J . Phys. Chem. 1988, 92, 6273-6278
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Kinetic Probe To Study the Structure of Mixed Aqueous-Organic Solvents. Kinetics and Mechanism of Nucleophilic Reactions of 1-Propanol and P(+)-Glucose with Ionized Phenyl Salicylate M . Niyaz Khan Department of Chemistry, Bayero University, P.M.B. 301 1 , Kano, Nigeria (Received: November 30, 1987) The reaction rates of propanolysis of ionized phenyl salicylate (PS-) studied at 0.01 M NaOH with different temperatures and varying contents of 1-propanol (1-PrOH) in mixed aqueous solutions reveal that the pseudo-first-order rate constants, k , , for the reactions of 1-PrOH with PS- show a nonlinear increase with increase of I-PrOH content until it becomes ca. 80% (v/v). An increase in 1-PrOH content beyond 80% (v/v) causes an essentially nonlinear decrease in k , values. The rate constants k l obtained within the 1-PrOH content range of 5% to ca. 60% (v/v) fit the relationship k l = k[ROHIT/(l K[ROH],), where k and K represent the nucleophilic second-order rate constant for the reaction of alkanol (1-PrOH) with PS-and the association constant for dimerization of monomers of ROH, respectively, and [ROH], is the total concentration of ROH. The rate constants k , for the reaction of D-(+)-glUCOSe with PS- follow the same relationship within the [glucose], range of 0.1-1.6 M at 0.05 M KOH. These observations could be qualitatively explained in terms of the structural properties of the mixed aqueous-organic cosolvent. It is proposed that the reaction of PS- with the OH group of organic cosolvent involves the preequilibrium formation of monomeric ROH from polymeric (ROH),, followed by an intramolecular general base-catalyzed nucleophilic attack by monomeric ROH on the carbonyl carbon of PS-. The activation parameters, AH* and a*, obtained for propanolysis of PS- are essentially independent of [ 1-PrOH] within the 1-PrOH content range 5-97% (v/v). The thermodynamic parameters, AH and AS, for the association constant of dimerization of 1-PrOH are 8.84 3.43 kcal mol-' and 24.2 10.9 cal K-' mol-', respectively.
+
*
Introduction Ionized phenyl salicylate (PS-)is known to undergo an efficient transesterification in the presence of a hydroxylic The pharmaceutical concern of this reaction is emphasized recently by Irwing and co-worker~.~JIt has been convincingly concluded that the mechanism of hydrolysis of salicylate esters involves intramolecular general base c a t a l y ~ i s . ~ -Many ~ enzymatic reactions are also known to involve intramolecular general base catalysis.l0*I1 We have recently proposed and supported that the hydrolyses of ionized salicylate esters involve an intramolecular intimate ion pair* (T).
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K-& T Just as the kinetic studies of hydrolysis of various compounds in mixed aqueous-organic solvents have been used as a reliable and effective tool in understanding the structure of water,'* we thought that the kinetics of alkanolysis of PS- might provide some informations about the structure of alkanol in mixed aqueousalkanol systems. The kinetic data on methanolysis and ethanolysis of PS- suggest that these alkanols exist in various forms such as monomeric, dimeric, trimeric, and in general polymeric forms in mixed aqueous solvent^.^ In the continuation of our study on (1) Irwin, W. J.; Masuda, Q. N.; Li Wan Po, A. Int. J. Pharm. 1984,21,
35. (2) Irwin, W. J.; Masuda, Q. N.; Li Wan Po, A. Tetrahedron 1984, 40, 5217. ( 3 ) Irwin, W. J.; Masuda, Q. N.; Li Wan Po, A. J. Pharm. Eiomed. Anal. 1985, 3, 241. (4) Khan, M. N. Int. J . Chern. Kinet. 1987, 19, 757. (5) Khan, M. N. J . Pharm. Eiomed. Anal. 1987, 5, 515. (6) Bender, M. L.; Kezdy, F. J.; Zerner, B. J . Am. Chem. SOC.1963.85, 3017. (7) Capon, B.; Ghosh, B. C. J . Chem. SOC.E 1966, 472. (8) Khan, M. N.; Gambo, S. K. Int. J . Chem. Kinet. 1985, 17, 419. (9) Khan, M. N. J . Mol. Catal. 1987, 40, 195. (10) Roberts, J. D.; Kanamori, K. Proc. Natl. Acad. Sci. U.S.A. 1980, 77, 3095, and references cited therein. ( 1 1) Quinn, D. M.; Elrod, J. P.; Ardis, R.; Friesen, P.; Schowen, R. L. J . A m . Chem. SOC.1980, 102, 5358, and references cited therein. (1 2) Engberts, J. B. F. N. In Water, a Comprehensive Treatise; Franks, F., Ed.; Plenum: New York, 1979; Vol. 6, Chapter 4.
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transesterification of PS- we have now studied the kinetics of the reactions of PS- with 1-propanol and D-(+)-glucose. The results and pertinent conclusion are described in this paper.
Experimental Section Materials. Reagent grade chemicals such as 1-propanol (1PrOH), phenyl salicylate (PSH), and anhydrous D-(+)-glucose (Glu) were obtained from E. Merck, BDH, and Aldrich, respectively. All other chemicals used were also of reagent grade and were obtained from BDH, Aldrich, or Fluka AG. The stock solutions of PSH were frequently prepared in MeCN and were kept below 0 OC if they were not in kinetic use. Kinetic Measurements. The reaction rates of propanolysis of ionized phenyl salicylate (PS-)in mixed aqueous solvents with water contents of
