A Kinetic and Mechanistic Study of the Alkaline Hydrolysis of Ethyl Acetoacetate by Acid-Base Determination of the p& of the Ester Rodrigo Paredes and Rogelio Ocampo Universidad del Valle, Apartado ABreo 25360, Cali, Colombia The hydrogen bonded to the carbon between the two carbonyls in p-ketocarbonyl compounds shows considerable acidity since the carbanion generated by its abstraction is stabilized bv hoth inductive and resonance effects ( I ) . Most of these ~ o ~ ~ o u are n dsufficiently s acidic that the derived carbanions can be generated in hydroxylic solvents such as water or alcohols, which have pK, values in the 15-20 range (2). The pK. values for carbon acids have been determined on the basis of the acid-base equilibrium: RHtB-*R-+BH
[ester],[OH-1,
--
[R-I
=
[RH][OH-]
3 K.
Time is1
Journal of Chemical Education
In [ester]
In [OH-]
l/[esterl
-6.40 -6.49 -6.59 -6.68 -6.77 -6.86 -6.95 -7.05 -7.09
-6.40 -6.49 -6.59 -6.68
603 661 724 794 87 1 955 1047
-6.77
-6.86 -6.95 -7.05
1148
-7.09
1202 1259
-7.14
-7.14
Table 2. Experlment2: 0.0100 M CHoCOCH2C02Etand 0.0105 M KOH Time (s)
pH
In [ester]
In [OH-]
1 In M a -X) -
60 216 566
1636 1803
11.34 11.32 11.28 11.24 11.20 11.16 11.14
-6.36 -6.44 -6.57 -6.69 -6.83 -6.96 -7.04
-6.12 -6.17 -6.26 -6.36 -6.45 -6.54
421 449 511 581 660 751
-6.59
2004
11.12
-7.11
802 856
912 1253
a-b
-6.63
sib-~)
Table 9. Experlment 5: 0.0100 M CHSCOCH2CO2E1and 0.0110 M KOH Time (s)
pH
In [ester]
In [OH-]
1 h-Ma-d -
a-b
afb-xi
(2)
The reported value for the pK. of ethyl acetoacetate is 10.7 (4). When the initial concentration of OH- used was slightly higher than that of the ester, the reaction remained first order as shown hv the fact that a plot of lu lesterl vs. time gaveastraight line whileaplot of lia - hln bio - r h ( b - x) versus time eave a curve Wables 2 and 3). When the straight line plots o f h [ester] and in [OH-] versus time were extended to intercept the in M axis, values corresponding to In [esterloand in [OH-10 were obtained. From these values the 72
KOH
(1)
where R H is the carbon acid and B- is a suitable base. The pK, of ethyl acetoacetate can be evaluated on the basis of the kinetics of the alkaline hvdrolvsis of the ester in which for each mole of OH- consumed-1 mol of ester is saponified. The kinetics of the reaction was studied by recording the decreasing pH of the reacting solution a t time intervals. For example for t h e reaction of 0.0100 M CH3COCH2C02Etwith 0.0100 M KOH (Table 1)aplot of in [ester] versus time gave a straight line while a plot of 11 [ester] gave a curve, thus indicating first-order kinetics (3). When equal volumes of 0.0200 M CH3COCH2C02Et and KOH were mixed (time 0) to nroduce the solution in which , starting pH was about lester],.,, = [OH-],",, = 0 . 0 1 0 d ~the 11.2 instead of the expected 12.00. This fact was interpreted as due to an initial very fast conversion of the ester &to its enolate, which rapidly establishes the ester-enolate equilibrium before proceeding further in the reaction. The starting ester and OH- concentrations ([esterlo and [OH-],,) can he evaluated hy extending the straight line of the plot of in [ester] versus time to intercept the in [ester] axis (time 0). When this was done a value of -6.34 was obtained for in [esterlo or in [OH-10, which correspond to startine ester or OH- concentrations of 0.00176 M. The starting enolate concentration [ e n ~ l a t eis] ~equal to 0.0100 0.00176 = 0.0082. The starting concentrations allow the determination of the pK, of the ester from eq 2. [enolate],
Table 1. Experiment 1: 0.0100 M CH,COCH2C02Etand 0.0100 M
Tabla 4. Calculated pKa's lor CH,COCH,CO,Et In Experiments 1,2, and 3 (25 OC) Experiment
[ester],.,
I 2
3
[OH-]mt
In [esterlo
in [OH-lo
pKa
0,0100
0.0100
0.0100 0.0100
0.0105 0.0110
-6.34 -6.36
-6.34 -6.11 -6.00
10.7 10.6
-6.50
10.6
pK, of the ester was calculated by eq 2. Table 4 shows the pK, values obtained in the three experiments performed. Simple esters such as ethyl propionate saponify by the B Amechanism ~ ~ showing second-order kinetics (5)since the rate-determinine sten is the formation of the tetrahedral negative intermidiat;. The kinetics of the alkaline hydrolysis of ethvl n r o ~ i o n a t ebv acid-base ootentiometrv was also studied Ln border to contrast its behavior to that of ethyl acetoacetate. For initial concentrations of 0.241 M ethyl propionate and 0.0100 M OH- (Table 5) a plot of l l a - b In b(a - x)la(b - x) versus time gave astraight line while a plot of In [ester] versus time gave a curve as expected of the second-order kinetics (3). Reruns This experiment illustrates the determination of the kinetic order for a simple reaction. Most esters show secondorder kinetics in their alkaline hydrolysis since they follow the B A mechanism ~ ~ (5). Since the experimentally determined kinetics for ethyl acetoacetate is first order, the ester must sanonifv bv a mechanism different from the classical B Amechanism. ~ ~ Three mechanisms have been proposed for the alkaline hvdrolvsis " - of ethvl acetoacetate which account for the first-order kinetics and some other experimental observations: ~
.~~ ..
~~~
1 . The Bw2 with an inert carbanion mechanism (6.7)involves the -~ -~~~
initial complete conversion of the ester into its enolate. Suhse-
- ~ - ~
nuant. hvrlrnlvria of enolnte resenerate low concentrations of 7---...,-...,... . .the ..~. ~
~
~
ester and OH-. These species react, in the rate-determiningstep, to form the negative tetrahedral intermediate that Lead to the products. 2. The keteneElcB mechanism (8)also involves the initial complete conversion of the ester into its enolate. However, in this case the enolate, in the rate-determining step, expels an ethoxide ion to form a reactive intermediate ketene that hydrolyzes in the alkaline medium to generate the products. 3. The cycJic enol mechanism (9, 10) first involves the establishment of a base-catalyzed k e h n o l equilibrium. Next, rate-determining nucleophilic addition of water to the cyclic end takes place to form an intermediate dipolar ion whose breakdown lead to the products (see figure). The existence of cyclic enols with deloealized bonding has been reported in the literature (11-13).
For first-order kinetics to be followed by mechanisms 1and 2. comnlete ionization of the ester to the enolate is required. -
-. - ~ - ~ . ~ ~
~
~
Experlmenlal A digital pH meter (Schat Gerate model CG 820) equipped with a combined electrode (Sehat Gerate type H61) was used for the pH measurements in experiments 1,2, and 3. In experiment 4 a Metrion V Perkin Elmer Coleman 80 pH meter equipped with Thomas high pH electrodeswas used. Redistilled water was used for the solutions. Experiments 1. 2, and 3 Bvmeans of volumetric oioets 100.0mL of 0.0200 M CHXOCHgC O solution ~ and 1lM.0 mi- of the KOH solution of the &roori-~ .. . ate roncentratiun (experiment 1.0.0200 M:experrment 2.0.U210 M: experiment 3.0.0220 MJ were poured into separate 200-ml. E r l ~ n meyer flasks. The flasks were stoppered and kept at room temperature (25 'C). An empty 400-mL beaker equipped with a magnetic stirring bar was olaced on too of s mametie stirrer. When the reaction was to he mn.ihe DHele$rndewas~ntroducedinto the beaker. Then simultaneously the two ~olutionrwere rapidly poured inw the beaker, the chronometer was started, nnd the magnetic stirring was turned on. pH readings were taken at time intervals. Tables 1,2,and 3 show the results ~~~~~~~~
~
~~~
~~
~
~
~
~
Experiment 4 An empty 200-mL hesker equipped with a magnetic stirring bar was placed on tap of s ma~meticstirrer.Eighty milliliters of a 0.0103 M KOH solution was poured into the beaker. and the magnetic stirring was turned on. he pH electrodes were introduced into the heaker, and 2.02 g of ethyl propionate (0.0198 mol) was added raoidlv to the alkaline solution. As soon as the addition was complete, the chronometer was started. pH readings werc taken nt time intervals. l'ahle 6 shows the results. The wltlme of r h solution ~ was measured at the end, arnuunting ro 82.3 ml.. ~
~
. ~ ~ ~ ~ ~ , ~~
~
~
~~
~
~
Table 5. Expwlmenl1: 0.241 CHoCH,M2Et and 0.0100 M KOH Time (s)
pH
[ester]
0 30 120 180 240
12.0 11.9 11.5 11.2 11.0
0.241 0.239 0.234 0.233 0.232
In [ester] -1.423 -1.431 -1.452 -1.457 -1.461
L i n - Ma-d
a(b- x)
a- b
0 0.96 4.86 7.83 9.80
~
T h e reported pK, value of 10.7 for ethyl acetoacetate (4) onlv allows 8W ionization a t the given initial concentrations ofekerand OH- ( 0 . 0 1 0 0 ~ )thusmaking . these mechanisms douhtful. On the other hand mechanism 3 does not require complete ionization of the ester to the enolate for first-order kinetics t o be followed, thus making this mechanism more plausible. Rate = k,[cyclic enol] [H,O] [H,O] =constant
k;
= k3[H201
Rate = k; [cyclic enol] But [cyclicenol] = [ester] Rate = k; K[esterl The experiment also illustrates the determination of the pK. of an enolizable 8-ketoester on the basis of the initial esterenolate eouilibrium. The exoeriment is m i t e easv and safe to perf or^. In addition to p H meter kquippLd with an electrode caoable of readinrr- nH's . in the alkaline region, common eq;ipment, glassware, and reagents are utilized. We have used it with good results in a 3-h laboratory as a physical organic chem6try experiment for our advanced undergraduate chemistry students.
a
The cyclic end mechanism.
Volume 67
Number 1 Janualy 1990
73
Acknowledgment We acknowledge the financial support of COLCIENCIAS and the interest and advice of Zvi Rappoport. Lltarature Cited 2nd ed,: Plenum: New Carey, F,A,: Sundberg, R, J , Aduoneed Ymk, 198k Part A. p 363. 2. Ref I , p 371. 2nd. od.: MeGrsw-HiU: New York. 1 9 6 % 465~ ~ 3. Bsrmw. G. M . P h ~ s i c dCh=-trx 4. Cram,D. J.findomenloL.ofCorbonion Chemi8try;Academic: New York. 1965;pp& 20.
74
Journal of Chemical Education
5. R S I, ~ pp 1214123. 6. Pratt.R. F.;Bmi~e,T.c.J.Am. Chem. Sac. 1910.92,5956-5964. 7. ~ o b f a a .S.; ~ ,~ ( z d y F. , J. J . ~ mC.h m . Soe. 1969,91.5171-5173. 8. Venkova Rao, G.;Bslakrishnam. M.: Venkataaubramanisn, N.; Subramanian. P. V.: Subramanian. V. Indian J. Chem. 1976; J4B, 465466. 9. Pandes. R.; Gil,J.Reu.Lolinoamar. Quim.,in press. 10. ~ ~ R.; oeampo, ~ R., d submitted ~ for pub~icatianr ~ . i n ~ m .ti^^^^^. quim. 11. Row1and.S. P.:Pearc~.L. Z.:Msck,C.H.: JanasenHJ. J. Cham.Soc. (B) 1968.40P 406. 12. Lowray, A. H.:Gnngs, C.; D'Antonio, P.; Karle, J. J. Am. Chem. Sor. 1911.93.63996403. n,M, S,SteticEiieefainOrg.nieChom raIry; Wiley:New 19S6;pp41513, 447
