J. Graham Dawber and Marguerita M. Crane North Staffordshire College of Technology Staffordshire, England
A
I I
Keto-Enol Tautomerization A thermodynamic a n d kinetic study
recent paper ( I ) in THIS .JOURNAL described a student project to study the effects of changes of solvent upon the keto-en01 tautomerization of acetoacetic ester. We have extended this investigation by measuring the extent of enolization of acetoacetic ester and acetyl acetone at various temperatures. This enables the thermodynamics of these two systems to be studied and compared. I n addition we have investigated the kmetics of enolization at various temperatures and hence obtained the activation energies for the enolization of these compounds. The experimental work was kept as simple as possible and could be carried out quite satisfactorily by students. There are many methods of determining en01 content, (I)-(7). The methods used in this work were (a) infrared spectroscopy to give a qualitative indication of relative amounts of keto and en01 forms, (b) analytical, involving bromination of the enol, ( c ) molar refraction. The infrared spectra of the purified liquids were measured a t room temperature as liquid films between NaCl plates with a Unicam SP 200 spectrophotometer. The spectra are reproduced in Figure 1, and their interpretation (8) is summarized in Figure 2. The spectra show qualitatively that acetyl acetone contains more of the en01 form than acetoacetic ester. This information is used in modifying the amounts of reagents in the analytical method of determining the en01 content. The analytical method used was a slight modification Solutions of the comof that described bv " Voeel (7). ~, pounds in methanol give the most reproducible results for the bromination method (I). Furthermore, the
equilibrium constant for enolization of amtoacetic ester in methanol is fairly close to the valuc for the pure ester itself. Hence this solvent was used for thc analytical determinations of en01 content. The excess bromine is removed by the additiou of di-iso-butylene, The successful measurement of the
-
150
/
Journal o f Chemical Education
Figure 1 . Infrared specfro of acetoacetic ester and ocetyl metone or liquid Rlmr
equilibrium en01 content depends on the rapidity with which this can be added. I n this work, the di-isobutylnie was added to several samples after varying intervals of timc. This enables not only theequilibrium en01 cont,ent to be obtained, but also allows the kinetics of the enolization process to be studied. The measurements were carried out at 20, 30, 40, and 50°C for acetoacetic cster and at 20 and 30°C for acetyl acetone. For t,he measurements with acetoacetic ester, a soluocetoacetic cster
Figure 3.
K=O 1735, 1710
crn:
3 0 H qC=O W p C 300? 1645 crn: cm:'
acetyi acetone
Determination of heat of enolir~tionof acetoacetic ester.
VNM %en01 (Z) = 20W-
where V is the volume of thiosulfate used and N is its normality, M is the molecular weight of the acetoacetic ester (or acetyl acetone), and W is the weight of the compound in the 25-ml aliquot. The percentage equilibrium en01 content, 20,is obtained by extrapolation of the results to zero time of addition of di-iso-butylene. The equilibrium constant is given by K = [enoll -=
[keto]
K=O 1720, 1700
crn:' Figure 2.
+
+-OH K = O 3000 cm:'
K=C
1610
crn.-I
Interpretation of IR spectra.
tion in anhydrous methanol, containing 15 g/l of ester, was convenient. Twenty-fivemilliliter aliquots of this solut,ioll were placed in stoppered flasks and equilibrated at, thc required temperature. Seven milliliters of freshly made methanolic bromine solution (8-10 g/l) at the rcquired temperature was added rapidly to each flask and the timer started. The excess bromine was removed after various intervals of time (3-120 see) by adding about 5 ml of di-iso-butylene to each flask to stop t,hc reaction. About 10 ml of 10% aqueouspotassium iodide was added to each flask, which was then warmed to about 30°C and allowed to stand for 30 min. The contents of each flask were diluted with wat,er and titrated with standard 0.1 N sodium thiosulfat,~, using starch indicator near the end-point. The en01 content of each sample is given (7) by Table 1.
The quantities have to be slightly modified for acetyl acetone. Twenty-five-milliliter aliquots of a methanolic solution containing 3 g/l is suitable. Figure 3 shows the plot of log K against the reciprocal of the absolute temperature for acetoacetic ester. AH for the enolization is calculated from the slope and is given in Table 1, along with other data for acetoacetic ester and also acetyl acetone. Comparison with data obtained by NMR spectroscopy (6) for related compounds in carbon tetrachloride (not methanol) indicate our results to be reasonable. If we assume that the bromination of the en01 is very rapid and that the enolization is the ratedetermining step, then, under conditions of excess bromine, the enolization process should follow first-order kinetics. The first-order equation,
will become,
where lcl is the specific rate constant for the enolization process and t is time. A graph of log,, (100 - Z)
Thermodynamic Data for Enolization
Table 2.
Temperature
"C
Acetoi~cet,iaEsler 20 30 40 50
Acetyl Acetone 20 30
6.41 6.19 5.98 5.84 80.8 76.96
0.0684 0.0659 0.0636 0.0620 4.21 3.34
ZQ (I00 - ZO)
k, (sec-'1
Acetoacetic Ester 20 50 Aeetyl Acetone 20
Kinetic Data for Enolization
15.2 X lo1 . 5 x 10-5 . 5 X In-' 119.4 X 10-6
6.114 X l W S 11x lo-=
El* (call ~:i,'ioo
Es* (call 13,750
...
... ...
3,iioo
i,iio
...
Volume 44, Number 3, March 1967
/
151
to calculate Arrhenius frequency factors and entropies of activation for enolization, and hence lead to discussions concerning the nature of the transition state in these reactions. The value of molar refraction as defined by
25
50
75
100
is independent of temperature, where n is refractive index, d is density, and M is molecular weight. I n principle, therefore, it should be possible to measure the change of enolization with temperature. Measurements of refractive index were carried out with a Bellingham and Stanley precision refractometer in conjunction with a sodium lamp, and the density measurements were made with a pyknometer. The en01 content is calculated (9) from
125
Time (recr.) Rates of endimtion of acetoocetis ester.
Figure 4.
3.1
3.2
1 Figure 5.
3.3
3.4
X 10-a
Arrhenius plot for ocetoacetic ester.
against t should be lmear and have a slope of -kJ 2.303. Figure 4 shows some results for acetoacetic ester. Table 2 gives the values of kl for both compounds a t various temperatures, along with the activation energies for enolization, El*. The values of El* were obtained from Figure 5 for acetoacetic ester, and by calculation for acetyl acetone. Smce AH = El* - Ez* the activation energies for the reverse reactions, Ez*, can also be calculated (Table 2). It would now be possible for the student to explain from his results the reason for the higher en01 content of acetyl acetone. The results could further be used Table 3.
Enol Content from Molar Refraction
Temperature "C
20
O/, En01 in acetoacetie ester from molar refraction
% Enol in acetyl acetone from molar refraction
152
/
Journal of Chemical Education
30
40
50
where R is the measured molar refraction, Rs is the calculated molar refraction for the pure en01 form, and Rn is the calculated molar refraction for the pure keto form. The refraction equivalents used to calculate Rs and RK were taken from reference (lo),and a figure of 1.80 used for exaltation of molar refraction due to conjugation of double bonds in the case of the en01 form. The results are given in Table 3. I t is seen that the values of en01 content differ considerably from those obtained by the analytical method. The reason for this is intramolecular hydrogen bonding in the en01 form (see infrared spectra) which has not been included in the calculation of Rs. Comparison of the two sets of r e sults would allow a calculation to be made of the refraetion equivalent for intramolecular hydrogen bonding, which would be expected to be temperature dependent. Other compounds suitable for study are suggested by Lockwood ( I ) , and possibly those listed by Allen and Dwek (6). Literature Cited (1) Loc~woon,K. L., J. CHEM.EDUC.,42, 481 (1965). (2) MEYER,K. H., Ann., 380, 212 (1911). (3) R u s s ~ ~ P .qB., J. Am. C h .Sac., 74, 2654 (1952). 141 MAUSER.H.. AND NICKEL.B.. Bm.. 97. 1745. 1753 (1964). \----,.
(7) VOOEL,A. I., "Elementary Practical Organic Chemistry, Part 111, Quantitative Organic Analysis," Longmans, Green and Co., London, N e w Yark, Toronto, 1958, p. 792. G., in "Physical Methods in Organic Chemis(8) ERLINTON, J. C. P.), Oliver and Boyd, try," (Editor: SCHWARZ, Edinburgh and London, 1964, p. 104. S., "Textbook of Physical Chemistry," 2nd (9) GLASSTONE, ed., The Mscmillan Ca., London, 1953, p 531. (10) MARON,S. H., AND PRUTPON,C. F., "Principles of Physical Chemistry," 4th ed., The Macmillan Co., New York and London, 1965, p. 692.
