I Keto-Enol Tautomerism of Ethyl Acetoacetate I
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I Keto-Enol Tautomerism of C. H. Ward1 Auburn University I ' Ethyl Acetoacetate Auburn, Alabama I Experiment in homogeneous equilibrium The concept of chemical equilibrium and the actual laboratory determination of equilib- rium 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 de- sirable to include a laboratory experiment on the de- termination of the equilibrium constant of a homog- eneous 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 equi- librium 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 mate- rially altered, thus yielding fictitious equilibrium con- stant 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 experi- ment. The ethyl acetoacetate system requires no catalyst, equilibrium is repidly attained so that the experiment can be easily completed in a 3-hr labor- atory period, and results of good accuracy and pre- cision 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 Ordinary ethyl acetoacetate, either pure or in solu- tion, is an equilibrium mixture containing mole- cules of two different strnctures: 0 0 II I1 CH3-C-CHrC-O-CHrCHs F? Keto form OH 0 I II CH-C=CH-C-0-CH,CHs End form The analytical method for determining the equi- librium 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 can then be found by difference. Such a direct ti- tration is called the Kurt Meyer titration (5). How- ever, the direct Kurt Meyer titration is not capable of high accuracy because the hydrogen bromide liber- ated 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 fol- lowed by addition of more than enough beta-naphthol to react with the bromine remaining after rapid bro- mination 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: 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 0 I Il cHs-4-cm-c-0-cHx-cHa + H+ + 21- - 0 0 The equilibrium constant is then evaluated from the analytical data, i.e., C d K., = K, = - 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 / 95
Transcript of I Keto-Enol Tautomerism of Ethyl Acetoacetate I
I Keto-Enol Tautomerism of C. H. Ward1
Auburn University I ' Ethyl Acetoacetate Auburn, Alabama I Experiment in homogeneous equilibrium
The concept of chemical equilibrium and the actual laboratory determination of equilib- rium 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 de- sirable to include a laboratory experiment on the de- termination of the equilibrium constant of a homog- eneous 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 equi- librium 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 mate- rially altered, thus yielding fictitious equilibrium con- stant 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 experi- ment. The ethyl acetoacetate system requires no catalyst, equilibrium is repidly attained so that the experiment can be easily completed in a 3-hr labor- atory period, and results of good accuracy and pre- cision 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
Ordinary ethyl acetoacetate, either pure or in solu- tion, is an equilibrium mixture containing mole- cules of two different strnctures:
0 0 II I1
CH3-C-CHrC-O-CHrCHs F? Keto form
OH 0 I II
CH-C=CH-C-0-CH,CHs End form
The analytical method for determining the equi- librium 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
can then be found by difference. Such a direct ti- tration is called the Kurt Meyer titration (5). How- ever, the direct Kurt Meyer titration is not capable of high accuracy because the hydrogen bromide liber- ated 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 fol- lowed by addition of more than enough beta-naphthol to react with the bromine remaining after rapid bro- mination 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:
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 0 I I l
cHs-4-cm-c-0-cHx-cHa + H + + 21- - 0 0
The equilibrium constant is then evaluated from the analytical data, i.e.,
C d K., = K, = - 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
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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 bro- minated 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 pre- pared once a week.
Procedure. Three different concentrations (approx- imately 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 con- stant temperature bath at 25°C for about 45 minutes during which time the samples are shaken occasion- ally.
Table 1. Typical Data for the Keto-Enol Equilibrium Constant Determination
Average Orig. vol. conc. 0.100 M Equi- ester NaaSlOa C d Cb, librium
(molarit") (ml) (molarity) (molanty) constant
Fifty-ml portions of each solution are then t,rans- ferred 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
used to measure the bromine solution and the beta- naphthol solution.
Fift,v ml of aonroximatelv 0.1 M ootassium iodide ~" L~ &~ " 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.
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 evalu- ation 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 com- parison, 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 con- centrations.
Literature Cited
(1) CROCKFORD, H. D., AND NOWELL, J. W., "Laboratory Manual of Physical Chemistry," John Wiley & Sons, Inc., New York. 1956. 0.92.
periments," 3rd . .
(2) LIVIN~STON, ROBERT, " P h y ~ i ~ o Chemical Ex ed., The Macmillan Co., New York, 1957, p. 137.
(3) DANIELS, F., ET AL., "Experimental Physical Chemistry," 4th ed., MoGraw-Hill I r, 1949, 3ook Co., Inc., New Yorl 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). 16) KOLTHOPF. I . M.. AND SANDELL. E. B.. 3rd ed.. "Textbook
of Quantitative Inorganic Analysis," The Maemillan Co., New York, 1952, pp. 592, 594.
' 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.
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