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The Journal of Physical Chemistry, Vol. 83, No. 23, 1979
(3) L. Davison and R. A. Graham, Phys. Rep., accepted for publication. (4) R. J. Eichelberger and G. E. Hauver in "Les Ondes De DBtonation", Editions du Centre National de la Recherche Scientifique, 15, Quai AnatoleFrance-Paris, VIP, 1962, p 363. (5) G. E. Hauver, J . Appl. Phys., 36, 2113 (1965). (6) M. de Icaza Herrera, A. Migault, and J. Jacquesson, C . R. Acad. Sci. Paris, 284, 503 (1977). (7) M. de Icaza Herrera, A. Migault, and J. Jacquesson, C . R. Acad. Sci. Paris, 264, 531 (1977). (8) M. de Icaza Herrera, Thesis, University of Poitiers, France, 1976, in French. (9) M. de Icaza Herrera, A. Migault, and J. Jacquesson in "High Pressure Science and Technology", Vol. 11, K. D. Timmerhaus and M. S. Barber, Ed., Plenum Press, New York, 1979, p 870. (10) E. Z. Novitskii, A. G. Ivanov, and N. P. Khokhlov in "Combustion and Explosion, Proceedings Third All-Union Symposium on Combustion and Explosion", Nauka, Moscow, 1971, p 579, in Russian. (11) F. E. Allison, J. Appl. Pbys., 36, 2111 (1965). (12) Y. Horie, Brit. J . Appl. Pbys. ( J . Phys. D), 1, 1183 (1968). (13) R. A. Graham, Phys. Rev. B , 6, 4779 (1972). (14) R. A. Graham, F. W. Neilson, and W. B. Benedick, J . Appl. Phys., 36, 1775 (1965). (15) Dupbnt Company, Bulletin H-2, Electrical Insulation Products Division, Dupont Film Department. (16) G. A. Bernier and D. E. Kline, J . Appl. Polym. Sci., 12, 593 (1968). (17) R. A. Graham, J . Appl. Pbys., 48, 2153 (1977). (18) R. G. McQueen, S. P. Marsh, J. W. Taylor, J. N. Fritz, and W. J. Carter in "High-Velocity Impact Phenomena", R. Kinslow, Ed., Academic Press, New York, 1970, p 293. (19) V. V. Yakushev, 0. K. Rozanov, and A. N. Dremin, Sov. Phys. JETP, 27, 213 (1968). (20) G. E. Hauver, Proc. Symp. Detonation, 5tb, 387 (1970). (21) D. Bloomquist, private communication.
Communications to the Editor (22) A. R. Champion and W. B. Benedick, Rev. Sci. Instrum., 39, 377 (1968). (23) W. J. Carter and S. P. Marsh, private communication. (24) G. E. Hauver, private communication. (25) A. R. Champion, J. Appl. Phys., 43, 2216 (1972). (26) R. A. Graham, Bull. Am. Pbys. Soc., 24, 711 (1979). (27) P. Yu. Butyagin, Russ. Chem. Rev., 40, 901 (1971). (28) A. Casale and R. S. Porter, "Polymer Stress Reactions", Voi. 1, Academic Press, New York, 1978. (29) A. Casale and R. S. Porter, "Polymer Stress Reactions", Vol. 2, Academic Press, New York, 1979. (30) P. Yu. Butyagin, A. A. Berlin, A. E. Kalmanson, and L. A. Blyumenfekl, Rubber Cbem. Techno/., 33, 942 (1960). (31) H. A. Larsen and H. G. Drickamer, J. Phys. Chem., 61, 1643 (1957). (32) G. A. Adadurov, I. M. Barkalov, V. I. Gol'danskii, A. N. Dremin, T. N. Ignatovich, A. M. Mikhailov, V. L. Tal'roze, and P. A. Yampol'skii, Polym. Sci. USSR, 7, 196 (1965). (33) L. V. Al'tshuler, I.M. Barkalov, I.N. Dulin, V. N. Zubarev, T. N. Ignatovich, and P. A. Yampol'skii, H/gbEnergy Cbem., 2, 73 (1968). (34) L. V. BabarB, S. V. Pershin, and V. V. Yakovlev in "Proceedings, Second All-Union Symposium on Combustion and Explosion", L. N. Stesik, Ed., 1971, p 305, in Russian. (35) I.Yu. Tsarevskaya, V. A. Kargin, V. N. Zubarev, V. I.Gol'danskii, P. A. Yampol'skii, and T. V. Fremel' in "Proceedings, Second AlCUnion Symposium on Combustion and Explosion", L. N. Stesik, Ed., 1971, p 301, in Russian. (36) P. A. Yampol'skii in "Proceedings, Second All-Union Symposium on Combustion and Explosion", L. N. Stesik, Ed., 1971, p 293, in Russian. (37) A. N. Dremin and 0. N. Breusov, Russ. Cbern. Rev., 37, 392 (1968). (38) G. E. Duvall and R. A. Graham, Rev. Mod. Phys., 49, 523 (1977). (39) S. Sheffield and D. Bloomquist, private communication. (40) D. Bloomquist and S. Sheffleld, private communication. (41) P. Harris, J . Appl. Phys., 36, 739 (1965).
COMMUNICATIONS TO THE EDITOR Chemical Oscillations during the Uncatalyzed Reaction of Aromatic Compounds with Bromate. 2. A Plausible Skeleton Mechanism
Sir: Koros and Orb6n1i2have recently reported oscillations when a number of aromatic compounds are oxidatively brominated by acidic bromate even without the metal ion catalysts previously thought necessary for such oscillating systems. These compounds can all be regarded as derivatives of phenol or aniline. All3 have a hydrogen attached to oxygen or nitrogen whose abstraction would generate a resonance stabilized free radical. All have a t least one free ortho position subject to bromination by Br, or HOBr. All have OH or NH, substituents and have at least one free ortho or para position so that oxidation could generate quinone or quinone imine structures. Investigations conducted to date have revealed very diverse behavior of different reacting s y ~ t e r n s . Thus, ~ various quinones, brominated derivatives, and oxidatively coupled products have been observed. However, these uncatalyzed oscillatory reactions are all controlled by bromide ion, and the critical bromide concentration is close to that observed during catalysis by metal ions.5 Although the detailed chemistry of any system will depend upon the organic substrate, we believe the oscillations observed with at least the polyphenolic compounds can be explained by the skeleton mechanism shown in Scheme I. In that scheme, HAr(OH), is an aromatic compound containing at least two phenolic groups, HAr(0H)O. is the radical obtained by hydrogen atom abstraction, HArO, is the related quinone, BrAr(OH), is the brominated derivative, and Ar2(0H)4is the coupling product. 0022-3654/79/2083-3056$01 .OO/O
Scheme I
+ Br- + 2H+ + HBrO, + HOBr HBrO, + Br- + Hf 2HOBr Br03- + HBrO, + H+ -+2Br0,. + HzO BrOz. + HAr(OH), HBrO, + HAr(0H)O. 2HBr0, Br0,- + HOBr + H+ HOBr + HAr(0H)O. + Br. + HArO, + H 2 0 Br. + HAr(0H)O. Br- + HAr02 + H+ HOBr + Br- + H+ + Br, + HzO Br2 + HAr(OH)z BrAr(OH), + Br- + H+ (K9) HOBr + HAr(OH), BrAr(OH), + H 2 0 (K10) Br0,-
-
-
-
-
-
Steps K 1 to K5 involve inorganic chemistry identical with that already demonstrated for metal-ion catalyzed bromate oscillations5 except the organic substrate replaces reduced metal ion for the 1-equiv reduction of the Br02. radical in step K4. If the system contains sufficient bromide ion, the stoichiometry of net reaction A is generated by the sequence K 1 + K2. Br0,-
+ 2Br- + 3H+
0 1979 American Chemical
Society
-
3HOBr
(A)
The Journal of Physical Chemistry, Vol. 83, No. 23, 1979 3057
Communications to the Editor
If the concentration of bromide ion is sufficiently small, the sequence K3 2(K4) will generate bromous acid autocatalytically until it approaches a steady state concentration6 of (k3/2k6)[H+][Br03-]. This concentration is about l o 5 times as large as the one when process A is dominants6 When such autocatalytic growth of H B r 0 2 occurs, residual Br- is rapidly consumed, and net process B is generated by the sequence 2(K3) + 4(K4) -C K5. Br03- + 4HAr(OH), H+ HOBr 4HAr(OH)O. + 2 H z 0 (B)
+
-+
+
Processes A and B generate the rather unstable species HOBr and HAr(0H)O. which will react further rather than accumulating. HAr(0H)O. is partly consumed in the coupling (or polymerization) reaction (see, e.g., K14). Studies of catalyzed oscillation^^^^ indicate the system will be particularly sensitive to oscillation if the products of reaction B generate almost exactly two bromide ions. The sequence K6 + K7 generates precisely this stoichiometry as shown by net reaction C. HOBr
+ 2HAr(OH)O.
-
Br-
+ 2HAr0, + H+ + HzO (C)
The standard reduction potential of HOBr to Br. is 0.70 V,9 almost exactly equal to that for the 2-equiv reduction of quinone to dihydroxybenzene. Presumably step K6 takes place by electron transfer to the protonated species H20Br+. The organic species undergo facile electron transfer as indicated by the reversibility of the quinhydrone electrode that was used to measure pH before the more convenient glass electrode was developed. Reaction C is therefore irreversible and well precedented. In catalyzed oscillators, the remaining HOBr is removed by bromination of enolizable species like malonic acid. Bromination of phenols and anilines is also well established. The stoichiometry of process D can be accomplished by the single step K10 or by the bromide-catalyzed sequence K8 + K9. HOBr + HAr(OH), BrAr(OH)2 + HzO (D)
-
The mechanism thus developed by steps K1 to K10 leads to the overall stoichiometry of reaction T which is 2Br0,-
-+
+
+
2H+ 2BrAr(OH),
4HAr02
+ 6 H z 0 (T)
+
generated by A + B + 2(C) 2(D). It is the free energy change of this irreversible process which drives the oscillations, and the stoichiometries are just those to generate an unstable steady state in a bromate driven oscillator. Although steps K1 to K10 are sufficient to account for oscillations, other elementary processes undoubtedly occur. The details of such processes will depend upon the organic substrate, but Scheme I1 contains some other steps that Scheme I1
-
+
Br.
+
BrO,. HAr(0H)O. HBrO, + HAr0, (K11) HAr(OH), Br- + HAr(0H)O. + H+ (K12) HAr(OH), + HArO, + 2HAr(OH)O- (K13) 2HAr(OH)O- ArZ(0Hl4 (K14) HArO. + HBrO, BrO,. + HArOH (K15) -+
HArO.
+ Br03-
+ H+
Ar(OHIz + BrO,. (K16)
are likely to be important. Steps K11 and K12 should be rapid and in competition with steps K4 and K7, respectively. Step K11 would reduce the amount of bromide subsequently formed by re-
action C, and step K12 would increase it. Oscillations would require approximate balance of the two effects and would help to explain why uncatalyzed oscillations occur during only part of the total course of the r e a ~ t i o n . ' ~ ~ ~ ~ The equilibrium of step K13 is well precedented. The equilibrium must lie well to the left, or direct reaction of HOBr with HAr(OH), would occur. However, equimolar mixtures of quinone and dihydroxybenzene are intensely colored, and the radical HAr(0H)O. is presumably responsible for the color changes observed during oscillations. Step K14 has been added to explain the observed coupling products and to prevent the buildup of quinone until reaction C produced bromide rapidly enough to prevent further oscillations. The mechanism developed above is applicable only to polyphenolic species and does not explain why phenol itself also generates oscillation^.^^^ Steps K15 and K16 suggest how phenol and its derivatives could be oxidized to oscillatory reactants. Hydrogen peroxide has a weak HO-OH bond and reacts by hydroxyl radical transfer with 1-equiv reducing agents such as organic radicals or ferrous ions. Bonds between oxygen and bromine are also weak, and the oxyacids of bromine should react with organic radicals by hydroxyl transfer. We have seen above that HOBr reacts by electron transfer instead, and HBrO, is always in very low concentration. However, bromic acid might well hydroxylate organic radicals by step K16 and thereby convert phenolic derivatives to dihydroxybenzene species that generate oscillations by the mechanism of Scheme I. Extension of the proposed mechanism to aniline derivatives is obvious. The quantitative modeling of any system will require detailed kinetic and spectrophotometric information not presently available. However, it appears that with minor modifications the skeleton mechanism presented here can provide a basis for qualitative modeling of all the uncatalyzed oscillator systems observed to date.1,2,4J1J2This mechanism also indicates there is no major difference in principle between the uncatalyzed bromate oscillators and the metal-ion catalyzed ones which are now well under~tood.~J" References and Notes (1) E. Koros and M. Orbln, Nature (London), 273, 371 (1978). (2) M. Orbln and E. Koros, J. Phys. Chem., 82, 1672 (1978). (3) Oscillations were observed with C6H5N(CH3),which has no such hydrogen, but in these acidic solutions it would be present as C6H5NH(CH3),+. Methyl substitution on oxygen usually generated a compound inert to oscillation. (4) M. Orbln and E. Koros, "Synergetics. Far from Equilibrium", A. Pacauk and C. Vidal, Ed., Springer-Verlag, Berlin, 1979, pp 43-46. (5) R. J. Field, E. Koros, and R. M. Noyes, J . Am. Chem. SOC.,94, 8649 (1972). (6) This argument approximates step K3 as irreversible. (7) R. J. Field and R. M. Noyes, J . Chem. Phys., 60, 1877 (1974). (8) R. J. Fieki and R. M. Noyes, Faraahy Symp. Chem. Soc., 9, 21 (1974). (9) W. M.Latimer, "Oxidation Potentials", 2nd ed.,Pretice-Hall, New York, 1952. (10) R. M. Noyes and R. J. Field, Acc. Chem. Res., 10, 273 (1977). (11) L. Kuhnert and H.Linde, Z. Chem., 17, 19 (1977). (12) L. Kuhnert, Z. Chem., 19, 63 (1979). (13) Department of Chemistry, University of Oregon, Eugene, Ore. 97403. Supported in part by the Alexander von Humboidt Stiftung and the U S . National Science Foundation. Institute of Inorganic and Analytical Chemistry L. Eotvos University
M. Orbhn" E. Koros"
H- 1443 Budapest, Hungary Max Planck Institut fur Biophysikalische Chemie 0-34 00 Gottingen-Nikolausberg Federal Republic of Germany Received June 12, 1979
R. M. Noyesls
