J . Phys. Chem. 1989, 93, 2388-2392
2388
Kinetic Study of the Initial Phase of the Uncataiyzed Oscillatory Reaction of Potassium Bromate and Gallic Acld Jing-Jer Jwo* and Eng-Fen Chang Department of Chemistry, National Cheng Kung University, Tainan, Taiwan 70101, Republic of China (Received: April 7, 1988; In Final Form: September 6, 1988)
Under suitable conditions, the uncatalyzed oxidation of gallic acid (GA) by bromate ion can exhibit damped oscillations in bromide ion concentration and color. The characteristics of oscillationsdepend on the concentrationsof gallic acid, bromate ion, and sulfuric acid as well as on temperature. Extensive study is made for obtaining the scope of this oscillating reaction. The observed rate law obtained with GA in great excess is d[Br-]/dt = k[H’]2[Br03-][GA] with k = (3.17 i 0.19) X M-’s-’ at 25 O C for [H2S0410= (0.400-2.00) M. The activation parameters AH* and AS* are 64.4 f 1.0 kJ/mol and -57.6 f 0.8 J/(mol K), respectively. Product analysis shows that bromination and decarboxylation reactions also take place during oscillations.
Introduction
Since the discovery of the first bromate-driven oscillating reaction by Belousov’ in 1958, there has been tremendous progress in the study of chemical oscillations. The introduction of the continuous-flow stirred-tank reactor (CSTR)Z-3as a tool for the systematic design of chemical oscillators has led to the discovery of many new oscillators. Three basic classes, namely, iodate, bromate, and chlorite oscillators, were distinguished in the known homogeneous liquid-phase oxyhalogen-driven oscillator^.^ Undoubtedly, bromate oscillators are by far the most thoroughly studied and understood chemical oscillating systems, especially the prototype Belousov-Zhabotinsky (BZ) ~ y s t e m . ~Generally, bromide ion acts as a control intermediate switching the system between an oxidized and a reduced state, corresponding to a low and high bromide ion concentration, respectively. Noyes5 formulated a generalized mechanism for bromate oscillators and divided them into five distinct classes, four of which consist of a metal-ion catalyst and a fifth “uncatalyzed” class. Babu et a1.68 studied a modified BZ reaction by replacing malonic acid with organic gallic acid (GA) and claimed that the observed oscillating reaction was catalyzed by cobalt and cerium ions. However, Koros and OtbBn9reported that this oscillating reaction was not catalyzed by Co(II1) ion. Their observations led to the discovery of the uncatalyzed class of bromate oscillators,’O*l’which do not require any metal-ion catalyst, and the organic substrate performs some of the functions of the metal ion as well. All of the organic substrates that give oscillations are phenol and aniline derivatives with at least one unblocked ortho or para position. OrbBn, Koros, and Noyes (OKN)I2 proposed a mechanism to rationalize the uncatalyzed oscillating reactions of bromate and dihydroxybenzenes. Herbine and Field13made a slight modification of the OKN mechanism. Both mechanisms simulate the experimental results very well. In the previous article, we reported that the (1) Belousov, B. P. ReJ Radiat. Med. 1958, 145.
( 2 ) DeKepper, P.; Epstein, I. R.; Kustin, K. J . Am. Chem. SOC.1981, 103, 2133. ( 3 ) Epstein, I. R. Chem. Eng. News, 1987, March 30, 24. (4) Field, R. J.; Koros, E.; Noyes, R. M. J. Am. Chem. SOC.1972, 94, 8649. ( 5 ) Noyes, R. M. J . Am. Chem. SOC. 1980, 102, 4644. (6) Babu, J. S.; Bokadia, M. M.; Srinivasulu, K. Bull. Chem. SOC.Jpn. 1976, 49, 2875. (7) Babu, J. S.; Srinivasulu, K. Proc. Ind. Natl. Sci. Acad. (India) 1976, 42A, 361. (8) Babu, J. S.;Srinivasulu, K. J . Chem. SOC.,Faraday Tram. 1 1977, 73, 1843. (9) Orbln, M.; Koros, E. Nature (London) 1978, 273, 371. (IO) Orbln, M.; Koros, E. J. Phys. Chem. 1978, 82, 1672. ( I I ) Orbln, M.; Koros, E. React. Kinet. Catal. Lett. 1978, 8(2), 273. (12) Orbln, M.; Koros, E.; Noyes, R. M. J . Phys. Chem. 1979.83, 3056. (13) Herbine, P.; Field, R. J. J. Phys. Chem. 1980, 84, 1330.
0022-365418912093-2388!301.50/0
bromate-gallic acid system was actually catalyzed by [Fe(phen),12+ ion and cerium ion was not a good catalyst in this system.14 Our recent observations show that even though the main features of the uncatalyzed BrOy-GA oscillating reaction are reasonably well understood, its kinetics still needs further investigation. In this paper, we report the kinetics and the scope of the uncatalyzed Br03--GA reaction. The rate law, the rate constant, and the activation parameters of this reaction are also given. Rationalization of experimental observations are made by applying the O K N m e c h a n i ~ m ’ with ~ ~ ’ ~slight modification. It is believed that the present work has a valuable implication in understanding the mechanistic details of the initial phase of the uncatalyzed Br03--GA oscillating reaction. Experimental Section
Materials. Gallic acid of Riedel-de Haen was 99% chemical pure. N-Bromosuccinimide was an extrapure chemical of Tokyokasei Kogyo, Japan. Other reagent grade chemicals were used directly without further purification. Water purified by Milipore Mili-RO 20 (reverse osmosis) was used in this study. Procedures. More than 1000 kinetic runs were carried out. The potentiometric method was applied in most of the kinetic studies. The bromide ion concentration was measured potentiometrically by an Orion 94-35 bromide-ion-selective electrode combined with an Orion 90-02 double-junction reference electrode. Both electrodes were connected to the Orion 701A Digital Ionalyzer, and the output signals were recorded on a YEW 3056 recorder, Japan. The potential of bromide electrode was calibrated frequently with standard KBr solutions, which were prepared with the same aqueous sulfuric acid solution as in the reaction solution. The temperature of reaction mixture was controlled to fO.l O C with a Haake F3 circulator. The reactant solutions were thermostated at the desired temperature before mixing. The mixing of reactant solutions was completed within 5 s. The absorbance of the color intermediate produced during the reaction was followed spectrophotometrically with a Hitachi 100-20 spectrophotometer. A Shimadzu GC-9A gas chromatograph, Hewlett-Packard 1084B liquid chromatograph, Hitachi GC-MS M-25 spectrometer, and Bruker WP 100 FT N M R were used in the analysis of products. The viscous red-brown products obtained by reacting 2.0 g of KBrO, and 1.0 g of G A in 150 mL of 1.0 M H2S04were extracted with ether after the completion of reaction. The analysis of these products by the HPLC method showed one major peak and at least three minor peaks. The ‘H NMR spectrum in deuterated acetone also showed three small peaks and a large sharp singlet -OH peak at 6.87 ppm which exchanges with D 2 0 to 4.75 ppm. There was no peak for the carboxylic group. In contrast, the IH N M R spectrum of pure (14) Jwo, J.-J.; Chang, E.-F. J. Chin. Chem. SOC.(Taipei) 1986, 33, 91.
0 1989 American Chemical Societv
The Journal of Physical Chemistry, Vol. 93, No. 6,1989 2389
Reaction of KBr03 and Gallic Acid
29
4 1.o
0 0 -6
1 1111 11 1111 0 6 12 18 24 30 t,min
Figure 1. Absorbance and log [Br-] vs time traces. [BrOC], = 0.0840 M, [GA], = 0.0160 M, [H2S04],= 0.800 M, 25 "C, 460 nm.
I
5 t,min
10
Figure 3. Absorbance vs time traces: (I) [GA], = 0.0100 M, [BrO3-Io = 5.00 X IO-' M, 25 "C, 500 nm; [H2SO4I0= (a) 2.00 M, (b) 1.60 M, (c) 1.20 M, (d) 0.800 M, (e) 0.400 M; (11) [BrO3-Io= 1.25 X M, [H2S04],= 1.20 M, 25 O C , 460 nm; [GA], = (f) 0.0100 M, (g) 0.0140 M, (h) 0.0320 M.
-21-----1
I el
20
u?
10
0 0
6
12
18 t,min
24
30
Figure 2. Absorbance vs time traces: 460 nm, 25 "C, [GA], = 0.0100 M, [H2SO4Io= (a-e) 0.800 M, (f) 1.20 M; 1O3[Br0 Br- > GA > C1- and the rate of oxygen exchange of bromic acid with water is very slow. The (19) (a) Barton, A. F. M.; Wright, F. A. J . Chem. SOC.A 1968, 1747. (b) Barton, A. F. M.; Loo, B.-H. J . Chem. SOC.A 1971, 3032. (20) Simoyi, R. H.; Masvikeni, P.; Sikosana, A. J. Phys. Chem. 1986, 90, 4126. (21) (a) Young, H. A.; Bray, W. C. J . Am. Chem. SOC.1932,54,4284. (b) Bray, W. C.; Liebhafsky, H. A. J . Am. Chem. SOC.1935, 57, 51. (22) Sigalla, J. J . Chim. Phys. 1958, 55, 758. (23) Birk, J. P.; Kozub, S. G. Inorg. Chem. 1973, 12, 2460. (24) Hoering, T. C.; Butler, R. C.; McDonald, H. 0.J . Am. Chem. SOC. 1956, 78, 4829. (25) Citri, 0.;Epstein, I. R. J. Am. Chem. SOC.1986, 108, 357.
2392 The Journal of Physical Chemistry, Vol. 93, No. 6, 1989
Jwo and Chang
0
0
t,mm
Figure 13. log [Br-] vs time trace for the BrOC-NBS reaction; [H2SO4I0 = 1.20 M, 25 "C. 103[GA]o: (a) 1.60; (b) 1.80; (c) 2.00; (d) 2.20; (e) 2.40; ( f ) 2.00 M. lO*[NBS]o: (a) 1.12; (b) 1.12; (c) 1.12; (d) 1.12; (e) 1.12; (f) 0.112 M.
activation energy of the Br03-+A reaction is comparable to that of the Br03--Br- reaction. The isotope exchange of the Br03--S03z- reaction by Taubez6showed that the oxygen atoms gained by the sulfite ion came from bromate. Therefore, it is generally proposed that these redox reactions occur by a mechanism involving replacement of an oxo group prior to electron transfer, Le., via an inner-sphere process. Barton and Wrightlga reported that the Br03--I- reaction was catalyzed by acetate and other carboxylate ions. The carboxylic group of GA may also show a similar catalytic behavior. One support of this argument comes from the much slower rate of bromide ion production in the bromate-pyrogallol reaction. For [ p y r o g a l l ~ l = ] ~0.1 10 M, [BrO [GA],, [Br-1 first increases, then decreases rapidly, and then increases more slowly (Figure 13d,e). If [GA], Z[NBS],, a nearly quantitative production of Br- ion is observed (Figure 130. Therefore, we consider that the bromination of GA by HOBr is less important as compared with reaction 53. As mentioned before, both HPLC and N M R data show that the products of the GA-Br03- reaction contain one major product and at least three minor products. On the basis of the product analysis, we suggest that the possible structures for the major product are P5-P7 and P3 in Figure 14. Partial support of these compounds comes from the mass data and the observation of gas bubbles generated during the oscillations. Compounds such as P1, P2, and P4 in Figure 14 are suggested as the possible structures for the minor products. The simplified derivations of some fragments are given in the following: m / e 201 (P3-Br-0, (P7/2)-0); 199 (P3-Br-HzO, (P5/2)-C02-H, (P6/2)-COz-H); 175 ((P5/2)-Br, (P6/2)-Br); 174 ((P5/2)-HBr, (P6/2)-HBr); 17 3 (P3-Br-CO); 121 ((P5/2)-COz-Br, (P6/2)-CO2-Br, (P7/2)-HOBr); 120 ((P5/2)-COz-HBr; (P6/2)-C02-HBr); 123 (Pl-COZH, P2-OH; (P4/2)-COH); 122 (Pl-COZH-H, P2-HZ0, (P4/2)-COH-H). The mechanism shown in Scheme I is by no means complete. Reactions that explain the decarboxylation and further bromination of products must be included to rationalize more fully the experimental results. However, we do believe that additions of these reactions do not affect the main features of the oscillatory Br03--GA reaction.
Acknowledgment. We thank the National Science Council of the Republic of China for the financial support. Very valuable comments and suggestions from the reviewers are most gratefully acknowledged. Registry No. GA, 149-91-7; BrO,-, 15541-45-4.
