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Langmuir 2000, 16, 72-75
Effect of Premicellar Complexes and Micellar Headgroup Size upon the Basic Hydrolysis of 2-(4-Chlorophenoxy)quinoxaline† Angela Cuenca Departamento de Quı´mica, Universidad Simo´ n Bolı´var, Apartado 89000, Caracas 1080-A, Venezuela Received March 22, 1999. In Final Form: October 1, 1999 Rates of the basic hydrolysis of 2-(4-chlorophenoxy)quinoxaline 1 were measured in aqueous cetyltrialkylammonium chlorides, C16H33NR3Cl, R ) Me (CTACl), Et (CTEACl), n-Pr (CTPACl), n-Bu (CTBACl). The shape of the rate versus surfactant concentration profiles depends on substrate concentration. At low substrate concentration, first-order rate constants go through maxima with increasing surfactant concentration, but at higher concentration, double-rate maxima are observed. The presence of single- or double-rate maxima is ascribed to reaction in micelles or in premicellar complexes. At low substrate concentration, micellar effects were analyzed by means of a mass-action model. Second-order rate constants, kM, for reactions in the micellar pseudophase increase with headgroup size.
Introduction Aggregation of surfactant monomers in aqueous solution can lead to a variety of organized assemblies including premicellar aggregates and micelles. Micellar systems alter reaction rates and equilibria and the effect is generally explained in terms of pseudophase models.1-3 According to the model micelles formation begins at the critical micelle concentration, the cmc, and it is assumed that all additional surfactant forms micelles with the monomer concentration remaining constant and equal to the cmc. Typically, the rate-surfactant concentration profiles for bimolecular reactions have a distinctive shape.4-6 Below the cmc, the observed rate should be the same as that in water. Once micellar formation begins, the rate increases rapidly to a maximum followed by a gradual but steady decrease in the rate. Quantitative treatments of micellar effects on reaction rates generally fit the data at high surfactant concentration but sometimes fail near the cmc because they are based on the pseudophase assumption.1-3 There is evidence that substrates and surfactants interact at surfactant concentrations below the cmc and that premicellar assemblies are involved in some † Part of the Special Issue “Clifford A. Bunton: From Reaction Mechanisms to Association Colloids; Crucial Contributions to Physical Organic Chemistry.
(1) Menger, F. M.; Portnoy, C. E. J. Am. Chem. Soc. 1967, 89, 4698. (2) Bunton C. A.; Savelli, G. Adv. Phys. Org. Chem. 1986, 22, 213. (3) Romsted, L. S. In Surfactants in Solution; Mittal, K. L., Lindman, B., Eds.; Plenum Press: New York, 1984; Vol. 2, p 1015. (4) Bunton, C. A.; Nome, F. H.; Quina, F.; Romsted, L. S. Acc. Chem. Res. 1991, 24, 357. (5) (a) Correia, V. R.; Cuccovia, I. M.; Chaimovich, H. J. Phys. Org. Chem. 1993, 6, 7. (b) Rodenas, E.; Dolcet, C.; Valiente, M.; Valeron, E. C. Langmuir 1994, 10, 2088. (6) (a) Cuenca, A. Int. J. Chem. Kinet. 1998, 30, 777. (b) Cuenca, A. Tetrahedron 1997, 53, 12361. (c) Cuenca, A.; Strubinger, A. Tetrahedron 1996, 52, 11665. (d) Cuenca, A.; Bruno, C.; Taddei, A. Tetrahedron 1994, 50, 1927. (7) (a) Bunton, C. A.; Cuenca, A. J. Org. Chem. 1987, 52, 901. (b) Cuenca, A.; Bruno C. Int. J. Chem. Kinet. 1994, 26, 963. (c) Cuenca, A. Int. J. Chem. Kinet. 1990, 22, 103. (8) (a) Bunton, C. A.; Bacaloglu, R. J. Colloid Interface Sci. 1987, 115, 288. (b) Bacaloglu, R.; Bunton, C. A. J. Colloid Interface Sci. 1992, 153, 140. (9) Cerichelli, G.; Mancini, G.; Luchetti, L.; Savelli, G.; Bunton, C. A. Langmuir 1994, 10, 3982.
reactions.7-10 Aqueous cationic micelles increase the rates of cyclization of o-(-OC6H4O)(CH)n-2X (X ) Br, I).9 In very dilute surfactants there are large rate increases; these rate increases are ascribed to the formation of substrate premicellar complexes of the substrate with the surfactant. The effect on the reaction rates of premicellar assemblies and nonmicellizing hydrophobic ammonium ions have been studied.10 These ions aggregate and increase rates of several bimolecular reactions,10a,b spontaneous dephosphorylation10c and decarboxylation.10d Qualitatively, hydrophobic quaternary ammonium ions behave like micellized cationic surfactants in speeding nucleophilic reactions, even though they form small aggregates. In the absence of surfactant, second-order rate constants for reaction of azide ion with N-hexadecyl-2-bromopyridinium ion increase very sharply with increasing substrate concentration.7a Aggregates of the very hydrophobic substrate are responsible for the marked dependence of the rate constant on substrate concentration. In the presence of hexadecyltrimethylammonium bromide micelles and below the surfactant’s cmc, the observed rate increase is due to self-association of the hydrophobic N-hexadecyl-2-bromopyridinium ion.7a In this report, the reaction of 2-(4-chlorophenoxy)quinoxaline (1) with OH- was studied over a range of surfactant concentrations above and below the cmc and in the presence of cationic micelles with different headgroup sizes. Recently,6a,b,11-13 there has been considerable (10) (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) Bunton, C. A.; Quan, C. J. Org. Chem. 1985, 50, 3230. (d) Biresaw, G.; Bunton, C. A. J. Phys. Chem. 1986, 90, 5854. (11) (a) Di Profio, P.; Germani, R.; Savelli, G.; Cerichelli, G.; Chiarini, M.; Mancini, G.; Bunton, C. A.; Gillitt, N. D. Langmuir 1998, 14, 2662. (b) Bazito, R. C.; El Seoud, O. A.; Barlow, G. K.; Halstead, T. K. Ber. Bunsen-Ges. Phys. Chem. 1997, 101, 1933. (c) Brinchi, L.; Di Profio, P.; Germani, R.; Savelli, G.; Spreti, N.; Bunton, C. A. J. Chem. Soc., Perkin Trans. 2 1998, 361. (12) (a) Del Rosso, F.; Bartoletti, A.; Di Profio, P.; Germani, R.; Savelli, G.; Blasko´, A.; Bunton, C. A. J. Chem. Soc., Perkin Trans. 2 1995, 673. (b) Bacaloglu, R.; Bunton, C. A.; Ortega, F. J. Phys. Chem. 1989, 93, 1497. (c) Broxton, T. J.; Lucas, M. J. Org. Chem. 1994, 7, 442. (d) Bijma, R.; Blandamer, M. J.; Engbers, J. B. F. N. 1998, 14, 79. (e) Bacaloglu, R.; Bunton, C. A.; Cerichelli, G.; Ortega, F. J. Phys. Chem. 1990, 94, 5068.
10.1021/la990338a CCC: $19.00 © 2000 American Chemical Society Published on Web 12/10/1999
Hydrolysis of 2-(4-Chlorophenoxy)quinoxaline
Langmuir, Vol. 16, No. 1, 2000 73
Scheme 1
interest on the effect of headgroup size upon reaction rates and micellar properties. The surfactants used in this study were cetyltrialkylammonium chlorides, C16H33NR3Cl, R ) Me (CTACl), Et (CTEACl), n-Pr (CTPACl), n-Bu (CTBACl). 2-(4-Chlorophenoxy)quinoxaline (1) is very hydrophobic, it binds strongly to micelles, and the reactions can be readily followed spectrophotometrically. Experimental Section Materials. 2-(4-Chlorophenoxy)quinoxaline (1)was synthesized in DMF from 2-chloroquinoxaline14 and 4-chlorophenoxide ion using Ag+ as the catalyst.15 The surfactants were samples used in an earlier work.6b In water, cmc values for CTACl, CTEACl, CTPACl, and CTBACl are 1.3, 1.2, 0.65, and 0.52 mM, respectively.13b These values were determined by variations in surface tension. Kinetics. Reactions were followed spectrophotometrically at 25.0 °C in a Perkin-Elmer Lambda II spectrometer. The basic hydrolysis of 1 was followed at 362 nm by using the λmax of the 2-quinoxalone. In some cases, the reaction was also followed by monitoring the appearance of 4-chlorophenoxide ion at 298 nm. Rate constants for kinetics followed at 362 and 298 nm were in agreement within 5% error. The rate constants are means of three measurements and agreed within 4%. The reactions were cleanly first-order and the rate constants, kΨ, are in reciprocal seconds. Solutions were prepared using deionized, distilled, CO2free water. The substrate was added in MeCN so that the reaction medium contained 0.1% organic solvent.
Figure 1. Variation of the first-order rate constant, kΨ, for the reaction of 5 × 10-6 M 2-(4-chlorophenoxy)quinoxaline (1) with OH- in CTACl: (b) with 0.012 M OH-; (2) with 0.03 M OH-. Curves are calculated.
Results and Discussion Reaction in the Absence of Surfactant. In water, 2-(4-chlorophenoxy)quinoxaline (1) reacts with OH- to produce 2-quinoxalone 2 (Scheme 1). The second-order rate constant, kw, is 1.23 × 10-2 M-1 s-1. Reaction in the Presence of Surfactants. Figures 1 and 2 show the reaction of 2-(4-chlorophenoxy)quinoxaline (1) with OH- in the presence of micellized hexadecyltrialkylammonium chlorides (CTACl and CTBACl) with different headgroup sizes and at low substrate concentrations (5.0 × 10-6 M). Rate data for CTEACl and CTPACl are available as Supporting Information. Reactions were followed at 0.012 and 0.03 M OH-. The rate-surfactant profiles for the reaction of compound 1 with OH- show three distinctive characteristics: (1) there is a rate enhancement below the cmc of the surfactant in water; (2) at higher concentrations of surfactant, the observed rates go through maxima; (3) the observed rates increase with the surfactant headgroup size at a given surfactant and OH- concentration. Reactions in CTEACl and CTPACl show the same general tendencies. Rate-surfactant profiles for the basic hydrolysis of compound 1 fit pseudophase model treatment at low substrate concentrations (5.0 × 10-6 M). The results can be described in terms of Scheme 2, where S is the substrate, OH- is the nucleophile, KS is the substrate-surfactant (13) (a) Bijma, K.; Blandamer, M.; Engberts, J. B. F. N. Langmuir 1998, 14, 79. (b) Bonan, C.; Germani, R.; Ponti, P. P.; Savelli, G.; Cerichelli, G.; Bacaloglu, R.; Bunton, C. A. J. Phys. Chem. 1990, 94, 5331. (c) Brinchi, L.; Germani, R.; Savelli, G.; Spreti, N.; Ruzziconi, R.; Bunton, C. A. Langmuir 1998, 14, 2656. (14) Castle R. N.; Onda, M J. Org. Chem. 1961, 26, 954. (15) Cuenca, A.; Lo´pez, S. E.; Garce´s, I.; Aranda, A. Synth. Commun. 1999, 29, 1393.
Figure 2. Variation of the first-order rate constant, kΨ, for the reaction of 2-(4-chlorophenoxy)quinoxaline (1) with OH- in CTBACl. Symbols as in Figure 1. Curves are calculated. Scheme 2
binding constant, Dn is the micellized surfactant, that is, [Dn] ) [D] - cmc, and k′W and k′M are first-order rate constants for reaction in the aqueous and micellar pseudophases, respectively. Subscripts w and M stand for the aqueous and micellar phases. The observed first-order rate constant is given by eq 1:1
kΨ )
k′MKS[OHM] 1 + KS([Dn] - cmc)
(1)
The first-order rate constants can be written as secondorder rate constants, kW′ and kM, with the concentration of OH- in the micellar pseudofase written as a mole fraction:
k′W ) kW[OHW]
(2)
74
Langmuir, Vol. 16, No. 1, 2000 OH k′M ) kMmOH M ) kM[OHM ]/([D] - cmc)
Cuenca
(3)
OH′w] is a molarity in terms of total solution volume. Rate data for reaction of OH- with compound 1 in solutions of cationic surfactants can be fitted using a pseudophase model (eqs 1-3), in which the distribution of OH- and Cl- between the aqueous and micellar pseudophases is written in terms of the mass-action-like equations (4) and (5):16 K′OH ) [OHM]/{([OHW]([Dn] - [OHM] - [ClM])}
(4)
K′Cl ) [ClM]/{([Cl′W]([Dn] - [OH′M] - [Cl′M])}
(5)
K′OH and K′Cl are assumed to be independent parameters. Table 1 gives the values of the parameters that best fit the experimental results for the reaction of compound 1 in cationic micelles and at low substrate concentrations. Solid lines in Figures 1 and 2 represent the values of kΨ calculated with these parameters by eqs 1, 4, and 5. The fit of theory and experiment is reasonably good. K′Cl, kM, and KS were treated as adjustable parameters. The values of kM and K′Cl are insensitive to changes in OH- concentration. Values of K′OH are from the literature (K′OH ) 55, 45, 25, and 12 for CTA+ and CTBA+, respectively).13b,16 kW and kM are second-order rate constants; the dimensions of kW are M-1 s-1, following the usual convention, but the concentration of the nucleophile in the micelles is written as a mole ratio so that the dimensions of kM are s-1 (eq 3). This concentration can be written as molarity in whose molar volume is VM, M-1. Estimations of this volume range from 0.14 to 0.37 L3,16 and in this report the lower limit was selected so that the second-order rate constant, kM, M-1 s-1, is given by
km 2 ) kMVM
Table 1. Parameters that Best Fit the Kinetic Results for Substrate 1 in Micellesa surfactant
102 [OH-T], M
K′Cl, M-1
103 cmc, M
10 kM, s-1
102 km 2, M-1 s-1
CTACl CTACl CTEACl CTEACl CTPACl CTPACl CTBACl CTBACl
1.2 3.0 1.2 3.0 1.2 3.0 1.2 3.0
125 125 110 110 60 60 50 50
1.1 1.1 0.9 0.9 0.5 0.5 0.4 0.4
2.8 2.8 4.1 4.1 5.6 5.6 7.9 7.9
3.9 3.9 5.7 5.7 7.8 7.8 11.1 11.1
a At 25.0 °C and with K ) 8400 M-1, k ) 1.23 × 10-2 M-1 s-1, S W K′OH ) 55, 45, 25, and 12 M-1 for CTA+, CTEA+, CTPA+, and CTBA+, respectively.13b,16 [Substrate] ) 5.0 × 10-6 M.
Figure 3. Reaction with 0.012 M OH- in CTACl: ([) with 9.0 × 10-5 M substrate; (9) with 5.0 × 10-5 M substrate; (b) with 5.0 × 10-5 M substrate and 0.012 M NaCl.
(6)
A change of headgroup in the sequence Me3N+, Et3N+, Pr3N+, and Bu3N+ increases the second-order rate constant, kM (Table 1). The increase in headgroup size results in a less hydrated micellar interface because the headgroups displace water from the interfacial region, relative to that of CTACl. These observations indicate that hydroxide ion nucleophilicity is accelerated by a decrease in polarity and water content of the interface. Additional experiments were performed with a 10-fold increase in substrate concentration (5.0 × 10-5 M) and at 9.0 × 10-5 M. Figures 3 and 4 show the effect of substrate concentration on the pseudo-first-order rate constants, kι, for reaction of 1 with 0.012 M OH- in the presence of CTACl and CTBACl. For these experiments first-order rate constants go through double maxima with increasing concentrations of surfactant. Figures 3 and 4 show that added 0.012 M NaCl suppresses the double-rate maxima. The observed double-rate maxima at higher substrate concentrations (5.0 × 10-5 and 9.0 × 10-5 M) are probably due to changes in the structure of surfactant assemblies. In dilute surfactant, below the cmc, there are surfactant monomers. As the surfactant concentration increases, submicellar clusters that are catalytically active form and their formation is probably induced by the substrate. As these aggregates incorporate the substrate, the rate increases until they become saturated and the first maximum is observed. As the surfactant concentration increases, micelles begin to form and they compete with (16) Bunton, C. A.; Gan, L. H.; Hamed, F. H.; Moffatt, J. R. J. Phys. Chem. 1983, 87, 336.
Figure 4. Reaction with 0.012 M OH- in CTBACl. Symbols as in Figure 3.
the clusters and bind the substrate. The rate increases again until the substrate is fully bound (second maximum) and starts to dilute as the surfactant concentration increases. The premicellar substrate complexes dissolve into normal micelles at higher surfactant concentrations (or with added electrolyte), and the unusual rate effects dissappear. Added NaCl induces two effects, rate inhibition due to competition between Cl- and OH- ions and promotion of micellar formation at lower surfactant concentrations. Anionic micelles of sodiun dodecyl sulfate (SDS) inhibit the reaction of 2-phenoxyquinoxaline6b and that of 2,7bis(diethylamino)phenazoxonium chloride6c with hydroxide ion and inhibition starts at SDS concentrations below
Hydrolysis of 2-(4-Chlorophenoxy)quinoxaline
the cmc in water; it was suggested that the substrate induces the formation of anionic aggregates that exclude OH-. The presence of double maxima has been observed for other micellar-catalyzed reactions.8,17 Reactions of nucleophiles with 2,4-dinitrochloronaphthalene in solutions of cetyltrialkylammonium chloride and bromide go through double maxima and the position of the rate extrema depend on substrate concentration.8 In neutral solution, a condition where the spontaneous hydrolysis of compound 1 is negligible, cmc values were measured in the presence (1.2, CTACl; 1.1, CTEACl; 0.67, CTBACl; 0.53, CTBACl) and absence (1.3 mM, CTACl; 1.2 mM, CTEACl; 0.65 mM, CTPACl; 0.52 mM, CTBACl) of the substrate (9.0 × 10-5 M). Substrate 1 should enhance the aggregation of all surfactants, probably by decreasing the chemical potential of the monomers; however, this enhancement neither explains nor predicts double-rate maxima. (17) Blasko´, A.; Bunton, C. A.; Hong, Y. S.; Mhala, M. M.; Moffatt, J. R.; Wright, S. J. Phys. Org. Chem. 1991, 4, 618.
Langmuir, Vol. 16, No. 1, 2000 75
The observed catalytic effect by premicellar substratesurfactant complexes or by micelles and the enhanced nucleophilic reactivity by the presence of micelles with bulky headgroups show the importance of hydrophophic forces in dealing with reactions carried out in colloidal systems. Acknowledgment. Support of this work by Decanato de Investigaciones Cientı´ficas, Universidad Simo´n Bolı´var (Grant S1-CB-410) and Consejo Nacional de Investigaciones Cientı´ficas y Tecnolo´gicas, Venezuela, is gratefully acknowledged. Supporting Information Available: Tables of rate constants for the basic hydrolysis of 2-(4-chlorophenoxy)quinoxaline (1) in CTEACl and CTPACl (3 pages) and substrate and NaOH concentrations as specified in Figures 1 and 3. This material is available free of charge via the Internet at http://pubs.acs.org. LA990338A
