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Surfactant Effects on Decarboxylation of Alkoxynitrobenzisoxazole-3-carboxylate Ions. Acceleration by Premicelles† Lucia Brinchi,‡ Pietro Di Profio,‡ Raimondo Germani,‡ Vittoria Giacomini,‡ Gianfranco Savelli,*,‡ and Clifford A. Bunton§ Dipartimento di Chimica, Universita` di Perugia, Via Elce di Sotto, 8, I-06123 Perugia, Italy, and Department of Chemistry, University of California, Santa Barbara, California 93106 Received July 13, 1999. In Final Form: November 10, 1999 Decarboxylation of 6-nitro-5-alkoxybenzisoxazole-3-carboxylate ion (1,OMe and 1,OTD, alkoxy ) MeO, n-C14H29O, respectively) is accelerated by cationic micelles of cetyltrialkylammonium bromide (CTABr, CTEABr, CTPABr, CTBABr, alkyl ) Me, Et, n-Pr, n-Bu respectively). The first-order rate constants kobs for reaction of 1,OMe increase monotonically with [surfactant] and become constant when the substrate is fully bound and kobs ) k′M. The values of kobs for 1,OTD increase sharply with increasing [surfactant] and reach well-defined maxima at [surfactant] at or below the cmc, before decreasing to values corresponding to kobs ) k′M. The magnitude of the rate maxima and the increase in k′M are in the sequence CTABr < CTEABr < CTPABr < CTBABr. The rate maxima are due to formation of premicellar complexes of substrate with one or a few surfactant monomers, but they “dissolve” in micelles at higher [surfactant]. A rationalization is provided for acceleration by very dilute surfactants.
Introduction Association colloids are assemblies of amphiphilic monomers with an ionic or polar head group and apolar residues.1 Self association in water, and some organic solvents, for example, diols and triols, generates micelles with head groups in contact with the solvent. Micelles can act as microreactors in influencing reaction rates and equilibria,2 and similar behavior is observed with other association colloids, for example, vesicles3 and microemulsions.4 Very dilute amphiphiles, for example, surfactants, do not significantly affect reaction rates of many reactions, but with increasing concentrations, micelles form and rate or equilibrium constants change.2 In micellar-accelerated spontaneous reactions, rate constants reach plateau values, but for bimolecular reactions with a constant concentration of reagent, they typically go through maxima. These changes of rate or equilibrium constants are described in terms of a two-state model with water and micelles behaving as discrete reaction regions or pseudophases.2-5 Micellization occurs at the critical mi* Corresponding author. E-mail:
[email protected]; Telephone: +39-075-585-5538. Fax: +39-075-585-5560. † Part of the Special Issue “Clifford A. Bunton: From Reaction Mechanisms to Association Colloids; Crucial Contributions to Physical Organic Chemistry”. ‡ Universita ` di Perugia. § University of California, Santa Barbara. (1) (a) Tanford, C. The Hydrophobic Effect: Formation of Micelles and Biological Membranes, 2nd ed.; Wiley: New York, 1980. (b) Fendler, J. H. Membrane Mimetic Chemistry; Wiley: New York, 1982. (c) Israelachvili, J. N. Intermolecular and Surface Forces; Academic Press: London, 1985. (2) (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) Romsted, L. S.; Bunton, C. A.; Yao, J. Curr. Opin. Colloid Interface Sci. 1997, 2, 622. (d) Bunton, C. A. J. Mol. Liq. 1997, 72, 231. (3) Chaimovich, H.; Cuccovia, I. M. Prog. Colloid Polym. Sci. 1997, 103, 67. (4) (a) Menger, F. M.; Elrington, A. R. J. Am. Chem. Soc. 1991, 113, 9621. (b) Chaudhuri, A.; Loughlin, J. A.; Romsted, L. S. J. Am. Chem. Soc. 1993, 115, 8351. (c) Holmberg, K. Adv. Colloid Interface Sci. 1994, 51, 137. (d) Schwuger, N. L.; Stickdorn, K.; Schomaker, R. Chem. Rev. 1995, 95, 849.
celle concentration, cmc, which in some systems marks the onset of the rate increase, but often rates increase below the cmc, either because reactants induce micellization or because other species, so-called premicelles, are kinetically effective. As a result, a “kinetic” cmc is often used empirically in fitting rate-surfactant profiles, and Buckingham et al. note physical evidence for the possible existence of premicelles.6 The term “premicelles” seems to be applied to any submicellar assemblies which form spontaneously or are generated by interactions with reactants. In the latter case it is not easy to distinguish between reactant-induced micellization and formation of premicelles.7 The change from monomeric surfactant to micelles occurs over a small range of concentration which is sensitive to ionic and nonionic solutes, and there is a dispersion of micellar size which is also sensitive to solutes. However, the simple pseudophase model, despite neglecting these complications, fits kinetic data for many reactions, except in dilute surfactant. Nonmicellar assemblies can influence reaction rates. Hydrophobic ammonium ions, which do not micellize, often increase reactivities, although generally concentrations are too low to allow physical identification of the assemblies.8,9 In a few reactions in surfactants which generate micelles, there are extrema in plots of rate constants against [surfactant] at concentrations near to, or below, the cmc.10,11 These extrema cannot be ascribed to reactant-induced micellization, which gives monotonic changes in observed rate constants. It is easiest to identify this kinetic behavior in spontaneous, unimolecular, reac(5) (a) Rodenas, E.; Vera, S. J. Phys. Chem. 1985, 89, 513. (b) Amado, S.; Garcia-Rios, L.; Leis, J. R.; Rios, A. Langmuir 1997, 13, 687. (6) Buckingham, S. A.; Garvey, C. J.; Warr, G. G. J. Phys. Chem. 1993, 97, 10236. (7) Drennan, C. E.; Hughes, R. J.; Reinsborough, V. C.; Soriyan, O. O. Can. J. Chem. 1998, 76, 152. (8) Kunitake, T.; Okahata, Y.; Ando, R.; Shinkai, S.; Hirakawa, S. J. Am. Chem. Soc. 1980, 102, 7877. (9) (a) Bunton, C. A.; Hong, Y. S.; Romsted, L. S.; Quan, C. J. Am. Chem. Soc. 1981, 103, 5784. (b) Biresaw, G.; Bunton, C. A.; Quan, C.; Yang, Z.-Y. J. Am. Chem. Soc. 1984, 106, 7178. (c) Biresaw, G.; Bunton, C. A. J. Phys. Chem. 1986, 90, 5849; (d) 1986, 90, 5854.
10.1021/la9909502 CCC: $19.00 © 2000 American Chemical Society Published on Web 12/03/1999
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tions where only one species is partitioned between water and micelles or other assemblies. For example, in bimolecular ionic reactions one has to consider electrolyte effects on the cmc and competition between reactive and inert ions for the association colloids.2 The pseudophase model predicts that overall first-order rate constants, kobs, for a spontaneous micellar-accelerated reaction should increase monotonically until all the substrate is micellar-bound.2 Conversely, for an inhibited reaction kobs decreases monotonically to a constant value. This behavior is fitted by eq 1:
kobs )
k′W + k′MKS[Dn] 1 + KS[Dn]
Scheme 1
(1)
In eq 1 k′W and k′M are first-order rate constants in the aqueous and micellar pseudophases, respectively, and KS is the association constant of substrate S with micellized surfactant (detergent) Dn, whose concentration is the total less that of monomer, given by the cmc.12 This kinetic form is observed for many spontaneous reactions, although often kobs increases at surfactant concentrations below the cmc, and occasionally values of kobs increase sharply with [D] < cmc but then decrease and follow eq 1 at higher concentrations.10 This anomalous behavior has been observed in two spontaneous reactions. The first is decarboxylation of 6-nitrobenzisoxazole-3-carboxylate ion (1,H) in a solution of didodecyldimethylammonium chloride,10a and the second is cyclization of o-(ω-haloalkoxy)phenoxide (2) with a long-chain alkyl tether in solutions of cationic surfactants10b (Scheme 1). These reactions have been studied extensively in a range of solvents and in the presence of association colloids, and their mechanisms are well understood.13,14 Typically their behavior in the presence of association colloids is described by eq 1. Decarboxylation of 1,H is accelerated by cationic and zwitterionic surfactants at concentrations well below the cmc, but values of kobs increase monotonically and we cannot distinguish between substrate-induced micellization and premicellar-assisted reaction. The same considerations apply to cyclization of analogues of 2 with n ) 3. Our speculation was that these “premicellar” rate effects on cyclizaton require a hydrophobic interaction between substrate and, at most, a limited number of surfactant monomers. We therefore prepared alkoxy derivatives of 1,H, with groups of various length with the aim of seeing (10) (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) Cerichelli, G.; Mancini, G.; Luchetti, L.; Savelli, G.; Bunton, C. A. Langmuir 1994, 10, 3982. (11) (a) Blasko`, A.; Bunton, C. A.; Foroudian, H. J. Colloid Interface Sci. 1994, 163, 500. (b) Bacaloglu, R.; Bunton, C. A. J. Colloid Interface Sci 1992, 153, 140. (12) Menger, F. M.; Portnoy, C. E. J. Am. Chem. Soc. 1967, 89, 4698. (13) (a) Paul, D. S.; Kemp, K. G. J. Am. Chem. Soc. 1975, 97, 7305. (b) Bunton, C. A.; Minch, M. J.; Hidalgo, J.; Sepulveda, L. J. Am. Chem. Soc. 1973, 95, 3262. (c) 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. (d) Patel, M. S.; Bijma, K.; Engberts, J. B. F. N. Langmuir 1994, 10, 2491. (e) Di Profio, P.; Germani, R.; Savelli, G.; Cerichelli, G.; Spreti, N.; Bunton, C. A. J. Chem. Soc., Perkin Trans. 2 1995, 1505. (14) (a) Mandolini, L. Adv. Phys. Org. Chem. 1986, 22, 1. (b) 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. (c) Cerichelli, G.; Luchetti, L.; Mancini, G.; Savelli, G.; Bunton, C. A. Langmuir 1996, 12, 2348. (15) (a) Lindemann, H.; Cissee, H. Justus Liebigs Ann. Chem. 1929, 44, 469. (b) Borche, W. Chem. Ber. 1909, 42, 1316. (16) 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.
rate extrema in decarboxylation (Scheme 1). This reaction is very sensitive to substituent electronic effects and is inhibited by alkoxy groups.13a We used two alkoxy substituents, OMe and O(CH2)13Me, and substrates are designated as 1,OMe and 1,OTD. So far as we know, hydrophobic benzisoxazole carboxylate ions have not been prepared13a,b,15 and synthesis was a key and difficult part of this work. The surfactants are n-C16H33NR3Br (R ) Me, Et, n-Pr, n-Bu, CTABr, CTEABr, CTPABr, and CTBABr, respectively. Decarboxylation of the unsubstituted substrate 1,H had been examined in these surfactants.16 Values of kobs increased monotonically, as predicted by eq 1, and the plateau values k′M increased with increasing head group bulk, which decreases the polarity in the interfacial region. The rate constants increased monotonically at [surfactant] below the cmc in water, which was not unexpected because the anionic substrates could induce micellization, and the increases do not necessarily show that substrate is associated with premicelles. Results and Discussion Reactivities in Water. Decarboxylation is significantly inhibited by a 5-alkoxy group, and we could not obtain values of kobs in water directly at 25.0 °C. On the basis of the value of kobs for reaction of 1,H in water at 25.0 °C and the substituent effects reported by Kemp and Paul,13a we estimate that kobs for decarboxylation of the 5-methoxy derivative will be approximately 2 × 10-7 s-1. Reaction of 1,OMe was followed at 40.0, 45.0, and 50.0 °C, and extrapolation by using the Arrhenius equation gives kobs ) 1.2 × 10-7 s-1 at 25.0 °C in water, but there is uncertainty in this value because of the limited range of temperature. We could not examine decarboxylation of 1,OTD in water because of its low solubility. The alkoxy groups have similar electronic effects, and kobs should be similar for the methoxy and tetradecyloxy derivatives. Kinetics of Decarboxylation in Surfactants. Introduction of a methoxy substituent decreases the rate constants of reaction of fully bound substrates in micelles (Table 1), as predicted,13a and surfactant effects are similar to those for the unsubstituted derivative,16 except that micellar rate enhancements are lower. However, the behavior of the tetradecyloxy derivative 1,OTD is strikingly different. The values of kobs are much higher than expected from the behavior of the methoxy derivative in dilute surfactant at concentrations well below the cmc and then decrease and become consistent with predictions based on eq 1 (Figure 1 and Table S1). The surfactant
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Table 1. Variation of kobs with Surfactant Structure and Concentration for Reaction of 1,OMea 103[surfactant], M 0.2 0.6 1.0 5.0 15 20 30 40 a
CTABr
CTEABr
CTPABr
CTBABr
surfactant
1,Hb
1,OMec
5.73 7.75 7.24 5.32
1.37 7.70 9.60 10.3 9.30
4.60 14.6 14.0 14.7 13.9
10.3 15.3 16.0 18.3
CTABr CTEABr CTPABr CTBABr
33 (100) 100 (330) 410 (1400) 850 (2800)
0.48 (24) 0.80 (40) 1.43 (71) 1.93 (96)
5.06 4.61
7.80 8.26
13.7 15.0
20.0 18.5
-1 -4 M substrate and obs, s , at 25.0 °C, with 1 × 10 NEt3; the estimated value of kW is 0.2 × 10-6 s-1.
Values of 106k
7 × 10-3 M
Table 2. First-Order Rate Constants for Decarboxylations of 1,H and 1,OMea
Figure 1. (a) Decarboxylation of 1,OTD in CTABr (9) and CTEABr (2) and (b) in CTPABr (1) and CTBABr (b). The lines are drawn to guide the eye, and the rate constant in water is very close to zero; see text.
concentrations at the rate maxima are in the sequence CTABr ≈ CTEABr > CTPABr > CTBABr. Values of kobs increase so steeply in dilute CTPABr and CTBABr (Figure 1a) that we can observe only the descending part of the plots because of very low solubilities. The rate maxima in CTABr and CTEABr (Figure 1b) are at a [surfactant] which allows us to obtan data for both the ascending and descending parts of the plots. Replacement of a neutral base, Et3N, by NaOH affects the values of kobs in very dilute surfactant but has little effect at higher [surfactant], where reaction is in fully formed micelles.2 These differences indicate that ions affect the forms of these rate maxima which are observed in very dilute surfactant (
