Wiley InterScience Backfile Collection 1832-2000
Chemistry and Pharmacology
The effects of inorganic and organic salts on the rates of methanolysis of ionized phenyl salicylate, PS-, were studied at 30°C in H2O-MeOH solvents. The observed pseudo-first-order rate constants, kobs, for methanolysis of PS- represent a non-linear incrrease with increase in methanol contents in mixed H2O-MeOH solvents. The observed data are explained in terms of the proposal that methanol molecules exist in monomeric, dimeric and in general polymeric forms in H2O-MeOH solvents. The rate constants, kobs, at alkanol [ROH (R = Me, HOCH2CH2)] contents of ca ≤ 55% (v/v), fit the relationship kobs = k [ROH]T/(1 + kA [ROH]), (1 + KA [ROH]), where k and KA represent the nucleophilic second-order rate constant for the reaction of ROH with PS- and the association constant for the dimerization of ROH, respectively, and [ROH]T is the total concentration of ROH. The rate constants, k, appear to be independent of total concentrations of inorganic salts [MX]T (M = Li, Na, K and X = OH, Cl), but the values of KA increase with increase in [MX]T and this increase varies in the order Li+ 〉 Na++ 〉 K+. The values of both k and KA show a decrease with increase in the total concentrations of organic salts, [R4MX]T (R = Et, Prn, Bun and X = Br, I). The effects of [MX]T and [R4MX]T on kobs versus content of ROH (%, v/v) profiles reveal the following inferences: (i) in the water-rich region of H2O-ROH solvents [at ca 〈 55% (v/v) ROH] the solvation shells of M+ and X- contain preferentially only water molecules whereas in the methanol-rich region [at ca 〉 55% (v/v) ROH] of solvents some cosolvent methanol molecules also enter into these solvation shells; (ii) the solvation shells of tetraalkylammonium ions, R4N+, contain some methanol molecules even in the water-rich region of the solvents; (iii) Li+ cause a methanol structure-making effect whereas Na+ and K+ ions show a methanol structurebreaking effect; and (iv) organic cations such as Et4N+, Prn4N+ and Bun4N+ reveal a methanol structure-breaking effect through an interaction mechanism different from that operating for Na+ and K+. Both k and KA show significant normal deuterium isotope effects in the reactions of MeOH and PS- which indicates the probable occurrence of proton transfer in the rate-determining step.
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