J . Phys. Chem. 1989, 93, 7802-7806
7802
Kinetics and Mechanism of the Ce(1V) Oxidation of Methylmalonic Acid in H,SO, Peter Ruoff* and Gunn Nevdal Department of Chemistry, Rogaland Uniuersity Center, Ullandhaug, N-4004 Stauanger, Norway IReceiced: March 17, 1989)
In sulfuric acid solutions Ce(1V) oxidizes methylmalonic acid by a sequential mechanism to carbon dioxide and acetic acid as the major products and with hydroxymethylmalonic acid and pyruvic acid as probable intermediates. Additional complexities
have been found due to the formation of polymeric material. Both H+ and sulfate ion decrease reaction rates independently and can be described as noncompetitive inhibitors.
Introduction
The use of ceric ions as an oxidant is of considerable importance and interest to many fields in chemistry.I4 One particular aspect is that the Ce(1V) oxidation of organic compounds which contain a reactive C-H bond, as in malonic acid, has been found to be an important component process in bromate-driven oscillators, which is a class of reactions where sustained concentration oscillations in time or space can be Our interest in the title reaction originated from an earlier describedg cerium-catalyzed bromate-driven oscillator, which even in a closed system can exhibit a variety of exotic dynamical behaviors such as excitability and b i ~ t a b i l i t y . ~ ~ ~ ~ The Ce( 1V)-methylmalonic acid (MeMA) reaction itself has not been studied before. In analogy to the malonic acid system, its stoichiometry is expected to be described by reaction R1 consuming 6 mol Ce(1V) per mole of MeMA. CH,CH(COOH)2
+
+
-
6Ce4+ 2 H 2 0 CH,COOH + 6Ce3+ + 6 H + + 2 C 0 2 ( R I )
We found that reaction R1 follows a sequential" mechanism but that only 4.7 mol Ce(1V) are consumed per mole of MeMA. Possible causes of this serious underconsumption are discussed below. Sulfate ions or H+ were found to retard the velocity of reaction R I , and double-reciprocal plots of initial rates indicate that these ions can be characterized as noncompetitive inhibitors. Mechanistic implications for malonic acid and bromomalonic acid systems and for the MeMA bromate-driven oscillator are discussed. Materials and M e t h o d
Kinetics. Kinetic experiments were performed at 25 OC (fO.l "C) using a Perkin-Elmer Lambda 15 spectrophotometer by following the reaction in a sulfuric acid solution a t the Ce(IV) absorption maximum at 320 nm with MeMA in excess over Ce( IV). Ce(1V) stock solutions for kinetic studies were made from Ce(S04)2.4H20(Merck, analytical quality, FRG) in different concentrated H2S04solutions (0.1-2.0 M). For each sulfuric acid concentration a Ce(1V) calibration curve in the range 10-250 p M was prepared at 320 nm. These calibration curves strictly follow Beer's law. Absorbance coefficients at 320 nm were found to increase slightly with increasing sulfuric acid concentration (Table I ; the data from which these absorbance coefficients have been calculated are given in the supplementary material (see Supplementary Material Available paragraph at the end of this article)). MeMA (>99% Fluka. Switzerland) stock solutions were also prepared in H2S04. All solutions were made with double-distilled water. Kinetic runs were started by rapidly mixing certain amounts of the MeMA and Ce(1V) stock solutions in a spectrophotometric cuvette which was stirred magnetically inside the spectrophotometer. *To whom correspondence should be addressed. Present address: Department of Chemistry, University of Oregon, Eugene, O R 97403.
0022-3654/89/2093-7802$01.50/0
TABLE I: Ce(1V) Absorption Coefficients c for Different Sulfuric Acid Concentrations" 0.1
0.25 0.5
4.1x 103 5.5 x 103 5.5 x 103
1.o 2.0
5.8 x 103 6.2
x
103
For calculation of c values see supplementary material.
Stoichiometry. For the stoichiometric studies a 0.1 M Ce(1V) stock solution was made from (NHJ2Ce(N0& (Merck, analytical quality, FRG) in a 1 M sulfuric acid. The stoichiometry of reaction R1 was investigated by determining the amount of Ce(IV) reacting with a certain amount of MeMA. Cerium(1V) was used in IO-fold molar excess over MeMA. After the mixed initial reagents had stood for approximately 20 h at room temperature ( ~ 2 3"C), the amount of unreacted Ce(IV) was determined iodometri~a1ly.l~Acetic acid was identified as a reaction product by smell and by 'H N M R spectroscopy. Carbon dioxide was detected as precipitated BaC03 by bubbling the evolving gas through a saturated Ba(OH), solution. A PdCI2 testI4 for detecting CO was negative. The amount of evolved gas was measured in a separate set of experiments using a gas buret which was attached to the reactor. The increase of the gas volume at the outer atmosphere pressure was measured. Correction was made for the water vapor pressure inside the vessel. Corrections made due to dissolved C 0 2 in the solution were found to be negligible. N M R Measurements. ' H N M R experiments were performed on a Varian XI-200 Fourier-transform spectrometer at the Department of Chemistry and Chemical Engineering at Michigan Technological University. In all our 'H N M R studies the 1 M sulfuric acid was replaced ( I ) Willard, H. H.; Young, P. J . Am. Chem. SOC.1930, 52, 132. (2) Smith, G. F . Cerate Oxidimetry; The G. Frederick Smith Chemical Co.: Columbus, OH, 1942. (3) Richardson, W. H. In Oxidation in Organic Chemistry; Wiberg, K. G.. Ed.: Academic Press: New York. 1965. (4) Ho , T -L Synthesis 1973, 347. (5) Noyes, R M. J . Am. Chem. SOC.1980, 102,4644 (6) Field, R. J. In Theorerical Chemistry: Periodicities in Chemistry and Biology; Eyring, H., Henderson, D., Eds.; Academic Press: New York, 1978; Vol. 4. (7) Field, R. J., Burger, M . , Eds., Oscillations and Traueling Waoes in Chemical Sysrems; Wiley: New York, 1985. (8) (a) Zaikin, A. N.; Zhabotinsky, A. M. Nature 1970, 225, 535. (b) Winfree, A. T. Science 1972, 175,634. (c) Jahnke, W.; Henze, C.; Winfree, A. T. Nature 1988, 336, 662 and references therein. (9) (a) Ruoff, P.; Schwitters, B. 2. Phys. Chem. (Munich) 1983, f35,171. (b) Hansen, E. W.; Gran, H . C.; Ruoff, P. J . Phys. Chem. 1985, 89, 682. (IO) (a) Ruoff, P.; Noyes, R. M . J . Phys. Chem. 1985, 89, 1339. (b) Ruoff, P. Noyes, R. M . J . Chem. Phys. 1986, 84, 1413. (11) Kasperek, G . J.; Bruice, T. C. Inorg. Chem. 1971, 10, 382. (12) Barkin, S.; Bixon, M. Noyes, R. M . , Bar-Eli, K. Inr. J . Chem. Kiner. 1978, 10, 619. (13) Jander, G.; Jahr, K. F.; Knoll, H. Mapanalyse; Sammlung Goschen, Band 221/221a; Walter de Gruyter: Berlin, 1969. (14) Bayer, F.; Wagner, G. Die chemische Analyse; Enke: Stuttgart, 1960; Band 39.
0 1989 American Chemical Society
Ce(1V) Oxidation of Methylmalonic Acid in H2S04
The Journal of Physical Chemistry, Vol. 93, No. 23, I989 7803
J
0 07
I
Time.Min
0
04
12
08
20
16
Figure 1 . Absorbance change (at 320 nm) against time of a Ce(1V)-
MeMA reacting system. Initial concentrations: [Ce(IV)] = 5 X M, [MeMA] = 0.2 M in 0.25 M sulfuric acid solution. The rapid decrease from high absorbance values to the value corresponding to the initial Ce(lV) concentration in the very beginning is probably due to the mixing process.
‘y, -5
,
10g[Cf1,,
,
-3
-4
-2
, .5r 0
,
,
,
-05
,
log [MeMAl. ,
-10
-15
0
5
10
20
15
Figure 3. Double-reciprocal plots between initial velocity uo and initial
MeMA concentration. Numbers e, n, and r in the following show the sulfuric acid concentration, number of experiments for each point, and the linear regression correlation coefficient, respectively. ( I ) c = 0.1 M, n = 3, r = 0.9995; (2) 0.25 M, n = 3, r = 0.9999; (3) c = 0.5 M, n = 4, r = 0.9982; (4) c = 1.0 M, n = 4, r = 0.9982; ( 5 ) c = 2.0 M, n = 3, r = 0.9993. Inset shows the slopes of double-reciprocal plots as a function of sulfuric acid concentration. In all runs, [Ce(IV)lo = 5 X M.
,
B
A
Figure 2. (A) Logarithm of initial velocity against logarithm of initial Ce(lV) concentration. Initial MeMA concentrations: ( I ) 0.3 M; (2) 0.2 M; (3) 0.1 M; (4) 0.05 M. All four slopes resulted in a Ce(IV) reaction order of 1 .O. (B) Same as A, but now varying MeMA concentrations. Initial Ce(lV) concentrations: ( I ) 5 X lo4 M; (2) 2.5 X IO4 M; (3) 1 X IO4 M; (4) 5 X M. The slopes resulted in an average MeMA reaction order of 0.73.
by a 1 M D2S04 solution (D2S04: Aldrich, WI; D,O: Norsk Hydro, Norway; all deuterated reagents were of analytical quality). The N M R measurements were performed at room temperature by adding (NH4)2Ce(N03), in 2-, 4-, or IO-fold molar excess over a 0.1 M MeMA D2S04 solution. Spectra were recorded approximately 20 h after initial mixing of the reagents. Spectra were also taken at the start of the reaction when (NH,)2Ce(N03)6 was in IO-fold molar excess over MeMA. Additional Spectrophotometric Measurements. We have also performed spectrophotometric experiments with a higher initial Ce( IV) concentration (0.05 M) using a Gilford 2400-2 spectrophotometer a t the Department of Biological Sciences, Michigan Technological University. In these experiments a special cuvette was used which allowed to study the Ce(1V)-MeMA reaction under an argon atmosphere. Other Reactions. We studied to some extent the reaction between pyruvic acid (CH3COCOOH, F‘yr) and Ce(IV), and lactic acid (CH,CH(OH)COOH, Lac) and Ce(IV). Pyr was of analytical quality (Aldrich, WI), while Lac (Mallinckrodt) was of commercial 86% quality.
Results Kinetic Experiments. A typical change of absorbance of a Ce(1V)-MeMA reacting system is shown in Figure 1. The determined reaction order with respect to Ce(IV) from initial velocities is 1.0 (Figure 2A). The reaction order with respect to MeMA was found to be 0.73 (Figure 2B). Double-reciprocal plots were found to be linear with high accuracy (Figure 3), which is consistent with a sequential mechanism of the form Ce(1V)
+ MeMA
k k-I
+ k2
[Ce(IV)-MeMA] Ce(II1)
other products ( M I )
and a Michaelis-Menten type of rate law =
i
/’
Figure 4. Double-reciprocal plots for Ce(1V)-MeMA reacting system in 1 M HCIO,. NaHSO, concentrations: ( I ) 0.25 M; (2) 0.5 M; (3) 1.0 M. Initial Ce(1V) concentration was 5 X M. TABLE 11: K , and k 2 Values for H2S0, Systems
[H2S041, M
KM,M 0.22 0.29 0.25 0.30 0.6 1
0.1
0.25 0.5 1 .o 2.0
k2, S-’
2.53 X 2.27 X lo-* 1.43 X 1.42 X 2.38 X
TABLE 111: K , and k , Values for HCIOA/NaHSOASvstems ~
[HCIO,], M 1 .o 1 .o 1.o
0.5 0.25
[NaHSOJ, M 0.25 0.5 1 .o 1.o 1 .o
KM. M
0.32 0.32 0.43 0.55 0.40
~~
k 2 , s-l
1.90 X 9.7 X 6.9 X 1.16 X 1.08 X
Our double-reciprocal plots show that increased concentrations of sulfuric acid result in a decrease of initial velocities. However, the intersection pattern of these lines is complex and does not seem to allow a clearcut classification of the type of inhibition as often done in enzyme kinetic work.Is W e have also studied the effect of sulfate and H+ separately. Figure 4 shows double-reciprocal plots as a function of different HS04- concentrations in a 1 M HC104, while Figure 5 shows the
v = -d[Ce(IV)]/dt
~~[C~(IV)IO[M~MAI/([M~MAI + k ~ ( 1)) where KM = ( k ,+ k 2 ) / k , ,[Ce(IV)lo is the initial Ce(IV) concentration, and c is the reaction rate.
( 1 5 ) (a) Cleland, W. W. Investigation of Rates and Mechanisms of Reactions. In Techniques of Chemistry, Bernasconi, C. F., Ed.; Wiley-Interscience: New York, 1986; Vol. 6. (b) Dixon, M.; Webb, E. C. Enzymes; Academic Press: New York, 1979; p 337 ff.
7804 The Journal of Physical Chemistry, Vol. 93, No. 23, 1989
'{0
,
1
,
,
,
/ 22 ,/
/HCIOL1 M,
IO
05
Ruoff and Nevdal
/'
L., ,,.-,Mi "
(4)
(3)
IMeMAI,
5
0
10
15
Zb
Figure 5. Double-recrprocal plots for Ce(lV)-MeMA reacting system i n 1 M NaHS04. HCIO4 concentrations: (1) 0.25 M; (2) 0 5 M; (3) I 0 M initial Ce(lV) concentration was 5 X M
t
0
Figure 7. 'H NMR spectra of MeMA-Ce(IV) system taken approximately 20 h after mixing of reagents. Initial concentrations: (A) 0.2 M Ce(IV), 0.1 M MeMA; (B) 0.4 M Ce(IV), 0.1 M MeMA; (C) 1.0 M Ce(lV), 0.1 M MeMA. Peak 1 is due to the methyl group of unreacted MeMA, peak 2 is an unknown compound, while peak 3 is the methyl group of acetic acid. Peak 4 is due to the residual water and has two "spinning side band" peaks. Because the sum of peak intensities of ( I ) , (2), and ( 3 ) in A-C has been found to be constant within a variation of about 596, peak 2 appears to be from a methyl group from a reaction product/intermediate of MeMA.
0 5.
mol MeMA.10'
moi MeMA-104 ,
,
,
2
,
,
,
,
,
6
0
Time Min
20
IO
Figure 6. (A) Number of moles of evolved C 0 2 ,n(COz),as a function of number of moles of initially available MeMA, n(MeMA), when Ce(IV) concentration was in 10-fold excess over MeMA. The solid linear regression line is given by the equation n(C02) = 0.0866 1.7183n(MeMA) with a correlation coefficient of r = 0.9986. (B) Number of
+
0
'
I2
'
ii
'
36
'
i'6
'
'
'
60
'
72
'
$4
' s b
i
moles of consumed Ce(IV), n(Ce(IV)), as a function of number of moles of initially available MeMA, n(MeMA). Same experimental conditions as in A. Each point consists of two parallel experiments. The solid line is described by the equation n(Ce(iV)) = 0.0022 + 4.7460n(MeMA) with r = 1.0000. influence of different HCIO, concentrations in a 1 M N a H S 0 4 solution. From Figures 5 and 6 it is obvious that increased amounts of HSQ4- or H + decrease initial rates. Tables I I and 111 show the calculated rate constants k , and K M for the different sulfuric acid and perchloric acid systems. Stoichiometry. The amount of evolved C 0 2 was found to be 1.7 mol per mole of initially available MeMA (Figure 6A). Figure 6B shows that 4.7 mol of Ce(IV) are consumed per mole of initially available MeMA, which results in 2.8 mol of consumed Ce(IV) per mole of evolved COz compared with the theoretical value of 3 mol for reaction R 1. N M R and Additional UV- Visible Absorption Experiments. 8Figure 7 shows IH NMR spectra for initial [Ce(IV)]/[MeMA] ratios of 2, 4,and IO approximately 20 h after reagent mixing. In all cases, acetic acid (HOAc) was identified as a product (peak 3). When Ce(IV) was not in excess (according to the stoichiometry of reaction RI), unreacted MeMA (peak 1) could be identified along with an additional unknown peak 2. When Ce(IV) is applied in excess, only the H O A c peak 3 is detected (Figure 7C). Spectra taken during the early part of the reaction show the same peaks as in Figure 7A,B. When increased amounts of oxidant are present, we also observe a downfield shift of peak ppm values for peaks 1-3. The downfield shift, however, is due to the
i
0
l
OW"'
J
[CdlMI, M 1:lO-i
'
005
2:10* 01
Figure 8. (A) Observation of "break points" (arrow and dashed line) in
the Ce(IV) absorbance in reacting Ce(1V)-MeMA systems. Initial Ce(1V) concentration is 0.05 M in all four runs. Initial MeMA concentrations: (1) 0.05 M; (2) 0.042 M; ( 3 ) 0.033 M; (4) 0.025 M. The residual absorbance after reaction is due to Ce(II1). The same break point is observed when the reaction solution (1.2 mL) is purged with argon for about 2 min in the cuvette and recording was started with a continuous argon stream above the solution. (B) Ce(1V) absorbanceconcentration relationship at 320 nm over an extended Ce(iV) concentration range. Note that break point in A is the same as in 8. presence of paramagnetic Ce(II1) acting as a shift reagent.16 The sum of the intensities of peaks 1, 2, and 3 has been found to be constant (within a 5% variation), which, together with their chemical shift positions, indicates that peak 2 originates from a methyl group of a reaction product/intermediate of MeMA. ( 1 6 ) Sohlr, P. Nuclear Magnetic Resonance Spectroscopy: CRC Press: Boca Raton, FL, 1983: Vol. 11, p 115 ff.
Ce(lV) Oxidation of Methylmalonic Acid in H2S04 Spectrophotometric records (at 320 nm) of these reacting systems show the occurrence of a breakpoint (Figure 8A), after which a rapid decrease in absorbance occurs. This breakpoint occurs always at the same absorption values (indicated by the dashed line in Figure 8A) and is not affected by the presence or absence of atmospheric oxygen. For a bromomalonic acid (BrMA)-Ce(lV) reacting system, Jwo and NoyesI7 made a very similar observation and explained their breakpoint due to a switching mechanism between radical consumption and radical production r e a ~ t i 0 n s . l However, ~ in our case we found that the breakpoint does not indicate a change in the kinetic situation of the system but reflects a sudden change in the linear relationship between absorbance and the Ce(1V) concentration at an absorbance value of about 1.76 (arrow in Figure 8B) which we think might be an instrumental effect. We do not known whether our breakpoint has the same origin as that observed by Jwo and Noyes." Additional Observations. A white precipitate was observed when the initial [Ce(lV)]/[MeMA] ratio was 4. At either lower or higher ratio values, only smaller amounts of precipitate are formed. The precipitate could not be dissolved in chloroform, water, acid, or base. The same precipitate was formed when pyruvic acid (Pyr) was allowed to react with Ce(1V) in a mole ratio of 1:2 (initial concentration of Pyr was 0.1 M). In addition we found the Ce(IV)-Pyr reaction to be considerably faster than a corresponding MeMA-Ce( IV) reaction with the same initial concentrations. N M R spectra indicate acetic acid as the only product in the Ce( IV)-Pyr system. The oxidation of lactic acid (Lac) by Ce(1V) under the same conditions as for Pyr did not show any form of precipitate and was found to be slower by several orders of magnitude than an otherwise identical MeMA system. 'H N M R spectra showed that formic acid and acetic acid are formed as products. Pyr and Lac solutions (both 0.1 M in 1 M D2SO4) which stood overnight showed that some carbon dioxide, probably due to a decarboxylation reaction, had been produced. However, 'H N M R spectra showed no significant changes in peak intensities during a 20-h period, indicating that only small amounts of either Pyr or Lac had decarboxylated.20
The Journal of Physical Chemistry, Vol. 93, No. 23, I989
1 L
?+OH
i3+ J
I Figures 4 and 5 indicate that sulfate ion or H+ can inhibit reaction R I , in agreement with earlier reports of Ce(IV) oxidation of different organic compounds in sulfuric acidic media.'z*22-27 (17) JWO,J.-J.; Noyes, R. M. J . Am. Chem. SOC.1975, 97, 5422. (18) Brusa, M . A.; Perissinotti, L. J.; Colussi, A. J. J . Phys. Chem. 1985, 89, 1572. (19) Brusa, M. A.; Perissinotti. L. J.; Colussi, A . J. Inorg. Chem. 1988, 27, 4414. (20) Hanson, R. W. J . Chem. Educ. 1987, 64, 591. (21) Cotton, F. A.; Wilkinson, G. Aduanced Inorganic Chemistry; Interscience: New York, 1962; p 881. (22) McAuley, A. J . Chem. SOC.1965, 4054. (23) Krishna, B.; Tewari, K. C. J . Chem. SOC.1961, 3097. (24) Treindl. L.; Dorovsky, V. Collect. Czech. Chem. Commun. 1982, 47, 2831.
C?
I
-w
cy3
HOOC-
H
F - COOH
Nethylmalnnyl radical
OH OHMeflA
'"3,
OH
C'
acetyl-
II
aldehyde
C HO'
'OH DHHP
pyruvlc acid
\? I
polymeric m a t e r i a l
CH3- C -OH 11 0
+
J/
co2
acetic acld
Figure 9. Proposed reaction scheme of the Ce(IV)-MeMA system.
The inhibitory effect by sulfate in these systems can qualitatively be explained by the lower oxidation potentials of the sulfatecomplexed cerium(1V) species (Ce(S04), and C e ( S 0 4 ) p ) present under our experimental condition^.^^^*^ Figure 4 indicates that dependent upon its concentration, sulfate can be classified as a noncompetitive or mixed inhibitor.15 This is consistent with the view that Ce(S04)2is much more reactive than Ce(S04)32-and that the intermediate (Ce(S04)32-,MeMA) slowly releases products only at high sulfate concentrations. Ce(S04)* + MeMA
Discussion The double-reciprocal plots indicate that the Ce(IV)-MeMA reaction follows a sequential mechanism as described by eq M I , where MeMA probably serves as a bidentate ligand to Ce(IV)"q'8-21 (complex I). Complex I is probably formed via the enol of MeMAZ' and then converted into a methylmalonyl radi~aI.'~.'~
cy3
HOOC- C - COOH
1805
Ce(SO4);-
+
MeMA
e
{Ce(S04)2*MeMA}
-
{Ce(S04)32-*MeMA}
Ce(1II)
slow reaction
This interpretation is further in agreement with the observation that increased initial concentrations of sulfate increase the induction period and can even eliminate the oscillations in Ce(1V)-catalyzed bromate-driven oscillator^.^^ The inhibition of H+ by sulfuric acid systems has been explained by Barkin et a1.I2 in terms of a less stable but more positively charged transition state which results in a decrease of the overall reaction rate. Complementary to this interpretation, our initial rate data of Figure 5 imply that H + inhibits reaction M I either by protonating the reacting Ce-sulfate complexes and forming unreactive or much less reactive Ce(IV) species, or by protonating the intermediate Ce(1V)-MeMA complex in reaction scheme M2, forming a complex reacting much more slowly. Figure 9 shows our proposed reaction scheme of the MeMA oxidation by Ce(1V). The first step involves a hydrogen abstraction and the formation of methylmalonic acid radicals. These radicals react further with Ce(IV) and water and form hydroxymethylmalonic acid (OHMeMA). OHMeMA can react further by two distinct modes: (a) Ce(IV) may abstract hydrogens from either (25) Hanna, S. B.; Moehlenkamp, M. E. J . Org. Chem. 1983, 48, 826. (26) (a) Sengupta, K. K.; Aditya, S. Z . Phys. Chem. (Munich) 1963,38, 25. (b) Vaidya, V. K.; Joshi, S. N.; Bakore, G. V , Z . Phys. Chem. ( b i p z i g ) 1987. 268. 364. (27) Madhava Rao, B.; Sastry, T. P.; Parekh, T. S. 2. Phys. Chem. (Leipzig) 1983, 264, 906. (28) Latimer, W. L. Oxidation Polentiah; Prentice-Hall: New York, 1952; p 295. (29) (a) Hardwick, T. J.; Robertson, E. Can. J . Chem. 1951,29,818. (b) Hardwick, T. J.; Robertson, E. Ibid. 1951, 29, 828. ( 3 0 ) Ruoff, P., unpublished results. ~
7806 The Journal of Physical Chemistry, Vol. 93, No. 23, 198' 9 the hydroxy group or the methyl group, or (b) O H M e M A can decarboxylate. In general, C-H rupture is preferred over 0 - H r ~ p t u r e , ~so' the first hydrogen abstraction in OHMeMA will probably occur at the methyl group. However, a second H abstraction a t this group seems unfavorable compared with an hydrogen abstraction at one of the (protonated) carboxyl groups. This second hydrogen abstraction at one of the C O O H groups makes decarboxylation favorable, leading to the enol form of pyruvic acid (Pyr). Once Pyr is formed it reacts relatively rapidly with acetic acid (HOAc) and carbon dioxide by consuming 2 equiv of Ce(IV).32 The decarboxylation route of MeMA leads to 1,l-dihydroxyprop-I-ene (DHHP). The acidity of our system makes addition
OHMeMA
DHHP
of water to the double bond of D H H P also possible, which leads by Markovnikov's rule to the hydrated form of acetylaldehyde (CHJOCHO). This compound would require two additional equivalents of Ce( IV) for further oxidation to pyruvic acid. Concerning H abstraction, OHMeMA will probably react more slowly than MeMA and should accumulate. We expect the methyl peak of O H M e M A to occur a t similar ppm values as for bromomethylmalonic acid (BrMeMA), Le., very near the HOAc peak. The same or almost identical methyl ppm values for BrMeMA and O H M e M A can in fact explain an earlier observation by Hansen and RuofPbs33that a t the beginning of the Ce(1V)-catalyzed MeMA bromate-driven oscillation a considerably higher [BrMeMA]/[HOAc] ratio than the expected 1.5 FKN34 value is observed. Thus, the earlier study by Hansen and Ruoff might have overestimated the "BrMeMA" peak, which probably was composed of both a BrMeMA and a O H M e M A contribution. We believe that our unidentified peak 2 (Figure 7) is due to the methyl group of an intermediate closely related to D H H P or perhaps acetylaldehyde. Due to the double bond in D H H P and in the enol form of pyruvic acid (Pyr), D H H P or Pyr will lead to polymeric material in the presence of Ce( IV)-induced radicals. We do not know if direct D H H P oxidation to HOAc will occur at a significant rate. Lactic acid (Lac) can be ruled out as a conceiveable intermediate because of several reasons. Lac reacts with Ce(IV) much more slowly than MeMA, and an accumulation of Lac should be observed. Furthermore, Lac as a major intermediate would require a total of 7.8 equiv of Ce(IV) together with the accumulation of formic acid.35 Since we neither observe an accumulation of lactic (31) Urry, W. H.; Stacey, F. W.; Huyser, E. S.; Juveland, 0. 0. J . Am. Chem. Soc. 1954, 76, 450. (32) Reference 2, Table LX. (33) Hansen. E. W.;Ruoff, P. J . Phys. Chem. 1989, 93, 2696. (34) Field, R. J.; Koros, E.; Noyes, R. M. J . Am. Chem. SOC.1972, 94, 8649. (35) Reference 3, Chapter VI.C, p 265.
Ruoff and Nevdal acid or formic acid nor a Ce(IV) consumption beyond 6 equiv, Lac seems not to be a likely intermediate in the Ce(IV)-MeMA reaction. From the data presented it is obvious that under our conditions reaction R1 does not proceed to completion. Finally, we wish to show that the proposed reaction scheme in Figure 9 not only can quantitatively account for consumed and produced amounts of Ce(1V) and COz,respectively, but also for relative 'H N M R peak intensities. To do so, we assume, as described above, that OHMeMA can react both by decarboxylation and with Ce(1V). The Ce(1V) route leads to pyruvic acid which is relatively rapidly oxidized further to acetic acid and CO,. Best fit to our observed amounts of consumed Ce(1V) and produced CO, is obtained when the Ce(IV) route (process R I ) consumes 70% of the MeMA, while process R2 consumes 30%. This is in agreement with relative intensities of the N M R peaks 2 and 3, representing methyl groups of reaction intermediates and products from MeMA. Peak 2 represents almost precisely 30% of the reacted initial MeMA, while peak 3, the acetic acid, is formed from 70% of the reacting MeMA. An alternative explanation for the reduced amounts of produced and consumed CO, and Ce(IV), respectively, could involve the formation of unreactive p ~ l y m e r i z a t i o n or ~ ~ condensation3' products by pyruvic acid (Pyr). For example, it is known that pyruvic acid in the presence of sulfuric acid reacts spontaneously acid (II).37 to a-keto-y-valerolactone-y-carboxylic
0
I1 It is obvious from the arguments above that there are still uncertainties about the mechanistic detours from reaction R1. Considerably more work is needed to understand these complexities in full detail. We hope that the data obtained in this study will be of help to do more quantitative modeling of the Ce(1V)-catalyzed MeMA bromate-driven oscillator in sulfuric acid systems.
Acknowledgment. P.R. thanks Ms. Aud Bouzga (University of Oslo) and Ms. Lorri Reilly (Michigan Technological University (MTU)) for preliminary N M R runs. Professor Wilbur H. Campbell and Professor Dean Luehrs are thanked for providing lab space and access to the scientific equipment at MTU. This work was supported by the Norwegian Research Council NAVF under grant no. D.38.40.028 and D.78.98.010, and by Norsk Hydro AIS, Norway. Registry No. MeMA, 516-05-2; Ce(lV), 16065-90-0.
Supplementary Material Auailable: Tables listing Ce( IV) absorbances and calculated molar absorption coefficients for 0.1, 0.25, 0.5, 1 .O, and 2.0 M sulfuric acid ( 5 pages). Ordering information is given on any current masthead page. (36) The Merck Index, 10th ed.; Merck: Rahway, NJ, 1983. (37) Beilsteins Handbuch der Organischen Chemie; Vierte Auflage, Dritter Band; Springer: Berlin, 1921; p 608 ff.
