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Langmuir 1996, 12, 2348-2352
Catalyzed Cyclization of 2-((3-Halopropyl)oxy)phenoxide Ions in Zwitterionic and Nonionic Micelles Giorgio Cerichelli* Dipartimento di Chimica, Ingegneria Chimica e Materiali, Universita` degli Studi de L’Aquila, Via Vetoio, 67010 Coppito Due (AQ), Italy
Luciana Luchetti* Dipartimento di Scienze e Tecnologie Chimiche, Universita` degli Studi di Roma “Tor Vergata”, Via della Ricerca Scientifica, 00133 Roma, Italy
Giovanna Mancini Centro CNR di Studio sui Meccanismi di Reazione, c/o Dipartimento di Chimica, Universita` degli Studi di Roma “La Sapienza”, P.le Aldo Moro 5, 00185 Roma, Italy
Gianfranco Savelli Dipartimento di Chimica, Universita` degli Studi di Perugia, Via Elce di Sotto 8, 06100 Perugia, Italy
Clifford A. Bunton Department of Chemistry, University of California, Santa Barbara, California 93106 Received July 3, 1995. In Final Form: January 22, 1996X Cyclization of 2-((3-halopropyl)oxy)phenoxide ion (halo ) Br, I; PhBr7 and PhI7, respectively) in micelles is a model for SN2 reactions of nucleophilic anions at micelle-water interfaces. Sulfobetaine micelles of dodecyldimethylammoniopropanesulfonate (C12H25N+(CH3)2(CH2)3SO3-, SB3-12) modestly increase rates of cyclization of PhBr7 and PhI7, and the effect is larger for the latter. The relative rate constants kI/kBr are indicators of medium polarity and their values indicate that the surface polarity of micellized SB3-12 is slightly lower than that of a trimethylammonium micelle and significantly lower than that of water. Changes in 1H chemical shifts of SB3-12 show that the micellar rate effects are not due to changes in micellar structure. Nonionic micelles of polyoxyethylene(23) dodecyl ether (C12H25(OCH2CH2)23OH, Brij 35) modestly speed up cyclization, but reaction appears to be in the water-rich palisade layer of the micelle in conditions such that the distinction between aqueous and micellar pseudophases is uncertain. Scheme 1
Introduction Association colloids, formed for example by the selfassembly of aqueous amphiphiles, can control overall rate and equilibrium constants. Micelles of aqueous surfactants are the most widely studied systems and treatments are generally based on pseudophase models.1,2 Water and micelles are regarded as distinct reaction regions, i.e., as pseudophases. The treatment of spontaneous unimolecular, or bimolecular, water-catalyzed, reactions is simple because the overall rate depends upon reactant distributions between the two pseudophases and the firstorder rate constants in each, as shown in Scheme 1, where subscripts w and m denote the aqueous and micellar pseudophases, respectively, k′w and k′m are first-order rate constants, KS is the binding constant of substrate (S) to micellized surfactant (detergent), Dn ) DT - cmc, where T denotes total surfactant and the cmc is assumed to give the concentration of monomeric surfactant.1,2 The first* To whom the correspondence should be addressed. X Abstract published in Advance ACS Abstracts, April 1, 1996. (1) (a) Fendler, J. H. Membrane Mimetic Chemistry; Wiley-Interscience: New York, 1982. (b) Sudholter, E. J. R.; Van der Langkruis, G. B.; Engberts, J. B. F. N. Recl. Trav. Chim. Pays-Bas 1980, 99, 73. (c) Bunton, C. A.; Savelli, G. Adv. Phys. Org. Chem. 1986, 22, 213. (d) Bunton, C. A.; Nome, F.; Quina, F. H.; Romsted, L. S. Acc. Chem. Res. 1991, 24, 357. (2) Menger, F. M.; Portnoy, C. E. J. Am. Chem. Soc. 1967, 89, 4698.
order rate constant is given by
kobs )
k′w + k′mKS[Dn] 1 + KS[Dn]
(1)
Equation 1 fits a great deal of data, although monomeric or premicellar surfactant may affect rates, especially with apolar substrates.1,3 The situation is more complicated if a second reagent is involved in the rate-limiting step, because k′w and k′m (3) (a) Germani, R.; Ponti, P. P.; Savelli, G.; Spreti, N.; Cipiciani, A.; Cerichelli, G.; Bunton, C. A. J. Chem. Soc., Perkin Trans. 2 1989, 1767. (b) Bacaloglu, R.; Bunton, C. A. J. Colloid Interface Sci. 1992, 153, 140. (c) Cerichelli, G.; Mancini, G.; Luchetti, L.; Savelli, G.; Bunton, C. A. Langmuir 1994, 10, 3982. (4) (a) Martinek, K.; Yatsimirski, A. K.; Levashov, A. V.; Berezin, I. V. In Micellization, Solubilization and Microemulsions; Mittal, K. L., Ed.; Plenum Press: New York, 1977; Vol. 2, p 489. (b) Romsted, L. S. In ref 4a, p 509. (c) Romsted, L. S. In Surfactant in Solution; Mittal, K. L., Lindman, B., Eds.; Plenum Press: New York, 1984; Vol. 2, p 1015.
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Catalyzed Cyclization
then depend upon concentrations of the reagent in each pseudophase.1,4 For some reactions the partitioning of the second reagent between water and micelles can be determined experimentally.1,4-8 However, even then there are questions regarding the meaning of concentration in the submicroscopic region provided by the micellar pseudophase. Another experimental approach is to trap a very reactive, and therefore unselective, intermediate at the micelle-water interface and to assume that selectivities are independent of environment. This method has been used with aryl cations generated by dediazonization,9 but it cannot be used in basic solution, which limits its applicability. A variety of theoretical treatments have been developed to estimate ionic distributions between water and micelles. For example, an ion-exchange model is based on the assumption that counterions compete at the micellar surface, as for an ion-exchange resin.1b-d,4b,c,10 Alternatively ionic distributions can be calculated by solution of the Poisson-Boltzmann equation in the appropriate symmetry, considering both ion-specific and nonspecific Coulombic interactions.11 On the basis of these treatments it is generally accepted that micellar rate enhancements of bimolecular reactions involving counterionic reactants are due largely to concentration of counterions at the micellar surface and that second-order rate constants in this region are similar to those in water, at least for reaction of nucleophilic anions. However, these conclusions depend on a variety of unproven assumptions regarding ionic concentrations at the micelle-water interface. First-order rate constants are concentration-independent and for an overall first-order reaction can be calculated unambiguously, provided that substrate distribution between water and micelles is known.1c,5 Values of k′m/k′w often differ from unity and may be larger or smaller by several orders of magnitude, depending on the reaction mechanism and surfactant charge.12 Therefore there is no a priori reason for assuming that second-order rate constants should be similar at micellar surfaces and in water. The similarity, or lack of it, could be due to our inability to estimate ionic concentrations at micellar surfaces. For example, if second-order rate constants are written with concentration expressed as molarity, comparison of these rate constants in water and micelles generally involves assumptions regarding the molar volume of the micellar reaction region, which is often identified with the Stern layer.1,2,4 These uncertainties disappear with reactions which are overall first order. We note the caveat that this region is assumed to be uniform, which is probably not strictly correct, in part because of (5) Sepu´lveda, L.; Lissi, E.; Quina, F. Adv. Colloid Interface Sci. 1986, 25, 1. (6) (a) Zana, R. J. Colloid Interface Sci. 1980, 78, 330. (b) Lianos, P.; Zana, R. J. Colloid Interface Sci. 1982, 88, 594. (7) Abuin, E. B.; Lissi, E.; Aranjo, R. S.; Aleixo, R. M. V.; Chaimovich, H.; Bianchi, N.; Miola, L.; Quina, F. H. J. Colloid Interface Sci. 1983, 96, 293. (8) (a) Bacaloglu, R.; Bunton, C. A.; Cerichelli, G.; Ortega, F. J. Phys. Chem. 1989, 93, 1490. (b) Blasko´, A.; Bunton, C. A.; Cerichelli, G.; McKenzie, D. C. J. Phys. Chem. 1993, 97, 11324. (9) (a) Chandhuri, A.; Romsted, L. S. J. Am. Chem. Soc. 1991, 113, 5052. (b) Chandhuri, A.; Longhlisi, J. A.; Romsted, L. S. J. Am. Chem. Soc. 1993, 115, 8351. (c) Chandhuri, A.; Romsted, L. S.; Yao, J. H. J. Am. Chem. Soc. 1993, 115, 8362. (10) Quina, F. H.; Chaimovich, H. J. Phys. Chem. 1979, 83, 1844. (11) (a) Bunton, C. A.; Moffatt, J. R. J. Phys. Chem. 1985, 89, 4166. (b) Bunton, C. A.; Moffatt, J. R. J. Phys. Chem. 1986, 90, 538. (c) Ortega, F.; Rodenas, E. J. Phys. Chem. 1987, 91, 837. (12) Bunton, C. A. In Nucleophilicity; Advances in Chemistry Series; Harris, I. M., McManus, S. P., Eds.; American Chemical Society: Washington, DC, 1987; Chapter 29.
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Langmuir, Vol. 12, No. 10, 1996 2349 Scheme 2
the way in which ionic concentrations change as a function of distance from the micellar surface.11,13 Our approach to this problem has been to study intramolecular cyclizations of 2-((3-halopropyl)oxy)phenoxide ion (halo ) Br, I; PhBr7 and PhI7, respectively) whose mechanism is, apart from molecularity, that of an SN2 displacement14 (Scheme 2). The mechanism of these reactions is well established, as are kinetic solvent effects.15 In particular, in water kI/kBr < 1 (kI and kBr are first-order rate constants for PhBr7 and PhI7, respectively), but kI/kBr > 1 in organic solvents.14,15 Protic solvents inhibit reaction by hydrogen bonding to phenoxide oxygen. It is offset by electrophilic assistance to bond breaking which will differ for bromides and iodides. As a result values of kI/kBr in various media may provide more useful information than the individual rate constants. Cationic micelles strongly bond PhBr7 and PhI7 and in the micellar pseudophase kBr < kI,14 consistent with water-micelle interfaces being less polar than water. However, k′m/k′w depends on the size of the surfactant head group. For small head groups, e.g., with CTABr (N-hexadecyl-N,N,N-trimethylammonium bromide), rate constants are similar to those in water, but they and kI/ kBr increase with increasing head group bulk.14 These differences can be ascribed to the effect of head group bulk on polarity at micelle-water interfaces.16,17 We use values of k′m/k′w and kI/kBr to monitor properties of micelle-water interfaces of formally neutral, zwitterionic, micelles of C12H25N+(CH3)2(CH2)3SO3-, SB3-12, and nonionic micelles of C12H25(OCH2CH2)23OH, Brij 35. Zwitterionic micelles of betaine surfactants behave similarly to cationic micelles in their effect on rates of spontaneous reactions.12,18 They increase rates of bimolecular reactions of nonionic substrates with moderately hydrophilic anions, e.g., Br-, and inhibit, but not suppress, reactions of very hydrophilic anions, e.g., OH- and F-.19 Consideration of electrostatic interactions indicates that these micelles should weakly attract anions and these (13) Gunnarsson, G.; Jonsson, B.; Wennerstro¨m, H. J. J. Phys. Chem. 1980, 84, 3114. (14) (a) 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. (b) Cerichelli, G.; Luchetti, L.; Mancini, G.; Savelli, G.; Bunton, C. A. J. Phys. Org. Chem. 1991, 4, 71. (c) Cerichelli, G.; Luchetti, L.; Mancini, G.; Savelli, G.; Bunton, C. A. J. Colloid Interface Sci. 1993, 160, 85. (15) Mandolini, L. Adv. Phys. Org. Chem. 1986, 22, 1. (16) We note that these generalizations have to be used cautiously in interpreting micellar rate effects on cyclizations of longer chain homologs of PhBr7 and PhI7. Under some conditions cationic surfactants speed these cyclizations17 but these rate enhancements are due to formation of premicellar complexes with substrates and micelles have only limited rate effects.3c (17) Wei, L.; Lucas, A.; Yue, J.; Lennox, R. B. Langmuir 1991, 7, 1336. (18) (a) Bunton, C. A.; Kamego, A. A.; Minch, M. J.; Wright, J. L. J. Org. Chem. 1975, 40, 1321. (b) Bunton, C. A.; Mhala, M. M.; Moffatt, J. R.; Wright, S. J. Org. Chem. 1985, 50, 4921. (c) Correia, V. P.; Cuccovia, I. M.; Stelmo, M.; Chaimovich, H. J. Am. Chem. Soc. 1992, 114, 2144. (19) (a) Bunton, C. A.; Mhala, M. M.; Moffatt, J. R. J. Org. Chem. 1987, 52, 3832. (b) Bunton, C. A.; Mhala, M. M.; Moffatt, J. R. J. Phys. Chem. 1989, 93, 854. (20) Baptista, M. S.; Chaimovich, H.; Politi, M. J.; Reed, W. F. J. Phys. Chem. 1992, 96, 6442.
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Cerichelli et al.
Figure 1. Variation of kobs for the cyclization of PhBr7 (b) and PhI7 (2) (left axis) and variation of ratio kI/kBr (0) (right axis) with [SB3-12].
Figure 2. Variation of kobs for the cyclization of PhBr7 (b) and PhI7 (2) (left axis) and variation of ratio kI/kBr (0) (right axis) with [Brij 35].
attractions have been treated theoretically.20 However, it is difficult to develop general experimental methods for the estimation of ionic concentrations at surface of these micelles and quantitative treatments of rate data. Nonionic micelles of surfactants such as Brij 35 have little effect on reactions of nucleophilic anions with some organic substrates, but they inhibit reactions of very hydrophobic substrates.1,21 We do not know the extent to which these rate effects depend upon transfer equilibria between aqueous and micellar pseudophases and secondorder rate constants in the latter. Cyclizations of PhBr7 and PhI7 are models of bimolecular anion-molecule reactions in nonionic micelles and should provide an answer to this question.
Table 1. Effect of Concentration of SB3-12 on 1H NMR Line Widtha
Results Kinetics. The dependence of kobs on [SB3-12] is shown in Figure 1. There is no rate effect at surfactant concentration below the cmc which is 2.1 × 10-3 in water22 and kobs increases at ca. 5 × 10-3 and 3 × 10-3 M SB3-12 for reactions of PhBr7 and PhI7, respectively, and corresponding values of kobs become constant at ca. 0.2 and 0.1 M SB3-12 (Table S1, supporting information). As expected, rate increases are much steeper with cationic micelles which interact strongly with these anionic substrates.14 The rate enhancements, k′m/k′w of ca. 2 and 5 for PhBr7 and PhI7, respectively, are larger than those given by cationic micelles of CTABr and DoTABr (N-dodecyl-N,N,N-trimethylammonium bromide) but are smaller than those given by surfactants with bulky head groups.14 Brij 35 also increases kobs, but to a lesser extent than SB3-12, as shown in Figure 2, and rate constants do not become strictly constant even with 0.1 M Brij 35, where k′m/k′w ) 1.2 and 2.5 for PhBr7 and PhI7, respectively (Table S2, supporting information). Although reaction of fully-bound substrate is faster in betaine sulfonate than in CTABr micelles, the binding is much weaker. The binding constant to micellized surfactant is given, to a first approximation, by the reciprocal of micellized surfactant concentration at half the micellar rate enhancement.12,21a For reactions of PhBr7 and PhI7 in CTABr, this concentration is less than 3 × 10-4 M, i.e., less than the cmc in water, probably because these anionic substrates induce micellization or premicelles catalyze (21) (a) Cordes, E. H.; Gitler, C. Prog. Bioorg. Chem. 1973, 2, 1. (b) Bunton, C. A.; Robinson, L. J. Am. Chem. Soc. 1968, 90, 5972. (c) Bunton, C. A.; Robinson, L. J. Am. Chem. Soc. 1969, 34, 773. (22) Frescura, V. L. A.; Marconi, D. M. O.; Zanette, D.; Nome, F.; Blasko´, A.; Bunton, C. A. J. Phys. Chem. 1995, 99, 11494.
[SB3-12], M
dioxane
N(CH3)2
CH2SO3- b
ω-CH3b
0.0004 0.0016 0.043 0.055 0.067 0.08c 0.40 1.0
0.7 0.7 0.5 0.5 0.6 0.9 0.5 0.7
1.7 1.6 2.4 2.4 2.6 2.6 2.6 2.7
1.5 1.4 2.2 2.2 2.4 2.4 2.4 2.4
1.5 1.5 2.2 2.5 2.4 2.5 2.5 2.1
a Values of line width, Hz, at 25.0 °C. b Line widths at the center of the triplet signal. c Poor field homogeneity in this experiment.
reaction.3c The corresponding concentrations are much higher for SB3-12, being approximately 6 × 10-3 M for PhBr7 and 7 × 10-3 M for PhI7. For reaction of PhI7 the surfactant concentration at half the maximum rate increase is >10-2 M, indicating that the anionic substrates do not bind strongly to these nonionic micelles. NMR Spectrometry of SB3-12. Chemical shifts and line widths change as micelles form. They may also change at high [surfactant] due simply to a medium change; e.g., for a molecular weight of 300, 1 M surfactant corresponds to ca. 30 wt % surfactant. We therefore focus attention on NMR spectra with 3.2, does not readily hydrogen bond to phenoxide oxygen, but it should solvate the transition state of the iodide more strongly than that of the bromide. Dipolar aprotic solvents have especially large effects upon values of kI/kBr as well as upon rates of cyclization.15 The highest value of kI/kBr ) 2.46 in micelles is with CTBABr, because the butyl groups reduce polarity and availability of water at the micellar surface (Table 2). Values are significantly higher than that in water and are similar to those in low molecular weight alcohols, and this generalization also applies to the betaine surfactants 3 and SB3-12. However, the value of kI/kBr ) 1.18 in Brij 35 is lower than those for the other surfactants and is indicative of a “wetter” reaction environment than that provided by them. The change from kI/kBr ) 0.6 in water to >1 in alcohols and at micellar surfaces illustrates the balance between initial state solvation, largely of phenoxide ion, and transition state solvation, which also applies at micelle-water interfaces. (28) (a) Ferreia, L. C. M.; Zucco, C.; Zanette, D.; Nome, F. J. Phys. Chem. 1992, 96, 9058. (b) Rodenas, E.; Vera, S. J. Phys. Chem. 1985, 89, 513. (c) Bunton, C. A.; Moffatt, J. R. Langmuir 1992, 8, 2130. (d) Rubio, D. A. R.; Zanette, D.; Nome, F.; Bunton, C. A. Langmuir 1994, 10, 1155.
Cerichelli et al. Table 2. Medium Effect on kI/kBra 104kobs, s-1 solventb
surfactantc
kI/kBr
C16H33NMe3Br C16H33NEt3Br C16H33NPr3Br C16H33NBu3Br C12H25NMe3Br C16H33N+Me2CH2CO2(C16H33NMe2)2(CH2)32Br C12H25N+Me2(CH2)3SO3Brij 35
0.60 1.56 1.55 1.47 3.03 1.34 1.63 2.00 2.46 1.24 1.75 1.53 1.50 1.2
H2O MeOH EtOH 2-PrOH MeCN (moist)
PhI7
PhBr7
1.40 2.33 1.58 1.02 8.98 5.80 22.0 15.0 320 107 5.50 4.10 10.0 6.10 26.0 13.0 41.0 16.7 3.75 3.02 9.02 5.16 15.8 10.4 7.10 4.85 ≈3.5 >2.9
a At 25.0 °C. b Reference 14a. c For fully bound PhX7, ref 14 and this work.
Conclusions The widely applied pseudophase model of the effects of association colloids on reaction rates and equilibria is satisfactory for dilute solutions of amphiphiles and reactant.1,2,4 The distinction between aqueous and micellar pseudophases begins to break down with moderately concentrated surfactant, especially when it has a high molecular weight, e.g., as with Brij 35, and especially if there is an extensive region, the polyoxyethylene, palisade, layer, which is freely accessible to water. Another example of this breakdown is provided by ionic micelles with added electrolyte. In dilute electrolyte there is a strong concentration gradient of counterions between the micellar surface and bulk water, which decreases significantly with more concentrated electrolyte.1,4b,c Except for very hydrophilic anions, e.g., OH- and F-, counterion concentrations at micellar surfaces change only slowly with increasing total [counterion], and the concentration gradient essentially disappears at high, ca. 3 M, electrolyte.9,28 As a result micellar rate enhancements of bimolecular, counterionic, reactions decrease sharply, relative to rates in water, with an increase in counterion concentration. Experimental Section Materials. Preparation and purification of 2-((3-bromopropyl)oxy)phenol and 2-((3-iodopropyl)oxy)phenol have been described.14a Dodecyldimethylammoniopropanesulfonate (SB312) and polyoxyethylene(23) dodecyl ether (Brij 35) are commercial materials (Aldrich). Kinetics. Reactions were followed at 25.0 °C in deionized, distilled, CO2-free water is described.14a NMR. 1H and 13C NMR measures have been carried out on Bruker AC300P instrument operating at 6.96 T.
Acknowledgment. Support of this work by CNR (Roma), the MURST, and the US Army Office of Research is gratefully acknowledged. Supporting Information Available: Tables of rate constants of cyclization in SB3-12 and Brij 35 and effect of concentration of SB3-12 on 1H and 13C NMR chemical shift (5 pages). Ordering information is given on any current masthead page. LA950534R
