Photolysis of a-Keto Acids and Esters
ce Study of Radicals of a-Keto Acids and Ester ehar, and R. W. Gessenden" Padistion Research Laboratories, Center for Special Studies and Department of Chemistry, Mellon Institute of Science, Carneyie-Adellon Univers!ty, Pittsburgh, Pennsylvania 15213 (Received October 16, 1972)
P'ubircation costs assisted by the Carnegie-Me/lon University and the U. S. Atomic Energy Coiltmission
Radicals of the type RC(OII)COOR', produced by photoreduction of a-keto acids and esters in 2-propar,ol i;olutions, have been studied by esr. In each of the five systems, glyoxylic acid, pyruvic acid, ketobutyric acid, methyl pyruvate, and ethyl pyruvate, the esr spectrum gave evidence for the existence of comparable concentrations of two different species interpreted as cis and trans isomers of the ~ ~ C ~ ~ ~ ~radicals. C O OAccurate R ' hyperfine constants are given for the pairs of isomers in all but the first case where resolution of the individual spectra was not possible.
Radicals of the type R,C(OH)COOR' have been detected in the reduction of the appropriate 2-keto organic acids or ester^^-^ and in the oxidation of the 2-hydroxy compounds.5~6The reduction of the carbonyl-containing compounds was carried out by either p h o t o e ~ c i t a t i o nof ~ ,the ~ carbonyl molecule which then abstracts hydrogen from the solvent RCQGOQR' %- RC0"COOR' (CH3),CITOK -P,e(QH)COOK(
(1) RCO*COOR' (CH&COH (2) or chemicallys uia one electron reducing agent such as
+
+
GO2E-l. RCOCOOR' t- &&I3 --+ RC(0H)COOR' The esr spectra of the resulting a-hydroxyalkyl radicals were observed in both aqueous and nonaqueous solutions. Although in most cases the experimental findings and the spectral assignments agreed, in some cases conflicting results were reported as well as differing interpretations of the spectra. For instance, various characterizafions were suggested for the radicals formed from ethyl pyruvate. Fujisawa, et ai.,2 generated the radical by photoreduction of the ester in 2-propanol. They measured values of 16.48, 0.82. and 2.14 G for the coupling constants of the a-methyl, -CHz--, and OH protons, respectively, and identified the free radical as CH3C(OH)CCbOC2Htj. On the other hand, Anderson, et aL,3 on reacting the ester with COzH, using a rapid mixing technique, reported the observation of the cis and trans isomers I and E. The splittings of the OB
I
II
methylene and hydroxyl protons measured by them were found to be fortuitously equivalent (a(CH3) = 17.0 G, a(CH2) = a((4I-I) = 1.7 for one isomer and a(CH3) = 16.5 6, a(CH2) = a(OH) = 1.5 G for the second isomer). However, not ai1 the features of the spectrum were fitted by these paramet,er,s, suggesting either the presence of some other radicals or misinterpretation of the observed
spectra. It has also been found7 that the esr spectra derived from the acids themselves (RC(O = H, CH3, or C2H5) exhibit anomalo among the hyperfine lines. Although the most obvious explanation for this phenomenon i s some form of exchange of the hydroxyl and carboxyl protons TBQ mechanism is apparent which would account for the observed pattern of line broadening. In the present paper we wish to present some results which help considerably in answering the questions raised by these previous papers.
Experimental Section The photolysis system and the esr apparatus was as previously described.8 Magnetic field measurements were made with a field-tracking nmr U h i t and frequency counter. The g factors were determined from measurements of field and microwave frequency (also by frequency counting) with account taken of the magnetic field difference between the esr sample and nmr probe positions. Photolysis was performed under flow conditions with i2 flow rate of about 1 ml/min. Pyruvic acid, methyl pyruvate, ethyl pyruvate, and 2-ketobutyric acid from Aldrich and glyoxylic acid from Pfaltz and Bauer were used without further purification. All solutions were d e o ~ ~ g ~ ~by a t bubbling ed with nitrogen prior and during the photolysis. Second-derivative spectra were taken to provide somewhat better resolution than obtainable in the ~ ~ s t - a e r ~ v a t mode. ive This fact accounts in part for the o ~ ~ s e ~ v a t iofo nmore structure than found by previous workers.2-*
Resylts and Diseussion Pyruoic Acid. The esr spectrum observed during the photolysis of a 5% pyruvic acid solution in 2 propaaol i s shown at the top of Figure 1. This sp with previous findings where the CH3 Supported in part by the U. S. Atomic Energy Commission. T. Fujisawa, B. M. Monroe, and G. 5. Hammond, J . Arne?. Chem. SOC.,92, 542 (197Q). N. H. Anderson. A. J. Dobbs, D. J. Edge, R. 0.C. Norman, and B. R. West, J. Chern. SOC.6, 1004 (1971). P. 8.Ayscough and M. C. Brice, J. Chern. Soc. 8, 491 (1971). W. T. Dixon. R. 0.C. Norman. and A. L. Bulsy, J. Chem. Soc., 3625 (1964). M. Simic. P. Neta. and E. Hayon, J. Phys. Chem., 73,4214 (1969). N. L. Arthur and R. W. Fessenden, unpublished results. D. Behar and R. W. Fessenden, J. Phys. Chem., 75,2752 (1Y71).
A. Sarnuni, D. Behar. and
778
Figure 1, Portions of second-derivative esr spectra observed during the photolysis, of pyruvic acid in 2-propanol: upper trace, 5% acid; lower tiace, 0.2% acid. Superimposed lines from two isorneric radicals are resolved in the lower trace. Each of the lines of first-order intensity three (Le., with l Z = a%) shows a partial resolution into the second-order components of intensity
ratio 1:2. Signal entiancement can be seen by comparing corresponding lines in the end groups. The lines in the high-field group are more intense. cal was produced photochemically2 (reactions 1 and 2) or through redox reaations of lactic acid5 and pyruvic acid3 in rapid-mixing experiments. In the above-mentioned cases2,3J the coupling with the carboxyl proton was not observed so thal the anomalous intensity pattern, seen at the top of Figmc 1, was not detected. Ayscough and Brice4 improved t tie resolution by reducing the acid concentration and were able to observe the splitting by the acid proton. The reason for this improvement is the reduction in the rate of exchange of the acid proton which affects the line widths. They4 do not, however, report any unusual broadening of the signals or irregular intensity ratios. In our experiment the splitting by the carboxyl protoon was observable even at 10% pyruvic acid. When the concentration of the acid was lowered to 0.2% further resolution was achieved (see the lower trace in Figure 1). Lines from two different radicals, rather than only one, are identified by the stick spectra shown in Figure I. Each spectrum can be described by three coupling constants (u(CHa), a(OH), and a(C0OH)) as given in T a b k I. On the addition of 50970 water and an increase in pH from 2 to 4 the spectra remained essentially unchanged. The close similarity between the magnetic parameters of these two radicals suggests the presence of two rotational isomers, cis and trans, A lack of free rotation about the CH& ( OH) OOB- bond as a consequence of partial double bond character would obviousiy account for the discrete spectra. The finding of two isomers in the case of pyruvic acid i s in accord with the recent suggestion3 of cis and trans isomer& for the radicals derived from ethyl pyruvate. Only very weak limes attributable to radicals of the type discussed above were found in experiments on neutral or basic solutions and no interpretation of these lines was possible. However, a more intense set of lines which consists of a 6.89-6 sieptet (g = 2.00455) did appear. The intenbity of this spectrum was independent of the flow rate suggesting that this radical is not a result of some secondary reactions. 'The intensity distribution of the lines is very close to l:fi:l5:20:15:6:1 which would be found for a radical with SIX equivalent protons. These magnetic
-e
The Journal of Physical Chemistry, Vol. 77, No. 6, 1973
a(OH1
210 G I
W. W. Fessenden
C
I 25 G+-;*
Figure 2. fsr spectra observed during the photolysis of ketobutyric acid in 2-propanol: upper trace, 10% acid; lower trace, 0.2% acid.
parameters fit well with the 6.84-G hyperfine constant ( g = 2.00470) found by Zeldes and Livingstong for the biacetyl radical anion. In their works they showed that very low concentrations of biacetyl present as impurities could be responsible for the appearance of the biacetyl radical spectrum. The biacetyl could be present as an impurity in the pyruvic acid or could be produced photolytically in some way. Similar results were obtained in the photolysis of 2ketobutyric acid. The unresolved spectrum of CH&H26(0H)COOH with 10% solute is compared in Figure 2 with the resolved one obtained with 0.2% solute. The measured magnetic parameters are summarized in Table I. The spectra are attributed to the two isomers of CzH5C(OH)COOH. When glyoxylic acid was photolyzed, the esr spectrum exhibited the same type of unsymmetrical intensity pattern as was found with the above mentioned acids at high solute concentrations. In this case attempts to achieve better resolution by decreasing the acid concentration were not successful. Nevertheless, it would be reasonable to assume that the rotational isomerism exists also in the case of the glyoxylic acid. Ethyl and Methyl Pyruvate Anderson, et a L , 3 produced CHaC(OH)COOC2H5 from ethyl pyruvate by reacting COzH radical with the ester. They claim that the spectrum obtained in the pH region 1-5 contains two quartets of quartets which can be attributed to the radicals I and IT. As mentioned before, they interpreted their spectra by assuming that in both isomers aI
