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The alkaline hydrolysis of esters in aqueous-organic solvent mixtures. The effects of solvents and of the activity coefficients of reactants on the kinetics of the alkaline hydrolysis of methyl acetate in aqueous dioxan, aqueous dimethyl sulphoxide and aqueous diglyme (bis (2-methoxyethyl ) ether) mixtures as solvents.

By Abdol Rassoul Kazempour

Abstract

Values of the rate constant for the alkaline hydrolysis of methyl\ud acetate in various aqueous-organic solvent mixtures (dimethyl sulfoxide\ud 0<x40.2, dioxane 0 <, x., < 0.2, methyl ethyl ketone 0<x<0.06 and\ud diglyme, i. e. ether-bis (2-methyloxethyl) 0x<0.10) have been\ud determined for the temperatures 15 0 C, 25 0C\ud and 35 0C\ud conductometrically.\ud To interpret these results the approach adapted is to experimentally\ud determine the activity coefficient of the ester (YE ) and the activity of the\ud water (aH20', mechanistically, at least one molecule of water is involved in the rate-determining step) and then to use the Bronsted-Bjerrum equation\ud to determine the residual activity coefficient ratio of the participating\ud ions, y (Yf\ud - for Oil the transition state).\ud Values of YE and aH\ud 20\ud have been determined by a transpiration method,\ud using gas-chromatographic analysis of the vapours of solutions of methyl\ud acetate in aqueous-organic solvent mixtures of dir. ethyl sulfoxide, dioxane,\ud methyl ethyl ketone and diglyme in the same composition ranges as above,\ud tetrahydrofuran 04x\ud org z<, 0.15, methanol, ethanol and tert-butanol in\ud t1h6e range 04x0.20'at 25oC. These results indicate that on changing org\ud the solvent composition YE varies by a larger factor than is predicted for\ud the ratio YOH-/yýO_ by the Debye-Iluckel approach, and hence is the\ud dominant factor in determining the effects of solvent composition on the\ud rates of the hydrolysis. This is in contradiction to the assumptions of\ud the electrostatic theories of Laidler and Eyring, and of Amis and Jaffe.\ud The gas-chromatographic results also indicate that whilst the concentration\ud of the water varies in each mixture studied, the activity coefficient varies\ud in the opposite way to produce almost constant values of aý, 0*\ud Using the transpiratioii/gas-chromatogralýlic method, the thermodynamic\ud properties of the ternary systems, methyl acetate-water-organic Solvcat,\ud using the organic solvents mentioned above (excepting, diglyme) have been\ud investigated, and the results indicate that the variation of *ýE with\ud solvent composition, for the dilute solutions of ester used, can be\ud estimated from the thermodynamic properties of the binary water-organic\ud solvent mixtures, using the Gibbs-Dahem equation.\ud Single ion activity coefficients in the literature for small negative\ud ions, to represent the OH_ ion, and for large ions, to rep-resent the\ud transition state ion, have been used to explain the experimentally fomd\ud variation of the residual activity coefficient -ratio with solvent\ud composition.\ud Hence, it is concluded that the importance of the parameters involved\ud in the hydrolysis of esters - an ion-molecule reaction - in aqueousorganic\ud solvent mixtures are in the order of\ud Ymolecule > aH\ud 20>\ud YOH_/YM+ -> (dielectric constant),\ud and that the nonelectrostatic effects -- thermodynamic effects - are more\ud important in these studies than the electrostatic effects.\ud From a preliminary investigation of the data in the literature the\ud thermodynamic approach also yields a valid interpretation of the effect of\ud solvent composition on the rates of the acid hydrolysis of esters.Ministry of Science and Higher Education of Ira

Topics: Methyl acetate, Alkaline hydrolysis, Aqueous-organic solvent mixtures, Activity coefficients, Bronsted-Bjerrum equation, Residual activity coefficient ratio
Publisher: Postgraduate School of Studies in Chemistry
Year: 1978
OAI identifier: oai:bradscholars.brad.ac.uk:10454/4352
Provided by: Bradford Scholars

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