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Micellar Charge Effects upon Hydrolyses of Substituted Benzoyl Chlorides. Their Relation to Mechanism† Clifford A. Bunton,*,‡ Nicholas D. Gillitt,‡ Marutirao M. Mhala,‡ John R. Moffatt,‡ and Anatoly K. Yatsimirsky*,⊥ Department of Chemistry and Biochemistry, University of California, Santa Barbara, California, 93106, and Facultad de Quimica, Universidad Nacional Autonoma de Mexico, Mexico City, DF 04510, Mexico Received January 28, 2000. In Final Form: May 2, 2000 Micellar rate effects on hydrolyses of substituted benzoyl chlorides, 1, depend on headgroup charge and electron donation or withdrawal by substituents. Micellized sodium dodecyl sulfate, SDS, inhibits hydrolyses, and first-order rate constants in the micellar pseudophase, k′M, decrease, relative to those in water, k′W, over a range of ca. 10, but in cetyl trimethylammonium chloride, CTACl, k′M/k′W > 1 for hydrolyses of 1,3,5-(NO2)2 and 1, 4-NO2, and decreases steeply with electron-donating substituents in the following sequence: 1,4-Cl ≈ 4-Br > 4-H > 4-Me > 4-OMe, over a range of >103. Cetyl trimethylammonium bromide and mesylate behave like CTACl. Fits to the Hammett equation give F ≈ 1 in SDS and F ≈ 4 in CTACl. Anionic micelles have higher interfacial polarities than cationic micelles, but micellar and solvent effects do not correspond because over a range of solvents, H2O to H2O-MeCN, 1:1 w/w plots of log k′W against σ go through minima with positive F for 1, 3,5-(NO2)2, and 4-NO2 and negative for the other substrates. The micellar effects correspond to differing extents of bond-making and -breaking in the transition state. Values of k+/k- (rate constants in CTACl and SDS) decrease strongly with increasing electron donation by substituents. Micellar rate effects in hydrolyses of benzyl bromide and 4-methoxybenzyl chloride are similar to those with the benzoyl chlorides. Although data were analyzed by a pseudophase treatment, application of transition state theory and reported micellar surface potentials allows estimation of local charge at the reaction center for hydrolyses of the benzoyl chlorides.
There is extensive evidence on the ability of aqueous micelles and other association colloids to influence reaction rates and equilibria, and concentration, or depletion, of reactants in the interfacial region have major effects on rates of bimolecular reactions.1 However, this region differs from water as a reaction medium,2-4 which can affect rates of spontaneous reaction. The effects depend on the transfer of substrate from water to micelles, the reaction mechanism, and properties of the interfacial region, e.g., local charge, and polarity and water content, which are lower than in water. Spontaneous reactions that are accelerated by decreases in solvent polarity, e.g., anionic decarboxylation and dephosphorylation, are micellar-accelerated, and most spontaneous hydrolyses that are accelerated by increases in solvent polarity are micellar-inhibited.1d,f,g,4-6 † Part of the Special Issue “Colloid Science Matured, Four Colloid Scientists Turn 60 at the Millennium”. ‡ University of California. § Deceased. ⊥ Universidad Nacional Autonoma de Mexico.
(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) Martinek, K.; Yatsimirsky, 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. (d) Bunton, C. A.; Savelli, G. Adv. Phys. Org. Chem. 1986, 22, 213. (e) Bunton, C. A.; Nome, F.; Quina, F. H.; Romsted, L. S. Acc. Chem. Res. 1991, 24, 357. (f) Tascioglu, S. Tetrahedron 1996, 52, 11113. (g) Bunton, C. A. J. Mol. Liq. 1997, 72, 231. (h) Romsted, L. S.; Bunton, C. A.; Yao, J. Curr. Opin. Colloid Interface Sci. 1997, 2, 622. (2) (a) Zachariasse, K. A.; Phuc, N. Y.; Kozankiewicz, B. J. Phys. Chem. 1981, 85, 2676. (b) Ramachandran, C.; Pyter, R. A.; Mukerjee, P. Phys. Chem. 1982, 86, 3198. (c) Novaki, L. P.; El Seoud, O. A. Phys. Chem. Chem. Phys. 1999, 1, 1957. (3) (a) Chaudhuri, A.; Loughlin, J. A.; Romsted, L. S.; Yao, J. J. Am. Chem. Soc. 1993, 115, 8351. (b) Romsted, L. S.; Yao, J. Langmuir 1996, 12, 2425 and references cited therein. (4) Buurma, N. J.; Herranz, A.; Engberts, J. B. F. N. J. Chem. Soc., Perkin Trans. 2 1999, 113, and references cited therein.
Comparisons of kinetic solvent and micellar effects on spontaneous hydrolyses provide evidence on the structures of interfacial regions, especially with regard to changes in the headgroups. This kinetic evidence complements physical evidence and that from the use of spectral, or other, probes.2,3 The polarities of the interfacial regions can be compared with those of bulk solvents in terms of ET(30) or effective dielectric constant, which agrees with independent evidence that these regions are partially depleted in water.3 Hydrolysis of methylnaphthalene-2-sulfonate follows the SN2 mechanism and is inhibited by decreases in solvent polarity and water content and by ionic and zwitterionic sulfobetaine micelles.7 Anionic micelles of SDS inhibit reaction more than cationic and sulfobetaine micelles, showing that micellar charge, as well as polarity, is important. Consistently SN11d,6,8,9 reactions are strongly micellar-inhibited, but inhibition by SDS is less than that by cationic or sulfobetaine micelles,9 although kinetic solvent effects are qualitatively similar to those on spontaneous SN2 reactions.10 Sulfobetaine micelles are (5) (a) Possidonio, S.; El Seoud, O. A. J. Mol. Liq. 1999, 80, 231. (b) Possidonio, S.; Siviero, F.; El Seoud, O. A. J. Phys. Org. Chem. 1999, 12, 325. (6) (a) Bunton, C. A. In Nucleophilicity; Harris, J. M., McManus, S. P., Eds.; Advances in Chemistry Series 215; American Chemical Society: Washington, D. C., 1987; Chapter 29. (b) Bunton, C. A.; Mhala, M. M.; Moffatt, J. R. In Surfactants in Solution; Mittal, K. L., Bothorel, P., Eds.; Plenum Press: New York, 1986; Vol. 5, p 625. (7) Brinchi, L.; Di Profio, R.; Germani, R.; Savelli, G.; Spreti, N.; Bunton, C. A. J. Chem. Soc., Perkin Trans. 2 1998, 361. (8) Menger, F. M.; Yoshinaga, H.; Venkatasubban, K. S.; Das, A. R. J. Org. Chem. 1981, 46, 415. (9) (a) Al-Lohedan, H.; Bunton, C. A.; Mhala, M. M. J. Am. Chem. Soc. 1982, 104, 6654. (b) Bunton, C. A.; Ljunggren, S. J. Chem. Soc., Perkin Trans. 2 1984, 355. (10) Ingold, C. K. Structure and Mechanism in Organic Chemistry, 2nd ed.; Cornell University Press: Ithaca, New York, 1969; Chapter 7.
10.1021/la000109k CCC: $19.00 © 2000 American Chemical Society Published on Web 07/12/2000
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Bunton et al.
Scheme 1
reactions depend on local concentration of the added reactant in water and micelles.1,14 Reactions are
formally uncharged, but they behave like cationic micelles in their effects on rates of hydrolyses and other spontaneous reactions.6,7,9 Spontaneous solvolyses of acyl halides do not fit readily into the concepts of the SN1-SN2 duality of mechanism developed for reactions at alkyl centers, or of the additionelimination mechanism of deacylation.11,12 Substituent effects, which are typically described by the value of F from the Hammett equation, vary in both sign and magnitude, depending on substituents, leaving groups, and the reaction medium. Solvents play a variety of roles, e.g., as nucleophiles, as general bases, and by interacting with developing ionic centers. There is extensive mechanistic work in this area, especially in delineation of the SN1-SN2 and addition-elimination reaction paths that were recently critically discussed by Kevill and Wang.12 Spontaneous hydrolyses of carboxylic anhydrides, diaryl carbonates, and acyltriazoles are micellar-inhibited, but, as for SN2 hydrolysis, inhibition is greater with anionic than with cationic, or sulfobetaine, micelles.1d,4-6,8,9a It appears that reactions in which there is extensive nucleophilic intervention by water in formation of the transition state are most inhibited by anionic micelles, probably because of development of negative charge in the organic residue. The situation for hydrolyses of acyl chlorides is complex, because cationic micelles accelerate hydrolyses when substrates contain strongly electron-withdrawing substituents but otherwise micelles inhibit reactions. Hydrolyses of aryl chloroformates and acyl chlorides are affected similarly by aqueous micelles.1d,5,6,9a A systematic study of micellar and solvent effects upon these spontaneous hydrolyses should provide evidence on both reaction mechanism and the nature of the interfacial region as a reaction medium. Micellar effects upon overall first-order rate constants, kobs, of spontaneous reaction are typically described as shown in Scheme 1.1 Substrate, S, associates with micellized surfactant (detergent), Dn, with an association constant, KS, k′W and k′M are, respectively, first-order rate constants in the aqueous and micellar pseudophases, and [Dn] is [DT] less the concentration of monomeric surfactant, which is assumed to be the critical micelle concentration, cmc, under the reaction conditions.13 First-order rate constants are given by
Substrates are designated, 1,4-Z or 1,3,5-Z2, where Z ) NO2, Cl, Br, H, Me, OMe. Some of the hydrolyses are too fast to be followed in water; we therefore followed them in H2O-MeCN and estimated k′W by extrapolation and also obtained information on kinetic solvent effects. Some data on hydrolyses of benzyl halides, 2-Z, Z ) 4-OMe, H, halide ) Cl, Br, are included for comparison. The surfactants are typically cetyl trimethylammonium chloride, n-C16H33NMe3Cl, CTACl, or sodium dodecyl sulfate, n-C12H25OSO3Na, SDS, but we used CTABr or CTAOMs in a few experiments. (OMs ≡ O3SMe). The pseudophase model treats water and micelles as distinct reaction regions (Scheme 1 and eq 1). Provided that the value of k′W is known, eq 1 can be linearized as13
kobs )
k′W + k′MKS[Dn] 1 + KS[Dn]
(1)
First-order rate constants of bimolecular, nonspontaneous, (11) (a) Kivinen, A. In The Chemistry of Acyl Halides; Patai, S., Ed.; Interscience: New York, 1972; Chapter 6. (b) Talbot, R. J. E. In Comprehensive Chemical Kinetics; Bamford, C. H., Tipper, C. F. H., Eds.; Elsevier: New York, 1972;, Vol. 10, p 226. (12) Kevill, D. N.; Wang, W. F. K. J. Chem. Soc., Perkin Trans. 2 1998, 2631 and references cited therein. (13) Menger, F. M.; Portnoy, C. E. J. Am. Chem. Soc. 1967, 89, 4698.
1 1 1 ) + (2) (k′W - kobs) (k′W - k′M) (k′W - k′M)KS[Dn] which, in effect, allows extrapolation to infinite [surfactant] but is unsatisfactory when k′W . k′M.8 A simpler extrapolation, which does not depend on k′W, uses data at high [surfactant] where KS[Dn] . 1, giving
kobs )
k′W KS[Dn]
+ k′M
(3)
i.e., as 1/[Dn] f 0, k′M ) kobs. These methods give considerable weight to data at high [surfactant] and another approach involves simulation of the dependence of kobs on [Dn], eq 1. All these treatments involve the assumption that the rate and equilibrium constants in eq 1 remain constant over the experimental conditions, and limitations of the treatment are considered later in the context of hydrolyses of specific substrates. Experimental Section Materials. Surfactants were materials used earlier;9 most of the substrates were commercial samples (Aldrich) and were recrystallized or vacuum-distilled. 4-Methoxybenzoyl chloride was prepared from the acid and SOCl2 and vacuum-distilled, bp., 91 °C at 1 mm. 4-Methoxybenzyl chloride prepared from the alcohol and HCl in Et2O and chromatographed on SiO2 gel with elution by petroleum ether (bp. 30-60 °C), was purified by vacuum distillation, bp. 82-83 °C at 8 mm. MeCN (spectral grade) was purified as described.15 Deionized H2O was redistilled, and reactions were in dilute HCl, 10-3 M, unless specified, to eliminate reaction with adventitious base. Kinetics. Hydrolyses were followed spectrophotometrically following the general methods described earlier,7,9 in a Beckman spectrometer or an HP 8451A diode-array spectrometer for the faster reactions. All reactions were at 25.0 °C. Substrate was added in MeCN, the reaction solution contained 0.2 vol % MeCN, and [substrate] was between 5 × 10-6 and 10-4 M, depending on the change of absorbance during reaction and its rate. We used (14) (a) Romsted, L. S. In Micellization, Solubilization and Microemulsions; Mittal, K. L., Ed.; Plenum Press: New York, 1977; Vol. 2, p 509. (b) Romsted, L. S. In Surfactants in Solution; Mittal K. L., Lindman, B., Eds.; Plenum Press: New York, 1984; vol. 2, p 1015. (15) Coetzee, J. F.; Cunningham, G. P.; McGuire, D. K.; Padnambham, G. R. Anal. Chem. 1962, 34, 1139.
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Table 1. Hydrolyses of Benzoyl Chlorides in Water and Ionic Micellesa k′M, s-1 substituent 3,5-(NO2)2 4-NO2 4-Cl 4-Br 4-H 4-Me 4-OMe
k′W,
s-1
0.20 0.053 0.214 0.190 1.41 5 ca. 11
Ks, M-1
CTACl >0.5b 0.11 (0.11)c 0.012 (0.011)d 0.009 (0.011) 0.004 (0.007) 0.0015 (0.002) 8 16 4 3.6 0.27 0.038 -0.2 (>-0.28) -0.25 (-0.36) -0.12 (-0.17) -0.11 (-0.16)
4-H 4-Me 4-OMe
0.13 (0.18) 0.29 (0.41)
