Values of the rate constant for the alkaline hydrolysis of methyl
acetate in various aqueous-organic solvent mixtures (dimethyl sulfoxide
0<x40.2, dioxane 0 <, x., < 0.2, methyl ethyl ketone 0<x<0.06 and
diglyme, i. e. ether-bis (2-methyloxethyl) 0x<0.10) have been
determined for the temperatures 15 0 C, 25 0C
and 35 0C
conductometrically.
To interpret these results the approach adapted is to experimentally
determine the activity coefficient of the ester (YE ) and the activity of the
water (aH20', mechanistically, at least one molecule of water is involved in the rate-determining step) and then to use the Bronsted-Bjerrum equation
to determine the residual activity coefficient ratio of the participating
ions, y (Yf
- for Oil the transition state).
Values of YE and aH
20
have been determined by a transpiration method,
using gas-chromatographic analysis of the vapours of solutions of methyl
acetate in aqueous-organic solvent mixtures of dir. ethyl sulfoxide, dioxane,
methyl ethyl ketone and diglyme in the same composition ranges as above,
tetrahydrofuran 04x
org z<, 0.15, methanol, ethanol and tert-butanol in
t1h6e range 04x0.20'at 25oC. These results indicate that on changing org
the solvent composition YE varies by a larger factor than is predicted for
the ratio YOH-/yýO_ by the Debye-Iluckel approach, and hence is the
dominant factor in determining the effects of solvent composition on the
rates of the hydrolysis. This is in contradiction to the assumptions of
the electrostatic theories of Laidler and Eyring, and of Amis and Jaffe.
The gas-chromatographic results also indicate that whilst the concentration
of the water varies in each mixture studied, the activity coefficient varies
in the opposite way to produce almost constant values of aý, 0*
Using the transpiratioii/gas-chromatogralýlic method, the thermodynamic
properties of the ternary systems, methyl acetate-water-organic Solvcat,
using the organic solvents mentioned above (excepting, diglyme) have been
investigated, and the results indicate that the variation of *ýE with
solvent composition, for the dilute solutions of ester used, can be
estimated from the thermodynamic properties of the binary water-organic
solvent mixtures, using the Gibbs-Dahem equation.
Single ion activity coefficients in the literature for small negative
ions, to represent the OH_ ion, and for large ions, to rep-resent the
transition state ion, have been used to explain the experimentally fomd
variation of the residual activity coefficient -ratio with solvent
composition.
Hence, it is concluded that the importance of the parameters involved
in the hydrolysis of esters - an ion-molecule reaction - in aqueousorganic
solvent mixtures are in the order of
Ymolecule > aH
20>
YOH_/YM+ -> (dielectric constant),
and that the nonelectrostatic effects -- thermodynamic effects - are more
important in these studies than the electrostatic effects.
From a preliminary investigation of the data in the literature the
thermodynamic approach also yields a valid interpretation of the effect of
solvent composition on the rates of the acid hydrolysis of esters. / Ministry of Science and Higher Education of Iran
| Identifer | oai:union.ndltd.org:BRADFORD/oai:bradscholars.brad.ac.uk:10454/4352 |
| Date | January 1978 |
| Creators | Kazempour, Abdol Rassoul |
| Contributors | Diaper, John |
| Publisher | University of Bradford, Postgraduate School of Studies in Chemistry |
| Source Sets | Bradford Scholars |
| Language | English |
| Detected Language | English |
| Type | Thesis, doctoral, PhD |
| Rights | <a rel="license" href="http://creativecommons.org/licenses/by-nc-nd/3.0/"><img alt="Creative Commons License" style="border-width:0" src="http://i.creativecommons.org/l/by-nc-nd/3.0/88x31.png" /></a><br />The University of Bradford theses are licenced under a <a rel="license" href="http://creativecommons.org/licenses/by-nc-nd/3.0/">Creative Commons Licence</a>. |
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