C. H. Ward1 Auburn University Auburn, Alabama
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Keto-Enol Tautomerism of Ethyl Acetoacetate Experiment in homogeneous equilibrium
The concept of chemical equilibrium and the actual laboratory determination of equilibrium constants are of fundamental importance in any undergraduate physical chemistry laboratory course. Since in practice most common chemical reactions are carried out in solution, it is highly desirable to include a laboratory experiment on the determination of the equilibrium constant of a homogeneous reaction in solution. At the present time, the hydrolysis of ethyl acetate is almost invariably studied to illustrate homogeneous equilibria in liquid systems (1, 8, 5). This system exhibits several rather serious shortcomings. The attainment of equilibrium is slow so that more than one laboratory period is required to complete the experiment even when a catalyst (usually concentrated hydrochloric acid) is used; furthermore, the concentration of catalyst required is so great that the system is materially altered, thus yielding fictitious equilibrium constant values; and results of low precision are commonly obtained due to factors such as evaporation losses. Tautomerism of ethyl acetoacetate provides the basis for a much more satisfactory laboratory experiment. The ethyl acetoacetate system requires no catalyst, equilibrium is repidly attained so that the experiment can be easily completed in a 3-hr laboratory period, and results of good accuracy and precision have been routinely obtained during the past. three years in this laboratory. A study of this system has the added advantage of lending physical reality to the rather abstract concept of tautomerism for the beginning student of organic chemistry. The Ethyl Acetoacetate System
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OH
0
+ Bra
+ HBr
The amount of the bromoester (and consequently the amount of en01 present in the original system) is then determined by addition of excess potassium iodide and subsequent titration of the liberated iodine with standard thiosulfate. 0
Ordinary ethyl acetoacetate, either pure or in solution, is an equilibrium mixture containing molecules of two different strnctures: 0
can then be found by difference. Such a direct titration is called the Kurt Meyer titration (5). However, the direct Kurt Meyer titration is not capable of high accuracy because the hydrogen bromide liberated on reaction of the bromine with the en01 form catalyzes the transformation of more keto form into enol and consequently the end point is not sharp and is in error. An indirect method which is more accurate is therefore usually employed and is so used in the present experiment. This indirect method involves adding excess bromine to the keto-en01 system followed by addition of more than enough beta-naphthol to react with the bromine remaining after rapid bromination of the en01 originally present. As a result of these two operations, all en01 in the original mixture is converted to the bromo derivative and the excess bromine is destroyed. The reactions are as follows:
0
Il
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+ H + + 21-
cHs-4-cm-c-0-cHx-cHa 0
-
0
0
I1
CH3-C-CHrC-O-CHrCHs Keto form
F?
OH
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0
The equilibrium constant is then evaluated from the analytical data, i.e.,
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K.,
CH-C=CH-C-0-CH,CHs E n d form
The analytical method for determining the equilibrium point of this system is based on the fact that the en01 form reacts exceedingly rapidly with bromine, while the keto form does not (4). Direct titration with bromine until bromination is no longer rapid can thus determine the en01 content, and the keto content
=
K,
C d
= -
Ct.,,
The Experiment
Reagents. 0.4 M ethyl acetoacetate in methanol, 0.1 M bromine in methanol, 10% beta-naphthol in methanol, 0.1 M aqueous potassium iodide solution, 0.1 M aqueous sodium thiosulfate solution (6),and methanol. The standard stock solution of ethyl Volume 39, Number 2, February 1962
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acetoacetate in methanol may be made by direct weighing of the solute; or a solution of approximate molarity may be prepared by rough weighing and the exact concentration det,ermined by adding excess bromine to an aliquot sample. The sample is allowed to stand for several minutes to allow for transformation of all keto form to the en01 form, which is then brominated and analyzed as noted below in the section on procedure. The stock solution may be stored for several months wit,hout change in concentration if bottles are properly sealed. The concentration of the bromine solution need only be known approximately, and so can be prepared by rough weighing of the bromine in a hood. Fresh solution should be prepared once a week. Procedure. Three different concentrations (approximately 0.4 M, 0.2 M, and 0.1 M ) of ethyl acetoacetate in methanol are prepared by dilution of the stock solution. The three samples are then placed in a constant temperature bath at 25°C for about 45 minutes during which time the samples are shaken occasionally. Table 1.
Orig. conc. ester (molarit")
Typical Data for the Keto-Enol Equilibrium Constant Determination
Average vol. 0.100 M NaaSlOa (ml)
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Calculations and Results
Equilibrium concentrations of the en01 and keto forms are then calculated from the analytical data for each of the solutions and the equilibrium constant calculated. It will be noted that t,he simple evaluation of the amount of 1% measures the amount of en01 (1 mole en01 = 1 mole 1%= 2 moles Sz03-2). Typical data are given in Table 1. For a rough comparison, a value of K. of 0.074 may be calculated from the data of Meyer (5) which was measured a t O°C wing ethyl acetoacetate solutions of unspecified concentrations. Literature Cited
(1) CROCKFORD, H. D., AND NOWELL, J. W., "Laboratory Manual of Physical Chemistry," John Wiley & Sons, Inc., New York. 1956. 0.92. (2) LIVIN~STON, ROBERT, " P h y ~ i ~Chemical o Ex periments," 3rd ed., The Macmillan Co., New York, 1957, p. 137. (3) DANIELS,F., ET AL., "Experimental Physical Chemistry," 4th ed., MoGraw-Hill I3ook Co., Inc., New Yorlr, 1949, p. 107. (4) W ~ L A N D G., W., "Advanced Organic Chemistry," John Wiley and Sons, Inc., New York, 1960, p. 670. (5) MEYER,K. H., Ann., 380, 212 (1911). E. B.. 3rd ed.. "Textbook 16) KOLTHOPF. I. M.. AND SANDELL. of Quantitative Inorganic Analysis," The Maemillan Co., New York, 1952, pp. 592, 594.
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Cb, C d (molarity) (molanty)
Equilibrium constant
Fifty-ml portions of each solution are then t,ransferred to 500-ml Erlenmeyer flasks. 0.1 M bromine solution in methanol is added to each aliquot (50 ml for the 0.4 M and 0.2 M samples, and 25 ml for the 0.1 M sample) with shaking, followed immediately by the addition of 10 ml of beta-naphthol solution. These two operations, addition of bromine and subsequent addition of beta-naphthol solution, should be completed in one minute or less. Graduated cylinders may be
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used to measure the bromine solution and the betanaphthol solution. Fift,v ~" ml of aonroximatelv " 0.1 M ootassium iodide solution are then added to each sample, and the samples are allowed to stand for 15 minutes with frequent shaking. The samples are then titrated with standard thiosulfate solution.' All titrations should be done in duplicate.
Journal of Chemical Education
' Shrch indicator solution is not used since high methanol concentrations prevent the development of the blue complex. Samples are titrsted to the colorless end point. If slightly impure (oxidized) beta-naphthol is employed, the solution st the end point will have a very faint brownish tinge. The end point tends to fade somewhat so that the titration is not complete until the end point color persists for at least three minutes.
