Effects of Headgroup Structure on Dephosphorylation of p-Nitrophenyl

Micellar Catalytic and Salt Effect on Oxidation of EDTA by MnO 4 −. Hisham J. Y. El-Aila. Journal of Dispersion Science and Technology 2012 33, 1688...
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Langmuir 1999, 15, 1067-1074

1067

Effects of Headgroup Structure on Dephosphorylation of p-Nitrophenyl Diphenyl Phosphate by Functional Oximate Comicelles Clifford A. Bunton,* Houshang J. Foroudian, and Nicholas D. Gillitt Department of Chemistry, University of California, Santa Barbara, California 93106 Received September 1, 1998. In Final Form: November 30, 1998 Rate constants of reaction of n-dodecyl [2-(hydroximino)-2-phenylethyl]dimethylammonium bromide (DHDBr) with p-nitrophenyl diphenyl phosphate in comicelles with inert surfactants, at pH such that the oximate zwitterion (DHD) is formed quantitatively, depend on the mole fraction of DHD to total surfactant. Second-order rate constants of reaction in the micellar pseudophase decrease in the following sequence of inert surfactant headgroups: phosphine oxide ≈ sulfoxide ≈ pyrrolidinone > Me3N+ > sulfobetaine > Bu3N+ > polyoxyethylene > OSO3-. Except for reaction in anionic comicelles these second-order rate constants vary within a factor of 5 and appear to depend on the local structure of the interface rather than on micellar charge per se. The simple relation between rate constants and nucleophile concentration given by the mole fraction of DHDBr applies reasonably well to mixtures of C16H33NMe3Br and the other inert surfactants. Reaction is relatively slow in comicelles of DHD and C12H25OSO3Na, probably due to different locations of reactants in the interfacial region. Samples of DHDBr prepared and purified by the literature method are contaminated by pyridine hydrochloride, which is removed by washing with aqueous NaBr.

Introduction Surfactants are amphiphiles that spontaneously assemble to form micelles with polar or ionic groups at the surface in contact with water.1 Reagents that are partitioned into the micelles can react in this interfacial region, which is regarded as a pseudophase distinct from bulk solvent. Depending upon reactant transfer and local rate constants, reactions may be accelerated or inhibited.2 If reactants are very hydrophilic, they remain in the water, and except for reactions between some inorganic ions,3 micelles exert essentially no rate effect.2a The treatment of spontaneous reactions is simple, eq 1, where S is substrate, Dn is micellized surfactant (detergent) whose concentration is that of the total surfactant, less that of monomer which is assumed to be the critical micelle concentration, cmc.1,2,4 Subscripts W and M denote the aqueous and micellar pseudophases, KS is an association constant with respect to Dn and k′W and k′M are first-order rate constants. The overall first-order rate constant is given by eq 1, where the quantities in squared brackets

kobs )

k′W + k′MKS[Dn] 1 + KS[Dn]

(1)

indicate molar concentrations in terms of total solution volume.2,4 Typically reactions that are favored by polar solvents have k′W > k′M, and conversely reactions disfavored by * To whom correspondence should be addressed. Telephone: (805) 893-2605. Fax: (805) 893-4120. E-mail: bunton@ ultra.chem.ucsb.edu. (1) (a) Fendler, J. H.; Fendler, E. J. Catalysis in Micellar and Macromolecular Systems; Academic Press: New York, 1975. (b) Fendler, J. H. Membrane Mimetic Chemistry; Wiley-Interscience: New York, 1982. (c) Tascioglu, S. Tetrahedron 1996, 52, 11113. (2) (a) Bunton, C. A.; Nome, F.; Quina, F. H.; Romsted, L. S. Acc. Chem. Res. 1991, 24, 357. (b) Bunton, C. A. J. Mol. Liq. 1997, 72, 231. (c) Romsted, L. S.; Bunton, C. A.; Yao, J. H. Curr. Opinion Colloid Interface Sci. 1997, 2, 622. (3) Lopez-Cornejo, R. L.; Jiminez, R.; Moya, M. L.; Sanchez, F.; Burgess, J. Langmuir 1996, 100, 3237. (4) Menger, F. M.; Portnoy, C. E. J. Am. Chem. Soc. 1967, 89, 4698.

polar solvents have k′W > kM. These observations are consistent with the interfacial region being less polar than water, but not markedly so, based on physical data.1,2,5 This assumption of distinct reaction regions is only an approximation because distribution of ions between water and micelles does not follow a step-function,6 and nonionic solutes may not be distributed uniformly in the interfacial region.7 These uncertainties are not a problem in calculating k′M (eq 1) because first-order rate constants of spontaneous reactions are independent of concentration. The situation is more complicated for nonsolvolytic, bimolecular, reactions, but k′W and k′M can be written in terms of the appropriate second-order rate constants and local concentrations. For reaction of substrate, S, with a reagent, N, e.g., a nucleophile, the first-order rate constant with respect to S is given by

kobs )

kW[NW] + km 2 KSNM[Dn] 1 + KS[Dn]

(2)

where kW and km 2 are second-order rate constants written in terms of local molarities, [NW] and NM in the aqueous and micellar pseudophases, respectively.2 This equation is based on the assumption that reactants are uniformly distributed in each reaction region, and that concentrations follow a step-function. However, if ionic distributions are analyzed in terms of the Poisson-Boltzmann equation (PBE) allowance is made for a changing spatial distribution of ions.6 In addition molarity is assumed to be an appropriate measure of local concentration. We can equally well write the local concentration of N as the mole fraction of bound N to micellar headgroups, giving eq 3, where kM (5) (a) Zachariasse, K. A.; Phuc, N. Y.; Kozankiewicz, B. J. Phys. Chem. 1981, 85, 2676. (b) Ramachandran, C.; Pyter, R. A.; Mukerjee, P. J. Phys. Chem. 1982, 86, 3198. (c) Quina, F. H.; Alonso, E. O.; Farah, J. P. S. J. Phys. Chem. 1995, 99, 11708. (6) (a) Bunton, C. A.; Moffatt, J. R.; J. Phys. Chem. 1985, 89, 466. (b) Bunton, C. A.; Moffatt, J. R. J. Phys. Chem. 1986, 90, 538. (c) Ortega, F.; Rodenas, E. J. Phys. Chem. 1988, 92, 2896. (7) Davies, D. M.; Gillitt, N. D.; Paradis, P. J. Chem. Soc., Perkin Trans 2 1996, 659.

10.1021/la981138m CCC: $18.00 © 1999 American Chemical Society Published on Web 01/13/1999

1068 Langmuir, Vol. 15, No. 4, 1999

kobs )

kW[NW] + kMKS[NM] 1 + KS[Dn]

Bunton et al.

(3)

is a second-order rate constant with concentration written as a mole fraction, but then kW and kM have different units. -1 However, kM and km 2 are related by eq 4, where VM, M ,

k M ) km 2 /VM

(4)

is the molar volume of the micellar reaction region. Its value is uncertain and estimates range from 0.14 to 0.37 M-1 for aqueous ionic micelles.2 It is convenient to write the second-order rate constant as kM so that concentration is defined unambiguously, except in relating second-order rate constants in water and in association colloids. Local molarities of some ions can be determined by trapping,8 or estimated by solving the Poisson-Boltzmann equation (PBE),6 and it is then convenient to write second-order rate constants as km 2 (eq 2). Changes in the partitioning of the reactants between water and micelles due to changes in substrate hydrophobicity, or the presence of competing ions, markedly affect overall rate constants and rate-surfactant profiles, which can be fitted quantitatively in terms of distribution models.2,9 However, calculation of second-order rate constants in the association colloidal pseudophase depends on assumptions involved in calculation of local reactant concentrations. Despite simplifications and assumptions the pseudophase model fits extensive data on bimolecular reactions, and generally km 2 ≈ kW, for reactions of nucleophilic anions.2,6,9 For reactions of some electrophilic anions, e.g., Br3-, HSO5- and IO4-, km 2 < kW in cationic and zwitterionic micelles,10 but these differences are understandable in terms of medium effects of the interfacial region and are not due to major breakdowns in the simple model. However, Davies et al. have successfully analyzed rate effects of anionic and nonionic micelles by using a multiple pseudophase model in which the micellar interface does not act a uniform reaction region. Instead there are a series of regions and the rate and equilibrium constants in eqs 2 and 3 are weighted averages of constants pertaining to each region.7 One test of the simple pseudophase model involves variations in the surfactant headgroups and changes in the properties of the interfacial region. Such changes can significantly affect rates of spontaneous, unimolecular, reactions, which are sensitive to the reaction medium and should be affected by changes in the micropolarity of the interfacial region.11 Kinetic solvent effects are often small for reactions of nucleophilic anions in polar, protic, solvents and this generalization also applies to SN2 reactions of Br- in cationic micelles where local second-order rate constants increase only modestly on addition of surfactants (8) (a) Chaudhuri, A.; Romsted, L. S. J. Am Chem. Soc. 1991, 113, 5052. (b) Chaudhuri, A.; Romsted, L. S.; Yao, J. J. Am. Chem. Soc. 1993, 115, 8362. (c) Romsted, L. S.; Yao, J. Langmuir 1996, 12, 2425. (9) Romsted, L. S. In Surfactants in Solution; Mittal, K. L., Lindman, B., Eds.; Plenum Press: New York, 1984, Vol. 2, p 1015 and references cited therein. (10) (a) Cerichelli, G.; Grande, C.; Luchetti, L.; Mancini, G. J. Org. Chem. 1991, 56, 3025. (b) Blasko, A.; Bunton, C. A.; Foroudian, H. J. J. Colloid Interface Sci. 1995, 175, 122. (11) Bunton, C. A. In Nucleophilicity; Harris, J. M., McManus, S. P., Eds.; Advances in Chemistry 215; American Chemical Society: Washington, DC, 1987; Chapter 29.

with sulfoxide or phosphine oxide headgroups.12 The small rate increases were ascribed to partial dehydration of Br-, as in nonmicellar reactions in aqueous DMSO and other aprotic solvents.13 Cyclizations of 2-((3-halopropyl)oxy)phenoxide ions, are, apart from molecularity, mechanistically equivalent to SN2 reactions.14 Their rate constants in a variety of association colloids are slightly higher than in water and increase modestly with increasing size of quaternary ammonium headgroups.15 Rate constants differ slightly in cationic and zwitterionic, betaine sulfonate, micelles due to effects on local rate constants in the micellar pseudophase and not to changes in transfer equilibria, which indicates that headgroup structure might modestly affect rate constants of bimolecular nucleophilic reactions. Synthetic cationic vesicles strongly accelerate anionic deacylations and effects were fitted by the pseudophase, ion-exchange, treatment.16 Second-order rate constants of reaction with OH- in the vesicles were slightly lower than in water, as for reactions in cationic micelles, but they were much higher for deacylation by thiolate ions. The use of functional surfactants simplifies quantitative treatments of rate data, because conditions can be selected such that both the substrate and the reagent, e.g., a nucleophilic headgroup, are wholly in the interfacial region. The local molarity then depends on the molar volume of the reaction region and, for comicelles, the relative amounts of micellized functional and inert surfactants.2b,17,18 We examined dephosphorylation of p-nitrophenyl diphenyl phosphate (p-NPDPP) in comicelles of n-dodecyl [2-(hydroximino)-2-phenylethyl]dimethylammonium bromide (DHDBr) (Scheme 1) with various ionic and nonionic surfactants (Chart 1), and quantitative formation of the oximate ion (DHD), shown as the E-isomer. This reaction had earlier been examined in comicelles of CTABr with oximate ion in excess over p-NPDPP, so that turnover of phosphorylated species did not affect the kinetics.17 Second-order rate constants in the micellar pseudophase were slightly lower than those of reaction of a similar, but nonmicellizing, oximate ion in water. Rate constants of the overall reaction depended on the local concentration of DHD in the micelles, i.e., on its stoichiometric concentration relative to CTABr, and not upon concentrations of the surfactants in terms of total solution volume. Oximate ions are effective dephosphorylating and deacylating agents.18a,19 Similar observations were made on dephosphorylation in comicelles of CTABr and N-phenyl (12) (a) Blasko, A.; Bunton, C. A.; Toledo, E. A.; Nome, F.; Holland, P. M. J. Chem. Soc., Perkin Trans 2 1995, 2367. (b) Foroudian, H. J.; Bunton, C. A.; Holland, P. M.; Nome, F. J. Chem. Soc., Perkin Tans. 2 1996, 557. (13) (a) Parker, A. J. Chem. Rev. 1969, 69, 1. (b) Buncel, E.; Wilson, H. Acc. Chem. Res. 1979, 12, 42. (14) Mandolini, L. Adv. Phys. Org. Chem. 1986, 22, 1. (15) Cerichelli, G.; Luchetti, L.; Mancini, G.; Muzzioli, M. N.; Germani, R.; Ponti, P. P.; Spreti, N.; Savelli, G.; Bunton, C. A. J. Chem. Soc., Perkin Trans 2 1989, 1081. (16) (a) Kawamuro, M. K.; Chaimovich, H.; Abuin, E. B.; Lissi, E. A.; Cuccovia, I. M. J. Phys. Chem. 1991, 95, 1458. (b) Chaimovich, H.; Cuccovia, I. M. Prog. Colloid Polym. Sci. 1997, 103, 67. (17) Bunton, C. A.; Hamed, F. H.; Romsted, L. S. J. Phys. Chem. 1982, 86, 2103. (18) (a) Fornasier, R.; Tonellato, U. J. Chem. Soc., Faraday Trans. 1 1980, 76, 1302. (b) Bunton, C. A.; Scrimin, P.; Tecilla, P. J. Chem. Soc., Perkin Trans. 2 1996, 419. (c) Scrimin, P.; Ghirlanda, G.; Tecilla, P.; Moss, R. A. Langmuir 1996, 12, 6235. (19) (a) Fina, N. J.; Edwards, J. O. Int. J. Chem. Kinet. 1973, 5, 1. (b) Camille, C. A.; Hansen, A. S. Phosphorus, Sulfur Silicon 1991, 57, 147. (c) Yang, Y.-C.; Berg, F. J.; Szafraniec, L. L.; Beaudry, W. T.; Bunton, C. A.; Kumar, A. J. Chem. Soc., Perkin Trans. 2 1997, 607. (d) Epstein, J.; Kaminski, J. J.; Bodor, N.; Enever, R.; Sowa, J.; Higuchi, T. J. Org. Chem. 1978, 43, 2816.

Effects of Headgroup Structure

Langmuir, Vol. 15, No. 4, 1999 1069 Scheme 1

Chart 1

tetradecanohydroxamate ion (1),17 RCO‚NPhO- where (1) R ) n-C13H27 and (2) R ) n-C11H23. Kunitake et al. had used DHDBr with inert surfactants in the deacylation of p-nitrophenyl acetate at pH such that the oxime was only partially deprotonated and the substrate was partially micellar bound.20 In the present work we use several inert surfactants with a variety of headgroups, but in all experiments the oxime is fully deprotonated. Under these conditions, with fully bound p-NPDPP, i.e., KS[Dn] . 1, eqs 2 and 3 simplify to

kobs )

kM[DHD] [DHD] + [Dn]

)

km 2 [DHD] VM([DHD] + [Dn])

comparisons of these rate constants have been made with other systems including deacylations18a and dephosphorylations by functionalized iodosobenzoates,21 which are very effective dephosphorylating agents,22 and by amphiphilic metallo complexes.18b,c Provided that the substrate is as hydrophobic as p-NPDPP, where KS ≈ 104 M-1, reaction is wholly in the micelles. Most experiments were made in comicelles of DHDBr and an inert surfactant, but in other cases we used DHDBr plus CTABr and another inert surfactant, Dn, (Chart 1). In these situations, with fully micellar-bound p-NPDPP, we write

(5)

where Dn is inert, micellized surfactant and VM is the molar volume of the reaction region which was originally taken as 0.14 M-1 for CTABr.17 To simplify the kinetic interpretation surfactant concentrations were well above those of the monomeric surfactant and it is then easy to compare rate constants, km 2 and kW, in micelles and water, respectively. Similar (20) Kunitake, T.; Shinkai, S.; Okahata, Y. Bull Chem. Soc. Jpn. 1976, 49, 540.

kobs )

kM[DHD] [DHD] + [CTABr] + [Dn]

(6)

These equations can be also written with the secondorder rate constant as km 2 , and concentration as a local molarity; cf. eq 4. (21) Bunton, C. A.; Mhala, M. M.; Moffatt, J. R. J. Phys. Chem. 1989, 93, 854. (22) (a) Moss, R. A.; Alwis, K. W.; Shin, J.-S. J. Am. Chem. Soc. 1984, 106, 2651. (b) Moss, R. A.; Kim, K. Y.; Swarup, S. J. Am. Chem. Soc. 1986, 108, 788.

1070 Langmuir, Vol. 15, No. 4, 1999

In discussing the results we use kM, which is calculated unambiguously, unless we are comparing rate constants in the micelles with those in water. Oximes can exist as E- or Z-isomers23 which may have different nucleophilicities. This question was not raised in the earlier work,17,20 but we used NMR spectroscopy23b to examine isomerism in DHDBr. In addition NMR spectroscopy showed that the material prepared originally17,20 was contaminated by pyridine. Subsequent experiments showed that dephosphorylations with some amphiphilic nucleophiles have complications, apart from impurities in the nucleophile.17 For example, hydroxamate24 as well as oximate ions23 can exist as geometrical isomers, which readily equilibrate in the hydroxamates, where the composition depends on the medium.24 In addition, location of the hydroxamate headgroup in the interfacial region of a micelle depends on its charge24b which affects rate constants of dephosphorylation by N-phenyldodecanohydroxamate ion (2).25 Triaryl phosphates are often used as nontoxic surrogates for nerve agents and there are several examples of the effects of association colloids on reactions of these substrates with a variety of nucleophiles, including oximates.18b,19c,d,21,22,26 Experimental Section Materials. A reported sample of DHDBr was originally prepared from N-phenacyl-N,N-dimethyldodecylammonium bromide by heating it with hydroxylamine hydrochloride at 60 °C for 20 h in 1:1 pyridine-EtOH (v/v).20 Volatiles were removed in vacuo, and the oil was treated with MeCN to precipitate pyridine hydrochloride.20 After removal of MeCN the product was recrystallized from EtOAc/EtOH to give a solid, mp 89-90 °C (sharp).20 This procedure was previously followed,17 the solid product had the expected melting point, and it did not appear to be necessary to confirm its purity. We followed this procedure in the present work and confirmed the earlier melting point. However, the 1H NMR spectrum (400 or 500 MHz in either CD3CN or CD3OD) showed that the sample contained significant amounts of pyridinium ion (400 MHz 1H NMR in CD3OD with TSP as standard: d, δ ) 8.901 ppm, 2H; t, δ ) 8.864 ppm, 1H; t, δ ) 8.136 ppm, 2H). Addition of KOD gave the 1H NMR signals of pyridine (400 MHz 1H NMR in Me-d4-OD plus KOD with TSP as standard: d, δ ) 8.531 ppm, 2H; t, δ ) 7.857 ppm, 1H; and a third signal, 2H, partially obscured by the DHDBr signal at ∼7.4 ppm). This material dissolved in MeCN and recrystallization from it gave material, mp 89 °C, with the same NMR spectrum. On the basis of integration of the 1H NMR signals, the molar ratio of pyridinium ion to DHDBr was ca. 1:1. The original method of purification20 did not remove the pyridinium salt, which was removed by washing with aqueous NaBr, in which we expected DHDBr to be sparingly soluble. Addition of 1.8 g of material, mp 89 °C, to 50 mL of warm 0.2 M aqueous NaBr initially gave a clear solution from which a colorless solid gradually recrystallized upon cooling. The 1H NMR spectrum showed that this solid contained no pyridinium ion, and we detected pyridine in the filtrate on addition of NaOH. The 1H and 13C NMR spectra of the purified oxime confirm its structure. This purification is feasible because DHDBr is sparingly soluble (ca. 10-3 M) in cold H2O. The sample of DHDBr was washed with a small amount of H2O and after drying in vacuo over concentrated H2SO4 it had mp 123 (23) (a) March, J. Advanced Organic Chemistry, 3rd. ed.; WileyInterscience: New York, 1985; Chapter 4. (b) Chan, A. S. C., Chen, C.-C., Lin, C.-W., Lin, Y.-C., Cheng, M.-C, Peng, S.-M. Chem Commun. 1995, 17, 1767. (24) (a) Brown, D. A.; Glass, W. K.; Mageswaran, R.; Mohammed, S. A. Magn. Reson. Chem. 1991, 29, 40. (b) Blasko, A.; Bunton, C. A.; Gillitt, N. D. Langmuir 1997, 13, 6439. (25) Bunton, C. A.; Gillitt, N. D.; Foroudian, H. J. Langmuir 1998, 14, 4415. (26) (a) Menger, F. M.; Gan, L. H.; Johnson, E.; Durst, D. H. J. Am. Chem. Soc. 1987, 109, 2800. (b) Yang, Y.-C.; Baker, J. A.; Ward, J. R. Chem. Rev. 1992, 92, 1729.

Bunton et al. °C (sharp). The material, mp 89 °C, which was isolated earlier17,20 appears to be a eutectic mixture or a molecular complex, but we did not examine its physical properties. Most of the surfactants were samples used earlier,17,24b except for n-dodecyl-2-pyrrolidinone (Surfadone) which was a gift from Dr. Yu-Chu Yang. The polyoxyethylene surfactants, C12E10 (Sigma) and C12E23 (Brij 35, Aldrich) were used without purification. The other materials were those used earlier, and reaction solutions were made up in deionized, distilled, CO2-free, water. Deprotonation of DHDBr. The apparent pKa of DHDBr in comicelles with CTABr had been estimated spectrophotometrically17 from absorbances which were assumed to correspond to those of the oxime and the oximate ion, on the assumption that the original material, mp 89 °C, was uncontaminated by acidic or basic impurities.20 The apparent dissociation constants were therefore remeasured with a purified sample of DHDBr by making repetitive scans over a range of pH with comicelles of 10-4 M DHDBr and 1.5 × 10-3 M CTABr or 1.3 × 10-3 M SB3-14 in 0.01 M carbonate buffer. On the basis of absorbances of the oxime at 245 nm or the oximate ion at 273 nm, respective values of the apparent pKa are 9.4 in CTABr and 9.3 in SB3-14 at 25 °C. These pKa values are lower, by ca. 0.6 pKa units, than those reported earlier for a sample that contained pyridinium ion.17 There was an isosbestic point at 255 nm and also one at 230 nm, which was not sharp, possibly because of the presence of a small amount of isomer.23 Kinetics. Reactions were followed spectrophotometrically at 25.0 °C as described.17 Complete deprotonation of the oxime by 0.01 M OH- was confirmed spectrophotometrically with the kinetic solution, except for p-NPDPP, in a short path length cuvette. Freshly made up solutions were used because DHDBr decomposes slowly at high pH. Reaction was started by adding p-NPDPP in MeCN in a spring-loaded Hamilton syringe to the stirred reaction mixture in an HP diode array spectrometer. The final reaction solution contained 0.4 vol % MeCN and 10-5 M p-NPDPP, and duplicate values of kobs agreed within 5%. Samples of DHDBr prepared by the original method20 contained pyridinium ion; reported concentrations were therefore too high, and calculated rate constants were too low.17 Surfactant concentrations were well above the cmc, we neglected concentrations of monomeric surfactant, and kobs depended on the ratio of DHDBr to total surfactant rather than on surfactant concentration. There is a minor contribution of reaction of OH- in cationic micelles, and a small correction was made for this reaction.17 Reaction of OH- with p-NPDPP is inhibited by anionic, nonionic, and zwitterionic micelles2b,21 and can be neglected. For 0.05 M CTBABr we corrected for reaction with OH- based on values of kobs ) 2.5 × 10-2 and 5.8 × 10-2 s-1 in 0.01 and 0.03 M OH-, with allowance for depletion of OH- by complete deprotonation of DHDBr. These small corrections are approximations, because DHD is a zwitterion, and it and Brinhibit reaction of OH- with p-NPDPP in cationic micelles, but the uncertainties so introduced do not introduce serious errors. Solutions of DHDBr and CTABr gave what appeared to be liquid crystals if the room temperature was below ca. 20 °C. Warming to 25 °C gave clear solutions, but to obtain consistent kinetic data we left solutions for 1 h to allow equilibration of the colloidal assemblies. We did not have this problem with the other surfactants, and DHDBr dissolved very readily in solutions of CTBABr. DHDBr which is almost insoluble in cold water is fully incorporated in the comicelles,17 values of kobs at a given [DHDBr]/ [CTABr] are independent of [surfactant] with DHDBr g 5 × 10-4 M and reactions were carried out with 1.4-1.6 × 10-3 M DHDBr, unless specified. NMR Spectroscopy. We examined 1H and 13C NMR spectra of samples of contaminated and purified DHDBr in CD3OD and DMSO-d6 by using Varian Unity (Inova) instruments, generally 400 MHz for 1H. Signals were assigned by 1H COSY and 1H-13C HETCOR spectra, and assignment of 13C signals was consistent with that predicted by the ACD/CNMR simulation (Advanced Chemistry Development, Version 2.04).

Results Structure and Configuration of DHDBr. We used NMR spectroscopy to examine the configuration of the

Effects of Headgroup Structure Table 1.

1H

Langmuir, Vol. 15, No. 4, 1999 1071

Chemical Shiftsa

Table 3. Effect of Inert Surfactants on Dephosphorylation in Cationic and Mixed Zwitterionic and Nonionic Comicelles with CTABra

solvent signal

DMSO-d6

CD3OD

CD3OD/KOD

ω-CH3 (s) CH2(7-11) (b,s) CH2(4-6) (b,s) CH2(3) (b,m) CH2(2) (b,m) CH2(1) (b,t) CH2(1′)b (s) N-CH3 (s) o-Ph (d) m-Ph (t) p-Ph (t) OH (s)

0.862 1.245 1.215 1.050 (0.950) 1.668 (1.548) 3.245 (3.145) 4.680 (4.504) 2.964 (2.974) 7.77 7.468 7.468 12.764 (12.102)

0.890 1.286 1.286 1.150 (1.102) 1.790 (1.695) 3.305 (3.211) 4.750 (4.501) 3.050 7.735 (7.650) 7.475 7.475

0.895 1.283 1.283 1.175 (1.035) 1.778 (1.625) 3.201 (3.106) 4.620 2.961 7.640 7.395 7.320

a δ , referred to TMS in DMSO-d and TSP in CD OD. b Prime 1H 6 3 designates CH2 of the imino residue; observable signals of the minor Z-isomer are in parentheses.

Table 2.

13C

Chemical

Shiftsa

position

δ, ppm

ω-CH3 CH2(4-11) CH2(3) CH2(2) CH2(1) CH2(1′) NsCH3 CdNb Cc o-Ph m-Ph p-Ph

14.656 30.2-33.2 27.355 (27.248) 24.009 (23.766) 67.476 (65.754) 56.014 (66.680) 52.980 (52.494) 148.910 (147.886) 136.887 (133.230) 128.178d 130.272d 131.076d

δ13C, referred to CD3OD at 49.15 ppm and positions as in Table 1; observable signals of the minor Z-isomer are in parentheses. b Imino carbon. c Quarternary carbon of Ph. d Minor signals are also observed at 129.862 and 129.908 ppm and as a lower shift shoulder to the signal at 131.076 ppm. a

oxime. The impure material, mp 89 °C, prepared as described,20 gave 1H NMR spectra with signals of pyridinium ion, or pyridine in KOD, and in addition to the major signals of an oxime, there were small signals in the aromatic and aliphatic regions. We saw these major and minor signals with purified DHDBr. The minor signals were not those of the ketone precursor, and both major and minor signals shifted on addition of KOD to CD3OD (Table 1). Samples before and after removal of the pyridinium salt had major:minor ) 85:15, and this ratio did not change on further recrystallization. The 1H NMR chemical shifts and peak areas are as expected (Table 1). In CD3OD two of the three aromatic signals of the major isomer merge but they separate on addition of KOD. Consequent decreases in chemical shift are largest at the o- and p-positions due to charge delocalization into the phenyl group. Some of the aromatic signals of the minor isomer are under the major signals. Deprotonation has little effect on signals of hydrogens distant from the oxime group, and, based on the NMR spectra, does not change the E-Z ratio. The major isomer has the E-configuration based on rules for 1H and 13C chemical shifts.23b The isomer with CH2 syn to imino OH has its 13C chemical shift lower (Table 2) and the 1H chemical shift higher (Table 1), than those of the minor Z-isomer with OH syn to phenyl, and in aprotic DMSO-d6 the 1H chemical shift of the OH group is lower in the Z-isomer, due to diamagnetic shielding by phenyl (Table 1). The OH signals disappear on addition of D2O to DMSO-d6. Steric interactions between phenyl and OH should decrease the acidity of the Z-isomer and the reactivity of

inert surfactant system 100% CTABrb 100% CTABr (this work) 100% CTBABr 11.6% SB3-14 24.7% SB3-14 51.5% SB3-14 72.4% SB3-14 100% SB3-14 22.9% C12PO 51.1% C12PO 75.2% C12PO 100% C12PO 41.9% C10SO 70.0% C10SO 78.8% C10SO 28.7% Surfadone 10.7% C12E10 24.7% C12E10 49.5% C12E10 75.1% C12E10 100% C12E10 23.9% C12E23 46.7% C12E23 75.9% C12E23 100% C12E23

[DHDBr]/[DT] 1/5

kM, s-1 [DHDBr]/[DT] 1/10

[DHDBr]/[DT] 1/15

2.69 3.83

4.07 3.93

3.58 3.85

3.78 3.75 4.35 5.94

3.45 1.75

2.00 4.13 4.14 4.33 4.02 3.79c 4.43 5.05 5.52 6.34 4.74 5.48 6.10 4.65 4.22 4.15 3.69 2.86 1.54 3.74 3.19 2.11 1.19

3.95 3.72 4.93 6.36

4.35 1.50

a Values of k , s-1, at 25.0 °C in 0.01 M NaOH, relative M concentrations are in mole percent and CTABr is the other surfactant. b Reference 17, values corrected by multiplying kobs by 1.12 for the presence of 12 wt % pyridine hydrochloride. c kM also equals 3.79 s-1 with 5 mM DHDBr and 0.1 M NaOH.

its oximate ion, and we assume that the major E-isomer is more reactive than the Z, consistent with the earlier observation of clean first-order kinetics even when DHDBr is not in large excess over p-NPDPP.17 However, we cannot exclude a minor contribution from reaction of the Z-isomer. Most of the 1H aliphatic signals of the oxime in aqueous surfactant are heavily overlapped by signals of the surfactants or HOD and are uninformative. However, chemical shifts in the aromatic region are very similar to those in CD3OD (Table 1) for example in 0.05 M CTABr and [DHDBr]:[CTABr] ) 1:10, δortho ) 7.78 (7.67), δmeta ) 7.43 (7.36) and δpara ) 7.34 (7.20) ppm. Values in parentheses are in 0.05 M NaOD. Corresponding values with [DHDBr]:[CTABr] ) 1:1 are δortho ) 7.71 (7.60), δmeta ) 7.40 (7.31), and δpara ) 7.32(7.16) ppm, and we saw minor signals of the Z-isomer with slightly lower chemical shifts. We obtained similar data in comicelles of DHDBr and both SB3-14 and C12PO. These results indicate that micelles do not affect the E-Z ratio of DHDBr. Reactivity in Comicelles of DHDBr. Values of kobs for reaction in comicelles of DHDBr and CTABr are higher than those reported earlier17 where the assumed concentration of DHDBr was too high, due to contamination by pyridinium hydrochloride. Earlier and present values of kobs agree, provided that we allow for dilution by pyridinium hydrochloride (Table 3). We varied compositions of the comicelles by adding a third surfactant, D, to mixtures of DHDBr and CTABr and used eq 6 to calculate kM (Table 3 and Figure 1). For mixtures of DHDBr, CTABr and other surfactants, D, and values of kM approximately follow eq 7, where kCTA M D kM ) kCTA M χCTABr + kM(1 - χCTABr)

(7)

1072 Langmuir, Vol. 15, No. 4, 1999

Bunton et al. Table 6. Salt Effects on Reaction in SB3-14 Comicellesa salt

kM, s-1

salt

kM, s-1

0.2 M NaCl 0.5 M NaCl

3.79 2.75 2.12

0.1 M NaClO4 0.2 M NaClO4 0.5 M NaClO4

1.51 1.02 0.97

a Values of k , s-1, at 25.0 °C in 0.1 M NaOH with [DHDBr]: M [SB3-14] ) 1:10 with 0.05 M SB3-14.

density of the anion and its increasing affinity for the micelle. These kinetic salt effects are similar to those seen with the micellar-mediated reaction of the hydroxamate ion (2) with p-NPDPP,25 which were ascribed to salt effects on the average location of the nucleophile in the micellar interfacial region.24b The ion-order is consistent with other evidence on micelle-salt interactions.2,6,9,27,28 Discussion

Figure 1. Effect of surfactant headgroups upon values of kM for dephosphorylation in oximate comicelles based on addition of inert surfactants to CTABr. Lines are drawn to aid the eye. Surfactants are as follows: CTABr (b); SB3-14 (2); C12PO (0); C10SO (9); Surfadone (4); C12E10 (O); C12E23 ([). Table 4. Effect of Inert Surfactants on Dephosphorylation in SDS/DHDBr Comicellesa inert surfactant system 100% SDS 50% C12E10 100% C12E10 50% C12E23 100% C12E23 50% S10O

[DHDBr]/[DT] 1/5 0.356 1.75

kM, s-1 [DHDBr]/[DT] 1/10

[DHDBr]/[DT] 1/15

0.239/0.246b 0.477b 1.54 0.459 d 1.19 0.79d/0.93e

0.219 0.559c 1.50 0.491c

a Values of k , s-1, at 25.0 oC in 0.1 M NaOH, relative M concentrations are in mole % and values for 100% C12E10 and C12E23 are from Table 3. b 10 mM DHDBr. c 6.67 mM DHDBr. d 2.0 mM DHDBr. e 4.0 mM DHDBr.

Table 5. Salt Effects on Reaction in SDS Comicellesa salt

kM, s-1

salt

kM, s-1

0.2 M NaCl 0.2 M NaClO4

0.246 0.316 0.324

0.2M CsCl 0.2 M CsClb 0.2 M Me4NNO3 0.5 M Me4NNO3

0.404 0.303 0.353 0.509

a Values of k , s-1, at 25.0 °C in 0.1 M NaOH with [DHDBr]: M [SDS] ) 1:10 with 0.1 M SDS. b [DHDBr]:[SDS] ) 1:15 with 0.025 M SDS.

kD M refer to reactions in CTABr and D, respectively. This linear relation fails with the polyoxyethylene surfactants where there is curvature in plots of kM against micellar composition in mole percent (Figure 1). The solubility of Surfadone in water is so low that we could use only dilute surfactant. Reaction is significantly slower in comicelles with SDS than with the other surfactants (Tables 3 and 4). Kinetic Salt Effects. Added salts increase kM in comicelles with SDS (Table 5). The effect depends on the salt cation, its affinity for SDS micelles, and its ability to reduce the negative charge density at the surface. The behavior is different for reaction in SB3-14 (Table 6), where the anion enters the micelles and generates negative charge. The inhibition increases with decreasing charge

Interpretations of quantitative treatments of the effects of micelles and other association colloids on rates of nonsolvolytic, bimolecular, reactions are difficult if reactants are very hydrophilic and the extent of their partitioning between the pseudophases is uncertain. Transfer equilibria of ions depend on competition with other ions for the association colloid,2,6,9 and if nucleophilic anions are formed by deprotonation, effects on the acidbase equilibrium must also be considered.1,2,29 We avoid these complications by using a hydrophobic substrate and a functional micelle, in conditions such that reaction is wholly in the micellar pseudophase. The rate data for reactions in comicelles of inert surfactants and the oximate surfactant DHDBr fit eqs 2 or 3 reasonably well over a range of concentrations. Comparison of second-order rate constants in the aqueous and micellar pseudophases depends on the assumption that concentration at the micellar surface can be written as a local molarity, and that a value can be assigned to VM. Literature values of VM range from 0.14 to 0.37 M-1 and are assumed to be independent of structures of reactants and surfactant.2 Within these uncertainties similar values of km 2 and kW have been estimated for a number of bimolecular ion-molecule reactions in nonfunctional and functional micelles and comicelles2,6,9 and these generalizations often apply to reactions mediated by vesicles, microemulsions and alcohol modified micelles.2,16,30 On the basis of VM ) 0.14 M-1 values of km 2 are slightly smaller than kW, which is 0.9 M-1 s-1,17 as compared with km 2 ≈ 0.56 for reactions in comicelles with CTABr (Table 3) and eq 4. Values of km 2 , calculated with VM ) 0.14 M-1, in C12PO or C10SO (Table 3) are very similar to that of kW, which is probably coincidental in view of uncertainties in the value of VM. The decrease in kM in going from CTABr to CTBABr was unexpected, because small or opposite effects had been observed for other reactions of nucleophilic anions.31 An increase in the size of the headgroup may increase the volume of the region at the micelle-water interface in (27) (a) Bunton, C. A.; Mhala, M. M.; Moffatt, J. R. J. Org. Chem. 1987, 52, 3832. (b) Baptista, M. S.; Cuccovia, I.; Chaimovich, H.; Politi, M. J.; Reed, W. F. J. Phys. Chem. 1992, 96, 6442. (c) Kamenka, N.; Chorro, M.; Chevalier, Y.; Levy, H.; Zana, R. Langmuir 1995, 11, 4234. (28) (a) Romsted, L. S.; Yoon, C. O. J. Am. Chem. Soc. 1993, 115, 989. (b) Brinchi, L.; DiProfio, P.; Germani, R.; Savelli, G.; Spreti, N.; Bunton, C. A. J. Chem. Soc. Perkin Trans. 2 1998, 2, 361. (29) (a) Hartley, G. S. Quart. Rev. Chem. Soc. 1948, 2, 152. (b) Romsted, L. S. J. Phys. Chem. 1985, 89, 5107, 5113. (30) Bunton, C. A.; Savelli, G. Adv. Phys. Org. Chem. 1986, 22, 213. (31) (a) Bacaloglu, R.; Bunton, C. A.; Ortega, F. J. Phys. Chem. 1989, 93, 1497. (b) Bacaloglu, R.; Bunton, C. A.; Cerichelli, G.; Ortega, F. J. Phys. Chem. 1990, 94, 5068. (c) Cuenca, A.; Bruno, C. Int. J. Chem. Kinet. 1994, 26, 963.

Effects of Headgroup Structure

which reaction occurs, and dilute the nucleophile in this region, and the effect may be characteristic of reactions involving functional micelles or comicelles. Micellar-bound ions are not significantly dehydrated,2 cf. ref 32, and on the basis of spectral probes, the properties of the interfacial region appear to be similar to those of low molecular weight, hydrophilic alcohols.5,33 Therefore reactions of nucleophilic anions should not be very sensitive to kinetic medium effects of micelles. The behavior of sulfobetaines is understandable in view of similar rate effects of these and cationic micelles in other reactions.2b,15,25 Sulfobetaine micelles are formally neutral, but they attract anions due to dipole-ion and specific ammonium-anion interactions.21,27,28b Thus they behave like hypothetical cationic micelles with a low fractional charge and limited affinity for anions. Values of kM are higher in C10SO or C12PO than in CTABr comicelles (Table 3 and Figure 1), probably because water molecules at the interface are hydrating the sulfoxide or phosphine oxide headgroups and are less effective in deactivating oximate ion, cf. ref 12, where analogy was drawn between this behavior and rate effects of dipolar, aprotic solvents.13 Initial addition of nonionic, polyoxyethylene, surfactants to CTABr slightly decreases kM (Table 3 and Figure 1) and the length of the hydrophilic polyoxyethylene headgroup is not very important, probably because the reaction center is close to the junction of the apolar tail and the hydrophilic headgroup. Water is not excluded from this region34 and will hydrogen bond to the oximate residue, as in other micelles. The rate constants, kM, in Table 3 are similar for reactions in cationic, zwitterionic and some nonionic micelles. If we assume that molarity in the interfacial region is the appropriate measure of concentration we could ascribe the lower rate constants in C12E10 and C12E23 to the large volume of the interfacial region, which is the palisade layer. The volume of this region is much larger in these nonionic micelles than that of the interfacial regions of other micelles. (In ionic micelles this region is often identified with the Stern layer.1,2,33) This explanation is the same as assuming that the value of VM (eq 4) depends on the structure of the headgroup, e.g., the differences in values of kM in CTABr and CTBABr and C12E10 and C12E23 (Table 3) may be related to different volumes of the interfacial regions. The considerably lower reactivity of DHD in comicelles with SDS, as compared with the other surfactants (Tables 3 and 4), is not due to incomplete deprotonation of the oxime (Experimental). This behavior is similar to that seen for the reaction of p-NPDPP with the amphiphilic hydroxamate (2), although here values of kM for reactions in CTABr and SDS differ by a factor of ca. 4025 rather than 16 (Tables 3 and 4). As found earlier for reaction of 2, values of kM increase on addition of nonionic surfactants and salts to SDS (Tables 4 and 5). The low rate constants in SDS, relative to other surfactants, are ascribed to differences in locations of the reactive centers, on the assumption that the hydrophobic substrate, p-NPDPP, is located, on average, close to the junction of the head and alkyl groups of the surfactant but that in SDS micelles the nucleophilic oxide ion will be preferentially in the more (32) Morgan, J. D.; Napper, D. H.; Warr, G. G. J. Phys. Chem. 1995, 99, 9458. (33) Cordes, E. H.; Gitler, C. Prog. Bioorg. Chem. 1973, 2, 1. (34) (a) Elworthy, P. H.; Florence, A. T.; Macfarlane, C. B. Solubilization by Surface Active Agents; Chapman and Hall: London, 1968; Chapter 1. (b) Jonstomer, M.; Jonsson, B.; Lindman, B. J. Phys. Chem. 1991, 95, 3293. (c) Romsted, L. S.; Yao, J. Langmuir 1996, 12, 3431.

Langmuir, Vol. 15, No. 4, 1999 1073

aqueous part of the interfacial region.24b This discrimination in position should be stronger with the anionic hydroxamate (2) than with a zwitterionic oximate. We note that values of kM increase with increasing [DHD]: [SDS], i.e., as the zwitterionic oximate ion is added to SDS (Table 4). The Concept of a Distinct Micellar Reaction Region. The pseudophase model involves the assumed existence of a discrete reaction region at the micellewater interface, and in treating bimolecular, nonsolvolytic, reactions we have to define concentration in this region. In principle local ion concentration can be calculated in terms of theoretical models which involve a variety of assumptions,2,6,9 and for some weakly basic ions, dediazonization trapping measures concentration directly.8 Most experimental methods measure the partitioning of ionic or nonionic solutes between water and association colloids,35 which gives concentration as a mole ratio of bound solute to surfactant headgroup, and it can be converted into a local molarity by using eq 4 with an assumed value of VM. The interfacial region is made up of headgroups, nearby segments of the apolar tails, associated ions and water molecules, and, in some cases, cosurfactant. Although this region is not uniform, especially in cosurfactant-modified or mixed micelles, the simple treatment, eqs 2 and 3, is reasonably satisfactory over a range of conditions. Our use of a functional surfactant removes uncertainty in the partitioning of the nucleophile between the aqueous and micellar pseudophases and the need for determining it experimentally or by use of a theoretical treatment. This problem of partitioning is acute for some very hydrophilic ions. For example, there appears to be no reliable experimental method for determining the distribution of OH- between water and micelles, and it is often estimated by using the ion-exchange treatment of competition between OH- and an inert anion.2,9 This method becomes indeterminate if the inert anion has a very high affinity for the micelle, and in some conditions rate-surfactant 36 profiles can be fitted by a range of values of kM or km 2. However, the use of functional surfactants does not eliminate the possibility that changes in headgroups may affect relative locations of p-NPDPP and the nucleophilic, e.g., oximate, residue in the interfacial region. There is curvature in the plots of kM against mole fraction in mixtures of CTABr and the polyoxyethylene surfactants (Table 3 and Figure 1) which could be due to nonuniform mixing. If p-NPDPP and oximate preferentially locate close to the quaternary ammonium ion, initial addition of C12E10 or C12E23 would not significantly decrease kM, as is observed. However, deviations from linearity are relatively small; e.g., with CTABr + C12E10 rate constants are within 25% of the predicted value of kM (eq 7) and are smaller for the other surfactants (Figure 1). The concepts of uniform mixing and “local concentration” appear to be useful in rationalizing reactivities, at least qualitatively. This generalization fails for dephosphorylations by amphiphilic nucleophiles in anionic comicelles (Table 4 and refs 24b and 25). Conclusions The basic assumption of the pseudophase model of effects of association colloids on reaction rates and equilibria is that the colloids, e.g., micelles, act as a distinct (35) Sepulveda, L.; Lissi, E.; Quina, F. Adv. Colloid Interface Sci. 1986, 25, 1. (36) Blasko, A.; Bunton, C. A.; Cerichelli, G.; McKenzie, D. C. J. Phys. Chem. 1993, 97, 11324.

1074 Langmuir, Vol. 15, No. 4, 1999

reaction region from bulk solvent.1,2 Overall rates depend on local rate constants and reactant concentrations. Ionic or polar reactants apparently remain in the water-rich interfacial region, rather than in the hydrocarbon-like core (for exceptions to this generalization as applied to microemulsions, see ref 37). Reactant distribution is, to a first approximation, assumed to be uniform in the interfacial region, as in bulk solvents with no ion-pairing or other reactant association. This assumption is probably no worse than those applied to reactions in mixed solvents, or solutions of moderately concentrated electrolyte, where second-order rate constants are usually calculated with concentration expressed as molarity in terms of total solution volume. Second-order rate constants of dephosphorylation by DHD in the interfacial region are similar for cationic, nonionic, and zwitterionic micelles, despite the probable nonuniform composition of this region. Our evidence indicates that a change of headgroup charge from positive to neutral, of itself, does not have major effects on reactivity in the interfacial region. Polyoxyethylene groups, which are very hydrophilic, slightly decrease reactivity, and phosphine or sulfoxide headgroups, which accept hydrogen bonds, slightly increase it. These effects on oximate ion nucleophilicity are as expected, if the interfacial region is regarded as a microscale solvent. The general conclusions are consistent with those based on (37) (a) Minero, C.; Pramauro, E.; Pelizzetti, E. Langmuir 1988, 4, 101. (b) DaRocha Pereira, R.; Zanette, D.; Nome, F. J. Phys. Chem. 1990, 94, 356.

Bunton et al.

micellar effects on rates of decarboxylation, SN2-like cyclizations and SN2 reactions where changes in headgroup charge have only modest effects on rates.2b,15,21 These generalizations fail for reactions of amphiphilic nucleophiles in anionic comicelles due to changes in locations,24b,25 but the situation is different for reactions of hydrophilic anions where local ionic concentrations in a defined interfacial region are estimated by using theoretical treatments and local second-order rate constants are then not very sensitive to headgroup charge.38 Although second-order rate constants in the micellar pseudophase depend on the surfactant headgroup, especially when it is anionic, concentration of nucleophile in the interfacial region is the main source of the rate increase over reaction in water. For example, if VM ) 0.14 M-1 with [DHDBr]:[CTABr] ) 1:10 the molarity of oximate ion in the interfacial region is 0.77 M, and much higher than that in the overall solution. These observations show that rate comparisons in water and association colloids can be very misleading if they are based on second-order rate constants calculated by using total concentrations. Acknowledgment. Support by the US Army Research Office is gratefully acknowledged. LA981138M (38) (a) Bunton, C. A.; Mhala, M. M.; Moffatt, J. R. J. Phys. Chem. 1989, 93, 7851. (b) Amado, S.; Garcia-Rio, L.; Leis, J. R.; Rios, A. Langmuir 1997, 13, 687.