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