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Kinetic probe to study the structural behaviour of mixed aqueous—organic solvents. Effects of inorganic and organic salts on the kinetics and mechanism of intramolecular general base‐catalysed methanolysis of ionized phenyl salicylate
Author(s) -
Niyaz Khan M.,
Audu A. A.
Publication year - 1992
Publication title -
journal of physical organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.325
H-Index - 66
eISSN - 1099-1395
pISSN - 0894-3230
DOI - 10.1002/poc.610050305
Subject(s) - chemistry , nucleophile , reaction rate constant , methanol , aqueous solution , base (topology) , kinetics , intramolecular force , medicinal chemistry , stereochemistry , organic chemistry , catalysis , mathematical analysis , physics , mathematics , quantum mechanics
The effects of inorganic and organic salts on the rates of methanolysis of ionized phenyl salicylate, PS − , were studied at 30°C in H 2 O–MeOH solvents. The observed pseudo‐first‐order rate constants, k obs , for methanolysis of PS − represent a non‐linear incrrease with increase in methanol contents in mixed H 2 O–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 H 2 O–MeOH solvents. The rate constants, k obs , at alkanol [ROH (R = Me, HOCH 2 CH 2 )] contents of ca ≤ 55% (v/v), fit the relationship k obs = k [ROH] T /(1 + k A [ROH]), (1 + K A [ROH]), where k and K A 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 K A increase with increase in [MX] T and this increase varies in the order Li + > Na + + > K + . The values of both k and K A show a decrease with increase in the total concentrations of organic salts, [R 4 MX] T (R = Et, Pr n , Bu n and X = Br, I). The effects of [MX] T and [R 4 MX] T on k obs versus content of ROH (%, v/v) profiles reveal the following inferences: (i) in the water‐rich region of H 2 O‐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, R 4 N + , 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 Et 4 N + , Pr n 4 N + and Bu n 4 N + reveal a methanol structure‐breaking effect through an interaction mechanism different from that operating for Na + and K + . Both k and K A 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.