NOTES
939
Keto-Enol Tautomerism in 0-Dicarbonyls Studied by Nuclear Magnetic Resonance Spectroscopy.
111. Studies of Proton Chemical
Shifts and Equilibrium Constants a t Different Temperatures1
by Jane L. Burdett and Max T. Rogers Department of Chemistry, Michigan State University, East Lansing, Michigan 48823 (Received August 3, 1965)
As a part of a study of keto-enol tautomerism in 0dicarbonyls by nuclear magnetic resoiiance (nmr) s p e c t r ~ s c o p ywe ~ ~have ~ investigated the temperature dependence of proton chemical shifts of various groups in a number of substituted p-diketones and 0-keto esters. The amounts of each isomer have also been measured as a function of temperature in several cases and free energies, enthalpies, and entropies of tnutomerization obtained. The nmr method has previously been used to obtain a value of the enthalpy of tautomerization from the equilibrium tautomer ratios determined from integrated nmr intensities of peaks in the acetylacetone ~ p e c t r u m . ~ Briegleb, et u Z . , ~ have studied the temperature dependence of the keto-enol equilibrium for several coinpounds from isothermal distillation studies and earlier investigations had been made6,' using bromine titration for analysis. Since the keto and enol isomers each give rise to a set of peaks in the nnir spectrum, integration of the areas of these provides a method for analysis of the mixtures without disturbing the equilibrium or requiring that either tautomer be isolated. In addition me looked for peaks characteristic of species such as the tmns-enol* and dienolg forms which have been reported but found no evidence for them indicating that less than about 3% of those species exists in any of the materials studied. Experimental Section Instruments. Measurements were made with the Varian ,4-60 nmr spectrometer, V-6031 probe, and V-6040 temperature accessory. Temperatures were calibrated using the ethylene glycol or methanol chemical shifts and are probably accurate to h 3 " . The percentages of keto and enol tautomer were found by integrating appropriate peaks in the spectrum of each and are accurate to about &2%. Materials. Ethyl a-cyanoacetoacetate, t-butyl achloroacetoacetate, a-chloroacetylacetone, ethyl a-
chloroacetoacetate, and ethyl a-bromoacetoacetate were synthesized in this laboratory by standard methods. 3 , 1 0 The remaining compounds were obtained from commercial sources, purified by recrystallization, fractional distillation, or gas chromatography, and dried. Sample tubes mere not sealed, and all samples contained TATS as an internal reference.
Table I: Chemical Shifts of the Enol Hydroxyl and of Keto and Enol a-Hydrogen Protons in 6-Uicarbonyls a t Various Temperature>
Temp range, OC
Compd
Acety lacetone Butyl acetoacetate &Butyl acetoacetate &Butyl a-chloroacetoacetate a-Chloroacet ylacetone Ethyl acetoacetate E thy1 berizoylaceta t e Ethyl a-bromoacetoacetate Ethyl a-n-butylacetoacetate Ethyl a-chloroacetoacetate Ethyl a-cyanoacetoacetate Ethyl a-ethylacetoacetate Ethyl a-fluoroacetoacetate Ethyl trifluoroacetoacetate Ethyl a-methylacetoacetate Hexafluoroacetylacetone Trifluoroacetylacetone hlethyl acetoacetate a-Rlethylacetylacetone e
a -11 to 87" -11 t o 33".
-11 t o 80 -11 t o 3 3 -33 to 80 -33 t o 87 -33 to 80 -33 to 80 3to87 -33 to 53 -11 to 33 -33 to 106 33 to 87 -33 to 33 - 11 to 87 -33 to 80 - 11 to 80 -33 to 59 -33 t o 8 0 -33 to 33 -11 to 1-G
-33 to 33".
Upfield Ad chemical shift, -----cps---Enol Enol Keto OH a-CH u-CH
1 2 2
32" 0
5 10 25 3 10 Ob 4b 8 11
3 4
4 G 0' .5 37
2 3 0
13d
3
5 5 16 10 6 5
10 2 16 0 3E 13 8 6
9 3 8
-11 to 80'.
33 to 80".
~~
~
(1) This work was supported by a grant from the National Science Foundation. (2) J. L. Burdett and M. T. Rogers, J . Am. Chem. SOC., 86, 2105 (1964). (3) M. T . Rogers and J. L. Burdett, Can. J . Chem., 43, 1516 (1965). (4) L. W. Reeves, ibid., 35, 1351 (1957). The ratios reported in this article were obtained from the averaged integrated intensities of at least five spectra: the latter were measured by weighing tracings of the spectra on bond paper: L. W. Reeves, private communication. ( 5 ) W.Strohmeier and G. Briegleb, Z . Saturforsch., 66, 1 (1951); W. Strohmeier and I. Hohne, ibid., 76, 184 (1952); G. Briegleb, R. Strohmeier, and I. Hohne, ibid., 8b, 219 (1953). (6) K. H.Meyer, Ber., 44, 1147, 2718 (1911). ( 7 ) P. Grossman, Z . Physik. Chem. (Leipzig), 109, 305 (1924). (8) M. I. Kabachnik, S. T. Yoffe, E. M. Popov, and K. V. Vatsuro, Tetrahedron, 12, 76 (1961); Z h . Obshch. K h i m . , 31, 2682 (1961). (9) D. N. Shigorin, Z h . Fiz. K h i m . , 24, 932, 954 (1950); 28, 584 (1954). (10) J. L. Burdett, Ph.D. Thesis, Michigan State University, East Lansing, 1963.
Volume '70,,Vumber S March 1966
NOTES
940
Table I1 : Thermodynamic Quantities for the Tautomeriaation Equilibria in P-Dicarbonyls (Pure Liquids) Temp range, "C
Compd
Acetylacetone a-Chloroacetylacetone Ethyl trifluoroacetoacetate Ethyl a-chloroacetoacetate a-Methy lacet ylacetone
A&'(33'),
-11 to 59 33 5 -33 -11
to to to to
80 80 105 89
Log
AF(33O),
AH,
oal/mole
Ke(33')
cal/mole
cal/mole
des
0,570" 1.15 0.780 -0.185 -0.285
-799 f 70 -1610 f 110 -1090 f 180 259 i 100 399 f 100
Values for gas phase tautomerization (ref 5 ) 0 ... - 1600 140 ... - 1060 0 ... 99 160 ... 1410
Acetylacetone Ethyl a-chloroacetoacetate
* A value of
' Values taken from graph of In K , vs. 1/T.
-2840 f 200b -5920 f 200 -3910 f 200 -875 =t100 - 1330 f 100
-6.66 f 0 . 8 8 - 1 4 . 1 2 ~1 . 0 1 - 9 , 2 1 3 ~1 . 2 4 -3.69i0.57 -5.65 i 0 . 4 5
- 1780 - 3770 - 3660 - 100
-0.66 -6.55 -13.8 -3.50
-2705 =I= 100 cal/mole was reported by Reeves (ref 4).
Kote thdt A H
. reported here is an average over the temperature range for each of the five compounds studied.
Results and Discussion H
R'
H
R
/
\/
0
C
\/\/ C
/I
0
C
I
R" I keto
'
