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
Year: 2010
Type: dissertation
Abstract: Values of the rate constant for the alkaline hydrolysis of methyl \nacetate in various aqueous-organic solvent mixtures (dimethyl sulfoxide \n0<x40.2, dioxane 0 <, x., < 0.2, methyl ethyl ketone 0<x<0.06 and \ndiglyme, i. e. ether-bis (2-methyloxethyl) 0x<0.10) have been \ndetermined for the temperatures 15 0 C, 25 0C \nand 35 0C \nconductometrically. \nTo interpret these results the approach adapted is to experimentally \ndetermine the activity coefficient of the ester (YE ) and the activity of the \nwater (aH20', mechanistically, at least one molecule of water is involved in the rate-determining step) and then to use the Bronsted-Bjerrum equation \nto determine the residual activity coefficient ratio of the participating \nions, y (Yf \n- for Oil the transition state). \nValues of YE and aH \n20 \nhave been determined by a transpiration method, \nusing gas-chromatographic analysis of the vapours of solutions of methyl \nacetate in aqueous-organic solvent mixtures of dir. ethyl sulfoxide, dioxane, \nmethyl ethyl ketone and diglyme in the same composition ranges as above, \ntetrahydrofuran 04x \norg z<, 0.15, methanol, ethanol and tert-butanol in \nt1h6e range 04x0.20'at 25oC. These results indicate that on changing org \nthe solvent composition YE varies by a larger factor than is predicted for \nthe ratio YOH-/yýO_ by the Debye-Iluckel approach, and hence is the \ndominant factor in determining the effects of solvent composition on the \nrates of the hydrolysis. This is in contradiction to the assumptions of \nthe electrostatic theories of Laidler and Eyring, and of Amis and Jaffe. \nThe gas-chromatographic results also indicate that whilst the concentration \nof the water varies in each mixture studied, the activity coefficient varies \nin the opposite way to produce almost constant values of aý, 0* \nUsing the transpiratioii/gas-chromatogralýlic method, the thermodynamic \nproperties of the ternary systems, methyl acetate-water-organic Solvcat, \nusing the organic solvents mentioned above (excepting, diglyme) have been \ninvestigated, and the results indicate that the variation of *ýE with \nsolvent composition, for the dilute solutions of ester used, can be \nestimated from the thermodynamic properties of the binary water-organic \nsolvent mixtures, using the Gibbs-Dahem equation. \nSingle ion activity coefficients in the literature for small negative \nions, to represent the OH_ ion, and for large ions, to rep-resent the \ntransition state ion, have been used to explain the experimentally fomd \nvariation of the residual activity coefficient -ratio with solvent \ncomposition. \nHence, it is concluded that the importance of the parameters involved \nin the hydrolysis of esters - an ion-molecule reaction - in aqueousorganic \nsolvent mixtures are in the order of \nYmolecule > aH \n20> \nYOH_/YM+ -> (dielectric constant), \nand that the nonelectrostatic effects -- thermodynamic effects - are more \nimportant in these studies than the electrostatic effects. \nFrom a preliminary investigation of the data in the literature the \nthermodynamic approach also yields a valid interpretation of the effect of \nsolvent composition on the rates of the acid hydrolysis of esters.
Author: Abdol Rassoul Kazempour
Language: English
Subfield: Organic Chemistry
Field: Chemistry
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W1507079892