K. 1. Lockwood
Lebanon Valley College Annville, Pennsylvania
Solvent Effect on the Keto-Enol Equilibrium of Acetoacetic Ester
The purpose of this investigation is to introduce the students to some of the factors that influence an equilibrium constant. The reaction chosen for study is the keto-en01 tautomerization of acetoacetic ester:
The variation of the en01 content with changing environment has been discussed thoroughly by Wheland (1). The value of the equilibrium constant (K,, = [enol]/[keto]) depends a t least upon concentration of the acetoacetic ester, the solvent, and the temperature. Thus, the necessity is demonstrated for stating the relevant conditions under which the equilibrium constant is evaluated. The keto-en01 equilibrium of acetoacetic ester has been studied since 1911. The first analytical procedure used was the method of bromination of the en01 (2); more recently, UV spectroscopic (3) and potentiometric (4) methods have been applied. The problem as presented to the students is to evaluate the equilibrium constant for the tautomerization of acetoacetic ester in methyl, n-propyl, and t-butyl alcohols a t two concentrations (0.5 M and 2.5 M ) of ester in each of the solvents. The procedure used is essentially the modified Kurt Meyer method as published by Ward (5). The equilibrium en01 concentration is found by utilizing its rapid reaction with bromine (added in excess). The excess bromine is destroyed immediately to prevent further consumption of the bromine by en01 produced by tautomerization of keto after the analysis has been started; this is done by the addition of excess beta-naphthol. The amount of bromine consumed by the en01 only is found by taking advantage of the peculiar property of the bromined en01 to oxidize iodide to iodine. The liberated iodine is titrated with standard sodium thiosulfate. Solutions of assigned molarity are prepared by each student by weighing distilled acetoacetic ester (of the best obtainable quality, distilled under reduced pressure immediately before use) into a volumetric flask and dilut,ing with the assigned solvent at 25'C. A solution is allowed to stand for 24 hours to permit the equilibrium to be established. A 10.00-ml aliquot of the acetoacetic ester solution is pipetted into a clean, dry glass-sloppered Erlenmeyer flask. Then, 10 ml of an approximately 0.2 IM solution of bromine in the assigned solvent is measured in a graduated cylinder and added to the reaction flask. If the resulting solution does not show the color of bromine a t this point, adjustments in quantities of reagents are necessary. The solution is
thoroughly shaken and immediately 10 ml of a 10% solution of beta-naphthol in the assigned solvent is added. Finally, 25 ml of an approximately 0.1 M aqueous potassium iodide solution is added and the sample allowed to stand for about 15 min a t room temperature. The liberated iodine is titrated with standard sodium thiosulfate without starch indicator. The table indicates the results obtained in a typical class; the standard deviation, s, of the results from the mean are included. Effects of temperature variation can be minimized, though not eliminated, if determinations are made by students working under nearly identical conditions within the same time limits. Results seem to he more reproducible with methyl and t-butyl alcohols as solvents than with n-prnpyl alcohol. Considerable variation of, and "fading" in, end points are experienced by students who use n-propyl alcohol (commercial reagent grade). Apparently bromine reacts either with the n-propyl alcohol or with some impurity that is present. The student data are collected and the effectsof the solvent on the position of the keto-en01 equilibria are discussed in terms of polarities of solute and solvent and in terms of the en01 content of the pure acetoacetic ester (6% enol, K,, = 0.064; ref. 1). The en01 is less polar than the keto tautomer because of intramolecular hydrogen bonding, which leads to partial intramolecular neutralization of dipoles. The dipole moments of the three alcohols used in this particular study are virtually the same [1.66 Debye units (6)]. The dielectric constants (methanol, 32.6; n-propyl alcohol,20.1; t-butyl alcohol,10.9) indicate that methyl alcohol should be best able to support the more highly polar keto isomer, while tbutyl alcohol should be poorest. Thus the K.,, as defined, should be highest in tbutyl alcohol. Further, as the concentration of the acetoacetic ester increases, the en01 content should more nearly approach that of the pure ester. The data presented may thus be interpreted, with K., = [enol]/[keto] for Acetoacetic Ester Tautomerization,
25°C Solvent
(alcohol)
Methyl n-Propyl tButyl
Concentration (molar) 0.5 2.5 0.5 2.5 0.5 2.5
Trials 7 18 20 14
7 8
Ken mean
s
0.0717 0.0730 0.176 0.109 0.148 0.132
0.00252 0.00371 0.00679 0.0276 0.00112 0.00642
Volume 42, Number 9, September 1965 / 481
reservations, to support these generalities. The figures for the mpropyl alcohol systems are in question because of difficulties with this solvent. The equilibrium constants for the methyl alcohol systems are so close to that of the pure ester that little change due to concentration effects could be anticipated. In addition to the concentration effects on the equilibrium, several other aspects of the keto-enol systems could he studied. Other solvents which might he investigated include acetic acid, dimethylformamide (DMF), acetonit,rile, or dimethyl sulfoxide (DMSO). The solveuts should be miscible with water to permit titration with aqueous sodium thiosulfate. Temperature effects on the position of the equilibrium could be studied. Among those compounds beside acetoacetic
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ester which have a high en01 content, 2-acetylcyclohexanone (7) and acetylacetone (2,4-pentanedione) might be suggested for laboratory study. Literature Cited (1) WHEL~ND, G . W., "Advanced Organic Chemistry," John Wiley & Sons, Inc., New York, 1960, pp. 669-93. (2) MEYER,K. H., Ann., 380,212 (1911). (3) RUSSELL,P. B., J. Am. Chem. Sac., 74,2654(1952). (4) MAUSER, H.A N D NICKEL, B., Be?., 97,1745, 1753 (1964). (5) WARD,C. H., J. CAEM.EDUC.,39,95(1962). E. E., JR., "Org~nicSolvents," (6) RIDDICK,3. A,, AND TOOPS,
Vol. VII of Weissberger's "Technique of Organic Chemistry," 2nd ed., Interscience Publishers, Inc., New York, N - .. v- ., -1 I )- - . (7) RILEY, T., AND LONO,F. A,, J. .4m. C h m . Sac., 84, 522 ?
(1962).
