12) 22 (-3) 24 (4)
Values of (kcal mol-1) and, in parentheses, ∆Sq (cal mol-1 K-1). b Calculated from data in ref 1d. a
∆Hq
Table 3. Variations of k′W or k′M with Temperaturea DNMeOPP2DNTDOPP2DNMePP2temp, b b b b °C H2O CTABr CTBABr H2O CTABr CTBABr CTBABrb 25.0 0.120 1.72 35.0 0.745 6.24 40.0 1.22 11.7
2.64 9.65 18.3
0.0970 0.605 0.865
1.09 3.59 6.46
1.56 6.20 11.0
1.09 4.31 7.73
a Values of 104k′ or 104k′ (s-1), with 2 × 10-5 M substrate in W M 3.3 × 10-4 M NaOH. b [Surfactant] ) 0.03-0.04 M.
Activation Parameters. The rate increases in cationic micelles are due to decreases in the enthalpy of activation, ∆Hq, which are partially offset by decreases in the entropy of activation, ∆Sq. (Tables 2 and 3). Values of ∆Sq are close to zero, as is typical of these reactions.26 Calculations were based on the Arrhenius equation, but very similar values were obtained by using the Eyring equation.27 Values of activation parameters for the hydrolysis of DNPP2- were calculated from the rate constants in ref 1d. (These values differ slightly from the earlier values which had a numerical error.) Temperature effects upon micellar structure and the transfer equilibria of reactants between water and micelles may affect activation parameters, especially if reactants are not fully micellar-bound.6 The activation parameters for hydrolyses of DNMePP2- and DNTDOPP2- are calculated from values of kobs for fully bound substrates, which eliminates temperature effects on transfer equilibria, and temperature-induced changes in micellar structure should have similar effects on the reactivities of all the substrates (Table 3). Interactions between the forming aryloxide moiety in the transition state and cationic micellar headgroups accelerate hydrolysis by decreasing ∆Hq, despite the consequent decrease in ∆Sq, which is most evident in the reaction of DNTDOPP2- (Table 2). Reactions in Very Dilute Surfactant. We saw rate maxima for the hydrolysis of DNTDOPP2- but not for the other substrates at [surfactant] < cmc with all three surfactants (Tables 4 and 5), because of reaction in premicellar complexes (values of the cmc in water are 8.9 × 10-4, 5.6 × 10-4, and 2.4 × 10-4 M for CTABr, CTPABr, and CTBABr, respectively28). This substrate is relatively soluble in water, where its hydrolysis can be followed, and kobs increases very sharply by a factor of approximately 20 in very dilute surfactant, decreases to a shallow minimum, and then becomes independent of [surfactant]. However, there are complications in very dilute CTABr and CTPABr where kinetics are not always first order, and in some cases precipitates can be seen (Table 5), probably resulting from formation of salts with the quaternary ammonium ion, which dissolve on addition of surfactant. All values of kobs given here were obtained in (26) Bunnett, J. F. In Investigation of Rates and Mechanisms of Reactions; Friess, S. L., Lewis, E. S., Weissberger, A., Eds.; Interscience: New York, 1961. (27) Lowry, T. H.; Richardson, K. S. Mechanism and Theory in Organic Chemistry, 3rd ed.; Harper and Row: New York, 1987; Chapter 2. (28) (a) Mukerjee, P.; Mysels, K. J. Critical Micelle Concentrations of Aqueous Surfactant Systems; National Bureau of Standards: Washington, DC, 1970. (b) Bacaloglu, R.; Bunton, C. A.; Ortega, F. J. Phys. Chem. 1989, 93, 1497.
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Brinchi et al.
Table 4. Variations of kobs with Surfactant Concentration for Reaction of DNTDOPP2- a 104[surfactant], M 0.8 1.0 1.1 2.0 5.0 5.6 6.0 10 50 100 150 200 300 400
Table 6. Variation of kobs with Surfactant Concentration for Reaction of DNMeOPP2- a
CTABr
CTPABr
CTBABr
104[surfactant], M
0.097 2.08 1.98 2.02 1.67
0.097 2.04 1.95
0.097 2.36 2.01
1.61 1.29
1.72 1.42
1.30 1.12
1.41 1.41 1.54
0.1 0.2 0.4 0.5 0.8 1.0 3.0 4.0 5.0 6.0 10 20 30 40 60 100 150 200 250 300 400 500 800
1.38 1.38 1.18 1.00 1.05 1.18 1.07 1.09
1.23
1.56
1.36 1.39
1.62 1.56
a Values of 104k -1 -5 M substrate obs (s ) at 25.0 °C, with 2 × 10 and 3.3 × 10-4 M NaOH. The apparent rate maxima are in bold type.
Table 5. Effects of Variations in [DNTOPP2-]a 105[DNTOPP2-], M 1.0 2.0 4.0 104 [surfactant], M CTABr CTPABr CTABr CTPABr CTABr CTPABr 0.1 0.2 0.5 0.8 1.0 1.1 2.0 5.0 6.0 10 50
2.28 2.17
1.98 1.97 2.19
2.18
b b 2.08 1.98 2.02 1.67
1.30 1.04
1.38 1.18 1.00
2.21 b b 2.04 1.95 1.61 1.29 1.30 1.12
b 2.15 1.89
2.24 1.99c 1.60
1.53 1.23 1.26
Values of 104kobs (s-1) at 25.0 °C and 3.3 × 10-4 NaOH, unless otherwise specified; in water, 104kobs ) 0.097 s-1. b Precipitation observed. c 104kobs ) 1.90 s-1 in 6.6 × 10-4 M NaOH. a
conditions such that no precipitates were seen and reactions were cleanly first order for up to at least 3 halflives. For the hydrolysis of DNMeOPP2- (and DNPP2-), we have the “normal” behavior in CTABr, with kobs increasing only slightly in dilute surfactant and then increasing sharply at or near the cmc in water. However, in CTPABr and CTBABr values of kobs increase at [surfactant] < cmc (Table 6 and ref 1d). These monotonic increases may be due to substrate-induced micellization and do not require postulation of reaction in premicelles, and without observation of rate extrema we cannot distinguish between these possibilities. Values of the observed maxima in kobs for hydrolysis of DNTDOPP2correspond approximately to [surfactant]/[substrate] ) 4 (Tables 4 and 5), but we cannot assume that the complex has this composition. Because of the limited range of [surfactant], we cannot locate the rate maxima precisely or assume that only one complex is involved. The substrates in premicellar complexes are more reactive than micellar-bound substrates, and the rate maxima disappear as micelles form and dissolve the premicellar complexes. These rate maxima show that the reaction of DNTDOPP2occurs in three distinct regions: water, premicellar complexes, and fully formed micelles. To this extent, our evidence is analogous to that from fluorescence spectra of a hydrophobic probe in cationic surfactants, which demonstrate that the probe can be in three locations depending on the structure and concentration of the surfactant.8 It is difficult to decide whether the anomalous rate maxima are due to complexation of the substrate with
CTABr
CTPABr
CTBABr 0.147
0.185 0.198 0.140 0.187 0.172
0.421 0.676 2.00
1.27 2.02 2.38 2.27
1.59 1.70 1.61
1.97 1.97
1.62 1.62 1.60 1.62 1.64 1.64 1.68 1.72
2.10 2.25 2.20 2.27 2.33
2.26 2.27 2.32 2.48 2.48 2.54 2.56 2.66
2.44 2.47 2.47 2.45
2.72 2.64 2.65 2.69
a Values of 104k -1 -5 M substrate obs (s ) at 25.0 °C, with 2 × 10 and 3.3 × 10-4 M NaOH; the value of k′W is 0.12 × 10-4 s-1.
Table 7. Effects of NaBr on the Hydrolysis of DNTOPP2- a NaBr, M 104[CTABr], M
0
0.005
0.01
0.05
0.8 1.0 2.0 6.0 10 100 200 400
2.08 1.98 1.67 1.38 1.18 1.05 1.18 1.09
1.65 1.93b 1.23 1.16 1.11 1.10 1.15 1.16
1.91 2.01 1.44
2.00 2.01 2.25
1.17
1.41
a Values of 104k -1 -5 M DNTOPP2obs (s ) at 25.0 °C, with 2 × 10 and 3.3 × 10-4 M NaOH; in H2O k′W ) 0.097 × 10-4 s-1. b Mean of 104kobs ) 1.78 and 2.07 s-1.
monomeric surfactant or premicelles or to substrateinduced formation of premicelles, although there is evidence for the existence of premicelles in the absence of other solutes.7 We therefore examined the effects of varying [DNTOPP2-] in dilute CTABr and CTPABr (Table 5). We were unable to make this test for decarboxylation because of the low solubility of the substrate and the limited absorbance change during the reaction.15 Unlike the situation for bimolecular reactions with aromatic substrates which exhibit double rate maxima,20,21 [substrate] has little effect on kobs. (Concentrations of lutidine (see Experimental Section) varied in these experiments, but it is readily water soluble and in low concentration should not affect rates.) Intramolecular hydrophobic interactions between the tetradecyloxy group and the reaction center of DNTDOPP2- do not accelerate hydrolysis in water by excluding it from the reaction center (Table 1) nor do they make the solubility inconveniently low and prevent our following this reaction in water, but they allow reaction to occur in complexes of substrate and small numbers of surfactant monomers (Tables 4 and 5). Dilute NaBr affects rate constants in CTABr (Table 7), but unlike the situation for bimolecular reactions,20,21 there is no simple pattern to these salt effects and rate maxima do not disappear. Values of kobs are sensitive to both [CTABr] and [NaBr], and we could not obtain consistent
Hydrolyses of Dinitroalkoxyphenyl Phosphates
Langmuir, Vol. 16, No. 26, 2000 10105 Chart 1
rate data with 0.1 M NaBr. The bromide ion perturbs structures of CTABr micelles and probably affects interactions of DNTDOPP2- with the various assemblies present in dilute CTABr, although we cannot give a simple explanation for these rate changes. Micellar interfaces are not devoid of water, although bound ions may be partially dehydrated,6,29,30 and paradoxically our results appear to indicate that the phosphate group is more hydrated in micelles than in premicellar aggregates. The tetradecyl residue could hydrophobically assist complexation and partially exclude water from the phosphoryl oxygens in the initial state, but these roles are not mutually exclusive. Niu et al. show that nonmicellar cationic surfactants with a tetradecyl or longer tail interact with a fluorescent probe and control its environment, and they provide an environment different from those of water or micelles.8 Charge is delocalized from the phosphoryl oxygens into the leaving aryloxide moiety in the transition state,2 and interaction of this moiety and ammonium ion headgroups accelerates hydrolysis.1 There is considerable evidence that quaternary ammonium headgroups interact favorably with arenes.31 Aryl phosphates probably bind to micelles with the aryl groups toward the apolar regions and the phosphoryl residue in the water-rich interfacial region with hydration of the phosphoryl oxygens. In premicellar or hydrophobic clusters, ammonium ion(s) can interact with the aryloxy group and the long alkyl groups can, at least partially, screen phosphoryl oxygens from water. These postulated interactions are illustrated in the drawings in Chart 1, although the alkyl groups are almost certainly not wholly in linear conformations. The interactions of phosphate dianions and alkoxy-6-nitrobenzisoxazole-3-carboxylate ions in premicelles are probably very similar (cf. ref 15), and the factors that govern premicellar rate enhancements are common to both systems. We draw the premicellar assemblies with two surfactant monomers per phosphate dianion, which satisfies charge neutralization, but the composition and conformations are probably variable (Table 5). Association between anionic dyes and very dilute cationic surfactants is well established32 and in some cases appears to be governed by Coulombic interactions, whereas in dephosphorylation (and decar(29) Morgan, J. D.; Napper, D. H.; Warr, G. G. J. Phys. Chem. 1995, 99 (9), 9458. (30) Soldi, V.; Keiper, J.; Romsted, L. S.; Cuccovia, I. M.; Chaimovich, H. Langmuir, 2000, 16, 59 and references therein. (31) (a) Bachofer, S. J.; Simonis, U. Langmuir 1996, 12, 1744. (b) Kreke, P. J.; Magid, L. J.; Gee, J. C. Langmuir 1996, 12, 699. (c) Bunton, C. A.; Cowell, C. P. J. Colloid Interface Sci. 1988, 122, 154. (32) (a) Vitagliano, V. In Aggregation Processes in Solution; WynJones, E., Gormally, J., Eds.; Elsevier: New York, 1983; p 271. (b) Reeves, R. L.; Harkawy, S. In Micellization, Solubilization and Microemulsions; Mittal, K. L., Ed.; Plenum: New York, 1977; Vol. 2, p 819. (c) Buwalda, R. T.; Janker, J. M.; Engberts, J. B. F. N. Langmuir 1999, 15, 1083 and references therein.
boxylation) hydrophobic interactions are of major importance. Precipitates of dye complexes are often seen in dilute surfactants. Our kinetic and similar data do not require the existence of premicelles in the absence of reactants, and they may form by hydrophobically induced association with the substrate, although this conclusion seems improbable from the results in Table 5. However, the affinity of micelles for the substrate is higher than that of premicelles, whatever their composition, but the latter are kinetically more effective; kmax/k′M ≈ 1.8, 1.4, and 1.5 for the reaction of DNTOPP2- in CTABr, CTPABr, and CTBABr, respectively (Table 4). This ability of very dilute amphiphiles, including surfactants, to influence reactivity should be of environmental concern in view of the widespread domestic and industrial use of amphiphiles. Experimental Section Materials. The surfactants were the materials used earlier.1d,14 The dinitrophenyl phosphates were made from the dibenzylphosphorochloridate33 prepared by the method of Atherton.34 They were isolated as the lutidinium salts. Details of the preparations, purifications, and characterizations are given as Supporting Information. Kinetics. The reactions were followed spectrophotometrically as described earlier,1d generally in 3.3 × 10-4 M NaOH, but in a few experiments we varied [NaOH] in the range of 10-4 to 10-3 M without affecting kobs.1a-d Reaction solutions were made up in CO2-free redistilled water, and aqueous phosphate esters were used so that their final concentrations were usually 2 × 10-5 M, although other concentrations were used in a few experiments. Most of the values of kobs were averages of 2-5 data points which were within 5% of the mean. NMR Spectra. NMR spectra were recorded on a DPX 200 MHz Bruker spectrophotometer. Proton chemical shifts are relative to internal tetramethylsilane, and 31P chemical shifts are relative to external 85% aqueous H3PO4.
Acknowledgment. Support of this work by Consiglio Nazionale delle Ricerche, Rome, by the Ministero dell’Universita’ e della Ricerca Scientifica, Roma, and by the U.S. Army Office of Research is gratefully acknowledged, as is a grant by the Fulbright Commission to L.B. Supporting Information Available: Syntheses and purifications are described in detail for the 2,4-dinitro-5methylphenyl phosphate anion 2,6-lutidinium salt, DNMePP2-, the 2,4-dinitro-5-methoxy phenyl phosphate anion 2,6-lutidinium salt, DNMeOPP2-, and the 2,4-dinitro-5-tetradecyloxy phenyl phosphate anion 2,6-lutidinium salt, DNTDOPP2-. This material is available free of charge via the Internet at http://pubs.acs.org. LA000799S (33) Ramirez, F.; Maracek, J. F. Synthesis 1979, 602. (34) Atherton, F. R. Biochem. Prep. 1957, 5, 2.
