Hydrolysis of Ethyl Acetate Derek Jaques
Royal M~litaryCollege of Canada K~ngston,Ontarlo, Canada
1
in Concentrated Sulfuric Acid A group experiment for advanced students
Ingold and his co-workers showed that esters can hydrolyze in acid solution by a uni- or a bimolecular process and that bond breaking can occur to give either an alkyl or an acyl ion (1). Thus there are four distinct mechanisms, AAJ, AAJ, AA12, and A A , ~ .There is overwhelming evidence to suggest that primary alkyl esters, such as ethyl acetate, hydrolyze by acyl oxygen fission ( 2 ) ,and so only the AA,l and AA,2 mechanisms will be considered further. In this exercise the effectof acid concentration on the rate of hydrolysis of ethyl acetate will be used to differentiate between the two possible mechanisms and to give information about the structure of the transition state complex. In order to save time and unnecessary repetitious experimental work, the author has found it convenient to give students rate data in the range 10dOyoaqueous sulfuric acid, and to ask each pair of students to determine one rate constant in the range 65-98% acid. The combined results of the whole class can then be correlated at the end of the series of experiments. This leads to a useful group discussion of the experiment and the questions given in a later section. Theory
where lSl,o,ac = [S(H,O)J
+ ISH(HSO)~+I
(8)
Combining eqns. (6), (7), and (8) yields
+
hs. = (kTlh)K*(aa,oY[SH(H~O)c+lfs~r~,o,.* (LSH(HL)).+I IS(HIOLI )f~*(n,o)~ (9) Taking logarithms and putting C gives
=
log(kT/h)K*
It has been shown that for ethyl acetate in sulfuric acid
(4) where Ho is the Hammett acidity function based upon the protonation of substituted anilines (6). It can readily be shown that
where
We can write the general mechanism for acid-catalyzed primary ester hydrolysis using fully hydrated species (3)
Substituting eqn. (12) into eqn. (10) and rearranging gives log k.sSa
where S represents the ester. It follows from transition state theory that the rate of reaction is
where K* is the equilibrium constant in eqn. ( 2 ) . Rearranging eqn. (4)
By experiment the observed rate is kob[Sltotaz
log a m
+C
(14)
It is assumed that the ratio of activity coefficientof the conjugate acid and t,he transition state complex is approximately independent of the medium because both species have the same charge and have similar structures. We can determine whether the reaction is AAJ ( r e O ) or AA,2 (r? 1 ) by measuring the slope of a graph of log kOb,( a + l ) / a versus log aa,o. The Series of Experiments
CH3C02Et
=
T
The hydrolysis is followed by estimating the ethyl acetate remaining after suitable intervals of time. The method is based upon the quantitative conversion of the ester to its hydroxamie acid by alkaline hydroxylamine
Substituting eqn. (5) into eqn. (3) gives
Rate
+ l)/a e
(7 )
+ NHIOH = CHaCONHOH + EtOH
The concentration of hydroxamic acid is determined speotrophotometrically by utilizing its ferric chloride complex (6). Each pair of students is provided with 2 matched 1-em cells, a Unicam SP500 spectrophotometer, s. sample of purified ethyl
Volume 48, Number 9, September 1971
/
623
,
,
"
~
,
,
ehloric acid, and an aqueous solution of concentrated sulfuric acid. They use the approximate concentration written on the bottle to calculate the times to take samples of ester solution during the kinetic run (Table 1). At some convenient time they determine the exact concentration of scid (7). The first pair of students are asked t o verify Beer's law for the complex and hence provides check on the solutions. Table 1.
Approximate Sample Times for Various Acid Concentrations
%
HISO.
Sample Times (mi")
%H?SO,
Figure 1. The effect of acid concentrmtion on the hydrolysis of ethyl acetate in aqueous sulfuric acid at 25-C 16, 8).
Verification o f Beer's Law
Pipet 1 ml of purified ethyl acetate into a 100-ml volumetric flask and make up to the mark with distilled water. Pipet 1, 2, 3, 4, 5, 6, 7, 8, 9, and 10 ml samples into ten 500-ml volumetric flasks and make up to 500 ml with distilled water. To a 4-ml sample of each solution in turn, add 2 rnl of 2 M hydroxylamine hydrochloride and 2 ml of 2.5 N sodium hydroxide. At this stage the solution should he alkaline. After standing for 10 min add 1.5 ml of 10 N hydrochloric acid followed by 1 ml of 15% (WIV) ferric chloride in 0.2 N hydrochloric acid. At this stage the solution should be acidic. Measure the optical density (D) a t 500 m r within 2 min af mixing using 8. blank solution prepared by substituting 4 ml of distilled water in place of 4 ml of diluted ethyl acetate solution in the above procedure. Plot the optical density against the volume of aqueous ethyl acetate solution used and hence verify Beer's law. The remaining student pairs carry out a kinetic experiment in which each pair uses a differentsulfuric acid concentration. NOTE TO SUPERVISOR. Experiments in the 80-907, acid region require a continuous 10-12 hr period which may not be available in many institutions. Where only a 3-hr or 6-hr period is available the students can concentrate on the appropriate acid region and the rate constants can he supplemented from the literature (4, 6 ) . Kinetic Run
Pipet 50 ml of aqueous sulfuric scid solution into a 100-ml stoppered flask and place in a 25'C thermostat together with a. flask containing the ethyl acetate. After attainment of temperature equilibrium, add 1 ml of ester to the acid and note the time. At the predetermined times withdraw 5 ml samples and pour onto 30 gm of ice which has been frozen solid with a Dry Ice/acetone mixture. Rapidly neutralize with 14y0 and 0.1 N sodium hydroxide using phenolphthalein as indicator. At this stage the ice should have just disappeared with the mast concentrated sulfuric acid solution. Mske up to 500 ml with distilled water and to a 4-ml sample add 2 ml of 2 M hydroxylamine hydrochloride and 2 ml of 2.5 AT sodium hydroxide. After standing for 10 mi", add 1.5 ml of 10 N hydrochloric acid, followed by 1 ml of 15y0 (W/V) ferric chloride in 0.2 N hydrochloric acid. Measure the optical density (D) a t 500 mp within 2 min of mixing using a blank solution prepared hy substituting 4 ml of distilled water for the 4 ml of sample in the above procedure. To obtain the infinity reading (D,) select one of the following procedures. The most convenient one will depend upon the availability of time. 1 ) Return to the thermostat a t a time a t least equal to that given in the last column of Table 1 and take a final 5-ml sample. 2) At some convenient time during the kinetic run, pipet 5 ml of reaction mixture into a stoppered bottle and suspend
624
/
Journal of Chemical Education
'- % p Figure 2. Relationship b e h e e n the rate of hydrolysis of protonoted ester and the octivily of water.
it in boiling water for sufficient time to ensure completion of the reaction. If you assume that the rate doubles for every 10°C rise in temperature you will err on the right side. From a gra-ph of log (D - D,) versus time, calculate the pseudo firshrder rate constant (k.s.). Results and Discussion
Each student is required to plot the following graphs: (i) Ic,,versus a/o H2S04(Fig. 1) and (ii) log k,,, ( a l ) / a versus log aaXo(Fig. 2) making use of the results of Tahle 2 (S), his own result, and those of the other members of the class. Each pair of students computes a different point for both graphs. Example: A typical student result is shown in Figure 3 from which k,,, = 2.44 X min-' in 86.5% aqueous sulfuric acid. . From Table 3 the value of H , is estimated as -8.37 and log as -3.14. By eqn. (11)
+
log a = 0.645 (-6.93
+ 8.37) = 0.929
Toble 2. First-order Rate Constants for the Hydrolysis of 0.2 M Ethyl Acetate in Aqueous Sulfuric Acid Solutions a t 2 5 ' C (8)
Table 3. H@ Values (5) and Logarithm of the W a t e r Activity Valuer (9) for the Concentrated Aqueous Sulfuric Acid Region
What is the mechanism in the 10-80% aqueous sulphuric acid region? Al. r = 2.1, therefore A&. Q2. And in the 85-98% region? A2. r = -0.2, therefore A d . Q3. The rate determining step for the A A , ~mechanism is therefore 1
where the hydration of protonatad ester and transition state complex are omitted for simplicity. What is a possible structure for the transition state?
which was first suggested by Lane (10).
Q4. How does the maximum in Figure 1arise?
A4. The rate of hydrolysis of ester increases with increasing acid concentration but decreases with decreasing water concentration, hence the net result is s maximum in the rate. Q5. Why does the unimolecular mechanism hewme predominant in the region of high acidity? A5. The concentration of "avdable water" for the An.2 mechanism is so small that a. mechanism which does not require "mililable water'' is favored. Once an acyl ion is formed it immediately dehydrates a nearby hydrated sulfuric acid molecule. Figure 3.
A typical student result in 86.5% aqueous sulfuric acid.
Literature Cited
and therefore log k d n
a!
+ l)/a
= =
8.49.
2.44 X
X 9.49/8.49 = 3.436
=
-2.56
This result is shown on Figures 1 and 2 as a cross. Finally the slopes of the two straight lines on the graph (see Fig. 2) are measured. After a class discussion of the experiment the students write up the experiment, tabulate the results of the whole class, and give written answers to the following questions. Suggested answers are given but they are not the only possible ones.
(1) INaom, C. K. "Structure and Meohsniem in Organic Chemistry," Bell and Co.. London. 1953, p. 754. (2) Sea, e.g.. Ref. (1) p. 767-782; K ~ n a a ~ w D., N.. AND LEISTEN, J. A. Pvor. Chem. Soc., 84 (1960). R.NA., D .J . Amw. Chem. Soc.. 89, 2686 (3) . . YATEB.K..A N D M C C ~ ~ L L A (mi). M. , F. AND DORSEY, G . F., J . Amw. Chcm. Soc.. (4) LANE.C. A,. C ~ U N O 90, 6492 (1968). (5) Jonomaos. M. J., AND H ~ n r n s D. , R.. J . Amer. Chsm. Soc.. 85, 878 (1963). (6) JAOUEB.D..J . Chem. Soo.. 3874 (1965). (7) Voaer.. A. I.. "Quantitative Inorghnic Analysis" (2nd sd.), Longmans, Green & Co.. 1951, p. 241. (8) Bmr, R. P., DOVDINO, A. L., AND NOBLE, J. A. J . Chcm. Soe., 3106 (1955). (9) Grmque. W. F., Hanauso. E. W., K o l r z ~ s nJ. . E., A N D RneLw, T.R. J . Amw. Chem. Soe.. 82.62 (1960). (10) L m n , C. A,, J. Amcr. Chcm. Soc., 86, 2521 (1964).
Volume 48, Number 9, September 1971
/
625
