Determination of the Relative Rates of Alkaline Hydrolysis of Esters by the Method of Competition Rodrlgo Paredes, Jimmy Oil, and Pamela Ocampo Universidad del Valle, Apartado ABreo 25360,Cali, Colombia The alkaline hydrolysis of an ester (saponification) is an irreversible reaction which normally takes place by a B ~ c 2 mechanism'
The rate-determining step is the formation of the anionic tetrahedral intermediate 1. Electron-withdrawing substituents in either the acyl group or the alkoxy group of the ester facilitate hydrolysis since the negatively charged tetrahedral intermediate, and the transition state leading to it, will be stabilized by electron withdrawal. Substitution of hydrogens with alkyl groups in either the acyl or the alkoxy . . groups of the ester retard hydrolysis since steric crowding in the transition state leading to the tetrahedral intermediate is stronelv -.resisted. Manv e x a m ~ l e sof these behaviors are known for different tvpes of esters'. L'tilization of the method oresented here oermit further illustration of the electronic and steric factok in the saponification of selected groups of esters.
Theoretical Approach The relative rate of alkaline hydrolysis for a pair of suitable esters is determined on the basis of the competition3 between equimolar amounts of the two esters for a limited amount of NaOH (0.80 times the molar amount of each
ester). I t is clear that in the competition the more reactive ester gets saponified to a larger extent than the less reactive one. Since the reaction is irreversible, the molar amounts of the saponified esters in the final reaction mixture is equal to the initial molar amount of NaOH. Also for each ester the molar amount of the saponified and unsaponified fractions is equal to the initial molar amount of the ester. The experimental procedure we have developed allows the separation of the unreacted mixture of esters. Analysis of this mixture by GC or NMR yields the molar ratio b of the unsaponified esters in the final reaction mixture. Assuming we use 0.025 mol of ester A, 0.025 mol of ester B and 0.020 mol of NaOH, the following relationships exist in the final reaction mixture:
where x , = moles of unsaponified
ester A
x, = moles of saponified ester A y , = moles of unsaponified ester B y , = moles of
saponified ester B
These equations can he simplified by adding eqs 1and 2 and substituting eq 3 in the resulting eq 5:
Equations 4 and 6 can be easily solved to yield the values of and y,. The values of 1.2 and y2 can then be obtained from eqs 1and 2. The relative rate of saponification of ester A to ester B is
XI
'Carey, F. A,: Sundberg, R. J. Advanced Organic Chemistry, 2nd ed.: Plenum: New York. 1984:p. 421. Morrison, R. T.: Boyd, R. N. Organic Chemistry. 4th ad.: Allyn: Boston, 1983:p 385. Ref 2,p 107.
rate of saponification of ester A = 3 rate of saponification of ester B y ,
Volume 65 Number I2 December 1968
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Experimental First, 0.025 mol of pure ester A and 0.025 moi of pure ester B are successively weighted in a flask. Five milliliters of CCL is then added to the ester mixture. The mixture is made homogeneous hy stirring. An NMR spectrum of the ester solution can he taken to check its composition. Then the required volume (to contain 0.020 ma1 of NaOH) of a standardahout 2 M aqueous solutionof NaOH is added to the total CC14 solution of the esters. The hetemgenous mixture is vigorously stirred magnetically at room temperature until all the NaOH is consumed. This is easily determined by stopping the stirring and measuring the pH of the upper aqueous layer with pH paper. When the pH is about 7 (30-70 min), the layers are separated by means of a separatory funnel. The CClr unsaponified ester mixture solution is then washed twice with 4-mL portions of water and then dried with anhydrous Mg.501. The molar ratio b of the unsaponified ester pair is then determined by gas-liquid chromatography or by NMR. For the ester pairs that we have used, NMR proton counting analysis is quite suitable since the n-hydmgens to the carhonyl of the malonic and of the propionic esters appear in distinct regions free of any interferences. Results We have used the previous method to measure the relative rates of saponification of the following three pairs of esters: Pair number 1
Esters
Relative Rate
Eto2CCH2CO2E1:CH3CH2CO2Et
CHJ
I
2:l
CHI
i
The relative rates for the group of the four esters are easily calculated since ethyl isobutyrate saponifies the slowest and therefore 1can he assigned as its rate.
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Journal of Chemical Education
Unalkylated eslers
Monoalkylated esters CHI
Malonic esters
E10PCH2C02Et
4
E ~ O ~ C C H C O ~ E3~ CHs
Propionicesters
CH3CH2C02Et
2
CHsCHC02Et
I
1
For the malonate esters the rate of saponification of the first ester group is much more rapid than that of the second. For e x a m ~ l ewhen . diethvl malonate is treated with an eauimolar amo"nt of KOH in ethanol, the only product forked is potassium ethvl malonate4. The water h:vdrolysis of the esters at room temperature is a much slower Drocess than the alkaline hvdrolssis. . . . and therefore it can be disregarded. We have used the determination of the relative rates for the three pairs of esters given above as a physical-organic experiment for our advanced undergraduate chemistry students. Each pair of esters is assigned to a group of two students for the determination of the relative rate of saponification. At the end of a three-hour ~ e r i o dthe three .erouos of students integrate their data for the dettrmination of the relative rates of saponification for the whole group of esters.
.
Acknowledgment We acknowledge the financial support of COLCIENCIAS for this work.
Rabjohn, N.. Ed. Organic Syntheses: Wiley: New York. 1963: Voi. 4, p 417.
