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Decarboxylation of 6-Nitrobenzisoxazole-3-carboxylate in Polyampholyte Latexes Kenneth W. Hampton, Jr. and Warren T. Ford* Department of Chemistry, Oklahoma State University, Stillwater, Oklahoma 74078 Received March 10, 2000. In Final Form: June 19, 2000 Polyampholyte latexes that contain 29/18 and 25/23 mol % of (styrylmethyl)trimethylammonium/ methacrylate units and styrene cross-linked with divinylbenzene are colloidally stable in 4 M NaCl solution. As catalytic media in basic solutions 0.5 mg mL-1 of the polyampholyte latexes increase the rate of decarboxylation of 6-nitrobenzisoxazole-3-carboxylate (6-NBIC) 60-115 times, while the precursor polycation latexes increase the rate 540-670 times the rate in water alone. The latexes are active catalysts in 0.67 M NaCl solution, but activity decreases as the concentration of added NaCl increases. Analysis of rate constants at varied particle concentrations indicates that the fractions of the 6-NBIC anions bound to particles are similar in the two types of latexes and that the differences in activity are due primarily to larger intraparticle rate constants in the polycation latexes than in the polyampholytes.
Introduction In aqueous media the rates of reactions of anions are increased by many cationic colloids and polymers, such as surfactant micelles,1-5 vesicles,6,7 microemulsions,8-10 polyelectrolytes,1,11-14 polymeric crown ethers ligated to metal ions,1515 ion-exchange resins,1,16-18 and polymer latexes.19-24 The colloids and polymers are called “catalysts,” although “catalytic media” is a more accurate description. Catalysis occurs either when the rate constant is greater in the colloidal phase or pseudophase than in water or, in the case of bimolecular reactions, when the local concentrations of the reactive species in the colloid are much higher than in the aqueous phase, even if the (1) Fendler, J. H.; Fendler, E. H. Catalysis in Micellar and Macromolecular Systems; Academic: New York, 1975. (2) Bunton, C. A.; Savelli, G. Adv. Phys. Org. Chem. 1986, 22, 213. (3) Quina, F. H.; Chaimovich, H. J. Phys. Chem. 1979, 83, 1844. (4) Quina, F. H.; Politi, M. J.; Cuccovia, I. M.; Baumgarten, E.; Martins-Franchetti, S. M.; Chaimovich, H. J. Phys. Chem. 1980, 84, 361. (5) Romsted, L. S. Surfactants in Solution; Mittal, K. L., Lindman, B., Eds.; Plenum: New York, 1984; 1015-1068. (6) Kunitake, T.; Okahata, Y.; Ando, R.; Shinkai, S.; Hirakawa, S. J. Am. Chem. Soc. 1980, 102, 7877. (7) Germani, R.; Ponti, P. P.; Savelli, G.; Spreti, N.; Cipiciani, A.; Cerichelli, G.; Bunton, C. A. J. Chem. Soc., Perkin Trans. 2 1989, 1767. (8) Mackay, R. A.; Longo, F. R.; Knier, B. L.; Durst, H. D. J. Phys. Chem. 1987, 91, 861. (9) Garlick, S. M.; Durst, H. D.; Mackay, R. A.; Haddaway, K. G.; Longo, F. R. J. Colloid Interface Sci. 1990, 135, 508. (10) Menger, F. M.; Elrington, A. T. J. Am. Chem. Soc. 1990, 112, 8201. (11) Fife, W. K. Trends Polym. Sci. 1995, 3, 214. (12) Zheng, Y.; Knowsel, R.; Galin, J. Polymer 1987, 28, 2297. (13) Yang, Y.; Engberts, J. J. Org. Chem. 1991, 56, 4300. (14) Lee, J.-J.; Ford, W. T. Macromolecules 1994, 27, 4632. (15) Shah, S. C.; Smid, J. J. Am. Chem. Soc. 1978, 100, 1426. (16) Ford, W. T.; Tomoi, M. Adv. Polym. Sci. 1984, 55, 49. (17) Tomoi, M.; Ford, W. T. Synthesis and Separations Using Functional Polymers; Sherrington, D. C., Hodge, P., Eds.; Wiley: Chichester, U.K., 1988; 181-207. (18) Helfferich, F. Ion Exchange; McGraw-Hill: New York, 1962; pp 250-319. (19) Ford, W. T.; Badley, R. D.; Chandran, R. S.; Hari Babu, S.; Hassanein, M.; Srinivasan, S.; Turk, H.; Yu, H.; Zhu, W. ACS Symp. Ser. 1992, 492, 422. (20) Ford, W. T. React. Funct. Polym. 1997, 33, 147. (21) Lee, J. J.; Ford, W. T. J. Am. Chem. Soc. 1994, 116, 3753. (22) Yu, H.; Ford, W. T. Langmuir 1993, 9, 1999. (23) Miller, P. D.; Copeland, S. L.; Sanders, R.; Woodruff, A.; Gearhart, D.; Spivey, H. O.; Ford, W. T. Langmuir 2000, 16, 108. (24) Miller, P. D.; Ford, W. T. Langmuir 2000, 16, 596.
Scheme 1
rate constant in the colloidal phase is not greater. These heterogeneous media may provide efficient methods of detoxification of industrial waste, pesticides, and chemical warfare agents,8-10,21,22,25-29 and they could be applied under field conditions. The rate of unimolecular decarboxylation of 6-nitrobenzisoxazole-3-carboxylate ion (6-NBIC, Scheme 1) is a sensitive probe of its environment, being 108 times faster in hexamethylphosphoric triamide than in water at room temperature.30-32 The reaction is slow when 6-NBIC is stabilized by hydrogen bonding and is accelerated by transition state stabilization in dipolar aprotic solvents. In water the rate is increased 200-300 times by micelles and by latexes containing alkyltrimethylammonium ions and up to 20 000 times by more hydrophobic cationic colloids.6,33-35 The most active of these catalytic media so far are polystyrene- and poly(2-ethylhexyl methacrylate)based latexes containing 15-40 mol % of (styrylmethyl)tributylammonium ions.23,24,35 Certain poly(crown ether)s15 and micelles of alkyltributylamonium ions27,36 are nearly as active. (25) Moss, R.; Alwis, K.; Bizzigotti, G. J. Am. Chem. Soc. 1983, 105, 681. (26) Yang, Y.-C.; Baker, J. A.; Ward, J. R. Chem. Rev. 1992, 92, 1729. (27) 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. (28) Moss, R. A.; Kotchevar, A. T.; Park, B. D.; Scrimin, P. Langmuir 1996, 12, 2200. (29) Scrimin, P.; Ghirlanda, G.; Tecilla, P.; Moss, R. A. Langmuir 1996, 12, 6235. (30) Kemp, D. S.; Paul, K. G. J. Am. Chem. Soc. 1970, 92, 2553. (31) Kemp, D. S.; Paul, K. G. J. Am. Chem. Soc. 1975, 97, 7305. (32) Kemp, D. S.; Cox, D. D.; Paul, K. G. J. Am. Chem. Soc. 1975, 97, 7312. (33) Bunton, C. A.; Minch, M. J.; Hidalgo, J.; Sepulveda, L. J. Am. Chem. Soc. 1973, 95, 3262. (34) Engberts, J. B. F. N.; Rupert, L. A. M. J. Org. Chem. 1982, 47, 5015. (35) Lee, J. J.; Ford, W. T. J. Org. Chem. 1993, 58, 4070.
10.1021/la0003591 CCC: $19.00 © 2000 American Chemical Society Published on Web 08/25/2000
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Cationic latexes are effective catalytic media for pnitrophenyl carboxylate and phosphate ester hydrolyses at concentrations of 0.1 M in this work would have little effect on the rate of reaction. However, complete coagulation and precipitation of the latex, which occurs with the 25/25N and 20/30N samples at [NaCl] > 0.5 M,40 would slow the rate markedly. Conclusions The polyampholyte latexes overcome the usual instability of charged latexes at high electrolyte concentrations. They can be used as catalysts at NaCl concentrations at least as high as 0.67 M, although the activities are reduced by ion-exchange displacement of the reactant anion by chloride ion. They are also colloidally stable in seawater, which is important for application to decontamination of waste and chemical warfare agents. The polyampholyte latexes merit further investigation as catalysts for other reactions in high salt aqueous media. Acknowledgment. This research was supported by the US Army Research Office. LA0003591 (42) Satterfield, C. N. Mass Transfer in Heterogeneous Catalysis; MIT: Cambridge, MA, 1970. (43) Liu, G.; Ford, W. T. Langmuir, in press.
